Erratic Idle

[Jim Cash <j.cash@xxxxxxxxxxxx>]

Erratic Idle ( not Erotic Idol)

I=92ve noticed a number of posts lately about "idle" problems.  Often thi=
s
for E28=92s ( I had one),  but for other models as well.

Some of the issues relate to major items such as "I know my head gasket
is leaking - can this affect the idle? - YES.

But more often there is a roughness, or slight surge at idle - which can
gradually get worse - or sometimes it might start all of a sudden.

There are all the normal things to check, but when talking to the BMW=20
mechanic who bought my 1982 528e/A  (when I traded two years ago)  he
mentioned something that I thought might be worth passing along.

He said the car was still running great ( 320k  km=92s - yes transmission
is still OK) , but that he did experience a period of "surging idle".=20
He said that "in his experience that is usually caused by a vacuum
leak", and while he had checked all the usual stuff (I=92ll list below) h=
e
was suspicious it was time to change the vacuum lines.=20
He said he pulled a few and "looked for any signs of moisture inside
them" - which indicates that air is getting into them - and then when
they cool the moisture in the air condenses.

He simply changed all the vacuum lines and "viola" no idle problems.

So if you are having a "surging idle" situation - the above may be a
good item to consider.=20



Before someone asks - the reason for this is relatively simple.  The
electronic control of the combustion process is tightly controlled by
many sensors, and it very dependent on the measurement of exactly how
much air (oxygen) is available for combustion.  This is measured by the
"air flow meter" (various versions of movable flaps, or hot wire
sensors, etc.)
But that only measures what is coming through it. If you have other
sources of air, like a leaky vacuum hose, then the mixture is "leaned
out" by the extra air. This is referred to as "unmetered air which will=20
affect the idle,  and then other sensors (O2, engine speed, etc) take
over and try to compensate to get the speed back into line.  They over
compensate and the idle starts to fluctuate up and down - you get the
picture.


If you have not tackled this issue before, here are the obvious items to
address (beside the vacuum lines already mentioned)=20

Idle Control Valve (electric air valve to control the amount of air that
bypasses the throttle) - make sure this is properly connected and is
clean inside. The movable vane inside must be clean and free to move.
Also check that the electrical plug is secure and making good contact.

Inside Throttle body - clean, with no residue affecting the full closing
of the throttle plate inside the body.=20

Sensors - all sensors can affect idle operation(as well as at other
times as well_ make sure all electrical connections are clean, dry and
secure.

Other sources of "unmetered air":
- - oil dipstick - must be tightly sealed in tube (remove while engine
running and you will see what I mean)
- - oil filler cap - clean all sealing surfaces and make sure cap is tight
- - same issue as dipstick.
(the above 2 items should be suspect if you suddenly notice an idle
surge problem)
- - cracks in the rubber air duct from the air filter to the throttle.
- - any loose vacuum lines or components - must be tight at all
connections.
- - check small plastic tube from intake manifold to the cabin air
temperature sensor - there is supposed to be a small "restrictor" in the
tube at the manifold end - this keeps air intake to the proper amount.
- - might help to check that the bolts are tight wherever there are
gaskets ( head cover, throttle body, oil pan, etc. (any leakage will let
in unmetered air)

Finally - some of the early E28=92s (82-84 maybe 85) had an electric
"purge valve" - located in the vacuum line beside the purge canister.
This was controlled by a "purge relay" located on the side of the fuse
box.  This was removed on a mod that came out in 84 or 85. The relay was
removed (socket left empty) and the vacuum hose was replaced with a
larger one that went into the throttle body - which I think was also
replaced at that time with one that was heated by connecting it to a
coolant hose. So if your car never got those mods that could be part of
your problem.

I also remember that my idle air flow valve, and its small computer
module (the one above the DME above the glove compartment) were upgraded
to a later version, on another mod about 1985 - but I have no records of
mod numbers, part numbers, etc. Any BMW dealer should be able to advise
whether your equipment is pre or post those mods.


Hope that helps.

Cheers

Jim Cash
London, Ontario, Canada
BMW CCA 102929



PS - What should you do if you see an endangered animal eating and
endangered plant?

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