Paul V - an admitted newcomer to our ranks asks:

>Subject: Another E28 Hesitation Problem - HELP !
>
>My 1987 535is (60,000 miles) consistently hesitates around 2100 rpm. It
>feels like the engine elctronics shut off for a split second, then power
>returns. This cycle repeats every 1/2 to 3 seconds as long as I stay in the
>2100 - 2700 rpm range. Below 2100 rpm and above 2700 rpm there's no problem.
>
>I've tried the following:
>
>Replaced the cap and Rotor.
>Replaced the Fuel Filter.
>Replaced the O2 sensor, but just in case I'm going to test it anyway using
>Mark Calabrese's write up in the October Roundel.
>Checked the fuel delivery rate as specified in the Bentley manual. Fuel
>delivery was well within specs.
>Checked the working fuel pressure as specified in the Bentley manual using a
>home made pressure guage (a water well pressure guage attached to a brass
>tee fitting using fuel line hose). The pressure was below Bentley's specs,
>but within BMW's factory manual specs. I replaced the fuel pressure
>regulator anyway.
>Checked for vacuum leaks in the intake ducting. Haven't yet checked the
>injector seals.
>Checked the coolent temp sensor, the throttle position switches, the air
>flow meter resitence, the idle control valve, all according to Bentley's
>troubleshooting techniques.
>Adjusted the valves, Replaced the spark plugs.
>Tried Filippo Morelli's suggestion of spraying water on the cap, coil, and
>spark plug wires to look for arcing. I didn't see any arcing, but the plug
>wires are ducted, so I didn't wet them down for their whole length.
>
>I'm a new subscriber, if anyone can suggest any archives to look into, or
>any other ideas, I'd appreciate it.
>
>Thanks,
>Paul V.
>1987 535is
>1980 733i

WOW - you've attacked ALMOST everything in a very well thought out manner. A few random thoughts:

1. Coil.. possible. Dunno where the voltage requirement peak is on these systems (I used to have an engine scope which would show the pulse train and let you see where max voltages are being generated), but USUALLY - ignition systems have some RPM where they produce peak voltage (has to do with the resonant circuit of the coil and ignitorwhich in this case is the Motronic). Mebbe a coil? One off a wreck would prove/disprove this idea. The coil COULD possibly be breaking down internally at peak voltage.
2. Motronic.. possible. Sounds like a random sort of failure (haven't heard of this, but anything is possible). If you can't track it down without help from someone here on the list - give me an EYell.. I have a spare '87 Motronic sitting in my garage from BOHICA ('87 535i). Could loan it to you for cost of shipping to/from..
3. Relays.. as fuel pump output peaks, the current required also would peak. LOTS of mystery problems on E28's have been solved by replacement of the main relay (controls Motronic and fuel-pump).. it is the one unique relay in the car (has an extra 87 terminal). Unfortunately, a netter recently bought a bunch of BOHICA junk from me, including my spare main relay (doesn't fit an E34). On BOHICA, when it got to intermittent running, I tested this relay as per Bentley, and it looked OK. Replacing it and the Motronic at the same time cured the problem.. dunno if which was the actual problem, but didn't feel like following the experiment to it's logical conclusion (at the side of the road in a rainstorm undoubtably).

Also - is your car automatic or manual? Have you checked and adjusted the throttle position switch? The RPM you mention is about where the injectors are supposed to be turned OFF on coast-down (decreasing engine speed) then they are turned back on at about 1,000 rpm.. seems sorta a coincidence that this is the RPM where you're having the symptoms.

Best, let us know how you make out!