Coolant System Problem

Coolant System Problem

So I was driving home yesterday, about ten blocks from home and suddenly it felt like it was getting very warm in the cabin. I didn't think anything of it and just shut the heat off. (it had been in the middle position)

By the time I was two blocks from home, all this steam started pouring out from under the hood, and the temp gauge started climbing like crazy. I quickly dumped the windows and cranked the heat on and coasted home.

So I shut the car off and popped the hood, and sure enough the top had blown off the coolant reservoir (and actually warped/distorted part of the reservoir, but it didn't rupture). The reservoir was empty. I filled the reservoir to slightly above the full mark and let the car cool down for half an hour. Then I started it up and let it idle for 5 minutes. Then I ran it at 3000 for about 60 seconds, and everything seemed fine. Temp gauge at normal position. Anyway by now it was dark and still pouring rain, so I called it quits.

This morning I pop the hood, and the reservoir is now empty. I open the rad cap, and it appears empty too. So I fill it with water, and idle the car for ten minutes. Temp is fine. By now the reservoir has dropped about 1 inch in water. No surprise. Then I drive it around the block a few times, and everything seems fine. Then I start driving to my friends garage, and after about 5 minutes of driving, the temp gauge starts climbing like crazy again. So I crank the cabin heat, turn around, and go home. When I get home, I pop the hood and the coolant reservoir is basically overflowing.

I am not a professional mechanic by any means, but I am no idiot. I would say I have a "intermediate" level of automotive knowledge, I do all the minor stuff myself. I bought my P5 second hand at 83,000 km and almost 4 years later it has 125,000 km on it. It's been really reliable, save a cooked coil pack and a sticky brake caliper.

Obviously there is a serious problem. For some reason coolant is backing up into the reservoir.

Anybody have any ideas why this is happening?

Your thermostat maybe sticking or not opening.

You should make sure you fill the radiator to full before filling the overflow.

I'm going to pull the thermostat, looks pretty easy to do. Is there any way I can test it for functionality? To rule out whether or not it's faulty?

My Bimmer friend says that it's probably a $15-$20 dollar part to replace. Is that accurate?

thermostat is under $10.
You will want to change your oil. The viscosity is gone if you've over-heated the engine.

I see how to test the thermostat, it's in the FSM. Didn't see it the first time for some reason. For 10 bucks it's an cheap fix - if it's the problem. .... I'm due for an oil change anyway....

Thermostats usually take a beating and should be replaced once in a while.

Its easy and cheap compared to the BMW because your car is Japanese

I put it in boiling water for 10 minutes and it didn't open. FSM says it's supposed to be fully open at 95 C. I don't think it's EVER been replaced. I have all the maintenance records from the first owner and he never replaced it.

Hurray for a quick/cheap fix!

and dont just use regular water, use distilled water, you must also have the right mix of coolant and distilled water.

check water pump condition.

BlueBomber said:

Ok so quick update, I was at the parts store getting the thermostat and the guy behind the counter told me to warm the engine up and then rev it and watch for bubbles in the radiator with the cap off. If bubbles appear then compression is escaping and the cylinder head gasket is toast.

So I replaced the thermostat and tried his advice. Sure enough bubbles. And then after revving it a few more times, it was like a BIG bubble would push it's way out (of the top of the rad with the cap off). Also, the tailpipe had a lot of steam coming out and was wet. This doesn't sound good.

Click to expand...

If your system wasn't full, you will have air in it and bubbles will keep coming out for a while.

Best thing is to check compression, only then will I start worrying about engine condition.

s*** I was editing my last post and deleted it by accident. Anyway, I talked to my friend he said don't worry it's just purging air from the system on the other side of the thermostat. I need to take it for a drive but I ran out of time - parenting chores and etc....

He said if the cylinder head gasket was toast it would be a smoke show. Hopefully I get a chance to drive it later tonight.

well it's working great! thanks everyone for the help.

Make sure to run the car with heater on so coolant fills that back up too or you may wonder where the coolant went from overflow next time you use heater. Head gasket coolant leak will not go away after startup like valve seal leak does (when they are marginal) and the smell will be completely different than burning oil is. Glad you got things sorted out.