cccscotland

It's not even as if it stalls. It sits with a nice healthy idle, and then just dies. It happens more often when the car is warmed up. Sometimes the engine management tries to catch it successfully, other times not. I've reset the DME, checked the DME relay, the distributor belt, cleaned and checked the ISV, checked the idle microswitch, but can find no issues with them.

When it dies, the ISV makes a clicking noise. The input voltage drops from around 7V to 1V or less when it stumbles. Whether this is causing the dying or is the management trying to keep it alive as it's dying I don't know. I'm tempted to stick a constant 12V across the ISV and see if the prob goes away -is that a good idea?

Whenever it dies it will start with no effort and idle happily for another minute or so, until it dies again. The engine will rev happily, but it also seems to have the same stumble under load, but with the momentum / extra revs it doesn't stall.

Any ideas? What shall I check next?
cheers in advance
Matt
1990 964 C4

John Boggiano

How did you check the DME relay?

Worth trying a new one anyway?

dougn

what you describe is how my car behaved when it had a problem with the primary side of the ignition system. a simple test would be to unplug the primary coil switch and let the car idle on the secondary side only....see if it still dies.

cccscotland

bridged the DME with wire and it ran exactly the same. Will try the coil test tomorrow and report back.

Red rooster

While you have your DME bridge in place check out the volts at the three power points. They should run at battery volts .
I saw a Porsche a little while back where the power feed from the battery was going bad intermittently. With sometimes 7 volts to run the management system it wouldnt idle !!!!!! or much else .

Good luck

Geoff

951 S

Matt,
Keep me posted as to what you find out. My car was stumbleing/ shutting off and the ISC valve was clicking before shuting off. I changed the ISV and it ran good for the weekend. Went to start it this morning and the problem started all over again. I'm running out of ideas, Ive replaced the battery, check the ground cable, new distributor (it did not run at all then), swapped DME relay with my 944 Turbo and reset the ECU twice.

cccscotland

Finally some progress on this. I reset the ECU and the problems went away for a month or so, but then came back with avengeance. So much so that I couldn't even start the car! So reading up the thread here I tried Dougn's test above - unclipping the prmiary coil switch and letting it idle on the secondary side only - and hey presto she ran! So did a little swapping around of switches and finally isolated the primary coil as the fault. £40 new coil from Euro Car Parts, a DME reset, and she's driving beautifullyagain. 1,000 miles on ( a big travelling week!) and not even a hesitation! Cheers all for your input, and especially dougn for being right on the money!
m

951 S

M,
That was the exact same problem I had! Was a tough one to figure out but I had the same symptoms and it was the primary coil.

dougn

glad you got it worked out

so here's another example where a car has some kind of issue on the primary side and freaks out but will still run on the secondary. the opposite never happens (not including a broken belt). there has to be something different between the primary and secondary sides