Coolant System Problem

[markd248]

So my 91 318i has recently started overheating. Symptoms were that the motor would heat up quicker than normal. Usually hitting the operating temp about 5 to 10 minutes faster than normal. The upper rad hose was very hot and pressurized and the lower hose was cold. It would overheat after driving in stop and go.

I quickly figured it was the thermostat. I've had it for 2.5 years now and never had a coolant issue, so I assumed it wasn't a bubble. I just tested the thermostat on the stove top and it worked just as well as the replacement that I bought.

So my question is, should I immediately assume that it is the water pump? Is there a way that I can test the water pump before pulling the fan apart, etc.? I understand that the water pump doesn't frequently fail, so I was wondering if there's something else I should troubleshoot before tearing everything apart?

[bmwman91]

Do you know how old the water pump is? Some of the water pumps for this engine have plastic impellers which are known for exploding. I don't know of any way to test it without removing it. If the impeller did blow up, you are probably looking at a full cooling system tear-down to back-flush everything a few times.

[Balleristic31]

I've had a similar issue with a very stiff upper hose with overheating issues on an m42 car before and it was in fact a blown up water pump. Take a look at what happened to the pump!


I would also do a quick block test (can be rented at any local auto parts store) to rule out a potential head/headgasket issue with exhaust gasses getting in and pressurizing the cooling system.

[markd248]

It has 234,000 miles on it, so I wouldn't be surprised if it is the water pump at this point. I've had it for the past ~30k miles and haven't done it, so I have no idea when the previous owner would have.

Anything else I should consider replacing while I'm in there? I have new belts already, so I can't think of anything else.

[ccsdo5]

Probably not a bad idea to also do the thermostat, they're cheap anyway.

[RCtheRed]

I recently replaced my water pump as well after it failed and the car over heated and started boiling coolant..... :(. After that I replaced the water pump, thermostat, fan clutch, and belts. Runs great now, and stays at half mark on the temp gauge after warming up.

[bmwman91]

If you do the thermostat, make sure to clean out the bypass groove that connects the 2 chambers. It makes bleeding the system MUCH easier.

Also, and this applies to just about any assembly that you bolt together, install the thermostat housing as shown below. The red stuff is gasket sealer, the green thing is the gasket and the gray stuff is the parts you are assembling.

[markd248]

I think I'm all good. I just took the car out for a 20 minute drive and it sat right at 1/2 way on the gauge. I'm going to drive it to work tomorrow to make sure. Thanks so much for everyone's help. New coolant, Tstat, and water pump did the trick.

[mjimport]

I love hearing these success stories, never gets old!

[markd248]

Alright so I spoke too soon...

Help me brainstorm what else it could be. It drove totally fine on my 20 minute drive to work, which includes about 12 minutes on the highway. Car behaved fine for about 10 minutes on the highway on the way home and then started heating up in traffic and then stayed too hot till I got home. I stopped twice to check everything out and here's what I have:

Upper Hose is hot. It doesn't feel awful though. I can wrap my hand around it and keep it there. Its definitely pressurized but I can squeeze it some. I compared it to another car that just stopped and it felt just as hot as the e30's, if not hotter.

Lower hose is cold, but again, about as cold as the other radiator hose that I just compared it to.

Please help me brainstorm what it could be and what I should check. I guess it has to be the radiator? I could pass coolant through it just fine yesterday though when I was filling it. Where should I look or what should I do to troubleshoot? Is it worth replacing the coolant temp sensor or taking a look at it?

[AndrewBird]

You may have an air bubble in the radiator. Jack up the front driver side of the car and run the engine up to operating temp with the radiator cap off. Fill with coolant as needed. Make sure the heater is on full hot. Fan doesn't need to be on, just temp knob.

[digger]

partially blocked rad?

[5Toes]

is the car low on coolant? did you drill a small hole in the new thermostat?

[markd248]

You may have an air bubble in the radiator. Jack up the front driver side of the car and run the engine up to operating temp with the radiator cap off. Fill with coolant as needed. Make sure the heater is on full hot. Fan doesn't need to be on, just temp knob.

Should I open the bleeder screw too?

is the car low on coolant? did you drill a small hole in the new thermostat?

No and No. I've heard conflicting opinions on drilling a hole in the thermostat, so I just gave OEM BMW the benefit of the doubt that they did it right.

[ccsdo5]

did you happen to test the thermostat before installing it? Probably just needs to bleed some more. Do as Andrew said and see if it goes back to center