engine runs but wont restart after a short shutdown

[79por911sc3.0]

I have a 1979 911 sc 3.0
It runs fine, and after driving 1/2 hour, can shut down and immediately restart right back up.
Problem: after running, and say shopping for half an hour, it wont restart. But if it sits for say two more hours, it will then start right back up and run fine.
Any ideas? Tired of sitting in parking lots waiting to go home.

Has new plugs, pop off valve has been re-sealed with epoxy to body and is new. Air box is not blown.

[LIRS6]

Welcome to Pelican.

Can you elaborate about your knowledge of the car ... how long have you owned it? How many miles on it? Any upgrades? Any work done recently? Where do you live (climate)? Have you done any testing (fuel pressures, fuel flow rate)? At a quick glance I'm wondering if perhaps the fuel accumulator might be at fault, but I don't want to jump to conclusions and see you spend $ needlessly.

Give us a full introduction to your 911.

Jason

[79por911sc3.0]

yes, thanks. I have it about 3 years, 70K orig miles. Its a 76 but was told the motor is a later model 3.0sc motor. no upgrades other than later model engine. Was told it was tuned just before I bought it. I havent done any testing.
I Would love to know myself what year engine it is.....

some recent pics if it helps, also noticed an open fitting on left side of air box....
where could I get a hose or parts diagram of engine? THANKS!!!

[boyt911sc]

Do you have a set of fuel gauge to do a fuel pressure test? When the engine is cold, the CSV (cold start valve) is energized and makes your motor starts and run. Once the motor is warmed up (above 113°F), the TTS (thermotime switch) will cut off the ground contact of the CSV and will be disabled until the motor cools below 113°F.

With the CSV off, your warm engine will rely on the residual fuel pressure to go get it running. If your residual fuel pressure is too low when the CSV is disabled, the motor will struggle to get it running again until it cools down. There are several known culprits causing residual fuel pressure loss. The 2 most common culprits are FP check valve and fuel accumulator.

Tony

[79por911sc3.0]

I dont have fuel gauges for testing, should I get some?
Should I replace FP check valve?
I do hear the fuel pump running upon warm try restart....

Also, can you tell me on the 7th pic if there should be a hose connected to the bib on the left side of air box? Or should it be plugged? Just noticed it recently and seems would let unfiltered air into intake....

[stlrj]

Originally Posted by boyt911sc

With the CSV off, your warm engine will rely on the residual fuel pressure to go get it running. If your residual fuel pressure is too low when the CSV is disabled, the motor will struggle to get it running again until it cools down. There are several known culprits causing residual fuel pressure loss. The 2 most common culprits are FP check valve and fuel accumulator.

Tony

So how do you check for residual pressure?

After shutting your warm engine, pop the engine lid, remove the air filter and put your hand under the sensor plate and push up on it to see if you can feel any resistance. If no resistance then you have no residual pressure and likely caused by a leaky FP check valve.

Cheers,

Joe

[LIRS6]

https://forums.pelicanparts.com/porsche-911-technical-forum/758788-cis-troubleshooting-dummies.html

Read through this - there is a wealth of information contributed by many to this excellent thread.

.. and listen to Tony (boyt911sc) - he is the guru of CIS

Jason

PS - When was the last time that the oil & filter were changed? - the filter looks like it has a lot of years on it ...

[79por911sc3.0]

will check plate for pressure.... thanks! Where is the best place to find all parts or hose diagrams? I only see the parts here on pellica but no full diagrams....

Can anyone tell me if a hose is supposed to be connected to bib on left side of air box in pic seven above or if it should be capped off?
Seems like unfiltered air can get into intake...

[timmy2]

Plug the hole on the airbox in your photo.
Download the parts diagrams directly from Porsche classic.

[boyt911sc]

Originally Posted by stlrj

So how do you check for residual pressure?

After shutting your warm engine, pop the engine lid, remove the air filter and put your hand under the sensor plate and push up on it to see if you can feel any resistance. If no resistance then you have no residual pressure and likely caused by a leaky FP check valve.

Cheers,

Joe

Joe,

The FP check valve is one of the culprits. After all these time you are still struggling to troubleshoot systematically. You use a pressure gauge to identify the culprit and avoid doing guess-work troubleshooting. What happens after you replaced the FP check valve without verifying it as the culprit and the problem still there? Change the FP? Change the fuel accumulator? You can not rely on luck all the time specially for correctly diagnosing the problem/s.

Tony

[boyt911sc]

Originally Posted by 79por911sc3.0

yes, thanks. I have it about 3 years, 70K orig miles. Its a 76 but was told the motor is a later model 3.0sc motor. no upgrades other than later model engine. Was told it was tuned just before I bought it. I havent done any testing.
I Would love to know myself what year engine it is.....

some recent pics if it helps, also noticed an open fitting on left side of air box....
where could I get a hose or parts diagram of engine? THANKS!!!.........(edited).......

79POR911,

Do these fuel injection lines been cut and mended? They look very scary to me to have such fuel injection lines in a 911. This is like having a ticking time bomb in your car. Replace them when you a chance before it is too late.

[stlrj]

Originally Posted by 79por911sc3.0

Can anyone tell me if a hose is supposed to be connected to bib on left side of air box in pic seven above or if it should be capped off?
Seems like unfiltered air can get into intake...

Picts?

[porsche930dude]

Id say 90% chance its the accumulator. the check valve is a crappy job but a good oportunity to change the fuel hose at the tank which is probably nessesary by the looks of your engine bay

[LIRS6]

Originally Posted by 79por911sc3.0

Can anyone tell me if a hose is supposed to be connected to bib on left side of air box in pic seven above or if it should be capped off?
Seems like unfiltered air can get into intake...

I took a look at mine - I think it may be for a hose to/from the charcoal canister, but I'm not 100 pcnt sure.

Jason

[LIRS6]

Originally Posted by 79por911sc3.0

Can anyone tell me if a hose is supposed to be connected to bib on left side of air box in pic seven above or if it should be capped off?
Seems like unfiltered air can get into intake...

I took a look at mine - I think it may be for a hose to/from the charcoal canister, but I'm not 100 pcnt sure.



Jason

[pmax]

Originally Posted by boyt911sc

79POR911,

Do these fuel injection lines been cut and mended? They look very scary to me to have such fuel injection lines in a 911. This is like having a ticking time bomb in your car. Replace them when you a chance before it is too late.

79por,

To be clear, boyt911sc is referring to this.

[79por911sc3.0]

it appears so! Did not realize it when I bought the car and that it was not ok to do. dohhh.....
But seems I have more problems....

[79por911sc3.0]

thanks. This diagram shows it being plugged, has every diagram and parts numbers you could think of....
http://www.***********.com/porsche-parts/911-78-83/porsche_911_parts.php

[79por911sc3.0]

Not sure but will add to the list oil filter change....

After reading your links for vacumn leak test, i tried removing the oil filler cap during warm idle, no change in RPM's...... bad
Then I put my hand over air intake breather hole and no suction......no change in RPM's
Obviously this is bad....Any suggestions?

[79por911sc3.0]

I really need to get this car right, I love it!

just sharin lol