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

1995 Infiniti G20 engine cools while car is running

My sister owns a 1995 Infiniti G20. We live in the high desert, so we get cold weather (though not as cold as the NE) While she’s driving the car in the winter, the temperature gauge drops from mid-way to “C”. I’ve seen it happen both at idle and driving as fast as interstate speeds. So far she has:

replaced the thermostat (twice just in case the first was bad)

checked the temperature coming from the heater - it’s around 140 when the engine temp gauge reads normal and less when it drops to C

checked to see if the fans in back of the radiator are running when the temp drop happens - they are not

She’s had all kinds of mechanics look at it from very good professional to shade-tree and the response is always the same: “hmmm”

She’s avoiding going to the dealership in town to have them hook it up to their computer because they are blatant crooks (is that redundant - dealer and crook?)

I thought it could be some sort of air leak in the system that is signaling the temp drop, but I’m an economist and not a mechanic and don’t have any basis for that thinking other than it seems it might be possible. Any ideas about what might be wrong?

Did she replace the thermostat with an OEM unit? Some of the stuff you get at the parts store is junk.

Yes. Both times

Perhaps the coolant temperature sender is faulty or the electrical connection to it is dirty/corroded.

Thank you - that’s a new one. I’ll call her right now and have her check it out!

You have found the answer to global warming! If you could just find out what the mechanism is that, contrary to the law of thermodynamics, can make heat disappear, it could be applied to the larger picture. Wow!
But, on a more mundane level, maybe something else is at work, here. The temperature indication is decreasing; but, is the engine temperature actually decreasing? Has anyone taken temperature readings (with an infrared thermometer or a contact thermometer) on the engine and radiator?
Is the cooling system full of coolant? Has the system been bled of air? Is the a/c compressor coming on when the cabin heater releases less warmth? Are air blend doors opening or closing?

the temperature gauge drops from mid-way to “C”. I’ve seen it happen both at idle and driving as fast as interstate speeds.

OK but how about the actual temperature? Somehow I suspect a gauge problem not a temperature problem.

Ups and downs happen when the thermostat opens and closes, which it does a lot on the open highway if you have a great cooling system. I have noticed the gage going up and down at idle too. Things may be normal unless the heater goes cold.

Thank you all - it is really kind of you to offer your help and advice.

She’s checked the temp on the heater inside the car with some special kind of thermometer (a mechanic friend had some special thermometer) and the air coming from the heater vent is indeed cooler by about 40 degrees. At least one mechanic thought that was enough of an indication that the engine was actually cooling.

There is no indication that the air conditioner is coming on that she can see. We are both such girls, that we can only tell if it is on if the light is on, so there may very well be something there that no one has checked.

I’m glad to see someone else (Thanks hellokit)thinks it could be air in the system - that makes sense to me from a physics point of view, but I don’t really understand how engines exchange air all that well. How do we check/find these mysterious “air blend doors”?

If it does turn out to be the solution to global warming, we will package it and sell it with Shameless Commerce getting the first shot at marketing!

I disagree, the t-stat is not a discrete device, but a continuous one. If you are driving and it is very cold, and it is not opened yet or it is very hot and it is all the way open, you can get fluctuations in temp due to engine load. You might see the temp change due to the discrete fan thermostat operation.

I think that there is a good possibility of air in the cooling system. There may be a bleed bolt on top of the thermostat housing.

Air in the coolant system is a good possibility. The 95 g20 engine has a short rubber coolant hose under the back side of the intake manifold that after time can start to leak. Usually it will be unnoticeable until you are having heating/cooling problems during the colder months (ask me how I know).

Check the coolant level (pop the radiator cap) and try adding coolant. The bleed valve is directly above the thermostat housing (it’s a 10mm bolt). Loosen the bleed valve and add coolant to the radiator until the bleed valve starts to weep. Pop the cap back on, tighten the bleed valve and make sure to top off the overflow bottle.

If that solves the cooling issue have your mechanic check for leaks. If your lucky it will be an easy hose to access and replace.

In answer to your questions about the blend doors, I don’t think it’s necessary to worry about these. It sounds like your mechanic has verified that the engine is actually cooling down. The blend doors are simply the doors that divert air through the ductwork, also directing where the incoming air is brought in from (the heater core or the ventwork), and have no direct connection to the engine’s cooling system in teh way you’re thinking.

The answer to the question of how engines excahange air is…they don’t. What they exchange is heat. The hot coolant flows through a mini radiator called a “heater core” through which air is blown. The air passing through the heated vanes of the heater core absorbs the heat. The air comiong out of the heater core is heated, the coolant exiting the heater core is cooler.

The air in the system that’s being alluded to is not in any way directly related to the air going through the ductwork and warming your feet. The car’s cooling system is a totally closed, fully fluid filled system. What some are suggesting is that this system could have an air bubble trapped in one of the higher spots, usually a hose, that’s affecting the ability of the system to operate properly, perhaps even affecting the ability of the thermostat to properly detect and respond to the engine’s temperature changes. As already suggested, most systems today have a “bleeder valve” at that high spot to allow the air in the system to be effectively bled out.

You’ve gotten a lot of good advice here and I can’t think of anything to add, but I thought your questions were good ones and deserved answers.