Diagnosed Misfires

earld1337

Driving down road. Van cuts off. Have replaced spark plugs and wires, cam sensor, and coil. Computer scan says misfire on 1 & 4. Van will crank, runs rough at low idle,
but drove 250 miles even with misfire and it ran fairly well, except on upgrades. Any suggestions on where to go now?

thecardoc3

If the pin is crimped properly to the wire at the PCM connector for coil 1&4, and you have checked the pin for fit at the PCM, then it is very likely that you do have a failed PCM based on what you have written here.

elias

Failed O2 sensor could be a factor .

thecardoc3

Misfire diagnostics require testing that confirms not just that you have spark, fuel, and compression you also have to prove that you are getting enough spark at the right time, the correct amount of fuel, and the compression has to include fresh air. (O2)

Were you using a basic code puller, or a full scan tool that will provide data stream and bi-directional controls? (Chrysler calls them ASM tests). Using a tool to test for spark like an ST-125 HEI tester (looks like a spark plug with no electrodes and has an alligator clip to ground it) you can prove if the ignition system can put out the 25,000v required to fire a spark plug while its in the cylinder under compression. It only takes about 800v to fire a plug outside of the cylinder, or from a plug wire to a metal surface and that often tricks people into thinking that they have spark when they really don't. If cylinders #1 and #4 aren't firing it is notable that they are controlled by the same ignition coil which makes a loss of spark to them more likely. The coil is driven by the PCM (fuel injection computer) which is why the ASM tests become important. If you can prove that you have lost spark to those cylinders, then the ASM test allows for pinpoint testing of the coil command circuit to prove why.

earld1337

The van has 274K miles and the front three cylinders has approx. 150psi with the throttle open, 125 psi with the throttle closed.

Using both the original coil and a new coil, at the connector I have gotten a solid light at pin 1, flashing light at pins 2 & 3, solid light at pin 4. I have found two conflicting schematics, one showing 3 as common and the other showing 4 as common. Since 3 flashes and 4 does not, I made the assumption that 4 is actually the common.

Cylinder break down appears to be pin 1 cylinders 1 & 4, pin 2 2 & 5, and pin 3 3&6.

Since the code thrown was misfires on 1 & 4, and pin 1 being a solid light, I took it that pin 1/wire was the problem. I traced the wire back to the PCM and the light was still solid.

I know of nothing else to test and now think the PCM is the problem. I did replace the ASD and fuel pump relays.

If anyone has additional tests or thoughts, your feed back would be appreciated.

thecardoc3

What year and engine? The ASM relay didn't need to be replaced, it also powers up the fuel pump and since the van ran that proved the relay was operating.
The ignition coils are driven by the PCM as you have seen, that's why the ASM tests are important. You get to test the operation of the ignition system without having to crank the engine. To generate spark the PCM grounds the coil negative lead for 6-8ms and then opens the circuit. You should have power to one terminal of the coil connector and the PCM then grounds one the other three when it is time to fire that coil. All you need to do is disconnect the coil connector and you would see power only one of the wires when the ASD relay is energized.

earld1337

thecardoc3, thanks for the comments. 1996 Chrysler Town and Country 3.3

I am not a mechanic in any form or fashion. I replaced the ASD as that was suggested in another forum where I asked this question. I have done as you stated and the #4 pin receptor is the power to the coil. All of the pins, except #1 flash when tested, #1 is a solid light. #1 controls cylinders 1 & 4 which are the cylinders the misfire is coding on. If pin 2 and 3 which controls cylinders 2 & 5 and 3 & 6, respectively, flash and pin 1, which controls cylinders 1 & 4 does not flash, and the scan shows the misfire at 1 & 4, to me it follows that the solid light on pin 1 does indicate the problem.

I tracked the wire for pin 1 back to the PCM and it is a also a solid light at the PCM. I ohms tested the wire from the PCM back to the connector on the coil and there is no breaks in the wire indicated.

I see no other alternative than to think I have a PCM problem, but was just wondering if any experts out there might have a different opinion.

Thanks

earld1337

Since my last post and prior to buying a PCM, which seems to be the consensus on my problem, I did the following:

1. Removed the plenum in order to have better access to the wiring harness which runs from the coil, under the plenum, and plugs into a connection with wires after the connection joining up with other wires and ends at the PCM.

2. The wiring harness is nearly 20 years old. It was very hard and the wires within the harness were stuck together - from age and heat.

3. I carefully separated all the wires and then checked each for any tears or abrasions. I found two wires, which coincidentally were also two of the wires which run to the coil connector which had visible wires showing through the insulation. I taped both with tape.

4. I then checked the four wires which run to the coil connector for resistance and there was not indication of a break.

5. I put everything back together and tried to crank the van. It will not start.

6. Prior to taking the above steps, checking the wires at the coil connector with a test light by turning the engine over, the four wires did the following:
Pin 1 was a solid light - this is the pin which controls cylinders 1 & 4 which are the coded misfires.
Pin 2 was a flashing light - controls cylinders 2 & 5.
Pin 3 was a flashing light - controls cylinders 3 & 6.

7. After taking the above steps, checking the wires at the coil connector with a test light by turning the engine over, the four wires did the following:
All four pins - nothing, like there is no power

However, if the four wires are jumped as above, except the ignition is just turned to on and the engine is not cranked - all four lights initially light but then go out.

I'm not sure what this means, but it would appear the PCM (perhaps) is sending some type signal but then does nothing further.

I did put a spark plug on a coil post and cranked the engine - no spark.

Any observation would be appreciated.