Allanr

I just acquired an 87 928 S4 and am doing a full service on the car. One problem I would like to have some suggestions on is fluctuations of rpm at idle. The idle rpm will vary by about 200-300 rpm and will occassionaly die if the idle gets too low, restarts promptly. Is this mostly likely to be a defective MAS or the idle valve? Thanks
Allan

ErnestSw

Sounds more like a vacuum leak or a weak battery.

geekapalooza

I had the same problem appear after owning my 87 S4 for a few months. In my case the remedy was a major vacuum leak @ the idle speed solenoid. Thus a full R&R of the intake was in order with replacement of all the old vacuum and emission hoses and connectors, knock sensors, clean and test idle speed solenoid, test and replace Idle Switch if necessary, intake and throttle body gaskets, etc...

If your S4 has never had this service, it's due regardless of how low the mileage may be. You can find an excellent how-to writeup at http://928oc.org/journal/intake1b.pdf

Cheers,

Garth S

It could well be a sticking disc in the idle stabilizer valve. Open the ~15mm vac line from the 'Y' connector just behind the throttle quadrant. Into the line that dissapears under the intake manifold (the ISV is directly across - buried!) spray a cleaning lube like LPS-2, or even WD-40. Start the engine, spray some more, shut down and let soak for the duration of 1-2 cool beverages, or overnight.
If the idle improves, hope it stays that way - at least, it may point to the problem. If it is a vac line/bad valve, then the full monty Geekapalooza hits is amongst the next, and more $/time steps.

John Speake

Try a battery reset, if the behavour is consistant. If not consistant, then it is probably a hardware problem.

Deb

Plse explain "battery reset". I know I'm going to hate my self for asking!

jserio

I think he is saying disconnect the ground on the battery.

John Speake

Yes, I should have explained better, or called it "battery reset". Just disconnect the ground strap under the edge of the tool tray for a few seconds, then re-connect. This forces the ECU back to defualt settings. This can sometimes help with idle problems.

Flott Leben

Garth,

Exactly which line are you referring to? Drivers side, by the throttle body, the "y" connector that also has a vacuum line to the brake booster attached to it? Which line do you spray the WD-40 into? To the left or right of the "Y?' Won't the car stall out with that line disconnected (since it has a vacuum line). Thanks.

Garth S

That is the line - the top left of the 'Y' as you face the block is the branch that crosses underneath the manifold to the well hidden ISV on the right side (passenger). The car may stall with it open, may not - no big deal, for all you're trying to do is assure the cleaning lube gets inhaled all over the valve internals.
I recall seeing a good write up on this somewhere - Nichol's Tips ?, or do a search of posts.

geekapalooza

http://www.jageng.com/topten.htm

Flott Leben

Thanks, guys!

Allanr

Thanks guys for all the helpful information. It is great to get feed back from people who know the 928.
Allan

Old & New

Allan & Flott,

The Jageng write-up sucks. He is oversimplifying the necessary procedure...

Notice that the air flow is into the intake manifold, away from the idle valve. The only way to get lubricant into the valve is to attach a foot-long flexible piece of 1/8" vinyl tubing to the end of the spray lubricant nozzle and feed it down the black hose toward the idle valve. Spray the heck out of it, maybe 15 seconds, to allow sufficient lubricant to accumulate in the valve to be effective.

As mentioned, idle hunting is just as likely to be caused by something other than a defective valve, so good luck.

Bill Ball

OLD & New:

If you leave the vacuum hose disconnected and start the car, the lubricant will be sucked into the idle control valve. Car may not run great while you do this, but it should help to get the lube where you want it.