1990 735il overheating

[mshill]

after this car is warm, at idol it overheats and cools off as i go down the road again. the repair shop changed the waterpump, $500 bill. still overheats. they tested for cracked head or blown headgasket. that looked good. brand new radiator in february along with thermostat. they replaced the thermostat again. it still does it. they said they tested the fan and fan clutch and they look good too. now what? any ideas?

[zmuff]

I would still lean towards the fan clutch not working properly. How did they test it? My 535i was doing exactly the same thing as yours, and although the fan was still spinning, it wasn't enough to cool things down when stopped or moving slow. There is a way to check with a rolled up newspaper, just gently put it up against the turning fan. As you increase the resistance, and bad fan clutch will stop, but a good one will keep turning and beat the newspaper. I could actually stop mine with my hand and spin it backwards while it was running. My new fan clutch MOVES a lot of air and it can be heard while driving until it releases.

If that's okay, there may be some air still trapped in your system, but that generally make you run hot all the time, so probably not.

Lastly, there is a small hose running down the left side of the engine compartment from the radiator to the expansion tank. When your car is cold, remove the cap and start the engine. As the engine starts to warm up, a small stream of anti-freeze will start to pee back into the expansion tank. If there is no flow there, your engine will eventually start to overheat. It's a small hole and clogs easily. Line from a weedeater works real good to unclog that little bugger.

These checks you can do yourself to save a few bucks.

Good luck!

[Bear]

I second the fan clutch as the culprit. My 1988 750 had this problem and even with the AC engaged and the aux fan spinning at top speed it was barely enough to keep the temp gauge out of the red (of course the V12 needss a lot o cooling). is the aux fan working good (turn on AC and see if that fan spins, also eleiminates the fan relay). you have a fan temp gauge screwed into the right upper side of the radiator and that controls the aux fan speeds (low/high), I did replace this item before the fan clutch but it did not help ( a $34.00 waste). Again, I vote on the fan clutch.

[dlog]

The fan clutch could be a possibility, but I have experience that all cars vary when it comes to the resistance a fan clutch puts off. When you first start your car up when its cold listen to the sound of the engine. You will notice as it warms up the noise from the fan clutch will start to quite down as the clutch disengages. Now as the car is heated listen for that same sound coming from the fan clutch to start back up again. Also you can tell by how much air its putting off. Put your hand near the fan shroud when it’s cold and compare the flow when it’s fully warmed up. They should be the same, if your car starts to overheat and you don't hear or feel a good flow of air coming from the fan, than you know something is wrong. So if you test those out and they come back ok, check to see when the car is warm and appears to be going past the half way mark if the acc. fan is kicking in.

If the acc. fan kicks on then that means that your coolant is really getting that hot and the cooling system is doing everything that it can to cool it down. If the fan does not come on, then that means that your engine is getting hot, but the coolant in the radiator is cool. Which would ultimately mean that you have some type of circulation issue? Check this by feeling the inlet and outlet radiator house from the radiator. They shouldn't be too much of a difference. Or the fan switch mounted in the side of the radiator is bad.

[Bear]

Sssuming you have AC, turn on the AC and the auxiliary fan should start runnig (low speed) and that checks the fan relay. With AC off start her up and let her idle, at about 195 degrees (somewhere just beyond 12:00 O'Clock on the temnp gauge the auxiliary fan should come on (low speed) let her idle and at about 3/4 on the temp gauge the auxilary fan should pick up speed (high speed), this will check that temperature switch in the radiator. DO NOT let her over heat. I still believe it is the fan clutch since she only overheats in traffic or at low speed and is OK at hgh speed or slomewhre above 45 MPH.

[Bear]

Go to http://www.nmia.com/~/dgnrg/page_2.htm
this is Don Gales website and deals exclusively withc the 735 and has great pictures and text on how to check the cooling system as well as other fices.

[mshill]

well i've replaced the fan clutch and i still have the problem. new radiator,thermostat,waterpump and fan clutch. they tested at the repair shop for any head leaks and according to tests its ok. yet they still want to pull the head. now what? is that my last resort? help please.

[Bear]

Some of these cars are very difficult to bleed air from the cooling system. Go to: http://www.nmia.com/~dgnrg/page_2.htm This is Don Gales website and he has a lot of invaluable info on your car to incude how to bleed the cooling system.

[dlog]

Do you know what test(s) were performed by your shop to test the headgasket?

I had a simular problem w/ my 735iL, and it ended up being the headgasket. My pops and I Knock it out in no time and saved mucho danero.

Mine would overheat and coolant would discharge from the overflow tube. Because the headgasket was shot it was alowing exhaust gasses to flow into the cooling system, thus causing air bubbles in the cooling system. Pretty much causing a vapor lock in the cooling system, not alowing the coolant to circulate.

[mshill]

they tested the coolant system and no exhaust is in the coolant. they also tested the cylinders for compression. it all looked good. i've been bleeding the air out of it. there is some air. not sure what to do next. i want to avoid the head work if possible.

[dlog]

Well it sounds like they did all they could do, maybe besides checking the cylinders for moister. But if you have any air in the system it will overheat no doubt. I experience this first hand. Follow Bear's Link and Don Technique will get all the air out of the system. I know it help me out a lot. Make sure to add it to your favorites, because you may have to visit for something else.

If you continue to overheat, continue to bleed the air if any present, and if the air doesn't go away chances are its a small blown headgasket.