Stumbling Bimmer.."the usual suspects"

["David A. Leonard" <dleonar1@xxxxxxxxxxxx>]

Re: the 535 with indigestion>>

The first thing I would try is this:
Get a can of Wurth Contact-ol, or Caig De-Oxit, or any other commercial
electrical contact cleaner.  (Try Radio Shack)

Remove clean and replace all of the wiring connectors to the entire engine
management system..Airflow sensor, O2 sensor TPS Switch (This one has given
me wierd "no idle control symptoms" on many occasions, always fixable for
several years at a time by cleaning contacts), all the relays contact pins
in the fuse boxes, the DME main connector, etc.

I like to put a dab of dielectric grease on all of them before reconnecting.

I do this to all of my things mechanical, cars trucks, boats, aircraft..it
gives you a baseline to start from when things start failing.


There are a lot of connectors that carry very low voltage signals around the
engine, and high resistance can cause wierd symptoms.

I would then disconnect the battery for a few minutes( this seems to reset
the check engine light pretty consistently, my 88 735I used to kick this
light when the car got really wet until I cleaned and greased the 02 sensor
wire connector.)

This sounds like a big project, but it can be done in 1/2 hr.

It might not fix your problem, but the Bosh FI handbook suggests that most
of the time, the problem is a connection.


Carefully inspect the rubber boots that hold the induction system
together..surging can be caused by a vacuum leak letting air in downstream
of the airflow meter..this " False air" can cause a lean condition. (Really
mess you up with a turbo!)  I have seen the boost rip subtlely at the joint
where the little tube runs to the AIC motor, a small crack at the joint can
make the air leak come and go.  The rubber is about $35.00

Where the problem manifests itself at highway speeds as well as Idle, I'd
look pretty closely at the ignition system.



How old are the distributor cap, rotor and ignition wires??..Inspect the
rotor and cap carefully, replace all this stuff if they are old, if not
clean them and soak them down with WD 40 to drive out any moisture.  An
arcing coil wire can shut you down like this.  So can a carbon track on the
rotor or distributor cap.  All of these are, in my opinion, more failure
prone than a DME. (These are standard Tune up parts for most cars, but as
they cost $150, we BMW pinheads usually run them much longer than we
should.)  I can't for the life of me figure out why they aren't available
aftermarket for $60.. Napa has replacement caps and rotors for about $60.

If you run the engine in the dark with the hood open and listen and look you
can often see and hear the arcing, particularly in high moisture conditions.
Some people like to mist  h20 on the wires to see if they can induce the
problem.

Again, this may not fix the problem, but high tension wires are definitely
on the list of "usual suspects"

Have you ever changed the fuel filter?


I would than get an FI pressure testing kit, and ascertain that the fuel
pump is putting out enough pressure. I am told in tank FI pumps can fail
slowly, but I have never replaced one.

Only after all this would I start throwing FI electronics at the car.
Bentleys has good troubleshooting info for the FI system, check the coolant
temp sensor resistance, and any of the other stuff they recommend before you
throw a DME at it.

In my experience it is almost always the ignition system that stops a car or
truck, (It is ALWAYS ice, bugs or gunk in the carburator on a snowblower).

Good luck, it is usually something simple like electricity leaking out where
it is not supposed to...

Dave Leonard

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