bayrrej

Starts fine cold, warm it up, turn it off and it will start fine for the next 5 to 10 minutes. After about 10 minutes it cranks w/ no start. After about 1.5 hours or so, it starts again. When it's running, it's runs perfectly.
New fuel dampers and FPR, new fuel, LH and EKF relays. newly rebuilt MAF, new injectors.
Added FP guage. Pressure holds for half hour or so -- check valve ok? When it won't start, the fuel pressure gauge won't move. Once the gauge shows the FP is running, it starts. Also will start when the FP relay is jumpered although it may take some cranking and a couple tries.
Thoughts?

hwyengr

No fuel pump on crank leads towards an intermittent crank position sensor (CPS). Does your tach bounce when you're cranking and it doesn't start?

bayrrej

Hmmmm... Good question. I failed to notice that. I'll check...

jb

bayrrej

I'm a little reluctant to note this, but since I've been monitoring the tach bounce, the tach has always bounced and it hasn't failed to start.
Not sure if that's some sort of weird quantum/copenhagen effect, but I'll keep monitoring.


________________
86.5 928s
79 911 euro Targa
84 928s euro (project)

zekgb

If tach bounces then CPS signal being produced, no bounce = no CPS signal, so check CPS and/or harness. FYI I'm chasing the same on my 85 now except it is a cold start issue for me and my tach isn't bouncing as engine cranks.

davek9

Check your fuses for good contact connections, and replace the main Relays as this could be burnt/hot contact issue.

hwyengr

Yeah, I forgot to mention that tach bouncing was good. The tach lead is more or less the same signal that tells the LH to turn on the fuel pump, so if no signal is going to the tach, no fuel pressure is being ordered.

bayrrej

Finally it failed to start today as the weather warmed again.
Bigger than life, the tach languished w/o a flicker.
Removed the air filter paraphernalia, tapped on the CPS a bit and tadah, the tach promptly woke up and the throaty roar resonated to much rejoicing.
New CPS on the way. Will see if this totally resolves the issue....

Mrmerlin

more than likely you need to do a complete intake refresh,
also swap in new ignition wires caps rotors the short TPS harness a TPS and ICV and fuel lines

bayrrej

Waitin' for the T-shirt.
Pretty much the ONLY thing not replaced was the CPS...
No doubt a smarter bear would've just replaced that as well...
Buuuuuttt, gotta leave somethin' for the Rennlist folks to diagnose...

Thanks for all the input...

ramcram

I'd be checking temp sensor II on the water bridge. Had exactly that problem and that's what it was.
It has 2 pins, one for each EZFF & LH, to tell them what temp the motor is. If this sensor fails, or the wiring is faulty, they can't tell that the engine is hot and give it cold start fuelling.
The motor will not start or idle when hot without this signal.

bayrrej

Just to finalize this thread:
New CPS, while a MASSIVE PITA to replace, totally resolved the start problem!