Jeff Bowlsby

Is this decel condition normal for a stock L-jet 1981 928? I have had other L-jet cars (not 928s) that do not behave this way.

As an example, when exiting the highway at an offramp, the engine RPM naturally reduces towards the stop...when it gets to about 800rpm then it instantly increases to about 1400rpm, and repeats that cycle (800-1400rpm) 2-3 times until the car comes to a full stop where it idles steadily at about 950rpm. Feels like standard manual transmission behavior while downshifting, but this car is an auto trans and seems odd to me.

GregBBRD

Doubtfully this is caused from the transmission.
That is probably an engine surge, most likely from an overly rich fuel mixture.

joejoe

My 81 auto does this a bit....have not found what it is .Mine is from about 700 - 1100 2 times then settles.

Jerry Feather

My 82 auto is doing exactly the same thing, and in addition I can hear a slight irregularity or surge in the idle most of the time. I think that is a symptom of the same issue. There is another thread about that here very recently. The thought there is that it probably a failed fuel pressure regulator that is letting fuel into the vacuum line. I think Greg is right since my mileage is kind of poor. I get only about 16 on the highway and about 10 or 11 around town.

Simon928

Yup, that other thread was mine. I know that I have to replace my FPR so that could very well be the problem. My surging issue began after an intake refresh though and the FPR hadn't been changed, so it could've just started acting up at the moment that I put everything back together, but it would be a pretty big coincidence. Jeff, did you do anything to your car recently that coincided with the beginning of your symptoms? I'm just wondering if there are any commonalities between our vehicles that could be causing similar problems. I'm not sure which parts besides the FPR and the computers that can be contributing to the rich mixture that Greg is describing.

PorKen

Sounds like the vacuum limiter? (UFO shaped can on the side of the throttle body.)

They are NLA and unnecessary for automatics - factory deleted 83/84 for autos - manuals unfortunately may need them.

Pics and more info - https://rennlist.com/forums/928-foru...l#post16234783

PorKen

You can test if it is the vacuum limiter by disconnecting and plugging the small vacuum line to it.

GregBBRD

Anything is possible, but a bad vacuum limiter usually just makes the idle stick high (generally fail open).....and will not open and close randomly, as the car returns to idle.
If the OP knows he has a bad pressure regulator (which would leak raw fuel into the intake system), this would be a good place to start.

GregBBRD

Your problem could easily be a bad vacuum limiter, as Ken mentions above. This is the "normal" failure mode, before the limiter fails completely.
I also agree with Ken that with an automatic, I'd probably remove the vacuum limiter and buy the two factory plugs to cover the openings. (999 703 171 40 and 999 703 172 40. I keep them in stock, if you need them.)