Porsche Enthusiasts Club Forum

Well, the Subject title says it all really. I spent all day today trying to fix it.

It starts OK, as it always has done, but will rev up to 1600 rpm, drop then rev up to 1600rpm. Trouble is, it's intermittent. This morning it started fine, bit only went for quarter of a mile. After I changed the DME it was fine (Hurrah! I thought) but it only went another quarter mile. At least I was half a mile closer to home. Eventually I crept home at 1500rpm.

The first time this happened was during some torrential rain and there was a substantial amount of surface water. It was in a country lane and it looked like a river. We stopped and after half an hour or so the rain had stopped and the fault had gone. We drove home OK. Haven't had much reason to drive her for the last month or so. On Thursday she was fine, drove about 10 miles or so for a quick check. Friday evening, she packed up after about two miles.

So. I've checked and re-checked the voltages on the AFM, and it seems to check out fine. Same with the throttle switches. Distributor cap is dry and clean. Checked the ICU/DME connector and it's whistle clean.

Can anyone offer any advice on where to check now, or where the problem may be? Being intermittent makes it doubly annoying.

(as an aside, during the wet drive the oil gauge also stopped working - goes straight to the top. I checked this today as well and a wire came off, so I assume that's the problem with that!)

Any help gratefully received..

Check the black cap on the AFM that covers the internal electrics, if someone in the past has removed it to change the position of the arms on the track, and not resealed it fully then moisture will get in and affect it's operation. This could explain the symptoms you get when it's wet / damp outside.

Thanks for your reply. I've ruled the AFM out of being the problem. At the moment, the cover is off, it's dry, clean and been checked and checked. I'm wondering if the signal isn't getting back to the DME though, even though the connectors are all clean.

I'm now wondering if any other sensors are affecting it?

As I understand it, the computer puts the car into coast mode when the throttle is lifted (activating the switch that tells it the throttle is closed) and returns the car to idle if it is above 1600 rpm, so I'm guessing that's what it's doing. It seems to be detecting the throttle is closed (by the Air mass sensor?) and setting the revs to idle.

Can't think what else to check though. What I have noticed is that the idle revs are normal when it's working OK, but are about 200 rpm lower when it's not working, it's just about ticking over compared to normal.

Have you checked the TPS?

http://forums.tipec.net/viewtopic.php?f=25&t=18014

Hi Jim. I have checked the TPS and it was switching fine. At the moment it’s ok, I’ve had it running until it warmed up &it’s still OK, which is somehow even more annoying.
I checked all the vac lines and cleaned the earth connections (that wasn’t much fun!). Investors checked and cleaned the Idle valve thing connections as well. It still running ok, but I haven’t found anything specific that I can say has cured the fault.
Next plan is to check the sensor connection back at the DME and maybe have a look inside for some dry joints.