Ignition vs post ignition woes?
 

I am going to embark on a search tonight on the topic, but I figured I would post my progress. 1984 911 3.4l rebuilt engine to be started for first time for cam break in. Starts every time on a dime, no concerning engine noise, but only runs 1-2 seconds before stopping. As it fails, another kick with the starter buys it another 1-2 seconds. Subsequent starts are not difficult, suggesting that the issue is not one of too much fuel and flooding.

Details:

- 1984 usa car just rebuilt
- 3.4l, 9.8:1, 964 cams, single plug, 66mm throttle body (3mm larger), open airbox, ssi's, m&k 2:1
- no concerning engine noises during startup
- Steve wong ssi chip
- position #4 for fuel quality switch (as per SW, -2.79 ignition correction)
- Euro dme
- both dme and chip starts an 85 car when swapped
-Both flywheel sensors testing at expected 1000 ohms
-Tach working
-Fuel pump working, tested with manual bypass of dme
-Plugs making a spark
- new head temp sensor

That is what i know off the top of my head. I will begin to research further, but insights to help direct diagnosis would be appreciated

Pelican Technical Article: Ignition Troubleshooting List...

Not sure this is an ignition issue however... The issue is keeping the engine running.

http://www.pelicanparts.com/techarticles/911_carrera_start_troubleshoot/911_carrera_start_troubleshoot.htm

Factory instructions

I believe that I would check the voltage to the fuel pump. You indicated that the fuel pump tests ok, but it seems as though you may be getting voltage to the pump only when the starter is cranking.

Do that would imply the startup circuit is fine... But this might be an issue with warmup regulation?

I know that during normal operation, the DME drives the fuel pump. But, I don't know if there is a different circuit that supplies FP voltage when the engine is cranking. SW undoubtedly knows this along with many of the other technical wizards who contribute to this forum. Hopefully, one of them will jump in here to help.

Interesting. My dme will run another 85 car just fine... Unless it is a wiring issue to my fuel pump from the dme, and not the dme itself.

Still digging. This is from the my car does not start sticky.

"Fuel pumps Should run for a second when key is turned on, but some continue to run."

Sounds like my issue but in reverse: when the ignition activation of the fuel pump would normally turn off, the presumed secondary circuit to keep it going does not activate. Or maybe it does but the mixture is too lean, or grounding issues bleed the signal, or there is a big vacuum leak... etc.

from the pelican no start diagnosis article:

"2. Remove the air cleaner cover and air filter, and spray some fuel into the intake. You will have to hold open the little "door" inside there with something. If it starts briefy then dies you probably have a fuel problem."

Given it starts every time without issue, it would make sense to me that this is a lean issue. Not enough fuel getting in, or a vacuum leak. Whatever the answer, I will buy a DME relay spare.

Did you get a chip from SW made for 964 cams? If not, it won't ever run right.

I would be curious to know more on this topic. I do understand why it would not be optimal until my car is dyno tuned. I want to be sure I am not missing something obvious.

I bought a stage 1 SSI chip, prior to going 3.4L with 964 cams (the upgrade was not part of my initial plans). SW knows this, and said just to use position #4 for the fuel quality switch due to the higher 9.8:1 compression. He said this SSI chip should be fine for engine break in and the initial dyno. I don't think he misunderstood my communications on the issue either given that he said I could also use the stock chip if I wished (a safer option still with less aggressive fuel maps).

I have paid to upgrade to a stage 2 chip so the chip will be dialed in once I have dyno data.

I'm far from an expert, but it seems to me if you connect an appropriate fuel pressure gauge at one of the fuel rails, that should tell you if the engine quits due to no fuel pressure after it initially catches. If that is the problem, then I would check voltage along the circuit to the pump.

You're probably fine then. I do remember my idle being as lean as 19:1 with Motronic. I can't believe the car would even run. It's a long story.

The final answer has yet to be determined, but my mechanic thinks it is the speed/reference sensors. They were crispy, and needed re-wrapping. They both tested at 1000ohms, but one was not producing the intended signal. Probably fragile from the heat/age and gave up the ghost during manipulation from the rebuild. Even if this is not the answer, fresh ones are a great idea in my mind. I have ordered two new ones (actually the 1987 bmw 535i versions that are about half the price, and differ only by a 1 inch longer wire).