Why is my car overheating?

Factual Questions
1

I have a 1996 Geo Tracker, 2WD, standard transmission, 4 cylinders.

Recently the “Service Engine Soon” light came on steady, which according to my owner’s manual means I have a problem with the emissions system. So, Tuesday I took it to a shop for a diagnostic.

They said the EGR valve is bad. So I got in the car at the shop and drove to a dealer, where I purchased a replacement EGR valve.

On the way home the car overheated. I stopped, let it cool, added coolant (it drank in about half a gallon). No leaks were evident. I continued home (a short distance) without incident.

Yesterday I bought a new thermostat, thermostat gasket and radiator cap.

Today I installed the EGR valve. Then I drained the cooling system, installed the thermostat and cap, filled the system with water, then started the car.

The fan was turning. The water pump was turning. no squeals were coming from the fan belt. The radiator was not leaking.

I turned off the engine, flushed the system, refilled it with the proper water/coolant mix, then got in the car for a short drive.

The temperature gauge crept steadily upward toward the red zone. I didn’t give it a chance to overheat, but it was clearly headed that way.

But more perplexing was this: after the car had warmed up, I turned on the heater. COLD air came out of the vents!

So, what’s going on here?

2

WAG: water pump problem.

3

This talks about a bad head gasket: http://www.crxguru.com/tech_notes/overheat.htm

4

Blown head gasket.

Some time in the near past, did the engine get hot? One or two episodes of over heating can cause the gasket to fail. When the head gets to warm it streaches the bolts holding it to the block. The gasket is no longer held tightly and combustion pressure can leak out, burning a path which might allow coolant to be “sucked” in to the chamber and then forced out the exhaust. Some times you can see a slight “fog” out the tailpipe, but not always.
Get it to competant shop. If it it a bad gasket, time and running it only makes things worse.

5

If your blowing cold air, then no water’s getting through the heater core. It’s possible the new thermostat is a POS. I assume the water pump is mechanical, so if it’s turning, it’s pumping. Is there still water in the system?

6

Possible heater core leak. On some cars, the heater core is in the car so you wouldn’t see a leak, it would get absorbed by your carpet. You would also smell a foul maple syrup type smell.

Like Violet said: Water pump. The fins could have eroded away meaning yeah, it’ll heat up in the motor and show a high temp, but the water is not circulating. Meaning it’s not reaching the heater core. Water pumps can still fail without warning (Leaking at the bearing, gasket, or that annoying squeal).

7

Booker57, the car has never overheated until that time on Tuesday, and I got it off the road promptly.

And I’m not losing coolant, so I don’t think it’s getting sucked into the chamber.

Would a bad head gasket explain the cold air through the vents?

8

I will discount the leaking heater core and blown head gasket. If the heater core is leaking, coolant is reaching the core and it would blow warm air. Also, a leaking core is rarely cause for an over heating problem. If the head gasket was blown, one of 3 things will happen, none of them generally cause overheating. If the gasket failure was between the combustion chamber and the coolant system, coolant will be drawn into the cylinder(s) and there will be steam out of the tailpipe. Also, if the radiator cap is remove, coolant will be pumped out through the radiator opening. You will also smell fuel in your coolant. If the failure is between the combustion chamber and the crankcase, you will get large amounts of blue smoke out of your tailpipe. You will also have lots of blowby out of your oil filler cap. If the failure is between the cooling system and the crankcase, the crankcase will fill with coolant and the engine will sieze.

I have a couple of guesses. My first guess is the water pump. With the vehicle at operating temperature, carefully remove the radiator cap. You should see coolant flowing at a steady rate. There should be no air bubbles. Another problem, and I have seen it done, the thermostat may be installed backwards. This will cause the overheating plus no coolant will reach the heater core. To test for this without removing the radiator cap, bring the engine up to operating temp and carefully touch the upper and lower radiator hoses. If the lower hose is hot and the upper hose cool. the thermostat is in backwards. The thermostat could be bad also. Throw the thermstat in a pan of water on the stove with a candy or meat thermometer. Somewhere around the thermostat rating, you will see the thermostat open. You may also have to burp the cooling system, on some vehicles the top of the radiator is below the thermostat. Vehicles made like this have a coolant system bleed valve to get rid of all the air.

9

Possible air pocket. When you refilled the coolant, did you have your heater on?

10

Your story fairly screams “air lock” to me. There’s not enough coolant in the system to flow through the heater, and there’s not enough to keep the engine cool. You need to make sure that all the air pockets are bled out of the cooling system and that it’s full of liquid.

Right now we have no way of knowing why it overheated before the thermostat replacement. It may have had a faulty thermostat, but unless you actually tested it that’s just a guess. I’m away from my books, but I’m thinking the coolant capacity on the Tracker is around 6 qts.–2 qts. low is significant, so that’s another possibility. There are several other potential causes, including the head gasket, but we don’t have any real evidence for them yet.

Best case: It first overheated because the coolant was low, the coolant was low because the radiator cap was faulty, the subsequent overheating was because of excess air in the system, and when you get it properly bled and filled everything will now work fine.

It’s possible you still have a coolant leak (it had to go somewhere). Some leaks are not readily visible, and sometimes the coolant evaporates or disperses before it’s seen. Keep a close eye on the level for a while, or better yet pressure test the system.

Be aware that the coolant level in the overflow jar is not a reliable indicator of the level in the radiator. With some leaks, instead of liquid being sucked back in from the jar, air will be sucked in through the leak. In those cases, the level gets low in the radiator and engine while it stays normal in the overflow jar. To be certain, always check the level in the radiator (cold engine, of course).

If you still have overheating when you’re certain that the cooling system is full of liquid, then we’ve got a different problem. Let us know.

11

Basically your coolant isn’t flowing. Could be due to coolant system blockage or bad water pump. More on the blockage…GM issue a service bulletin to their service departments on vechiles using Dex-cool anti-freeze. The dex-cool turns to sludge. The bulletin metions 1996-2000 Blazers and Jimmys. Maybe your vehicle was using dex-cool too. Here is the link:

http://www.geocities.com/b_gillie/TSB990602012.htm

12

After taking in all the preceding answers, which seemed to imply nobody knows anymore about cars than I do, I have my own theory.

It’s aliens.

Green, beady eyed aliens with a bizarre taste for blowing head gaskets.

Just a theory.

13

You are seriously suggesting GaryT (just for starters) doesn’t know more than you do about cars? I don’t know as much about cars as him, but I know enough to know that he knows a hellava lot. Either you know a hellava lot too, or you know nothing about GaryT.

Or maybe your entire post was a joke, and not just your final comment :wink:

14

Now that you mention it, I had some aliens in my gas tank once. I just put in some 110 octane boost. Cleared it right up!

15

What I’m about to type doesn’t meet all of the criteria, but just in case you run up against something like this in the future…

I owned a Cutlass with one of those sideways engines. The computer controlled the fan. Then the computer began to fail. The Service Engine Soon light appeared. The fan would cut off when the engine got hot. After spending a bunch of money on shit that didn’t affect the problem, repair shop #3 finally got it right and replaced the computer. $300 for that damn thing.

16

I side with RACER72 and GARY T, They both know what they are talking about.

Also, in my opinion you don`t have enough coolant in the system to force the hot water into the heater core. I had this happen to me on an 88 thunderbird. Added water and the heater worked again. The cooling system works best when completely filled and pressurised properly (good cap and hoses clamped properly/no leaks).

17

Another vote for racer72 & GaryT. If your water pump is sucking air, there will be no circulation. BTW, I ALWAYS put a new thermostat on the stove to test it before it goes in. I have had a few bad ones.

18

I think its an air pocket, too.

Park the car nose-high on a steep driveway and fill with extra coolant, and run the heater.

19

If its cold then I take the radiator cap off & then start the car & watch the water in the radiator to see that it circulates. Have to watch it for a few minutes as it warms up. When the thermostat opens (did you put it in the right direction?) another channel of water should show up. I don’t know if you can do that with your car as some might have to have the cap on to work.

20

Color me stupid! After reading racer72 and Gary T’s posts last night I realized that I’d filled my overflow coolant tank (with about 1/2 gallon) but neglected to fill the radiator itself. So I did so (another gallon went in).

Today I drove the car around on some errands. The heater worked just fine and blew hot air just like it’s supposed to. So probably, I had an air pocket. A HUGE air pocket.

…I drove a total of probably ten miles, on surface streets, not highways. Much of that time was spent idling at intersections. The car never overheated. The needle of the temperature gauge climbed to about the middle of the normal range and stayed there.

Which, interestingly, is still cause for concern, because prior to these troubles the needle always stayed at the low end of the “normal” range. Should I be worried?

Thank you all for your feedback and advice.