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Re: 1988 M3 (rough idle) LONG REPLY (archive)

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Posted by Ron Man on July 18, 2000 at 13:29:19:

In Reply to: 1988 M3 (rough idle) posted by dss on July 18, 2000 at 07:52:01:

I got this M3, it runs good for one exception, it has a rough low idle. The idle goes up and down, not drastically though, but enough to bother you.

What do you think is the problem?

Typical things to check are (in this order):

1) INTAKE GASKETS. Spray water or carb cleaner between them and the throttlebodies, and the head. If the idle changes, they're leaking!
Replace them, or if they "look good"
get the E28 M5 paper gaskets and reuse you
old thorttlebody gaskets. Remember to use Hylomar sealant.

2) TEMPERATURE SENSOR. This thing is located
in the cooling rail above the header. You are
interested in the rear-most sensor, the one with the blue plastic housing. Check it's resistance with the motor *stone cold*, It should read about 2.0 ohm's (possibly mill-ohms...not sure on this one... I can
check a new one and let you know.)

3) METAL ELBOW'S: There are three. One is on the
intake plenum itself. One is at the rearmost
end of the throttlebodies, one is in between the two throttlebodies. Check to see if they
rotate. If they do, remove them, clean both
mating surfaces with carb cleaner and re-attach
using JB-Weld or similar product.

4) INTAKE BOOT. This is the rubber boot between the air flow meter and the intake plenum. They crack! Just happened to me on my own car.

5) IDLE CONTROL VALVE. This sits under the throttlebodies. It should move freely. If it
dosen't, clean it inside with carb cleaner.

6) MOTRONIC UNIT: This is by default. If all
above check out, try swapping your control unit with a friend. Just give him a deposit first!

These are the most common sources of a bad idle.
Of course, check all rubber hoses in the vapor control system too, as they feed back into the intake. Any cracks or loose hoses here will translate into an intake leak.

Ron Man

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