I have been having problems with my 87 S4 idling properly....changed the air filter....did better tho it still surges....but now my gas mileage has gone south................is there any way to lean it out?????I suspect I have problems with the mass air flow but that is all "rocket science" to me...timing belt, clutch I will fix...but I am a lost ball in the high weeds when it comes to this.........HELP!!!!!!!
Tom Taylor


It might be leaking in air somewhere.
Also, there is an idle regulator which is activated by the LH which might be non-functional if you have problems idling.
No, there aren't any ways to lean it out. It's all computer controlled by the LH, and should always do the right thing. However, leaking air means the MAF don't really tell the LH the true story, which mess things up.
And a non-functional idle regulator is obviously something bad as well. :-)



It's not too difficult, really. #1 issue we shade-tree mechanics must get over is fear that our 928 from the Black Forest is mysterious and complex.
I'll never forget about a month after I acquired my 928 that I realized, "it's a car!" :)

One of the first issues I had was a surging in the idle. Often our car's issues are simple. As Johnny posted, it probably has an air leak somewhere.
That was my issue. My PO had made some "improvements" to the Porsche design.... :) If you have the original rubber hoses under the hood then it's a good chance one of them has disintegrated. They sort of turn to an oily play-dough consistency after 15 years. Take the air box off and look around. Some mechanics have a smoke machine - they force smoke into the MAF and look for it coming out somewhere. You can do something similar - block off the MAF by covering it with a baggie or grocery bag with a rubber band.
Unhook rubber hose from the brake booster and blow into it. Listen for an air leak. Or if you are a smoker you can huff and puff into the hose.

If you don't find a leak, disconnect the electrical plug from the MAF, and start the engine. If it runs "better" (as in no longer surging) then the MAF may be the problem, and then it would be good if you can find someone else to swap MAF's and try a known good one.

Usually I think a bad MAF just causes poor or rough running. Surging is usually an air leak. There is air getting into the engine that the computer doesn't know about, but other sensors see an imbalance and so the computer tries to correct for it. Up and down go the RPM's.

There is a good book on the Bosch fuel injection that is worth getting, even for "mechanics" like me and you.

Hey, I turned 190,000 miles last night! However I think I may have a knock sensor issue...

John Pirtle
87 a/t 190.1k (not running like a scalded cat) http://members.rennlist.com/pirtle


A vacuum leak would tend to make the fuel mixture more lean. The main problems for running too rich are

- Defective Temp II sensor

If the LH "thinks" that the engine is cold, it will go into cold start enrichment mode. The steps to check the Temp II sensro are outlined on my website www.electronikrepair.com in the diagnostics section.

- Leaking fuel pressure regulator

Unplug the vacuum line from the fuel pressure regulator. If gas leaks out the diaphragm has a leak and you need to replace the regulator.

- Defective MAF

A quick and easy test is to remove the plug from the MAF and start the engine - this foces the LH into the limp-home mode. If the engine runs better, your MAF is probably bad. If you know any other 928 owners, the MAF from any 1985 to 1995 will fit your S4. Try plugging in the known good MAF to see if your problems go away.

- Defective LH fuel injection controller

One of the failure modes of the LH is to shoot too much fuel into the engine. The easiest test is to plug in a known good LH into your car, let it run for a while (burn out all that extra fuel) and see if the drivability improves.

Good luck,
'93 GTS
'87 S4
'79 Euro 5-liter track beast