Overheating: What Have I Missed?

[jlevie]

– take the lid off the coolant reservoir and coolant spills out everywhere. That seems like it means something.

Yes it does mean something. It means that there was air under pressure in the cooling system. That air could be from a failure to fully bleed the cooing system, or it could be from a head gasket leak that is pumping air into the system. Since the overheating occurs after a day or more of driving, I'd have to say the the air is from a head gasket leak.

There are three ways to verify this. One is to pressurize each cylinder with 175psi air for a few minutes while monitoring cooling system pressure with a sensitive gauge. A rise in cooling system pressure indicate a leak. If there is a negative result on a cold engine the test must be repeated with the engine hot. The next way is to remove the cap when the engine is cold to relieve any pressure. Then drive the car for a bit and let it completely cool down. If the hoses are stiff once the engine is cold, air has been pumped in. While easy this test is a bit iffy in that a tiny leak may relieve pressure as the car cools. The last way is to fully bleed the system (procedure below), then drive the car (not allowing it to overheat), and re-bleed. Release of a significant amount of air then, or during a subsequent bleed/drive cycle, is evidence of a head gasket leak.

You can check for exhaust gases in the coolant with a chemical test kit. But that test can give negative results even when a leak is present. If the test doesn't give a positive result, another test is required to prove that there isn't a leak.

1) Using ramps or a jack get the front of the car 1' or more higher than
the rear. That will make the radiator and bleed screw the highest point
on the engine and facilitate removal of air.

2) With the bleed screw open, add coolant until no more air comes out of
the bleed.

3) Leave the filler cap off, or at least loose, and set the heat for max
temp and fan speed. Leaving the cap loose will prevent air that's still
in the system from causing a "coolant fountain" once the engine heats
up. Start the engine and allow it to warm up to operating temp. As it
warms up occasionally crack the bleed screw to release any air and top
up the coolant as necessary.

4) Once the engine is at temp bring it up to 2000-2500rpm for a few seconds
several times. Then crack the bleed until no more air is released. At
this point the heater should be throwing lots of hot air, which
indicates that the heater core is filled with coolant. You may have to
repeat this a few times to get all the air out.

5) Drive the car a bit, allow it to cool back down, and recheck the
bleed for air. Over the next few days you may get very small amounts (a
few bubbles) of air out of the bleed screw.

[IAmGary]

As it turns out my particular e30 is fine. My cluster thermometer lies and / or is displaying every little hiccup and variation in temperature as evidenced by the fact that I can tap the glass and it usually tells me something I like better. Old solder joints, loose gauge, who knows for sure.

As long as it's not in the red then I'm not going to worry too much and I may install an aftermarket thermometer.

[jlevie]

An erratic temperature gauge is frequently cause by a poor connection where the gauge mounts to the PCB in the cluster. Reflowing the solder there and at the fuel gauge is the long term fix.