C4 cranks but won't fire (start)

[amr89c4]

Greetings,

I am new to this forum as well as a new 964 C4 owner. The car is a mint '89 model with 48,600 miles on the clock. I picked the car up Aug 31 and it has been running flawlessly for 9 days and about 600 miles.

However, this morning the car failed to start. What I mean is that it cranks perfectly but it has not fired. Not one pop. The tech needle is not moving during the crank, so I'm inclined to believe I have no ignition for whatever mysterious reason.

I picked the car up in Indy and was visiting a friend in Ohio on my way home to TX, I'm relying on her shop to get me going. I was headed to the airport this morning and had no time to troubleshoot before leaving.

I am very well versed in EFI and digital ignitions and am pretty familiar with the C4 systems as I studied the system as much as I could considering there are no readily (that I have found) available service manuals specific to the C4, prior to purchase. I am not, however a Porsche technician so am seeking some opinions.

In the outboard systems I am familiar with, a failed CPS (crank pos sensor) which I assume failsafes on this engine the same as our outboard and could cause this issue, as well as a main power relay or DME/fuel pump relay in the case of the Porsche. Worst case is an ECM failure, I suppose.

Someone suggested a failed fuel pump, but I find it hard to believe the car would not at least fire once or start and run ragged for a second or two. The car was running fine when I turned it off Saturday evening. Since I was trying to get to the airport this morning, I was unable to verify spark or fuel pressure.

Of course, not being where the car is makes it tough to diagnose. Do you folks have any thoughts? Have you ever heard of a car running fine and then 36 hours later, cranks fine, but won't start? The shop is supposed to check it out later this week and I return to the car on Saturday AM.

Any ideas or experiences would be appreciated.

thx,

Lynn
89 Guards Red C4 coupe

[ischmitz]

First I would check the fuse box to make sure it's not something really simple. Next on the list is the DME relay. The DME relay is the most likley culprit. They do go bad and leave the engine dead as a door nail. Many recommend to always have spare handy.

From what I understand there was a bad badge of these relays that plagued a lot of different cars with Bosch Motronic. Something about poor solder made the connections brittle inside and would eventually lead to complete failure.

[Don Plumley]

+1 on DME relay.

You can also try to unplug/plug the harness on the DME (under the seat).

[amr89c4]

Thanks guys...It was the DME/Fuel Pump Relay. Bad solder joint.

[porschedude996t]

Originally Posted by amr89c4

Thanks guys...It was the DME/Fuel Pump Relay. Bad solder joint.

By all means change it to the later 993 relay. The 964 relays over heated and would open and the car would die. I did it to me several times until I figured it out with the help of another forum. It never did it again to years after changing. I tested it on the bench and it would open and cycle open and closed.

The one you want is 993.615.227.00. Pelican parts carries them for $26.50.

[bigchillcar]

keith - the 993 relay fits where the 964's relay should go under the hood?

[kurquhart]

Originally Posted by bigchillcar

keith - the 993 relay fits where the 964's relay should go under the hood?

Yes, the 993 part supersedes the 964 relay. I installed the 993 relay and keep my old 964 relay in the tool box as my spare.

[bigchillcar]

Originally Posted by kurquhart

Yes, the 993 part supersedes the 964 relay. I installed the 993 relay and keep my old 964 relay in the tool box as my spare.

thanks, kris. had i known that i wouldn't have bought a 964 relay a couple months back. good to know.