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Chevrolet Blazer Heating and Air Conditioning

Just changed the water pump and the thermostat. Did step by step in accordance with a Chiltons back yard mechanic book. Got all put back together (with no spare parts) was connecting the negative battery cable back up and there was much clicking coming from the passenger side firewall area. Started the vehicle, let it warm up, put the air on max hot and fan speed hi, checked for leaks (none noted) and took it for a test drive. Air exiting the ventilation got extremely hot. Turned the climate control down to 66 (external ambient air temp was 53) and there was no change in ventilation air. Cycled the air positions (floor/dash vents, floor/defrost, floor, dash vents etc.) and cycled the fan speed. No change in temperature. Returned to house, left engine running, checked for leaks (none noted) and turned off the engine. Left it off for 5 minutes, started it back up, turned on ventilation and extremely hot air again, then it went to a cooler temp and right back to desert hot again. There is a clicking or popping type noise behind the dash somewhere. Any suggestions to what the problem may be? Is this a dealer service only fix? In advance thanks for any posts. 4.3 liter 4WD 121000 Miles, 24000 miles with brand new GM motor.


  • Is this an LT with auto temp (temps marked on dials and auto positions)? If so theres an electric blend motor that can fail and the noise is it searching around for temp. Rotate the dails a bunch of times and hope its buildup crap on them and they clean off, if not plastic gear maybe stripped - use the dealer for this small shops do not understand this one and charge tons while dealer sees them alot. If standard controls check the vac line under hood drivers side that ends in a plastic line that runs to the control head under dash but of thats bad default is defrost.
  • Yes, this is a LT. Will try your advice this evening. Thank your very much for your time and assistance. Will post results.
  • I also have an LT which does the same thing but if I turn the dial all the way to cold and the fan all the way up it finally kicks cold air out. When I run the heater it is fine but when I turn everything off it still kicks hot air out. Although I do not have the clicking or popping noise it still will not stop when I turn the heat off. I occasionally here a water sound like coolant is still coming into the heater core. Do you think it is similar to SSABIE? thanks in advance.
  • Yes, and you also have a partially clogged heater core with the classic noise - air in the system can also make the noise so check that the lower intake gasket is not leaking.
  • stobarstobar Posts: 110
    it is blowing blows cold aire and the temperature gauge keeps going up and down. what the heck is going on?
  • Thermostat probably bad. Time for a radiator flush, new thermostat and Stant cap. Is the fan clutch OK (sounds like a jet if stuck on).
  • rev7rev7 Posts: 4
    My 99 Blazer LS won't blow air out of the floor vents. When you put the selector to floor it defaults to the defrost. The defrost works well, the dash vents works well, ac blows cold and the heat blows hot. I had the pressure checked and it is within range.

    It's pretty cold here in Iowa and we could use some heat blowing on our feet. Any help would be great.

    I'm not for sure if there is a switch or a lever that moves the door that directs the air flow. I'm guessing that I have to take the dash apart to fix this - Is that something that is somewhat easy to do? Or is this going to cost me alot to have some shop fix it.
  • repairdogrepairdog Posts: 948
    Default for a bad vac line is defrost so open up the hood and on the drivers side check the rubber vac lines (5/32") from the top of the engine by the PCV over to the cannister and the other assorted ones and replace the rotten out ones - the final end one in the series is the plastic one to the heater selector. Probably will have to use some pliers to grab them and they are a black mess due to rot. The cannister one is often bad too and with alittle time a new piece can be put on (its in the fender and a feel to fit job).
  • I've already replaced the thermostat, which made no difference.

    The car blows cold air until the temperature gauge gets up around 260, then suddenly drops down below 200.

    Would my next step be to flush the cooling system? I don't see why it would be running hot though.

  • repairdogrepairdog Posts: 948
    Hope it wasn't going to 260 before you did the thermostat! How about a new Stant cap - appears that the old one is not holding pressure. Then could be a clogged radiator. Dex cool should be flushed and changed 3/36 and that GM 5/100 info was crap. Air in the system would be another reason for a temp buildup as steam was hitting the temp sender mounted on the drivers side head between the rear 2 plugs. Make sure the heater core flows and see if the 2 hoses entering the firewall are hot and squeeze and see if water flowing. Sometimes due to the high mounting of the heater core air gets trapped in it and you hear gurggling.
  • it was staying right around 200 before this week, when i noticed the problem and replaced the thermostat. i'll go ahead and replace the radiator cap as well, and if that doesn't help then i'll look into flushing the radiator/cooling system.

    Thanks for the help.
  • I have a 94 S10 Blazer and it stopped throwing heat replaced the control panel assembly (used) and just replaced the heater blower motor the motor engages and runs but still not blowing like it should..the only time you can feel some of the heat coming in is from the force of the air going down the idea what to do HELP ANYONE!
  • repairdogrepairdog Posts: 948
    Heater core could be clogged (flush it), the resistor that runs the fan speed could be bad (believe up on underhood housing so check it replace if necessary), or the control door (heat vs air etc) not moving so check that and the vac lines underhood drivers side - if defrost only thats default for no vac - look behind glove box for the door vac switch. This all assumes engine temp running at normal 195 so good thermostat.
  • Ok...So I've got no heat blowing into my cab, only cold air, when temperature gauge is hot, and even colder air when the temperature gauge is cold. I've replaced the thermostat and flushed the entire coolant system. Flushed the heater core and there were no restrictions in the flow of water. In-feed to the heater core is hot and the outlet is significantly cooler. Where should I start to look from this point?
  • jlflemmonsjlflemmons Posts: 2,242
    The baffle door in the duct work. Sounds like it is stuck in the "all cold" position. It is basically just a door that mixes the air coming through the heater core with the air coming through from outside.
  • Do you have an LT with auto climate? If so luckly you also has an electric blend door that has a plastic gear that often strips out. Get the dealer to look at it - they do these and will actually be cheaper.
  • I do have an LT, however it does not have the autoclimate. As for the baffle door, is there an easier approach to getting access tot his without having to rip out the entire dash. The Haynes book that I currently have has been useless.
  • on this topic...

    mine is a 2001 Blazer and recently it started blowing a smell of coolant into the cab. worse than that - more like CROTCH and coolant. its disgusting. later it started blowing really humid, damp air into the cab - so much so that i could hardly see out of REALLY foggy windows. its winter, so i'm using the heat. but get this, the heat works. very well in fact. if i turn it over to a cooler temperature, mist starts coming out of the vents.

    just to get around, i'm riding with all windows cracked to keep the humidity in the cab down.

    everyone i've talked to says its the heater core. just got back from a dealership who ran diagnostics and gave me an estimate. it says that an intake gasket leaking coolant is causing heater core failure. calls for intake gasket replacement (59.83 parts and 492.00 labor), heater core replacement (137.03 parts and 492.00 labor).

    my thing is (and i'm probably really mistaken here) that the heater core is obviously working if i'm getting heat. so why does the core have to be replaced? fix the leak, replace the intake gaskets really should be all thats needed here, right?
  • Does anyone know if a 2001 GMC Jimmy has a Heater Control Valve? If it does, could someone point me to where it might be located. If there isn't one...I have no idea why I'm not getting any heat. The flapper that controls the incoming works fine, the thermostat was changed, the radiator cap was changed, the heater core was flushed and then the entire system was flushed, all debris has been cleared away from the air there anything else that I'm missing? I guess I'll have to cave and take it in to get it looke at...Ugh!
  • Yes, thru the glove box you can get to that blend door.
  • No valve - the coolant always flows thru the core (if not clogged). Is the system in defrost cause thats default with a rotten vac line. Then if Auto climate add the electric blend door thats got a palestic gear.
  • OK - go but a 3/4 to 5/8" adapter at the auto store they all sell them about $3 and pull the heater hoses at the firewall and bypass it - no heat of couse until the core is done and yes they gave you the going price. If you are not going to fix now flush that leaky core with water so the smell will lessen. Then refill the reservior and see if it remains stable. Refill the radiator and the res when cold. Fill res to the hot line and check at same temp everyday to see.

    If it drops and the hoses/water pump do not leak, and lifters tap on startup, most likely the lower intake and again thats the current rate.

    Dex cool requires changing 3/36 and the GM BS of 5/100 causes this. Air in the system creates a mess and the wrong mix (50% correct) causes a voltage diff that eats the core. The GM cap was crap design also so get a Stant plus change the thermostat at the same time. Hope the waterpump doesn't start to leak soon too as all the crap in the system often gets in the seals and bearing.
  • Hey. I have a 98 Blazer that won't blow hot air. Changed heater core, thermostat, flushed the radiator. Now tore out the dash and replaced the actuator only that isn't the problem. The flapper sticks. Any advice on how to fix this problem? Thanks.
  • When you had the heater box apart would have been the time assuming you reinstalled it correctly. You should be able to move it thru the glove box. If in defrost thats the lose of vac default so look under hood at the drivers side last plastic line in the rubber series and see if all 5/32 lines are good to it. If an LT with climate control (temps on dials and auto postions) also have the electric blend motor that is another door.
  • Thanks! Got it fixed now!
  • delbiagedelbiage Posts: 1
    I think I might be having a similar problem.

    I have a 1999 Blazer, Trail Blazer pacakge with auto climate control.

    The system will blow cold, then I'll hear something squeak in the dash and all of a sudden it blows hot. Every 10-15 iseconds t switches, hot to cold. The cycle repeats when the engine is running, regardless of what position the temperature selector is at.

    Is that flapper box bad? Or could it be the control unit? Any advice?
  • repairdogrepairdog Posts: 948
    The electric blend motor (witha plastic gear) is probably trashed and I would strongly suggest the dealer on this - many other shops do not understand these and will actually cost more to determine and change - its behind the glove box and cycles the door between the AC and heater core based on the temp selected.

    Try rotating the knobs to see if the contacts are dirty and this may clean them and see if running at all cold or hot stops the cycling. I never use that auto fan position cause I hate it ramping up and down.
  • jtripjtrip Posts: 1
    Air went cold then hot a couple of times then stayed on hot,
    replaced actuator because it was clicking and new climate control but did not work.
    I talked to dealer and they said they haven't heard of this which I don't believe,if anyone can help Id appreciate it.
    hot air unless I manually turn the door to cold.
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