Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Coolant Temp Fluctuating

I have a 2002 Chevy Impala with the 3.4L engine. The car has 180K+ miles on the original engine. The coolant temperature started fluctuating recently. Going from 200 degrees up to 240 sometimes 260 degrees.

I recently had the check engine light on with the P0420 code which indicates a bad catalytic converter, I had that replaced and I was hoping that maybe the lack of exhaust flow through the converter was causing the overheating. It is still fluctuating however after the converter replacement.

The coolant temperature seems to go up when the car is idling but when moving it starts to go back down to normal. I had the cooling fan assembly replaced last year after one of the fans failed. These are less than a year old and are both operating as expected.

A little history on the engine. It originally had that crappy DexCool in it that then ate away at the cheap plastic head gasket causing oil to leak into the coolant system. This caused the coolant to turn into ‘mud’ within the radiator and eventually the head gasket failed completely and I had to have that replaced along with putting in ethylene glycol to replace the DexCool. Now when the coolant system was flushed, I am not completely sure that all that ‘mud’ was flushed out of the radiator. So the radiator may not be operating at full capacity. In fact, driving through the mountains this summer going up inclines, the car would tend to start overheating.

So thoughts on what might be the issue, radiator, thermostat, sensor, water pump, something else? I’m pretty sure it’s not the sensor because sensors don’t fail in this manner (they fail either high or low). I’m leaning toward the radiator because once I get some air flow across it, the coolant temp gets back to normal.

Also, the A/C is sometimes working great and blowing really cold and other times it is just slightly cold.

Very, very, very rarely have I seen it actually be a bad sensor… although it usually gets the blame first.

I doubt if you have a blown head gasket, but with a kit a lab test (checking the coolant for evidence of hydrocarbons) is easy and cheap to do, so I’d do one just to be on the safe side. If you feel the test is giving you a questionable result, a cylinder leakdown test is well worth doing. If with the radiator cap removed and the engine running bubbles are coming up out of the fill hole, focus on the headgasket. The bubbles would be combustion gasses blowing through the breech and migrating up to the cooling system high point… the fill hole.

T-stat is a real good possibility. A new one is dirt cheap. I’d change it.

Air in the cooling system is also a possibility. Often people forget to turn the heat on and turn the key to ON when they purge the system. On many cars there’s a diverter valve on the firewall that diverts the coolant through the heater core when the heat’s on and shuts the line when it’s off. If the valve wasn’t open when the system was purged, there may have been air trapped in the heater core and it may still be in the system causing havoc… and maybe even contaminating your new coolant with the evil Dexcool.

The radiator can be checked by scanning it with an infrared thermometer. Cold spots suggest clogged tubes.

I too am leaning toward the radiator.

Other possibilities are a missing radiator shroud, a missing splash shield (they can direct cooling air), and even eroded impellers on the water pump. You can do a flow test on the pump.

Yes @MTB, eroded pump impellors are a good possibility that are often overlooked.

I’m going to mention something which is not particularly helpful

I suspect that in the past, when the oil and coolant mixed, it was not the head gaskets that failed, but rather the intake manifold gaskets

Intake gaskets are very well known for failing on the 3.1 and 3.4 engines

the head gaskets . . . not so much

FWIW . . . many guys have misdiagnosed it, and as a result done far more work than was really needed

I’m going to possibly add further insult to injury . . . in my experience Dexcool is just fine, as long as it’s changed every 5 years, as you’re supposed to do anyways

I have no idea about OP, but we all known plenty of people that neglect their vehicles, and then talk smack when they don’t get away with it

Kind of like those plugged heater cores, due to 15- and 20-year old original coolant. And then the guy says the manufacturer built a junky car

I’m not suggesting that happened here, BTW

I’m going a bit out on a limb here… but isn’t burning coolant (or oil, for that matter) a potential cause for a catalytic converter failure?

Perhaps the engine is still burning coolant somehow, which might contribute to a lower coolant level and higher temps.

Or I could be wrong…

I agree with your skepticism of the issue being bad sensor. I worked as an avionics engineer for nine years and sensors just don’t fail in this manner.

I had a blown head gasket previously. I sure hope that isn’t the case again. I will do the hydrocarbon test within the coolant. Should give me some piece of mind if negative.

Thermostat - I agree that replacing this is simple. I’ll probably do this first and see what the outcome is.

Air in the system - I guess it’s possible but I would have thought this would have reared its head a long time ago when they system was flushed (a few years ago). Thanks for the info on the heater core and the heat needing to be on when purged. I was not aware of that and I’m not sure if they did this the last time it was flushed. God I hope there isn’t any more DexCool in there.

Radiator - Still my prime suspect. Do I need to replace it or can it be thoroughly cleaned?

The water pump isn’t that old (about 5 years and 50,000 miles on it) but I guess anything is possible.

Like I said, the heat goes back to normal while moving. I suppose this could be caused by better airflow over the radiator or by the pump spinning faster due to the increase in RPM. I could probably test this by just sitting in park and seeing if the temp increases past normal then rev the engine and see if the temp goes down.

The head gasket did indeed fail. I saw the original when it was removed from the engine.

DexCool is known to not play well with plastic. The head gasket on the 3.4L is plastic. The DexCool slowly erodes the plastic until it fails. GM lost a huge class action suit regarding this.

The DexCool was suppose to be good for 5 years or 100-150K miles but many owners experienced sludge in their radiators as low as 15K miles. Mine was nasty at only 45K miles and nowhere near 5 years.

I do a pretty good job of maintaining my car; that’s why I’ve gotten it to 180K+ miles. But having a head gasket slowly failing for years tends to screw with all the other hardware downstream.

You are correct that coolant and/or oil is not good for the catalytic converter and neither is uncombusted fuel. The coolant level appears to be adequate.

If the radiator is bad, it’ll need to be changed.
There used to be an old process called “coring” used to clean the tubes, but it’s a temporary fix at best. I don’t know if anybody still does that. Too labor intensive.

With so much ‘stuff’ within easy reach I have found that installing a long piece of hose on the engine heater outlet and capping the heater inlet, and with the radiator full, the cap off and engine cool start the engine and watch the volume of water flowing out of the engine. Because the thermostat is closed the flow should be substantial even when the hose is lifted 2 feet above the fender. Pressure will be quite low though…

If you want to try that be aware that antifreeze will damage paint if not quickly washed off. And never attempt it when the radiator housing is too hot to hold on to.

Could be as simple as a radiator cap, get a pressure test. Loosing coolant? Maybe a temp gun to see if all the radiator is working, a new thermostat never hurts.

The head gasket on your car is not plastic. The original intake manifold gasket was plastic. That must be what you were referring to.

1 Like

Agreed . . . clearly the intake manifold gaskets were replaced, not the head gaskets

Fluctuating coolant temp gauge possibilities

  • Radiator fan(s) not working or not spinning fast enough. When the engine overheats, the fans should be spinning like banshees. Are they?

  • Cooling system losing pressure. Have the system pressure tested, inexpensive test. Might be just a faulty radiator cap. The test might show where’s there’s a coolant leak too. A two-fer!!

  • Thermostat sticking. Remove it for inspection. If it looks ok, put in a pan of water heating on the stove and make sure it opens and closes at the correct temp, and to the correct dimensions.

  • Faulty sensor or gauge (don’t count on it, but such a thing is possible).