Rough running and erratic tach

[JBurer]

Thought someone here might have a suggestion on what is causing a rough running issue with my 964 Turbo.

I took the car out for a spirited drive over the weekend and it ran well. On the way back home, I start to notice a little rough running... the car dies as I'm releasing the clutch from a stop (in the middle of an intersection). Restarts no problem and I limp it up the road.

Pop the rear and see the O-ring on the intercooler showing. Figure it's a vacuum leak that is causing the issue. I limp the car the rest of the way home and put the IC back on correctly with a new O-ring. Car fires up fine, but still has the erratic tach / rough running problem.

I have two videos uploaded of the car idling here. I started the car cold about 3 minutes before I started the first video. The second video is of the car dieing a minute or two later while at idle -

YouTube - 964T - Rough running
YouTube - 964 Turbo Rough running2

The car has an AEM standalone engine management system and an MSD ignition amplifier. Assuming this was a bad MSD unit, I replaced the old box with a new one but the problem persists.

- Battery voltage is in the 12-13 range at idle
- TPS looks to be acting normally
- Closed loop O2 control is not active below operating temp, so the issue isn't the wideband oxygen sensor
- I can hold revvs at 2k rpms and still have the erratic jumping around (this leads me to believe the problem is not an idle air control motor or idle control related).

Seems like this has to be ignition or ignition control related, but I am not familiar with the Porsche ignition systems. Does this look like anything you guys have seen before?

Thanks in advance!
Best,
John

[lucittm]

John,
Did you replace the EZ69 with the MSD system? I believe the engine speed signal originates in the EZ69, but I could be wrong. My schematic is not clear about showing where the tach is wired.

I would check the +12 volts going to the coil when the engine dies. Mine had a wiring problem that caused that voltage to be lost.

Good Luck,
Mark

[JBurer]

Hi Mark,
Thanks for the reply - the system looks to consist of the MSD 6A amplifier and a Blaster 2 coil. Is the EZ69 the designation for the OEM coil?

I actually got the car running correctly. As bizarre as this sounds, I pulled the boot off of the coil pack, plugged it back in and the car regained it's normal idle. Took it out for a drive, and she's back to 100%. Go figure (?).
Best,
John

[lucittm]

John,
According to the Bentley manual, "Technical Data Without Guesswork" for the 1989-1994 Porsche 964 models, all the turbo cars came with the EZ69 digital spark control unit that sends trigger pulses to a special coil called an ignition transformer. I don't think a Blaster coil would work on my car.

What year is your 964 Turbo? You mentioned coil pack, you have an external ignition module? Mine is built into the coil. I'm glad to hear you found the source of the problem. So, just a coincidence or do you think you bumped the coil wire boot at some point?

Thanks,
Mark

[JBurer]

Mark,
This particular car (a '91) has been converted over to EFI - from what I can tell by tracing the wiring, the ECU controls an MSD 6A and a Blaster 2 coil. I'll have to inquire with the builder if any of the stock ignition components still remain.

I'm pretty sure I didn't bump anything while in there - the problem actually started after a 45 minute drive. When I pulled the boot, it was fairly difficult to get off - I only removed it because I was planning to pull the coil for more diagnosis work... just happened to get the itch after plugging the boot back on. I hope all of my Porsche ownership experiences are this easy!!
Best,
John

[lucittm]

John,
I have had some pleasant experiences like yours and also some that made me tear my hair out. Overall, it has been almost too good to be true.

I would like to hear more about your car since many of the folks I have talked to about upgrading my car says that it cannot be done. I really believe that there are not enough of our cars to create a demand for after-market high performance parts. I actually like the EZ69 digital ignition system, but I wish there was an easy way to go to 12-plug ignition.

I wonder if your EFI/ECU has a non-stock crankshaft position sensor? When you talk to the builder, see if they have any dyno or other HP estimates.

Thanks,
Mark