RE: M42 Stumbling -- Cam Timing?

["Shaun Dustin" <sdustin@xxxxxxxxxxxxxx>]

Tom,

I don't know what I'm going to do with it yet, but I sincerely appreciate
your help. Your posts help me look at the problem from a different angle.

I got to thinking about the cam timing, and I think if it were something
mechanical like that, it would be consistent; if one of the cams or the
crank had slipped a little bit, it should happen all the time within a given
set of conditions, and it doesn't. I'm not ruling it out, just moving it to
the backburner. The fault is too inconsistent in my mind for that to be at
the top of the list.

This morning on a hunch I popped the hood to look at the coolant temp sensor
on the block that provides input to the computer (before, I mistakenly
called the fan relay the coolant temp switch because when I ordered a
coolant temp switch at the dealer they sold me a fan relay and called it a
coolant temp switch). Anyhow, the spring clip that holds the wiring harness
to the sensor was missing. I tugged on the connector, and the idle speed
went way up. I pushed it back down, and the idle smoothed out a little bit.
Drove it to work, and it ran smoother than it has in two months.

I don't know that I'm out of the woods yet. It's not as smooth as it could
be, and as you indicated the Cat might be having problems because of the
over-rich mixture that's been running thru it for the last few weeks. I'm
going to redo what I thought I paid the BMW dealer to do, and go through all
the contacts and sensors very carefully again. I missed the coolant temp
sensor the first time through and may well have missed something else. I'm
also going to look at the position sensor, per your suggestion, particularly
where it plugs in to the black box under the intake manifold. The symptoms
sure seem to point to an intermittent fault in something that the computer
uses to adjust timing.

The throttle position sensor is an interesting idea. The throttle does catch
a little bit sometimes, but I thought it was just a sticky cable.

The thing that's odd about all this in my mind is that it sure seems like an
intermittent fault in the sensors providing feedback to the computer; I
would expect that any sensor errors or out of spec conditions in inputs to
the computer would be recorded and reported as error codes, but the mechanic
said there were no stored codes.

Thanks again for your help. I sure hope I can get this worked out. It's a
good little car.




Shaun Dustin

- -----Original Message-----
From: ToMoyer@domain.elided [mailto:ToMoyer@domain.elided]
Sent: Wednesday, March 05, 2003 5:46 PM
To: sdustin@domain.elided
Subject: RE: M42 Stumbling -- Cam Timing?

Shaun,
  I know I've written one or two times before on your problem and suggested
that you check and/or replace items which you have done, these items now
checking ok or are new can be ruled out as the problem, especially since you
had it at the dealer, they would have shown up bad when on the diagnostic
computer.  I also believe that I stated that another thing to check was the
crank position sensor.  It is located on the front of the engine, right side
down near the crank pulley.  This is a sensor/trigger for the computer and
if I'm not mistaken helps the computer adjust the timing electronically.
This is the next major step I would take, but I would have the dealer do it
as I don't believe it is a simple remove and install item (I have not done
it and little is written on it in my one Hayes Service Manual that covers
the M42 motor).
  The other possible cause that comes to mind, which I stated previously ,
is that your CAT can be partly clogged from too many miles with a bad O2
sensor.  When the CAT goes bad, some of the ceramic material inside the CAT
will break/burn off and fall to the bottom of the CAT housing.  If enough
falls to the bottom, the outlet and/or inlet to the CAT becomes restricted
or clogged, effectively choking the engine because there is too much exhaust
back pressure and poor exhaust flow from the engine thru the exhaust system.
You can very easily check to see if your CAT is a suspect.  You don't even
have to raise the car to do so.  Slide far enough underneath the car to
reach the CAT, basically directly under the passenger front seat area.  With
either a dead blow hammer or a rubber mallet, lightly hit the CAT housing.
If you hear "rattling" from the inside of the CAT, the CAT has at best
started to self-destruct.  If the car has been driven too many miles with a
bad O2 sensor or more time than normal between O2 sensor replacements, the
CAT will start to go bad.
  The other thing that could be causing your problem I know I also mentioned
previously.  That is the throttle position sensor.  It is located on the
throttle body and sends signals back to the computer on how far open or
closed the throttle plate is.  It is basically a resistive coil and will
eventually wear out.  There are resistive values for closed and wide open,
but I don't have them right at my finger tips.  Again, though, this is
something that the dealer should replace as it does have to be adjusted
precisely.
  Now it is possible thru many miles that the timing chain and its tensioner
are worn out and that because of the extra slop in the chain the cam shafts
are not perfectly timed to the crank and pistons.  It is also possible thru
lots of miles that the lobes on the camshafts are worn too flat which will
not open the valves all the way and cause more or less overlap, poor fuel
intake to the cylinders and poor exhaust escape from the cylinders, and
cause running problems.  I forgot how many miles you said were on your
318is, but I can tell you that on my '91 318is there are 143,568 miles and
there is little to no sign of slop in the timing chain (timing chains and
their tensioners often last many, many miles before causing a problem
because they are continually oiled when the engine is running) and I have a
very crisp, clean and sharp throttle response at all rpms.
  Other than what I have mentioned above, and with out driving the car or
seeing it and doing a few tests, I can think of nothing else off the top of
my head that would be causing your problem, other than bad fuel injectors
(either dirty and restricting flow of gas into the cylinders, or nozzle too
worn/too big letting too much fuel into the cylinders).  Again, on my 318is,
the original injectors are still in place.  Every time I change the oil I
add a can of Jectron Fuel Injection Cleaner.
Hope this helps you finally find your problem, happy motoring,
tom
BMWCCA Member
former ASE Certified Tech
ISCET Certified Electronics Engineer & Tech
'84 318i
'85 318i
'89 325ix
'91 318is

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