V2Rocket

In the past few weeks I have noticed maybe 3-4 times a momentary shutdown (maybe 1 second without engine power, tach needle drops, then catches itself and runs fine) while driving.

~2 weeks ago, after driving over 200 miles in one day the car started, made it down the driveway, and shut off. I jumpered fuel pump, swapped DME relay and fired it and drove it all the way back home (150 miles) without issue.

On Monday while doing some work on the 944, while running, it cut out suddenly. I cranked it over, it fired and died. It will crank and crank and crank but never fire.

I have fuel at the rail, the rotor is firmly attached to the shaft, I believe I have spark (have not verified but wires are all in good shape, coil is good shape, plugs are pretty new). Fuel pump runs when jumpered, swapping relays does no good because the car won't fire (which activates the fuel pump).

I swapped my NA-tune setup back to original AFM/DME, still nothing. I removed the column and checked ignition switch against a spare I carry, no change. Just cranks and cranks.

Every time so far I've had a tach bounce but now it is getting intermittent with cranking. Sometimes it bounces, sometimes it doesn't. The times it does bounce, it bounces again when I stop cranking. '

I suspect the speed or reference sensor wires - the lower one on the bracket has a visible crack in the wire and what appears to be exposed wiring. Wiggling the sensor doesn't help but has in the past.

...but am I missing anything else?

Tiger03447

Dear Spencer: if you have visible wiring showing from the reference sensors, you may have a bad lead going to the sensor..the sensor itself may be fried, but if you ohm it, do it with wiggles on the wire to see what happens..you might have one of those dreaded "Perry" things going on, but I hope not...Please let me know what the correct ohms are supposed to be with the early cars, reference sensors..I need to check mine..Thanks

V2Rocket

I found a video showing how to check the sensor outputs at the DME plug by checking the pins for each sensor, ohms while sitting and AC volts while cranking. Oddly both sensors seemed to check out OK.

divil

You could still have problems with them even if that test passes. The wires are shielded, so maybe interference could be an issue if you are seeing it exposed? Also, I think if the signal and shield wires are internally shorted together, it will test just fine with the connector floating because neither wire goes anywhere. Hook it up the DME, and the shield is grounded, so now your signal is grounded too. I think that's what was happening in Perry's case. With all that said, when you see the tach bounce, that is coming from the DME, so I think it must have been getting good signals when that was happening.

odonnell

http://www.lindseyracing.com/LR/Pors...4-HAR-REF.html

Although you could make your own pretty easily (and a lot cheaper) I would imagine:

http://www.eagleday.com/noname18.html

They have good stuff on their website, I've also had this place bookmarked for months:

http://www.efiexpress.com/catalog/in...b37f5d2fd9256b

A full harness rebuild would be cool. I have URLs for places to get all new connectors in a spreadsheet if there's any interest, I made it a while back when I was seriously considering doing mine.

V2Rocket

It's the sensor side of the wiring that is cracked - the harness is fine.
Interestingly, over the last almost decade of being around these cars and working on several dozen of them, I've never seen a ref sensor car-side plug broken or crumbling. 100% has been the sensor side of the wiring/plug.

FRporscheman

If it cut out suddenly, it sounds like something electrical.

Verify your spark. If you can run the fuel pump with a jumper, and you do have spark, see if the engine will start with the jumper.

odonnell

Also- how is your fuel pressure? 2 bar through and through? Too high of a pressure can cause the DME injector drivers to shut down in self preservation as I'm sure you know.

Tiger03447

Thanks for the video, Spencer..I'll give it a shot..Tiger

V2Rocket

I will attempt to fit a FP-gauge today without emptying my recently-filled tank
But the "pull the vacuum lines" test revealed no fluid in the lines so I think the FPR/FPD are OK.

Engine does not start with jumper installed. It did last time this problem came up but now just won't.

The intermittent/lack of tach bounce leads me to believe speed sensor. I was already planning on an AOS-reseal and manifold-off air/vac system overhaul so I'll just take care of the sensor(s) while in there.

I pull my intake manifold off a lot more than most do so I probably over-stressed the sensor wire at some point recently.

V2Rocket

Left work early today, feeling pretty sick.

But not too sick for 944 work.

After a leisurely 2 hours I got the intake manifold, under-manifold hoses, and AOS out of the way and removed the sensor...

Here is the prime suspect. Speed sensor (rear sensor, bottom plug). I hope this is the answer!
I am going to be rebuilding all the under-manifold hoses but that will be for another thread.

Tiger03447

Dear Spencer: I am sworn to secrecy about your recent illness..glad to hear that you are still moving and vertical. However, if that "prime suspect" isn't the demon in your car, it sure looks like it could be one..needs to be overhauled at any rate..hope that this is REALLY the culprit! Get better and hope this cures "all"the ills taht pervade both you and the car..

V2Rocket

Swapped sensor out, along with completely replacing every hose under the manifold to eliminate potential/existing vacuum leaks.

Still no dice. Tach seems to bounce intermittently, sometimes it will sometimes it won't. Sensor gap was not touched, just straight swap of sensors. They are plugged in correctly.

Definitely smells like fuel is spraying after cranking, fuel comes out of the rail nut, but no fire. Hard to check for spark as I don't have a helper to turn the key while I hold a plug in the coil wire.

V2Rocket

Fuel pressure is 40psi with fuel pump jumpered.
It does bleed down to 20 after half an hour or so with everything off, suspect the fuel pump check valve, but that comes later.

Checking the ignition coil per clark's testing, with ign on there should be 12v at each coil post relative to ground (battery neg), but on each post I am only seeing 1.94V?
Ohm test appears ok...what is happening?

jeffrsmith

I had a similar issue last year, found that I had intermittent readings when testing the speed and reference sensors. Replaced the harness to the DME for speed and reference and still no change, although I had good readings for the sensors. Found I had a power issue, replaced/repaired power wire from Central Electrics to the DME, no change. Replaced DME - magic, everything worked. Along the way I also replaced the coil, which did not change anything.

You can buy a spark tester light that plugs inline with the HT coil wire to the dizzy cap. Makes life easy for testing spark by yourself.

Do you have access to a spare DME to swap to see if the driver in the DME went bad? I would start there before messing with anything else.