1988 no start after short stop

[tkd0706]

Took my 1988 out for a drive and stopped to fill up the gas tank. After fueling tried to start it up ... cranks strong but doesn't fire. Tried 3 or 4 times then started to read these forums on my phone to find the problem. Fuses are all fine, so I started to think it was the fuel pump/DME relay. Called my friend who has an 86 targa and he took his relay out of his car and drove out to me. So car was sitting for a good 40 minutes to an hour. Swapped the relay and started right up. Problem solved or so I thought.

Ordered two brand new relays so I could have a spare and he could have a new one. Drove out to his house about 5 minutes away. Parked the car and chatted for a few minutes, asked him if he wanted to drive it and feel the G50 transmission. Tried to start it and same thing... cranks fine but won't fire. So I thought maybe letting it sit for a while would make the problem go away like it had done before. So while waiting we take my old relay (which we thought was bad and caused the issue in the first place) and plugged it into his 86...fired right up. So the relay was good anyways. (note that at this point his old relay was still in my car).

After waiting a good 15 minutes or so went back to the car and it fired right up. So for now what I know is that this seems to happen after the car sits for a short amount of time after running, but when given a little bit longer it'll fire right up.

Any suggestions on where to start to try to find the real root cause?

[911obgyn]

A quick check when it doesnt start is spraying starting fluid into intake, if it fires a bit its a fuel issue.
I would carry a meter and check reference and crank sensor ohms when it doesnt start
Also head temp sensor. You can bypass head temp sensor with a paper clip or short piece of wire and if it starts thats what it is. Ref sensor and crank sensors may have intermittent problems before they go completely.

[tkd0706]

Thanks! Googled your suggestion and found this thread also references a couple of hot no start conditions that were caused by the reference sensor and crank sensor. That's where I'll start

https://forums.pelicanparts.com/porsche-911-technical-forum/893183-reference-sensor-testing.html

[tkd0706]

Update for those who might see this in the future. Tested the reference sensors with an oscilloscope and they looked a little bit noisy so replaced both sensors and the head temperature sensor. This did not fix the issue. Checked how many ohms the fuel pump was drawing and it was low, I believe 9 ohms to a 12 or 13 ohm spec (don't quote me on this). Replaced the fuel pump and also rooted around and found some loose connections in the ignition system +car has a twin plug set up). Don't know which was the problem but after doing this the car has run better than ever.