cold start problem

[911tracker85]

did some searching but did not find any threads that match my symptoms.

1986 951 modified.

rebuilt the engine last year and had been driving to break it in. put about 300 mi on it.

after it was sitting for a week or so had trouble getting it started. once started seemed fine and drove it around the block. a week later would not start, barely sputtered then nothing more.

so let it sit for the past couple months but back to it.

in full disclosure I think I ran it out of gas. put in several gal and now gauge reads almost a 1/4 tank. still would not start nor even sputter.

so I spray some gas into the afm and it runs/sputters. spray some more gas and after some sputtering it starts running. very slow idle at first then settles down to a normal idle. give it some gas and all seems normal. let it run a bit, turn off, and it restarts normally.

but next day back to the same symptom, crank but nothing. again spray in some gas and it sputters to life and after a few seconds idles and seems to run normal.

I tried with the DME relay jumpered but when reinstalling the DME relay still would run.

it just seems like it takes some priming to get it running and then it comes to life.

wonder what I'll find today.

[soxnail]

fuel leaking back into the tank and the fuel rail runs dry after it sits....
Sox

[911tracker85]

what I initially thought. but even when I jumper the DME relay and hear the fuel pump running it wont start.

I am considering connecting a fuel pressure gauge to the fuel rail to check pressure.

I dont fully know how the speed/reference sensors trigger the fuel injectors. just seems like when you get the engine slightly running on the gas into the afm it finally triggers the injectors.

off to do more diagnostics.

[mytrplseven]

When you try to start the engine do you see the tach needle move slightly? If not you need to check the speed and ref sensors.

[911tracker85]

from talking to Dave Lindsey I checked the engine temp switch that signals a cold start to put more fuel into the start-up process.

as I had to replace the fuel inj wiring harness due to damaged wiring I checked the connector on that temp switch. when I cut back the rubber boot found the insulation inside had all cracked and it was shorting. re-wired another connector I had and it started immediately. engine not totally cold, so the proof is if it starts this morning. but optimistic.

[911tracker85]

well did not work. did get a little sputtering.

next pull that connector again and check ohms. need to research what the values should be, cold and hot.

[nize]

was there any fuel related work? there is a one-way check valve to maintain pressure when the engine is off, and when that fails it takes some time to prime the pump and build up fuel pressure to fire up the motor.

an easy way to check is to attach a fuel pressure gauge to the fuel rail and see if it maintains pressure overnight.

[911tracker85]

that was my first suspicion and am considering doing that, but even when I jumper the DME relay and hear fuel pump running still no start when cold.

so still think it is something to do with temp sensor.

to refocus on my issue it is only a COLD START problem. once started and when warm it fires right up.

found info on the DME temp sensor. appears when cold the resistance should be 4-7k ohms and resistance goes down as temp rises.

off to do some testing with this data.

[soxnail]

Sensor specs...
Had to submit as multiple pics as the site resolution looses data for large pics. The junction of the pics is where the data line is a duplicate, I.E. 33 degC is repeated etc.
Sox

[911tracker85]

thanks.

waiting on new temp switch.

[911tracker85]

success

installed new temp sensor and starts every time cold or hot.

took a nice drive, car runs great. so put on a little more breakin miles and retention the TB/balance belts