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Chevrolet/Geo Metro



  • 1996 1.0 5 spd
    Been on this one for months- starts fine but can't give any throttle before it warms up, or it floods out.
    Checked the voltage at the Coolant Temperature Sensor by backprobing.
    It has three wires:
    Light Green/Black, which is common and goes to the MAP, TPS, and IAT. Gray/White, which is the signal to the ECM.
    Yellow/White, which is not shown on wiring diagram, but I assume is the input from the ECM.
    LtGr/Bk is ground- no voltage.
    Gy/Wt reads 2.34V when cold, decreases with rising engine temp and finally goes to 0.5V when hot, at which point the cooling fan kicks on and takes it back up to around 0.65V. (With two different CTS units- same within .2V)
    At anything over 1.0V, the engine will flood and stall if the throttle is opened even slightly.
    These values seem reasonable if the temperature was around zero- but it's 80.
    What does not seem reasonable is the voltage on the Yellow/White wire which I believe should be the standard 5V input from the ECM, but reads a steady 9.2V, regardless of temperature and even when not running with the ignition on. If the input voltage is nearly twice what it should be, then the CTS could have exactly the correct impedence, but the output signal would be much higher than the ECM expects, and it would compensate with a mixture richer than Bill Gates.
    Is it possible that something is creating a field that the wire is having additional current induced in it somehow? My DMM read nearly a volt with the positive probe held in the air near (2 inches) the sensor wires.
    I hope this is the gremlin I have been looking for since April, but I don't have a clue how it could get 9.2V.
    Maybe something a little over 12v, yes- but not 9.
    Anyone see this before?
    Hints? My crystal ball keeps saying to try again later....
  • zaken1zaken1 Posts: 556
    Hi there: Since the amount of change you see in the CTS is in a range which seems reasonable; and the CTS ground is common with the TPS, IAT, and MAP; it would seem likely that the problem may be in one or more of those other devices; and not in the CTS. Also, I would not pay too much attention to the voltage reading you get on the Yellow/White wire; as the no load reading from a power source, when measured with a high impedance meter, may be greatly different from the voltage the computer 'sees' from that source. This is because your meter is grounded to the engine; which is electrically connected in common with the alternator, battery, and ignition system. The 5 volt signal from the computer, however, is a closed loop which is isolated from those other components. As an example, I have a plug in power supply for a phone answering machine; which has a rated output of 9 volts DC. But when I measure the open circuit output voltage from that power supply with my DMM; it reads something like 13 volts!!! However; as soon as it is connected to the designed load; that voltage drops to 9 volts. Because of this; plus all the inductive input and transients the meter will pick up; I would only become concerned about the supply voltage if you read more than 5 volts across a given sensor; rather than measuring voltage between one side of the sensor and an engine ground.

    But there is one other factor here which proves this point: The CTS resistance is highest when it is cold, and drops as the engine warms up. This means that the voltage signal the computer "sees" from the sensor is LOWEST when the engine is cold; and becomes higher as the engine warms up. So if your supply voltage was too high, the computer would get a signal that the engine was hotter than it really is. And under that condition, the engine would not go rich when throttle was applied. Instead, it would go too lean. But you claim the engine is flooding.

    Actually, I believe you are confusing the behavior of a flooding engine with one that is going too lean and starving for fuel. An engine which is too rich is typically insensitive to small changes in throttle opening; and will easily accept sudden openings in throttle position. A rich engine will also run much better when it is cold than it does when it is warmed up.

    On the other hand; an engine which is too lean will stall out if the throttle is opened when cold; but will run much better after the engine temperature comes up to normal. So I believe this is what your engine has been doing. Hopefully this will give you a better sense of where to look for the source of the problem.

    Here's a simple test you can do to prove this: Disconnect the plug to the IAT sensor, and then try running the motor. I bet you'll find that the cold stalling is gone. The IAT sensor is just like the CTS. Its resistance is highest when the air temperature is cold. So when you disconnect the IAT plug; the computer thinks the air temperature is below zero; and richens up the mixture.
  • zaken1zaken1 Posts: 556
    One other thing could also be going on here; if you don't see the oil actually coming from the valve cover, but can see it running down the engine below the distributor; chances are the oil is coming from the distributor housing rather than the valve cover. There is an o-ring on the distributor body which can become damaged or disintegrate over time, and a torn gasket between the distributor mounting flange and the cylinder head can also cause an oil leak.
  • Uh oh.
    The voltage most definately decreases with increasing temperature, which means the resistance is increasing. WTF? How could both my sensors work backwards?

    And the fan comes on when the voltage hits half a volt- shouldn't it be coming on with some increased voltage? If I disconnect the CTS the fan comes on, which implies that a zero volts signal received triggers the fan circuit.

    Tried disconnecting the IAT- no change.
    (It is receiving an index of 4.95V from the ECM)

    Tried measuring the voltages accross the signal and common ground wires, rather than grounding to the battery- same.
    It read 2.65V after sitting all night, and decreased as the engine warmed to a minimum of 0.50V, at which point the cooling fan came on. Maybe I drew the wrong conclusion from results, in that the signal of 0.5V is interpreted as zero, the same as if the sensor was disconnected, and the ECM thinks it's a malfunctioning sensor and defaults to cooling fan ON?

    I'll go and get another sensor today and see what it does. I could swear that when I tested the impedence on that unit it decreased with heat.
    I am disturbed that the wiring diagrams only show two wires, rather than the three I have. It implies other info may not be precise, either.

    Anyone with an FSM for 1996 have a table of voltages through a temperature range?

    My head hurts. Feels like I am dealing with things that violate accepted laws of physics- which implies I may be wigging out in a rather interesting fashion....
  • zaken1zaken1 Posts: 556
    I have seen temperature/resistance charts for those sensors; which are in the FSM. And they show a resistance of many thousand ohms (perhaps 5,000 to 15,000 or more) when the sensor is cold; decreasing to a few hundred when the sensor is hot. This is true for both the CTS and the IAT sensor.

    In view of this; it sounds to me like you are interpreting the voltage drop ACROSS the sensor as the amount of signal the computer sees. But this is just the opposite of what is happening. The higher the resistance of the sensor is at any time; the more the voltage will be dropped across the sensor. And the more voltage that is dropped across the sensor; the lower the voltage will be that reaches the computer.

    Think of it this way: If there is a 5.0 volt supply to a sensor circuit, and the sensor has enough resistance to drop 3.5 volts across it; the computer will only see the remaining 1.5 volts. IT WILL NOT SEE THE 3.5 VOLTS YOU ARE MEASURING ACROSS THE SENSOR. Similarly, the lower the voltage drop there is across a sensor; the higher the signal voltage will be at the computer. We know this much; sensor resistance decreases with heat; which will produce less voltage drop across the sensor. So if you are measuring lower voltage at the sensor as the engine warms up; you must be measuring the voltage ACROSS the sensor; rather than the voltage in series with the sensor. And that is the source of your headache.

    Furthermore, I believe the engine temperature sensing circuit is connected in SERIES through the CTS and IAT; rather than in parallel. I also believe the engine load sensing circuit is also connected in series through the TPS and MAP. And this would further confound your attempts to measure the circuit dynamics.

    But I see one thing that is not right in what you measured: Disconnecting the IAT should NOT have resulted in no change. If it did; this indicates either a short across the IAT sensor, or else the computer was not responding to the change in IAT signal, because it was in "limp in" mode as the result of a trouble code having been set. Does your "check engine" light come on when you first turn the key on, and does it then go out after the motor is started? If it doesn't come on at all; or if it stays on all the time; then the computer is in limp in mode, and will not respond normally to sensor inputs. In this case, you'll need to clear the trouble codes with a code scanner (assuming your 1996 model has an OBD II engine control system) Trouble codes can only be cleared by disconnecting the battery if you have the earlier OBD 1 system. And you can't tune the motor when it has a trouble code set in the computer. I know scanners are expensive; but there is no easy way around this. Actually, some people say that turning the ignition key on and off about 15 times in succession will clear OBD II trouble codes.
  • Pulled the CTS and put it in the freezer- resistance at 0 degrees F was around 8000 Ohms. Put it in hot water and watched it fall to233 Ohms at 212 degrees F. So it is working properly. It could be the thermister is betwwen the signal to the ECM and ground, which would make the voltage decrease with the resistance, but I would be seeing the opposite.
    Fine. At last some logic in the chaos.
    In looking at the wiring diagram i see the O2, MAP, IAT, TPS are all in fact connected to the CTS.
    So how is one to correctly troubleshoot these components? AAAARRRGGGHH.
    Removing the IAT had no effect because (as you pointed out) it had set a fail code when I disconnected the CTS, and was in limp mode.
    (BTW) I've found that an easy way to clear the codes is to first disconnect the negative battery cable, then ground the positive cable to the chassis for a few seconds, thus draining the various capacitors- including those pesky ones in the airbag system. Beats a trip to O'Reilly to borrow their OBDII reader.
    (Personally, I think they should be built in with readouts available in real time)
    Back at it tomorrow, with a new TPS which seems to have a much better and smoother resistance curve than the last two. Got my fingers crossed. Again.
    Thanks Zaken! I thought I was witnessing a miracle gone horribly wrong....
  • zaken1zaken1 Posts: 556
    FYI; the only way to conclusively test sensors is to pull them and test them individually, as you did with the CTS.

    Now, back to today's problem; if I were working on a vehicle that acted the way you describe, I would begin with the simple and direct approach: The symptoms you described sound just like what happens whan a TPS is adjusted too lean (set too far counterclockwise). So, before doing anything else, I would loosen the two mounting screws; and turn the damn thing clockwise until there was a noticeable improvement in the running. Forget the adustment specs; those specs are only valid when the engine is all new and stock. As soon as the compression changes, or any of 1,000 other factors changes; the TPS setting goes off. I have had to reset mine all over the range, after changing other things on the motor. And it ran great in all sorts of different settings, when they were appropriate.

    It would only be if resetting the TPS as far as possible did not correct the problem, that I would start thinking about defective sensors or other exotic issues.

    If you set the TPS too rich; the consequence will be that the fuel cut does not come in during decelleration. And that is fairly obvious, as you slow to a stop in gear. Normally, the fuel should come back on as you get down below about 10 mph; and the car will feel different when that happens. But the other consequence of too rich a TPS setting is that the fuel mileage drops severely; and the exhaust pipe becomes wet and sooty.

    One owner wrote in here some time ago; complaining that his exhaust pipe always looked rich. So I finally helped him to overcome his fears about disturbing the TPS setting; and explained that it has to be done by the "seat of the pants." A few weeks later, he wrote in to thank me for going against what everybody else had said, and reported that it took a series of tries and road tests before he finally got it right; but his car now is peppier than ever, the pipe is brown and clean, and the fuel economy is great.
  • TPS settings don't have any effect on the stalling problem- only warming the engine will allow it to run.
    I connected a known good CTS and immersed it in boiling water, thinking to imitate a warm engine and started it. Died as soon as I opened the throttle. So the coolant temerature sensor or it's circuit is not the culprit.

    Okay, what else is temperature sensitive?
    What about the Early Fuel Evap Heater (EFE) in the plate under the Throttle Body?
    Fuel system component, temperature related, has a 30A fuse and a relay.....
    It's on the car, in the wiring digram, and utterly without mention in Chilton or Haynes. AllData says it preheats the Fuel/Air and reduces the time in open loop, until engine reaches operating temperature when it is deenergized by the ECM.
    Resistance should be 0.5 to 3.0 Ohms. (seems low for a heater) Checks at 2.6 Ohms across the leads, and open from either lead to battery ground.
    Connect unit and measure voltage: Voltage coming in while running is 13.4V with a drop to 1.2V on the ground side. (12V drop) Tht's a heckuva drop for 3 Ohms. Sounds reasonable that it might be the culprit.
  • zaken1zaken1 Posts: 556
    Well; it sounds like the time for a little more education: The supply voltage in any series electrical circuit will ALL be dropped across the sum total of the resistances of the loads in that circuit; regardless of how much or how little resistance there is in the circuit. The amount of voltage drop across any single element will be proportional to the percentage of the total circuit resistance which it constitutes. Regardless of whether there is one, or six, or one hundred resistive elements in a 12 volt circuit; the entire 12 volts will always be dropped across the whole circuit. Suppose there are six resistive elements in a particular series circuit. If each of those six loads have the same amount of resistance; then each element will drop 1/6 of the total circuit supply voltage (which works out to 2 volts drop across each element). If there is only one resistive element in the circuit; then the entire 12 volt supply voltage will be dropped across that one element (regardless of whether the resistance of that element is 1/10 ohm, 5 ohms, or 10,000 ohms). If there are unequal resistances in a series circuit; then the supply voltage will be dropped across those loads in proportion to their resistances: Suppose there is a 5 ohm and a 10 ohm load in series with a 12 volt supply. In this circuit, the total circuit resistance is 15 ohms. The 5 ohm load makes up 1/3 of the 15 ohm total circuit resistance; so it will drop 1/3 of the supply voltage (which works out to 4 volts). The 10 ohm load makes up 2/3 of the 15 ohm total circuit resistance; so it will drop 2/3 of the supply voltage (which works out to 8 volts). 4 volts + 8 volts = 12 volts; which makes this a legally functioning circuit, because the entire supply voltage is used up.

    I hope you can see from this that the voltage drop across a 3 ohm load has NOTHING to do with the amount of resistance of that load. It is solely determined by how much ADDITIONAL resistance is in series in that circuit. If the 3 ohm heater is the only resistance in that circuit (which is what I would expect in a heating circuit) then the entire 12 volt supply (minus what is dropped across the plugs and wiring harness) will be dropped across that 3 ohm resistance. Applying Ohm's Law (Current = Voltage divided by Resistance); 12 volts / 3 ohms = 4 amps. 4 amps current at 12 volts is 48 watts of power (Power = Voltage X Current). 48 watts of power is about what a single 12 volt headlight draws. That is not at all unreasonable for a heater that has to bring a steady stream of fuel and air up to a temperature high enough to vaporize the fuel. It probably sounds high to you because most other current draws in automotive electronics are much smaller than that. But I expect the rear window defroster grid draws at least that much. And the starter motor draws 20 to 30 times that much!!! So your test of the EFE heater tells me that it is working right.

    I appreciate your perception that this appears to be a temperature sensitive problem; but I assure you that the root cause will turn out not to be someting directly related to temperature, and will instead turn out to be related to excess leanness, or sudden changes in air/fuel ratio when the throttle is opened. I'm glad you tested the TPS and eliminated that as a potential cause. I can accept that; and this information helps narrow down the focus.

    What it now sounds like to me is that the EGR valve is opening too soon, or too suddenly, when the throttle is opened. This would happen if one or more of the EGR vacuum control devices were bypassed or missing. (there should be an exhaust back pressure tradsducer and an electrical vacuum switching solenoid in the vacuum lines to the EGR valve). It would also happen if the ignition timing had been adjusted without first disabling the electronic advance by shorting the check connector wires.
  • zaken1zaken1 Posts: 556
    In addition to improper EGR valve regulation (the EGR valve essentially acts as a massive timed vacuum leak into the intake manifold), two other vacuum related issues would be a vacuum leak in the hose to the brake booster from the intake manifold, or the use of the wrong model PCV valve, or a PCV valve that was not plugged into its socket. An additional EGR issue could come from using manifold vacuum, or the wrong throttle body vacuum spigot, to supply the EGR circuit (it should come from the spigot on the front of the throttle body which is closest to the passenger side). A purge circuit issue could be caused by using manifold vacuum or the EGR spigot to supply the purge control valve (it should come from the port on the front of the throttle body which is closest to the driver's side).

    You can eliminate the EGR valve as a factor by simply disconnecting and plugging its vacuum hose; and seeing if that changes the stalling behavior. However; the EGR valve may also be sticking partly open; and may thus never close fully. And that would continue whether the vacuum hose was connected or plugged. It should be possible to reach underneath the valve actuating diaphragm, and manually raise and release it to see whether the spring returns it all the way down. If you manually raise the valve while the motor is cold and idling; it should make it stall.

    And if none of these things work, it is probably time to reset the dreaded idle air bypass.
  • 96 Metro 1.0/5spd
    Check engine light flashing occasionally while driving. Flashes for a couple minutes, then reverts to steady on for a while, then starts flashing again.
    Does this indicate it is busy setting a code? Is the brainbox freaking out?
    I'm currently getting a fairly frequent #1 cylinder misfire code, but I've never seen the light flash while driving before.
    Don't find any mention of this in forum archives or in manuals.

    Currently have bad compression in #1, and am waiting on parts to arrive to do a ring and valve job. Hopefully I won't have to replace the ECU too.
  • zaken1zaken1 Posts: 556
    My check engine light has flashed on occasion; when the air/fuel mixture was too lean in one particular load range; but was OK at other loads. What you describe sounds pretty normal; particularly considering the low # 1 cyl compression. I would expect it to go away after the compression is fixed. I seriously doubt that there is a problem with the computer.
  • zaken1zaken1 Posts: 556
    I just wanted to add that when one of these motors reaches the point where it is leaking compression past the pistons; just replacing the rings is often not enough to fix it. The reason for this is that a major part of the leakage takes place from the ring gooves in the piston becoming enlarged by wear. And that leads to compression leakage on the inside edge of the rings, through the ring grooves. In addition, when the ring grooves become worn; the increased clearance erodes the support the grooves normally provide to the rings; which allows the rings to twist and flex. And that raises havoc with the seal between the rings and the cylinder wall. So the right way to fix the problem is to replace both the rings and pistons; once it has been determined that the cylinder bores are still straight enough to support new pistons.

    These little motors make much more economy and power for their size than an old Ford flathead or Chevy 6; and as a result they are far more critical and demanding about precision in mechanical tolerances and tuning adjustments.
  • 96 1.0/5
    Valve job, rings, bearings. Compression test shows 200 in all three.
    Replaced: MAT, Distributor, Coil, Cap/rotor/wires/plugs, PCV and
    Throttle body with TPS, Injector, Fuel Pressure Regulator, ISM.
    Thoroughly clead all carbon from pistons and EGR tube in intake manifold.

    Set a code for EGR flow after 40 miles- had the vacuum lines reversed on the modulator (the label on mine was gone). Just to be sure, I replaced all the vacuum lines, the modulator, and both the solenoid valves.

    Runs great, except for two problems:
    1. Cold starts are the same as on the last engine/TB, etc- will not accept ANY throttle without dying until warmed up. ( I now suspect either the TPS, which is a three wire without an idle switch in it) or the fuel vapor purge system, which I find difficult to fathom. I just don't want another burned valve in a couple thousand miles....

    2. Air in the cooling system- this one REALLY scares me, as I don't seem to be able to clear it out. It will idle warm with an occasional bubble in the radiator filler neck, but when I race the engine, I get lots of bubbles, with an occasional surge- as if there were a pocket of steam forming somewhere and suddenly venting with the increased flow. The bubbles do not smell like exhaust, and there is no greasy film on top of the water (haven't put antifreeze in yet, as I don't want to waste it).
    Could this be a failing water pump, or is it more likely to be a cracked block?

    Is there a flow spec for the water pump? Some kind of sniffer for the cooling system?
  • zaken1zaken1 Posts: 556
    You say you replaced all those parts; but were they new or used replacements? If they were used; there might be compatability issues. And even if they were new, there still might have been errors in listings or model selection. There were significant changes in the TPS and distributor over the years. I tend to feel that you still don't have the right TPS. Also; the distributor for the 1996 model should not have a vacuum advance. If it is the right type (made for electronic advance, rather than vacuum advance) the timing MUST be set with a strobe while the check connector terminals are shorted to disable the electronic advance. Otherwise your timing will end up too far retarded.

    Does your engine have an electrical EGR vacuum switching valve unit in series with the exhaust back pressure transducer? If the VSV is not in the circuit; the EGR valve will get far too much vacuum; which will create all sorts of undesirable leanness. Do you have a vacuum hose routing diagram, so you can be sure the hoses are correctly routed?

    Regarding the evap cannister purge; the purge control valve on top of the cannister (at least on the earlier models) is activated by a vacuum signal from the purge port on the throttle body. There is some sort of temperature sensing "wax valve" plumbed into that vacuum line; which bleeds air into the signal line when the engine is cold, to reduce the strength of the vacuum signal to the purge control valve. If that air bleed function is defeated; it would result in the purge valve opening when the engine is cold; which would cause severe hesitation.

    On the earlier throttle bodies; there are three vacuum spigots, coming from ports which are all located at slightly different heights. The spigot closest to the distributor is intended for the purge system. The port which supplies that spigot is the highest of the three on the throttle body wall; which means it will be the last one to supply vacuum as the throttle is opened. The spigot furthest from the distributor is used to supply the EGR vacuum signal. It is the lowest (earliest opening) of the three ports. And the spigot in the middle was used to supply vacuum to the distributor advance (on those cars with vacuum advances). That spigot is typically capped on cars with electronic spark advance. If you get vacuum from the wrong spigot; it can really mess up the engine operation, particularly with regard to setting EGR codes.

    Have you tried setting the heater temperature valve control to maximum heat; in order to purge all trapped air from the heater core? And do you have a thermostat installed in the cooling system? Running without a thermostat can create weird imbalances in coolant flow. You can check for pressure build up in the cooling system with a radiator pressure tester. Install the tester in the filller neck, and pump up just enough pressure to lift the needle off the peg. Then start the engine while it is cold; and watch the pressure gauge. There should be no significant increase in pressure for at least the first 5 minutes of running (or until the temperature gauge reaches normal and the thermostat opens).
  • Most of the RPL parts were used, but bench tested okay.
    All of the TPS units (5) are the same- 3 wires, same as my harness.
    Input Voltage from ECU, Signal to CPU, and ground. Signal always goes through the potentiometer.

    Distributor is correct- with no vacuum advance and the pickiup coil bench tests okay.

    EGR is a bit different: Vacuum from TB base goes to a VSV, which goes to the "P" side of the modulator. The "Q" side of the modulator goes to a second VSV, which controls actuating vacuum from another spigot on the TB, which is connected to the
    EGR through this second VSV. All connections are true to the diagram on the hood.

    The third and lowest spigot goes to the purge system. I think I will try plugging the line to the purge system and see what it does to a cold start.

    I have a new thermostat which functions as expected, and have "burped the system from the high point on the back of the TB. Spot on with the pressure check- I hadn't thought about it in that way. If the engine is cold, there should be no increase in pressure until the coolant starts warming up, so an increase would have to come from combustion gasses leaking into it. I sure hope not- I really don't want to tear the whole thing down again. Sigh. At least I have a couple other blocks to start with, and might as well jump on whichever one has cylinders in better shape. (BTW, the piston ring grooves appeared to be undamaged, although I didn't bother to mike them- I was hoping for a quick 'n' dirty rehabilitation, not a perfect remanufacture)

    Thanks for the good advice....
  • I have a 1996 geo metro I am restoring and the vacuum hoses are missing. Its a 1.3 liter. Does anyone have a diagram of these hoses?
  • I have 95 metro 1.0 engine with manual 5 speed it starts and runs as long as i give it gas and drives normally. just replaced fuel regulator had head done a few weeks ago ran fine for a week then this O2sensor gave readings of .4- to 1.2 volts seams a little high since most i read say up to 1 volt TPS was smooth resistance from bottom of reading to top dont remember what the readings where exactly idle control motor is working after took it off and cleaned it and surrounding stuff. also the fuel milage has dropped 5-7 mpg since head gasket was done not so much worried about that as the wont idle (dies every time come to stop sign) since i drive 55 miles to work then same home kind of pain driving it this way i am new here so any help would be appreceated()
  • sorry i ran everything togther-- still new to forums
  • zaken1zaken1 Posts: 556
    Sounds like you have a bad vacuum leak somewhere, or your vacuum hoses are not connected properly; or you have a fouled spark plug or bad plug wire or defective distributor cap; or the ignition timing has gone off; or have lost compression in one or more cylinders. Sorry I can't be more specific.
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