Question about 1996 GMC Sierra

1 Answer

MY TRUCK IS BOILING OVER I have changed the water pump, thermostat, and temp. sensor, but it is still boiling over at 100 degrees but it should not boil over until about 107 degrees (that's when the fans should cut in.) When I removed the temp. sensor plug, the fans cut in immediatily which indicates to me there is no circuit interuption. When I turn on the air cond. the fans also cuts in immediatily. If I crossover the fan relays the fans also start. Do anyone have any suggestions

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  • 17 more comments 
  • quinlanterry Jul 14, 2009

    Yes the coolant is fine, but i don't know why the hose from the water pump to the top of the radiator has no pressure? I can squeeze the hose together with very little force. The rad.cap is rated for 15psi, and i already changed it.

  • quinlanterry Jul 14, 2009

    I flushed the core and there didn't seem to be any restriction, but i only used a garden hose, is there a better way to flush the rad. I think that the powertrain control module (pcm) controlls the low, and the high speed cooling fan relay.

  • quinlanterry Jul 14, 2009

    I done a pressure test and it held the pressure fine. There is no sign or any coolant in the base, or exiting out through the exhaust, or there is no coolant leaking on the groung.

  • quinlanterry Jul 14, 2009

    There is lots of heat comming from the heater, and the air cond. is cold

  • quinlanterry Jul 14, 2009

    I am going to do a compression test next to see if the compression is allright. I already did a pressure test and it held its pressure no leaks. So if the compression test is ok will that eliminate a head or head gasket problem?

  • quinlanterry Jul 14, 2009

    If it was a head gasket shouldn't there be more pressure on the rad hoses? I seem to have no pressure on the rad hoses.

    A s far as the pcm goes that's what i was thinking, but wanted to rule everything else out first. According to the wireing skematic there is a high and low speed cooling fan relays there. Does this also command the air cond. fans to start up also, or are they controlled by something else.

    I think you could be right about the boiling being pressure returned to the bottle, but why ain't there any pressure on the rad hoses?

  • quinlanterry Jul 14, 2009

    No I didn't put the thermostat in up-side down, actually it done the same thing without a thermostat in it. I got the schematic from Terra Nova Motors (document# 1572328). I f you unplug the temp.sensor wires, the fans are supposed to start up to protect your engine.

    I think you are right when saying that it might not be a temp. problem, but where is the pressure coming from, without building pressure in the rad hoses.

    If it is not a temp. problem how come i am able to drive my truck for 8-hours on the highway, towing a trailor, with a bike in it ,and another in the box of the truck without boiling over. As soon as i hit traffic it boiled over. Would it heat up before this if it was a head gasket.

    Would a compression test elimenate any head issues.

  • quinlanterry Jul 14, 2009

    I am going to try a compression test now so i will let you know the results later.

  • quinlanterry Jul 14, 2009

    what should the compression of a 4.8 gmc engine

  • quinlanterry Jul 14, 2009

    What should the compression be?

  • quinlanterry Jul 15, 2009

    When you are talking about temp. are you talking about farenhite or celcious?

    I performed the pressure test and all 8-cylinders held at 170-180 psi. Then I let the truck run until it overflowed and with a therometer I took the temp. which was 235-240 degrees farenhite. There was still no pressure on the hoses. I was able to remove the cap on the overflow bottle just before it boiled over, which I should not be able to do. At what temp. should the fans start running?

    PS: Have I got you stumped yet?, because I am.

    Hope you can give me a solution, because I am out of ideas.


  • quinlanterry Jul 15, 2009

    I changed the temp. sensor again to eliminate it being faulty.

    I unpluged the pcm to check for cracked wires-no luck.

    I then connected my test light yo the ground on the battery, and touched all the pins on the wireing plug going to the pcm, and when i touched the pin belonging to the fans they started running. Is this normal? Have you got any other suggestions? any other suggestions

  • quinlanterry Jul 15, 2009

    I spent another day trying to find a solution, but no success. I have done everything except change the pcm. So I droped the truck off to the dealer so they can check teh pcm, because i don't want to buy one if i havent got to. They are very expensive.

    I will let you know what i find out.

    Thanks for your help


  • quinlanterry Jul 16, 2009

    Good afternoon D

    I had my truck over to the dealer and they couldn't tell me exactily what was wrong. So they said it must be the heads/head gaskets. They performed another blow down test and there was no leaks, so i don't know how they can justify saying it must be in the heads. They said that it boiled at 102

    degrees celcious.

    I wonder would the problem be in the overflow bottle itself? What do you think, should I tear the engine down to check the heads. Very expensive.

  • quinlanterry Jul 17, 2009

    The GM dealer said they checked the pcm and the whole wireing system and said that it is working fine, and that the coolant is overflowing before it reaches the correct temp. for the fans to cut in.

    If it is the heads/gasket, why isn't there any pressure on the hoses? I can remove the rad cap anytime i want when the truck is running, and there is no pressure.

  • quinlanterry Jul 17, 2009

    I think I may have found the problem. The hose coming from the rad. was going into the overflow bottle at the cap, and the overflow line was below the cap where it didn't have to build pressure to blow off, whereas if was on to the connection under the cap, it would have to build to 15psi inorder to blow off. Which would explain why there wasw no pressure being built up.

    What do you think?

  • quinlanterry Jul 17, 2009

    I had the truck running all afternoon, and the fans were cutting in and out with no issues. As far as I can see I have found the solution. I was just not willing to except that there was a head issue without any proof. I am glad that I didn't. The fix was so simple, so I guess that's why I missed it, but the GM tech's. shouldn't of missed it.

    I am thinking that I may of had them off when I replaced the thermostat when I was up salmon fishing. Maybee that was my original problem, or the temp. sensor.

    The picture of the bottle that you sent me was simular but not the same. Anyway thanks for your help.


  • quinlanterry Jul 17, 2009

    I should of seen it long before this, but i wasn't even thinking of something that simple. I must of seen and followed all the hoses and wireing 100 times in my head and I missed it. Only because I was so determined, and stubborn and not willing to accept the dealers oppinion on it being the heads is when I found the problem. I can rember saying to my wife last night that they had no proof to support their theory and I was going to find the problem to prove them wrong, because in my mind I couldn't accept their reasoning for their conclusion, and I didn't believe them.

    Life lesson: Don't overlook the obvious

    Thanks again


  • quinlanterry Jul 18, 2009

    The fix ya team was very helpfull in troubleshooting, we found the problem, which ended up being A very minor repair.



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  • 15,935 Answers

Is your coolant at the correct ratio for your ambient temp??
Check that out before going much further.

Posted on Jul 14, 2009

  • 19 more comments 
  • blueextc3221
    blueextc3221 Jul 14, 2009

    Then maybe your radiator core is clogged up.

    I'd have to see a print to tell if your ECM controls the fans or not.

    I'll see if I can find one and report back.


  • blueextc3221
    blueextc3221 Jul 14, 2009

    Yes - the easiest way is with a infrared thermometer.

    All areas should be the same temp. If there are areas that are cooler - you have blockage.

    A blown head gasket would cause your problem... any oil in the coolant?

    When you replaced the pump - did you properly bleed the air from the system??

    This is also a way your problem can develop.

    If you turn on the heater - is it hot? If not - the heater core may have blockage.

    Check them all out :D

  • blueextc3221
    blueextc3221 Jul 14, 2009

    Well - if the coolant is "boiling over" at 100 degrees - the problem is with

    1. The coolant...(Concentrate level is most likely to blame)....

    2. A faulty head gasket....

    If you replaced the radiator cap then I doubt that was the problem...there is something else causing the pressure to be too great and thus flow (boil) out.

    You could have a bad head gasket, which allows the pressure from the combustion chamber to flow into the cooling circuit creating to great of pressure for the cap, and thus, flows out.

    Just capping that line cripples the functionality of your cooling system, it is designed to make use of the overflow tank, you shouldn't just disable it like that.

    IF your head gasket IS going out, the blocking the overflow tank will cause EXTREME pressure to build up, now your going to blow the radiator and your engine. Cheaper to fix the head gasket and keep the old radiator then replace a radiator and a head gasket.

    On a cold start, stand behind your car, someone starts the car, smell for coolant. When the car gets cold, the higher pressure in the cooling circuit could cause coolant to flow in the engine, so on a cold start, you may smell it. After it starts though, the coolant may not flow into the cylinders.

    Get a coolant tester kit to check for exhaust gases in your coolant, you can find them at your automotive stores.

    You may not see white smoke coming from the tail pipe after the car has warmed up because the coolant may not flowing into the engine combustion chamber while it is running...but you might see white smoke.

    You have troubleshot the electrical connections - so you KNOW that the fans work.

    You KNOW that if you bypass the sensor the fans work...

    So that leaves you with a faulty

    3. Faulty PCM... there is no fix for this - it must be replaced.


  • blueextc3221
    blueextc3221 Jul 14, 2009

    Coolant shouldn't BOIL at 100 degrees....period.

    a 50/50 mix should give you 225 degree boiling point.

    Are you sure it is BOIL and not pressure release (from blown head gasket)??

    Is the BOIL you are experiencing air released in the overflow tank?

  • blueextc3221
    blueextc3221 Jul 14, 2009

    Fluid takes the path of least resistance (coming out the overflow bottle)

    The fan relays are independant of the blower motor. One does not work in conjunction with the other.

    If the relays are not recieving the "turn on signal" when the engine is up to temp - the only thing left to blame is the PCM. It gets the signal from the temp sensor to activate the fan relays.

    Do you have this schematic in a manual, or did you find one online?

    I am unable to come up with one.

  • blueextc3221
    blueextc3221 Jul 14, 2009

    Pressure does not develop in the hoses until the heat makes the coolant expand. (did you put the thermostat in upside down?? :D )

    It is interesting, that you say the fans kick on when you UNPLUG the sensor.

    That indicates to me a faulty power-to-ground (broken/cracked power wire). Because the PCM looks for a 0 voltage to enable the relays (verified when you unplug) - yet the sensor is never giving the 0 voltage because it is not tripped.

    Which goes back to the whole TEMP thing.

    Being a new sensor - it would STILL be my opinion that the engine is not getting up to temp. What you are seeing "boiling" is not really boiling... it is air.

    The sensor wont trip the fans until approximately 180-200 degrees.

    If you're only getting 100-107, thats why the fans arent on :D

    You can check the proper operation by probing the wires of the ETC with an ohm meter. (multimeter set to ohms 20 multiplier.

    You will see (at cold) a nominal resistance (usually 5 ohms or so. WAY out of the ballpark so the fans dont start.

    As the engine heats... the resistance will drop.

    as the resistance goes below 1 ohm (usually .85 or so) is when the fan kicks on.

    If your resistance is not ever reaching that point (but the coolant is "boiling") there are problems elsewhere.

  • blueextc3221
    blueextc3221 Jul 14, 2009

    yes a compression test on all cyls would eliminate a cracked head or blown gasket.

    when you are driving - air is going over the radiator keeping the coolant cool. (the fans suppliment under extreme load)

    when idling in traffic - the fans are supposed to do that job.

    so that goes back to the etc being faulty, having faulty wires, or the PCM not giving the relay the signal to turn on.

  • blueextc3221
    blueextc3221 Jul 14, 2009

    9.5 : 1

  • blueextc3221
    blueextc3221 Jul 14, 2009

    Step 1
    Warm up the engine in the GMC Sierra before beginning the test, since a compression test performed on a cold engine will be inaccurate. The battery should also be in good condition, since it will spin the engine several times.

  • Step 2
    Remove all eight spark plugs from the V8 engine after you turn it off. Models containing a V6 engine have only six spark plugs. Disable the ignition system to eliminate the possibility of electrical shock while performing the compression test.

  • Step 3
    Place the compression tester into one of the spark plug holes and crank the starter so that compression builds in that cylinder. Write down the compression number and continue to the next spark plug hole. Repeat the procedure for each of the six or eight cylinders, recording each number.

  • Step 4
    Compare the compression numbers to manufacturer specifications. If the GMC Sierra contains a 4.3L V6 engine, the ideal compression ratio is 9.2:1. The ratio varies by engine size in trucks with a V8 engine. If it contains a 4.8L or 5.3L engine, 9.5:1 is ideal. If it has a 6.0L engine, the ratio should be 9.4:1.

  • Step 5
    Evaluate the difference between the recommended compression ratio and the test results. An ideal reading falls within 15 PSI. The numbers should be within 10 PSI of each other as well. You will need to retest any cylinder with a low reading.

  • Step 6
    Add a tiny amount of motor oil to the cylinder with the low reading before running the compression test again. If the resulting number is higher than the first, the cylinder has worn cylinder walls or piston rings. If the number remains the same, it has worn valves or valve seats.
  • blueextc3221
    blueextc3221 Jul 14, 2009

    Also, as a "pre-test", remove radiator cap while engine is running BUT NOT HOT YET. A blown headgasket will leak cylinder pressure into the cooling system; it will show up as anything from small bubbles in the coolant to coolant spurting out of the radiator filler.

    The head gasket may have been damaged originally from the ECT failure and overheating.

    To do the pressure drop test - (not sure if you know) you'll need an air compressor and fitting for the spark plug hole.

    Put the air line into one of the spark plug holes and blow air into each cylinder in turn, with the rad cap off.

    When air bubbles start coming out of the rad cap opening, that's the one that has the bad head gasket seal. If none of them leak, the head gasket is fine.

    Let me know what you find out.


  • blueextc3221
    blueextc3221 Jul 15, 2009

    I guess I shouldnt assume everone here is in the US.

    Im talking Farenheit :D

    That could explain our problem (somewhat).

    The fans should kick on as the engine approaches operating temp. 180F (ish)

    if the fans never came on - then you're looking at electrical.

    Retarded Ignition timing can cause overheating - but again - your fans should be coming on - and arent.

    Have you tried testing the sensor like I suggested before - to see if the resistance is dropping as the coolant heats up?

    If the sensor goes below 1ohm - it should kick the fan on.

    When you unplug the sensor - the fans turn on... that indicates the PCM is operating as it should at infinite resistance - but it may be mis-interpreting the voltage and not activating the fans.

    The only way to test this is to get your DMM out and set it to Ohms (20 multiplier)

    Run the truck up to operating temp - and watch the resistance drop.

    If it never drops - you have a faulty sensor.

    If the resistance drops below .8ohms and the fans dont kick on - the PCM is at fault.

    Not having "pressure" in the hoses is indicative of air being introduced. Either from not bleeding properly after the pump change - or a crack in the system somewhere. Air is compressable - fluid is not. any air cavity would take up the minimal 15PSI.

    Test the sensor, and let me know!

  • blueextc3221
    blueextc3221 Jul 15, 2009

    Right now it's 10:45pm EST.
    I'm heading to bed :D
    Please hold off any further reply until after 6AM - as FixYa only gives us 4 hours to reply to a comment left - or it goes back into the general pool.

    If you feel you've exhausted all the troubleshooting available, and have no more questions, please accept the answer to remove it from the work queue.


  • blueextc3221
    blueextc3221 Jul 15, 2009

    yes - as i explained before - your sensor is variable.

    as the engine heats up - the sensor loses resistance.

    any reading below .85 ohms the fan kicks on.

    what you did was provide a path to ground (instant) so the PCM sees the ground and activates the fans.

    instead of checking with a multimeter - you just replaced the sensor. that pretty much rules out the sensor now. 2 in a row is unlikely defective.

    but to be certain the PCM is at fault - you should check the resistance drop.

    Make SURE the PCM isnt seeing the voltage drop.

    It likes the INFINITE ground (O ohms on the unplug) but it may be misinterpreting the .85Ohm and below signal somehow and keeping the signal from the relay under ANY voltage.

    Otherwise your last shot is just replacing the PCM.

    Good Luck!

  • blueextc3221
    blueextc3221 Jul 15, 2009


    let me know :)

  • blueextc3221
    blueextc3221 Jul 17, 2009

    A head problem was one of my first suggestions - after thinking on it for a few days.... if you did the pressure test while the engine was cold (most likely) sometimes a warped head will make an OK seal - but once the engine heats - the metal expands and warps.

    You may want to try to torque down the bolts to see if it helps any - but it looks like thats where the problem is (if the PCM is doing its job correctly).

    The overflow bottle is just there to catch the expanded coolant.... no other job there.

    Get your tools out :D

  • blueextc3221
    blueextc3221 Jul 17, 2009

    as i stated earlier too, the head gasket failure may have been AFTER the ECT sensor died on you and it overheated.

    Im afraid you're in for a big repair.

    Thanks Again

  • blueextc3221
    blueextc3221 Jul 17, 2009

    Still wouldnt explain why those darn fans won't kick on though!

    You may have multiple problems.

  • blueextc3221
    blueextc3221 Jul 17, 2009

    The system pressure is going somewhere. By way of trapped air, or leak.

    You indicated you have a new pump, new radiator cap, new thermostat, - system holds pressure (when cool)... all the problems associated with no pressure in the system.

    It must be going somewhere!

    The only thing left unchecked is the Headgasket.

    If there is no visible external leak, then the leak is internal. That could include a blown head gasket

    or internal corrosion, and may indicate a serious problem.

    The only positive check for that is to look inside. Metal expands when heated - and a weak spot in the gasket may only be evident after heating up.

  • blueextc3221
    blueextc3221 Jul 17, 2009

    if I read that correctly - you're saying the lines were reversed in the overflow bottle?

    had they been removed for something in the past?

    Is it possible to post a picture of what you are talking about??

    Your coolant bottle looks like this....CLICK HERE

    The tube by the filler cap is for overflow purposes - so it does not get all over the engine - and dumps to the ground.... so if your radiator was hooked to THAT port - yes - there would be no head to build pressure against... basically your system is open to the air in that instance.... and all heat escapes there. Im surprised the dealership didnt catch that either... (they're looking right at it :D)

    I was not aware your lines had been messed with... of course it would help to be in front of the vehicle :) My information is based solely on your decription.

    Hope this solves your problem !!


  • blueextc3221
    blueextc3221 Jul 17, 2009

    No problem - glad you caught it :D

    We knew the pressure was escaping somewhere - but not knowing the hoses were disconnected at one point was a big factor in determining the solution.

    I'LL remember to ask in future sessions about tampering with the hoses ;D

    Thanks again for using FixYa!!

  • blueextc3221
    blueextc3221 Jul 18, 2009

    No problem.

    Please use the ACCEPT button to take this question out of the queue.


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