1998 740i overheating, no leak...i need help - Bimmerfest - BMW Forums

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7 Series - E38 (1995 - 2001)

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Old 12-27-2010, 10:00 AM
Gryphon202 Gryphon202 is offline
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Location: Cheverly, MD
Join Date: Dec 2010
Posts: 1
Mein Auto: 1998 740i
1998 740i overheating, no leak...i need help


I'm a new member to bimmerfest yet I have been using it for a while and proven to have a great community of bimmer lovers. I just need a lil help....I previously had a coolant and oil leak and rough idling... i did all the following and still she overheats.....I can hear the boiling sound from the vslley pan after a while....

1) Changed all Gaskets.... head gasket is fine(no water mixed), valve covers, intake, trottle body, o-rings for accumulator and water pump.
2) new water pump & new belt
3)new valley pan with gasket and top
4) new ignition coils ( i need to change my fuel injectors too, bad mixture codes came up)
5)oil changed and new filters

all hoses are fine and car no longer leaks fluid of any kind... thermostat was changed by the dealer about 6months ago (could this be my problem, can i test it?)

I am not a mechanic of any kind but i'm not affraid of getting into it either... i did all the work myself and could use some tips ...thanks....

*** also had code p0150 come up.... bad cat sensor? can i test to see to my cats are the reason my car is overheating?

Last edited by Gryphon202; 12-27-2010 at 10:32 AM. Reason: adding
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Old 12-27-2010, 12:43 PM
balance balance is offline
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Location: NC
Join Date: Feb 2009
Posts: 766
Mein Auto: 97 740iL
I would check the fan clutch next. WITH THE MOTOR OFF, put your hand on the radiator fan (that connects to the water pump), and try turning it like a wheel. Does have a smooth resistance like it is riding on gel, or does it feel gritty or get stuck in areas?

The next thing I would check for is a clogged radiator. It would be alot easier with a laser thermometer since there is almost no room around it on these cars, but if you can, check to see if one side of the radiator gets hot while the other side stays cool. If you have a clogged radiator, this will cause an overheat, but radiators don't usually get clogged unless the coolant hasn't been replaced in a while and corrosion has built up. How old was the coolant before the overheating started?

After that, I would do a compression test to make sure that it is not the head gasket. Oil and coolant don't have to be mixed in order for the head gasket to be bad. There could be a loss of compression from the cylinder to the coolant system, and exhaust smoke could be escaping into the antifreeze. Try doing a compression test, or opening the expansion tank cap while the motor is running to see if you see bubbles in the coolant or you smell exhaust fumes. This would usually cause a coolant leak, but it wouldn't hurt to check it anyway.

Considering that you've changed the most common parts that would cause an overheat (thermostat, water pump), I would try bleeding the system to make sure that no air was in it. When you turn on the heat to full blast, does hot air come out of the vents? How long has the car been overheating (days, months)? Did the overheat happen after a certain event, or repair? How long does the motor take to overheat?

It is possible that you got a bad part (thermostat, water pump), which is why I asked if the overheat started after a repair. If the thermostat is stuck closed, this will cause an overheat, if the water pump impeller has separated from the housing, this will also cause an overheat.
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Old 12-27-2010, 02:47 PM
ridgeback ridgeback is offline
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Location: Helendale, CA
Join Date: Dec 2010
Posts: 3
Mein Auto: 1999 740IL
I agree with Balance, because of your previous situations with the car I would start off with doing a compression test. I noticed that you had mentioned that you did the work yourself. Now when you installed all of your gaskets, did you torque them and in sequence? Also, when the thermostat was changed 6 months ago did the dealer change out the sensors? It's a dumb question but there are some questionable dealerships and only install the basics and not what is supose to be included. If your compression comes back good and the radiator is good then the only other option would be the thermostat.
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