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Re: 535is Driveability

Hi Matt.

I'll give this a stab, since I've had similar problems, and although I
still am not absolutely certain about the resolution, I have made
progress, and I think I have one final component to replace to solve it
forever... 

In my case, the problem could be *induced* by using the cruise control. 
My Tips Rep suggested that dirt and water had accumulated on the bottom of
the gas tank, and that during relatively constant throttle - cruise
control made this easier (as you said, "steady speed is maintained"), the
fuel flow pattern is such that it kicks up this dirt and water that enters
the fuel line, and exhibits the bucking you've noticed.  Otherwise, it
simply sits on the bottom of the tank.

I could "cure" it by varying the throttle for some period of miles, and
it eventually works its way through.

As with you, this happened, on and off, for awhile.  At first, at 150k, I
drained the original tank.  This cured it for awhile, but it came back,
mainly since the tank was wearing out. In fact, I found a bunch of dirt
and water in the tank, confirming his suspicions.  (Horrible job - I
didn't use a respirator and was sick for days - dumb, I know). 

It was always worse in the wintertime - my Tips Rep theorized that the
combination of oxy fuel and water freezing to ice and blocking the in-tank
pump was the reason.

Last April, I *replaced* the tank (it was badly rusted on the main seam
and the filler neck, and Hendrick gave me a great deal), and the in-tank
pump, as well as the filter again and much of the fuel line.  Again, it
subsided for awhile.  I had removed the "fuel mufflers" in the engine
compartment last summer (93) since they had started leaking and BMW
doesn't suggest replacing them, just bypassing them (the part is NLA).  I
was looking for any component in the fuel system that might, due to
gravity, be susceptible to accumulating water.  I think that is the key to
the solution.

In my car, the problem is back again.  This leaves the main fuel pump and
the fuel pressure regulator.  Both are original (229k).  I bought a main
pump and haven't put it in yet. I probably will at the 240k Major Svc. II
(x 4 8^)), when I replace the fuel filter.  I figure I'm on borrowed time
anyway with both components since they're original, so I'm writing this
off as a maintenance item. 8^)

But let me make one thing clear:  THIS IS A FUEL PROBLEM.  Don't waste
your money replacing electrical components, since they're fine.  Perhaps
Jim C. could confirm or deny this, but I believe that Motronics are more
susceptible to bad gas than other FI systems.  Since I have learned how to
"induce" and "cure" the problem, I am convinced it's a fuel problem.  I
just hope I don't have to drain the bloody tank on a regular basis. 

I hate oxygenated fuel!

I hope this helps a bit.

Regards,
 .steve_been_there_done_that.
1983 528e
229k

p.s. Your injectors are probably fine.  The bouncing idle is common while
the crap is in the fuel lines.  Once the crap has passed through, the idle
returned to normal.  I had that problem too.  Techron or dry gas always
"cured" the problem, temporarily. 
- ------------------------Mobilitaet ist Leben------------------------
Steven J. Bernstein                       Home: bernstein@domain.elided
P.O. Box 11242                            Work:    bernstein@domain.elided
Hauppauge, NY 11788 USA      __________   Home:     (1) 516 696 2841
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