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940 940 engine speed surge at constant throttle

HarryK

New member
Joined
Jun 14, 2016
Location
Asheville, NC
Hi all,

Revisiting the infamous wandering rpm issue on my 1992 740 Turbo AW71 (with 1994 940 block).

I just repaired a misfire on cylinder 2 due to a sagging IPD performance plug wire that was shorting on the heat shield. This is the second one in the set that failed in this way. The car has been running with the misfire for a while now and it was causing the engine to shake and run very rich.

Anyways, now that the engine is running smoother I tested the car with some spirited driving which resulted in a single instance of stalling (while coming to a stop), and now the engine rpms fluctuate during idle AND constant throttle input.

They fluctuate by no more than 200rpm in about 1-to-3 second intervals mmmmm-nnnnn-mmmmm-nnnnnn lol. This happened at idle and at constant throttle input even over 2000rpm.

While at idle I unplugged the IAC and the MAF and nothing really changed (this did result in respective fault codes on the OBD1 box). No other codes shown besides 3-1-1 faulty speedo. I also replaced the disintegrating vacuum line going from the intake manifold to under the dash with no real changes. Most of the other vacuum lines have been taken care of, and the PCV and throttle body were replaced/cleaned in the last year/5000 miles. I also tried a new VEMO MAF a month ago and the car wouldn't even run with it installed. I know the radio suppression relay connector is failing (female part of the connector completely falls apart when unplugged).
Spark plugs are new (2 IPD performance wires remain, 2 new duralast wires - mismatched bc I threw away the thread adapter things when I installed the spark plugs last week.) but they were run while the engine was misfiring for about an hour total.

Any possible causes for the surging rpm aside from other possible vacuum leaks?

Thanks for reading,
Harrison
 
I have the issue of fluctuating idle too in my 97 940T, but I've never found the issue after years of trying and replacing different stuff.
It's most noticeable when I come to the first stop after starting the car and driving away, revs dip down to like 500-600 briefly and then go up to where they should be, but keep fluctuating by like 100-150.

I gave up at some point and just accepted it.

No help I know, but following if someone has some suggestions.
 
I have the "dropping to 500 (or even less) when coming to a stop and then getting to normal idle" a lot, but since I found and addressed a large vacuum leak (hose from intercooler pipe to throttle body was not shoved flush to the throttle body, leaving a gap underneath it), the idle has been very smooth.

I have had funky idle before due to distributor being full of crap though
 
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