Automobile Cooling System

  • #1
1989 GMC truck 4 cyl 2.5 liter

The overheat light comes on after about a minute's driving. I have flushed the system, replaced the hoses, replaced the whole radiator, and the coolant temperature sensor and the thermostat(tried two separate new thermostats).

I tried removing the thermostat altogether, and this has been the only thing that worked: the overheat light no longer comes on.

The water pump seems to be working fine, although I haven't dismantled it to inspect the impeller.

Is there a chance the overheat warning is a false signal due to purely electronic considerations?

Answers and Replies

  • #2
There's always a teeny tiny chance it could be some oddball electronic problem.

But its far more likely the engine is running lean because of a vacuum leak (could be a $1 piece of vacuum hose) or a bad MAP (manifold air pressure) or O2 sensor or EGR (exhaust gas reciruclation) and a few others. If the engine runs lean it will be hotter and then the problem has little to do with the cooling system itself and more or less the engine.

Open the hood and inspect the vacuum lines, they're mostly just small 1/4" lines running off the intake manifold to various parts of the truck. There is going to be a canister (on newer S10/15s its near the brake booster mounted to the hood) and there is going to be a large 1/2" line going to the master cylinder if you have power brakes.

EGR is failure prone and a little tougher to test, same with O2 sensor (oxygen sensor). Typically with that vintage its cheap to replace the O2 for like $30, the EGRs vary in price.

Much easier with a diagnostic computer (and especially 1996+ OBD-II) but a decent neighborhood mechanic (if they exist anymore) should be able to assist in trying to fix an overheating problem. It could even be a vacuum leak in the HVAC control system inside the cab of the truck, so its a lot of small things to test and look for and someone knowledgeable on all the systems and how they interact is of great help. But you can start with obvious things like vacuum hoses and maybe O2 and EGR if you're so inclined to risk guessing instead of paying for diagnosis and likely an educated guess.
  • #3
Electronically controlled engines tend to show this kind of cause and effect failure more so than carbed engines. A sensor failure (as Cliff indicated) can and will manifest itself as a secondary problem. Do you have a check engine light lit? Any excessive divergence between required fuel and delivered fuel will cause a check engine light (engine running lean/rich). In fact, any sensor that effects emissions is 'supposed' to trip the check engine light if a problem is detected. This particular law took effect in 86 for cars and 89 for trucks and light trucks IIRC.

Don't rule out your water pump---it's usually the second thing to go(first being the sensor and third being the fan). Also, did you replace the correct engine temp sensor. Paging through decades of underhood time I seem to recall your truck having two temp sensors. One was for the dash indicator if you have one and the other was for the ECU/fan control---I don't remember if your engine has a clutched fan or an electric fan.

Put the thermostat back in (make sure it is oriented correctly) and run the engine until just before it over heats (I'm guessing you don't have a dash water temp indicator---a good investment) and check to see if the fan is spinning fast enough to cool the engine (either clutch engaged or electric fan is on). The fan should be spinning about the speed of a house fan on high if temps approach an overheating condition. Some electric fans have two modes while others simply go a full throttle when needed.

If the fan is A-OK. Pull the engine codes from your ECU. You'll have to look into how to do this yourself, but it usually invloves using a jumper wire and counting the check engine light flashes. Any codes stored in the ECU will be displayed for you indicating if the ECU itself detects a problem other than the over temp condition.

Well, good luck.

[edit]This might help:
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  • #4
That's an awfully short period of time to heat up. A damaged/disintegrated impeller is not an unreasonable possibility.
zoobyshoe said:
The water pump seems to be working fine, although I haven't dismantled it to inspect the impeller.
  • #5
Wow, thanks everyone for the excellent leads on how to proceed!

Couple things to mention:

I did replace the O2 sensor about a year and a half ago, so it is probably not that. I will have a look at the vacuum hoses, and EGR etc.

The "service engine soon" light is not on. (It comes on when I start it, of course, just to prove it is working.) I already figured out how to check the codes and nothing came up.

The fan is a clutch fan and operates off a serpentine belt. The belt tension seems to be right on. I have watched the fan running several times, and it looks great.

I checked the new thermostats in hot water with a thermometer and they were right on as well. Yet the difference in flow between the restriction of an open thermostat and the complete absence of a thermostat seems to be enough to keep the overheat light from coming on.

This, in conjunction with your collective advise not to dismiss the pump, makes me lean toward that as the problem.

I will get a new gasket and set aside some time to take it off and have a look.


  • #6
Oh yeah, one more thing too - a loose radiator cap will leak pressure and can cause a steam bubble to form (boiling point is much lower without pressure) and this can be really bad for the cylinder head. You said you replaced the whole radiator but who knows... :smile:

I don't know about the pump being the problem, typically the seal leaks or the bearing goes out. I guess its possible for the impeller to fall off or freewheel but it shouldn't work ok without the thermostat then too.

Another thing you might want to try as a band-aid to assist until you find the problem is the water wetter products from Redline or Lucas - I didn't scientifically test it but it does seem to help in my TransAm to keep it a little cooler when the car is moving (no change while sitting in traffic when the computer doesn't turn the fans on).

While thinking about this, does your thermostat have a small hole in it to ensure it doesn't get air trapped under it? Owning one of the infamous LT1 engines with its reverse cooling I'm always on top of the proper fill technique to remove all air possible from the system. If air or steam is trapped under the therostat it'll basically stay closed.
  • #7
I have the two thermostats in front of me and cannot find any holes. They are two different brands. (I am also sure I installed them in the right direction, spring side down.)

I'm not sure what exactly could be wrong with the impellor without it making noise, but I'll never know for sure it's OK unless I look at it.
  • #8
Your engine may have a cooling system bypass around the thermostat to prevent a vapor lock situation. Not at all uncommon. One minute seems really fast to overheat. Let the light come on and feel of the top radiator hose. If it truly is overheated it should be hot to the touch even if the water pump isn't doing a darn thing. The warm coolant should make its way out of the top of the engine on its own. Years ago it was not uncommon for liquid cooled engines to not have a water pump at all.
  • #9
You're right. One minute is a psychologically warped memory. The light only came on in one minute when the car was already warmed up. From a morning cold start it took about five minutes or more depending.

None of the hoses ever feels overheated. Several times I was so convinced of it being a false electronic signal I opened the radiator with my bare hands and found the water in it to be no hotter than what comes out of a hot water tap, certainly not boiling over.
  • #10
If it never feels overheated, and the radiator doesn't boil, then you might want to check and make sure the that niether the sensor nor the wire running to it are making direct contact with the engine block. The lack of heat in the hoses and radiator may just be because there's no circulation happening, but it could be that the engine is not overheating at all, only the sensor is. (Not exactly the same as an ellectrical malfunction, since the sensor really is giving you an accurate reading of the heat it is receiving).
  • #11
Yup, I thought of this, too. The sensor is new, as I said, and in replacing it I saw that the plastic shielding around the wire was pretty crumbly, so I replaced it. (You can buy like a yard of this stuff for a couple dollars.) It made no difference, though.
  • #12
At the auto parts stores around here they have non-contact infrared temperature sensors that you point at the part and it will give you a reading. They are like $100 but most store employees are ok with you borrowing one of them for a minute to take out to the parking lot to "try it out". Then at least you'd have a general idea how warm the block and cylinder head are when the light comes on and you could point it at the sensor too to ballpark things.
  • #13
That may be a good idea, but I've never used one. How would I know what temp is too hot for the block?
  • #14
Point it at the part of the block where the temp sensor is. Most newer vehicles are running hotter nowadays. With a pressure system and the correct mixture of anti-freeze an engine can easily tolerate 220 degrees F with no trouble. Also point it at the thermostat housing.
  • #15
If your engine is really overheating (not a sensor, wiring or electronics problem) you may have a blown head gasket.
  • #16
Have you tried asking this question on This seems like a general enough question that someone on that forum would know the answer, or you could call the Car Talk guys, themselves, and might even get on their radio show. (I was kind of surprised to learn that both of those guys were MIT graduates - somehow, funny auto mechanics just isn't the first thing you think of when you think of MIT).
  • #17
I seriously doubt it's a head gasket. If it were, he would notice a lot of pressure when he removed the radiator cap.
  • #18
I seem to be OK so far with the "band-aid" of simply having removed the thermostat altogether. I don't expect its good for the engine to do that indefinitely, and I am going to have to get out there and work on this at some point. It is really good to have all these leads to try.

I prefer never to take a car to a shop unless I already know what needs fixing and it's something I can't do myself.

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