I have an issue with my cooling system -- it overheats like crazy. The problem is, it only overheats with the heat on right now. I'm normally running the A/C and have it on full cool in the summer, and everything is fine. But after a hard rain a couple days ago, it was pretty cool outside, and I decided to switch the dial 5-6 clicks over from full cool (with the A/C still on).

The guage showed 5 bars on the water temp (with the A/C still on), so I immediately took notice. I switched the cooling dial back to full cool, and within 20 seconds it was down to 3 bars again. No problem since then, driving around on full cool. I'm taking the car into the dealership tomorrow to take a look at it, but I was wondering what could be the problem? Why would this only happen when the heat is turned on, and stop when I turn it back to cool?

Thanks!

 

Update -- drove home with A/C off, temp on full cold, no overheating. I think that means it's probably not an air bubble or thermostat issue, because the fans seem to still be turning on.

 

Your problem is very odd.

Usually turning the heat on will reduce engine temps, and using the AC will raise them.

Is the gauge reading the only reason you say its overheating? Its it over flowing or steaming?

I would plug in the OBD2 and watch the real temps.

 

It's not overflowing or steaming, even when it was at 5 bars. I took it to the dealership, they said that (while they didn't take it apart) they believe it is a bad thermostat. They didn't check the temps w/ an OBD2, and didn't check for leaks in the system, so I don't know if either one of those would be the issue. I drove it home fine, didn't overheat at all.

That is a great idea though, I will have to get a reader from Discount Auto or somewhere and check the temps personally.

 

Once again, dealer is PATHETIC.

Dont go back there, you will be sorry.

After explaining your situation, they should have realized your problem is contradictory, and read the temp with the OBD2.

There are few mecahanics left in the world. None of them are at any dealer. Well, slows2k is one.

I can sit here and rattle off guesses all day just like the dealer.

go buy the cheapest scanner that reads datastream, and fix your own car

 

Or get a BT dongle and app if you have a suitable smartphone.

 

Doesn't the heater core use coolant to heat the element? Maybe its diverting all the coolent into the heater core other than the radiator. Just a guess, I'm not a mechanic or anything.

 

AFAIK:
The ECU uses only 1 sensor to get an idea of the engine coolant temp.
And that is the ECT, mounted at the back of the intake manifold.

I'm guessing the dash gets its signal from the ECU and shows bars accordingly.
So reading the ECU with an ODBII reader should produce the same values, given in C or F instead of bars.
If not than I think you have a problem with the dash temp gauge.
If so than you really have very hot coolant - at least around the ECT and as long as the ECT is generating the correct voltage.
An AP1 dash showing 5 bars means the coolant temp is above 118C (244F).
Modifry has figured out the bars - real temp relation:
2nd bar = above 55C
3rd bar = above 72C
4th bar = above 112C
5th bar = above 118C
6th bar = above 127C
7th (red) bar = above 134C = engine meltdown

If the coolant system has no air in it and builds-up some pressure I doubt you can get a local - around the ECT - boiling coolant pocket that would reach 118C or higher for the 20 seconds that were mentioned.

 

That's kind of what I'm thinking. It is impossible for the cooling system to heat up and cool down 46 degrees Celsius in a matter of 20 seconds. I would think it is a sensor issue, but it baffles me that it is related to the temperature dial for the A/C & heat system.

 

Can you simply reproduce the 5 bars?

COLD ENGINE! : remove the radiator cap, do you see coolant?
Is there coolant in the overflow reservoir?

An OBDII reader will show you what the ECU is getting from the ECT.
You can compare it to the dash bars to rule out an issue between ECU - dash.
But it won't tell you if the ECT is telling the truth without an independant temp reading.

Long shot: you could compare it (a bit ) with the IAT (Intake Air Temp) value a while after a hot shutdown.
But its not really accurate.