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From digest.v4.n827 Thu Jul 11 01:14:49 1996
From: Don Eilenberger <>
Date: Wed, 10 Jul 1996 22:22:47 -0400
Subject: RE: E28 Idle Surge

Scott asks:

>Subject: E28 Idle Surge
>Hi everyone. My new to me '87 535is (5spd) with 109K occasionally
>experiences an idle "surge" once or twice a day. I'll pull up to a light,
>foot on clutch, and the idle will surge over and over again between 900 and
>1200rpm. Next light, nothing (other than a rough idle I haven't dealt with
>yet). Any suggestions? TIA.

The surge you're feeling is the engine trying to NOT stall.. since it only does it intermittently, it isn't a problem with the Idle Control system. Your '87 has the newer system which is run directly off the Motronic, and the only maintainence needed on it is to occasionally (I try to remember to do mine about every 12,000 miles, but sometimes I forget) clean the actual valve. It's the black/silver thing hanging off the back of your intake tube between the airflow meter and the throttle body. It can be removed fairly easily and then cleaned with some carb cleaner. A spritz of WD40 after cleaning assures smooth operation.

OK - what your REAL problem is, is the crappy idle. The car it actually trying to stall, and the ISC system is preventing it by bumping up the intake air, hence your surge..

To cure the crappy idle (mostly) [I spent LOTS and LOTS of time and $$$$ on this.. believe me!):

  1. Open up the gap on your plugs - instead of the specified 0.028" use 0.032-0.035"
  2. Open up the gap on both your intake and exhaust valves by approx 0.002" LARGER than spec. The intake makes the biggest difference.
  3. Use premium gas - it has the most fuel-injector cleaners in it.
  4. A dose of Techron won't hurt
  5. Make certain your ignition components are up to snuff.. figure plug replacement at 20,000 mile intervals, plug wires at 60,000, and cap and rotor at 50,000. If any are marginal, you will have problems. Use the Silber (Silver) Bosch plugs spec'd for the car.. they are designed specifically for this engine (there is NO cross-reference in the bosch to any other engine). Use Bosch cap and rotor and factory wires (they are available at a 'reasonable' price [ > $100] from several vendors in Roundel).
  6. If you can beg/steal/borrow a gas analyzer, try setting the CO setting on the bottom of the air-flow meter. I personally found this had very little effect - the CO at the exhaust is basically unmeasurable, you've gotta go to the tap on the exhaust manifold.. you want it 1.5-2% at idle. Richer (ie 2%) is better for idle - if you can attain it (I couldn't).

Once this is all done - the car won't idle perfectly (a failing of the '87 emissions system) but it will idle acceptably and not surge or stall. The '87 system was apparently a 'transition' system, '88 and on idle fine, but the '87's were ones the dealers said "they all do that.." about.

Somewhere, someplace there is a FAQ with my quest for a smooth idle on the '87 535 BOHICA.. anyone know who has it??

Phil Marx - any additional ideas???

Don Eilenberger
Spring Lk Hts, NJ, USA
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