Starting issue when warm?!

adam928 · 08-09-2017, 04:21 AM

Hi all! And greetings from the U.K.

I have an 83 928s, it starts immediately when cold, and again, if I switch it off when hot, and restart it, starts right up.

If I leave it 10 mins, and come back to it, it's a nightmare to get going again! Once it's going, runs and drives great!

Took it for its mot today, and refused to start after waiting for my time slot! Battery drained, and did start after I jump start!

Any ideas anyone?! I was wondering weather a temp sensor was gone, but really don't know?!

Cheers, adam

SwedeInSiam · 08-09-2017, 05:36 AM

I guess you have a 4.7L CIS M28.11/12 engine (same as my 1980 S).
They are known to have problems with staring when hot and been turned off for a few minutes up to an hour. It's due to vapor lock in the fuel lines on top of the engine (boiling fuel). Normal hot start procedure would be to keep the gas pedal depressed half way or more during cranking (longer when usual). This is when everything works normally. If you have problem with a bad accumulator or leaking injectors then the fuel will boil earlier and you will have more problem to start the car. Need gauges on the CIS to check for residual pressure to confirm this.

honerboys · honerboys's Garage

hey adam - funny, .. i was just thinking of starting a thread on this - my car does the exact same thing - i believe you would not have CIS injection and do have the electronic injection system like my 84 (4.7) - i didn't think these had any warm start temp sensor, but...? - mine has never not started, but requires about 20 seconds of cranking when warm .. when it's cold it barely needs 2 seconds - anyone out there have any ideas - thanks

928gene928 · 928gene928's Garage

This may not be the case, however; I did have the exact same problenm some time ago. After changing a few parts, and a lot of checking, it turned out in my case to be the fuel injection relay. I guess after all the years, some of the contacts were worn, and would cause the problem you described.

adam928 · 08-09-2017, 08:31 AM

Mine is not with electronic fuel injection! You can also smell fuel in the exhaust fumes, so does this also indicate leaking injectors?!

honerboys · honerboys's Garage

hey 928gene - i replaced the relay - still the same

stepson · 08-09-2017, 11:57 AM

Honer,
Put a switched jumper in place of the fuel pump relay. Turn the fuel pump off prior to turning off the ignition.
I had a similar issue and what was happening was that the injectors were leaking fuel into the cylinders after I shut the ignition off. By turning the fuel pump off prior to turning off the ignition, it should prevent this leakage and the car should start right up.

Good Luck

9two8 · 08-12-2017, 11:29 AM

Replace the fuel pump check valve, had the exact same thing with mine last year, replaced it and warm/hot restart issue is gone.., it's cheap and easy to install !

My car below 80 928S 300BHP

adam928 · 08-12-2017, 12:07 PM

Think the check valve may be the first thing to try then!

9two8 · 08-13-2017, 08:04 AM

Quote:

Originally Posted by adam928

Think the check valve may be the first thing to try then!

Here's a detailed how to...,

https://www.928gt.com/t-fcheck.aspx

adam928 · 08-13-2017, 09:12 AM

Thanks for that! I've ended up ordering one from the U.S. Too! They're loads more money over here, and not that easily found! I'll change the filter while I'm there too

adam928 · 09-06-2017, 04:24 AM

Well, check valve and fuel filter changed, whilst down there!
Unfortunately, still the same, where would you go now?! Rebuild wur, injectors?