joreekie

Just had the ECU for the fuel injection replaced!
Car runs fine and smoth over 1000 rpm but when hot the idle speed "surges" 500-700-500-700 rpm and if I dont push into neutral will stall ...... 1990 S4
any ideas ....... thanks John

Garth S

Has the car been driven since the LA replacement? - if not, there appears to be an adaption period before the idle calms down - same thing on my 993: disconnect the battery, and it will stall at every occasion for several miles. Always back to normal by the next morning.
There remains the possibility that a second problem was masked by the circumstances that led to the LH replacement .... and they are now creating a
Anyway, drive it a bit if not done so already .... sometimes, disconnecting the battery and restarting the sequence helps.

SwayBar

Your idle control valve could be sticking. Do a search, there is a lot of information on the subject.

Bill Ball

Disconnect the battery briefly, if you haven't already. Some have said 90+ require Spanner or Hammer idle adaptation rather than just driving. I'm not sure about that. Where are you located? Maybe someone nearby has a Spanner. That can be used to test the idle stabilizers as well as other circuits and to run the idle adaptation.

John Speake

If the idle is surging, it suggests that the ISV valve is operating, and the LH ECU is driving it, searching for a condition so the loop can settle. But in your case it doesn't.

You may have an airleak into the inlet system, so it can't settle. Check inlet vacuum.

Sometimes with an aged MAF you do need to use a diagnsotic tool to adapt the idle, even after driving the car long enough to adapt.

joreekie

Did not appear to have the problem before LH ECU failure ..... is this possible?

John Speake

The battery disconnect could have caused this.....where did you get the LH from ?

Bill Ball

Hi John: What do you mean the battery diconnect would cause this? Isn't that the way you would allow this to reset? He has a "new" LH. Wouldn't you disconnect the battery?

joreekie

In reply to John Speake ....... I am in the UK .... Edinburgh. Got the LH from ATP Hednesford.... it was my original one that they "rebuilt".

1990 S4 Auto (Black/Black)

John Speake

Hello Bill,
I have seen this happen in practise on some cars. It may be that the O2 loop adaption and the idle adaptation fight each other. With some cars resetting idle with a Spanner/Hammer is the only thing that works.

Whether there is some underlying reason, I don't know. An aged MAF would make adaption more difficult to achieve.

Hello joreekie. ATP do a good rebuild - the LH is unlikely to be the problem. Check inlet system vacuum.....get a Porsche specialist to do the idle adaption with a diagnostic tool.

Make sure the throttle closed switch is closing OK, you should hear a click when the throttle is just opening (use lever on side of inlet plenum), and then listen for another click as the throttle plate closes.

joreekie

Just to complete the thread ...... and thank all for thier contributions.

I started the car and then disconnected the battery and drove the car for 10 minutes ...... lots of messages and flashing warning lights but when the battery was re-connected the problem was solved. Idle steady at 600 rpm and does not stall ......... performance appears to have improved as she is more responsive between 1000 and 2000 rpm ...... 2000 + rocket ship !!! God I love this car.

SteveG

Have you read your owners manual? You should NEVER disconnect the battery while the car is running.

Bill Ball

Eeek! I hope he didn't do that based on my message.

Schocki

Dude you are so lucky! Disconnecting the battery fries your alternator immediately under normal circumstances.

Jim bailey - 928 International

Trial by fire !!!!