C4ProjectCar

When I first bought the car, I had an overheating problem. It seemed to go away after a short while, though. I took my car to get it smogged, but they wouldn't smog it because it was too hot. Again, the problem seemed to go away. A little later I had my car running, went inside for a second and when I came back out the temperature gauge was pegged and coolant was spurting out of the overflow tank.

I thought it was overflowing because it was overheating (now I suspect it may be overheating for the same reason it's overflowing), and I thought it was overheating because of air in the coolant lines. I took off the radiator cap, filled up the reservoir, and then started the car. Instead of the level going down like it is supposed to as the air pockets are replaced with coolant, it began to rise.

My dad grabbed the only thing he could think of - a large funnel, about 6" across and 8" tall - and stuck it in the reservoir so as the level rose it would fill the funnel instead of pouring out on the floor. We decided to wait it out to see if the level would decrease, but it just kept rising. I increased the RPMs to 2000 like the instructions said, but that didn't cause the level to go down. I had to turn off the engine because the funnel filled entirely with coolant and was about to overflow. It had about a quart of coolant in it.

What would cause that to happen? I was thinking maybe an obstruction in a line or perhaps a stuck thermostat?

It's a 1990 base model.

don hall

Possible head gasket rupture allowing exhaust gases to enter the coolant

C4ProjectCar

Thanks for the idea. I'll see if I can get some of the stuff you use to detect hydrocarbons in coolant.

EDIT: There's none of the tell-tale white exhaust smoke. And those combustion leak test kits are pricey. Can a head gasket rupture be ruled out by the lack of white smoke or do I need to pony up for a test kit?

John A. Marker

Are the plugs new? If not, then pull #7 and #8 and look at the white porcelain. A plug that has been in and run for awhile will have a light tan color. A blown head gasket that is not leaking into the oil will shoot a small stream of coolant at operating temperature when the system is pressurized into the cylinder. It will basically steam clean the cylinder and the porcelain on the plug will be white and clean. So if one plug is tan and the other is clean.....more than likely a head gasket failure.

Note that the L98 coolant passage on #7 and #8 is very close to the cylinder wall and the most likely area to fail.

Have you ruled out the thermostat? When ever I put in a new thermostat, I drill a small 1/16" hole in the housing and position the thermostat so the hole is facing the front of the car. This allows air to pass thru before the thermostat opens. I also jack up the front end of the car as high as I can get it (be sure to block rear wheels). Air will rise to the highest point.

summerst

Had similar problem with my 1991. Turned out that the plastic upper tank was leaking -- there was no visible leak (as I envisioned anti-freeze running out the side) -- it was not leaking coolant but rather it could not hold pressure. I initially thought my mechanic was nuts but I reluctantly replaced the tank (with a new one) and it solved my problem. Engine ran cool again. If you search this forum I believe that you will find others with a similar problem with their upper tank.

C4ProjectCar

The plugs are new. The previous owner had them replaced shortly before he sold it. I pulled the plugs a couple times a while back and I don't recall seeing a difference in color.
Nonetheless, I may pull them again to make sure.
I know an air pocket at the thermostat can give an incorrect temp reading, but I'm not sure how it would cause the overflowing coolant. Could you elaborate?
Okay, I'll put the front end on my ramps next time I burp the air.

Thanks, I'll look into that. Although would a leaking tank cause coolant to overflow with the cap off? I'm afraid I don't have much knowledge of how the cooling system works.


Thanks for the help, everyone!

John A. Marker

The question is....is the thermostat actually opening. It the stat is not opening, your going to see a very quick rise in temperature. This in turn could be forcing the coolant uo and out the system. You could pull the stat and put it in a pan of water and bring the water to a boil and see if it opens.

C4ProjectCar

That was my hypothesis. Since a thermostat is only $5-$10, I'll just replace mine.
I plan to check the old one anyway to see if that was my problem.

79C3tnd

I really think that if you replace the thermostat, flush the system, install a new cap, and then make sure you purge the air out correctly, that your problems will disappear. I had a similar problem, and it was related to air in the system. You can always replace the tank if needed, but I would hold off on that until you see your results from the above.

C4ProjectCar

I'm hoping...
As a side note, when you purge the air do you increase the speed of the engine as soon as you start or do you wait until coolant starts circulating?

79C3tnd

Add your coolant, then start and let the engine idle until it gets to the opening temp of your thermostat. Take the engine up to about 2000 rpm, which will cause the coolant to flow and the coolant level will drop as you are looking into the filler. Fill to the neck and install the radiator cap, and then fill the overflow 1/2 way. It also helps if you can park the car on a sloped driveway (get the radiator higher than the engine) since stock the engine sits higher than the radiator.

C4ProjectCar

Thanks for the instructions. Let's hope it doesn't overflow again when I do that.
I have ramps so I'll pull up on those. In fact, I'll already be up on them to change the oil filter. That works out well.

AirForceOne!

I have a 90 with the same problems and it was simple as a bad radiator cap! Don't buy a cheap one, get a new GM replacement! Trust me, I just went through the same situation!

C4ProjectCar

Bad news on the overheating front. After replacing my thermostat and testing my radiator cap to make sure it was good (thanks anyway for the tip, AirForceOne), I started the procedure to burp the air. I put the front wheels on ramps. It was a repeat performance. While air seemed to be bubbling out out the coolant, the level kept rising up into the funnel we inserted. Once it got near the top, I shut it off, but not until it just began to boil and quite a bit splashed out/overflowed. At no point during the test had the temperature gauge risen to an abnormal level (it actually seemed to be under 200 the whole time, but the water boiled so that can't be right). It went from bad to worse when a small fire ignited on the engine block near where it had spilled. We put out the fire easily, but I'm perplexed what could have burned. Does 50/50 coolant/water burn?

Anyway, now I'm probably a gallon low on coolant and no closer to fixing my problem. The only thing that makes any sense is a blown headgasket, but a lot more gas was coming out of the coolant than I think a blown headgasket could let in. And there is no white smoke from the exhaust.

Needless to say, I'd really appreciate your continued help as I sort this out.

cudamax

Pull the T-stat and forget about lifting the car. It bleeds out without that. Don't run in a garage, put it outside and hold it at about 1800 rpm and fill the system. Keep the rpm's up, don't let off or all the coolant will fly out. Just cap it off with the rpm's up and take it for a ride and see if the overflow tank overflows.
If so, your getting combustion in the coolant system otherwise ur ok.
Never let off the rpms quickly. Coolant flys out everywhere

don hall

Since you have replaced the t-stat and cap, what else could cause
these symptoms..... head gasket rupture allowing exhaust gases into the coolant.

White smoke out the exhaust, or coolant in the oil only occurs if the
ruptured gasket allows coolant entry into the combustion chamber, and, or, coolant into the oil passage.

Exhaust gases into the coolant creates excessive pressure and rapid heating in the cooling system which displaces coolant.

You have two alternatives to remedy your head gasket:

1. Chemicals: Thermagasket Plus or Blue Devil (I have no experience with either product).

2. Replace the head gasket.

C4ProjectCar

Thanks for the info. If there was a hole in the gasket between the exhaust passage and the coolant passage, wouldn't coolant seep through while it's not running causing white exhaust on startup?



I took it for a test drive, and the temperature was fine the whole time. I got it home, shut it off, and then remembered I needed to take a video of a squeak so I immediately restarted it. After waiting for the squeak to resume for a minute, the fan kicked on so I checked the temperature. The gauge was nearly at the 'danger zone', probably 240 or so, which is way too high. What temperature is the first fan supposed to kick on? I think around 220?

Shag

So lets recap....you took it for a drive and it was fine. Temp was normal. Sitting at a idle is when the temp rising to overheating?

C4ProjectCar

Pretty much.
Since it only overheats when it's stopped, and it seemed like the fan kicked on later than it should have, I'm thinking maybe an issue with the coolant temperature sensor. My hypothesis is the following: When it's moving, there's enough air flowing past the radiator that the fan coming on too late is not an issue, but when it's stopped the lack of a fan causes it to overheat.

The only thing that throws a wrench into my theory is the fact that I tested the coolant temperature sensor below the throttle body on the front of the engine and the resistance was roughly where it should have been. I'm guessing there are two, though - one for the gauge, and one for the ECM. I tested the one I thought was for the ECM. So maybe the sensor for the gauge is just off, making it look like it's hotter than it really is?

I hope my musings aren't too confusing to you.

Shag

I had the same problem with my 89. My issue was timing. My timing was off causing my engine temp to climb and fans not turning on at 223. in any case, you don't have to jack the car up, leave the cap off when burping the system. Top off your radiator. Warm up your engine then shut it off. take a 20 minute break to cool it off. Take the cap off and top it off again. Repeat. Nothing fancy. good luck bro.