Mystery overheating

[Krueger]

Hey guys, so here is the run-down, I have (12/88) 89 325is that I bought recently and i cannot keep it from overheating. The thermostat operates properly, the water pump is working, and there are no visable leaks Anywhere.

When I drive the car it will get all the way past the mark after center, then shoot to the mark under center and gradually make it's way almost to red then cool down to center then gradually heat up again, and it will do that a few times before it doesn't cool off and I shut the car off (I've been stranded quite a bit)
I know the fan clutch, aux. Fan work. I've changed the temp sensor, flushed the system, and bought a new radiator and I'm still at square 1 during the little amount of time when the car is not running hot the heater blows hot air but otherwise it's ice cold. Frankly I've spent more on coolant and destilled water than I have on gas and I now could bleed my coolant system in my sleep. Before I got this car I was driving an 86 325e and I tell you what that sucker was a tank. HELP ME PLEASE

[mr.vang]

have you check the oil? maybe a blown head. did you bleed with full heat on?

[Pussy30]

Not saying you didnt do it right. But MAKE SURE its bled all the way, if your getting pockets of air somewhere your not gonna keep it cool.

[jlevie]

What that sounds most like is a head gasket leak that is pumping air into the cooling system. As the air bubble gets larger, the engine temp rises and the heater stops producing heat. When the cooling system pressure gets high enough the cap lists venting the air and some coolant. With the amount of air in the cooling system reduced the engine temp comes back down and the heater starts producing heat. A loss of coolant with no leaks, especially with random overheats, is almost a diagnostic fro a leaking head gasket.

[Krueger]

Well the coolant loss is just from spewing it out the overflow, although I've been told the head gasket might be the culprit, is there any way to tell for sure that is what it is? I've been told to check my oil to see if it looks frothy or the color of coffee with cream. So I changed the oil and what came out was straight black. And since the oil change it just looks like oil. Compression test maybe?

[jlevie]

If the coolant loss is from the overflow the cause (with the other symptoms) is almost certainly the head gasket or a cracked head. A simple DIY check for a head/gasket leak is to fully bleed the cooling system (procedure below), drive the car just enough to see the temp begin to rise, then shut off the engine and let it cool down. Release of a significant amount of air on a re-bleed, then or later, is evidence of a head gasket leak. Another check, even before bleeding, is to see if the hoses are stiff and under pressure after the engine has gotten cold. If they are, there's a head gasket 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 2000-2500 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.

[redhatpat]

It could just be a bad thermostat.

[Krueger]

I'm thinking it's my radiator cap am I crazy? I know my head isn't blown.

[Krueger]

I drove it around town and it ran cool for a good hour before it got hot.