hot start issue why ???

[gorskined]

so i read bunches of threads about the cis systems for Porsche Ferrari VW's and the fuel related issues and the hot start problem basically its the not holding of fuel pressure after the car has been started and is hot , o-ring leaking injectors check valves fuel accumulator. and so on .. but i cant find an explanation to exactly what is going on and why this happens when it looses the fuel pressure.

i can understand the injectors washing out a cylinder if they are dripping by and that causing it to start hard but other than that once the pumps come back on the system is primed in about 2 seconds . When my car had this problem it was caused by a bad check valve. it started great cold, ran good but once shut down and sitting for 5 minutes would not restart even though pressures were perfect once the pumps came on . once fully cooled car stared right up again.
at fist i thought vapor lock but that shouldn't be an issue seeing as the fuel pressure is good when pumping and bypassing fuel . i even ran the pumps for a few minute so all the lines were cold still no start until the motor was actually cold .
what is actually going on inside the fuel system to cause this hot start problem when it looses pressure when hot but starts with no residual pressure when cold ? im just curious.

[Black_Hat]

Fuel accumulator

https://www.pelicanparts.com/More_Info/91111019734.htm?pn=911-110-197-34-M14&SVSVSI=599&DID=7261

[Black_Hat]

test for bleed down of pressure over time if out of spec replace fuel accumulator

[Alan L]

The idea of the accumulator is to keep the fuel pressurised when you have a hot system.
Why? Not because it needs the pressure to get started. As you have observed - it starts cold from zero pressure. But the pressure is there for a hot system to stop the fuel vapourising and causing vapour locks.
What happens when the fuel cools and the pressure drops? - no vapour locks. Cold fuel system.
It must have been an active/real issue for them to go to the trouble to design and fit another bit of kit in the fuel line.
You do hear of carburetted cars suffering this also - particularly race cars. Pigs to start when hot (they don't have fuel accumulators), but fine when cold. You sometimes see race cars that have had a sudden off and stalled. No restart after extensive cranking. Vapour locked. An hr later, fires right up.
You only have to look at the bubbles returning to the fuel tank on the return line from the FD to see how easily it degasses. That process is very sensitive to both pressure and temperature. One compensation for higher temps is to keep the fuel under higher pressure.
Thats my take on it.
Alan

[gorskined]

Originally Posted by Alan L

The idea of the accumulator is to keep the fuel pressurised when you have a hot system.
Why? Not because it needs the pressure to get started. As you have observed - it starts cold from zero pressure. But the pressure is there for a hot system to stop the fuel vapourising and causing vapour locks.
What happens when the fuel cools and the pressure drops? - no vapour locks. Cold fuel system.
It must have been an active/real issue for them to go to the trouble to design and fit another bit of kit in the fuel line.
You do hear of carburetted cars suffering this also - particularly race cars. Pigs to start when hot (they don't have fuel accumulators), but fine when cold. You sometimes see race cars that have had a sudden off and stalled. No restart after extensive cranking. Vapour locked. An hr later, fires right up.
You only have to look at the bubbles returning to the fuel tank on the return line from the FD to see how easily it degasses. That process is very sensitive to both pressure and temperature. One compensation for higher temps is to keep the fuel under higher pressure.


Thats my take on it.
Alan

Alan

The off gassing and vapor lock make sense. The part that baffles me is the pump can flush out the system with cold fuel in just a few seconds and the car still wont start .. yesterday i put about 100 miles on it . I stopped for about 30 minutes and still had 26 psi of residual fuel pressure in the system. i tried to start it and it wouldn't start . i over road the pump safety switch ran the pumps until everything in the fuel system was cold. still wouldn't start.
Then I manually depressed the fuel sensor plate for a good 3 seconds with the pump on it started . stalled two times but then ran perfect . not sure what I'm missing the hot start issue is intermittent.

Regards Ned

[flightlead404]

Originally Posted by gorskined

Alan

The off gassing and vapor lock make sense. The part that baffles me is the pump can flush out the system with cold fuel in just a few seconds and the car still wont start .. yesterday i put about 100 miles on it . I stopped for about 30 minutes and still had 26 psi of residual fuel pressure in the system. i tried to start it and it wouldn't start . i over road the pump safety switch ran the pumps until everything in the fuel system was cold. still wouldn't start.
Then I manually depressed the fuel sensor plate for a good 3 seconds with the pump on it started . stalled two times but then ran perfect . not sure what I'm missing the hot start issue is intermittent.

Regards Ned

but it doesn't flush it all out. It can only flush to the WUR. From then on, including the metal lines to the injector, it's not flushed.

[Alan L]

With near 2 bar residual press after 30 min, you should not have a hot start issue. That is quite a way above spec. Which makes you wonder how you get such a pressure.
Do you have a permanent gauge mounted?
What residual pressure do you start with at shut down?
The fact that after priming the injectors, it fires does sound like a vapour lock issue. For sure a fuel issue. But your residual pressure is the mystery bit. That is set by the FA. Generally from memory they start around 2 bar. There is a small bleed hole in them to slowly bleed fuel off as the engine cools.
Alan

[gorskined]

Originally Posted by Alan L

With near 2 bar residual press after 30 min, you should not have a hot start issue. That is quite a way above spec. Which makes you wonder how you get such a pressure.
Do you have a permanent gauge mounted?
What residual pressure do you start with at shut down?
The fact that after priming the injectors, it fires does sound like a vapour lock issue. For sure a fuel issue. But your residual pressure is the mystery bit. That is set by the FA. Generally from memory they start around 2 bar. There is a small bleed hole in them to slowly bleed fuel off as the engine cools.
Alan

yes i have a dash mounted aem gauge and two actual gauge in the engine compartment one for system and one for control pressure . i installed that jegs check valve just before my accumulator. the jegs check is rated for 1 dpm( drip per minute) over 100 psi i also have the factory bosch check on the pump. When i shut the car down the pressure drops to just above 30 psi it actually holds pressure for quite some time I've seen it in the low 20s after an hour .pretty sure my drip back pressure loss is only dripping back through my regulator, as designed if i shut the car off and remove my return line i will get a drip every 5 or 10 seconds well within recommended limits . also my car doesn't have the delay run cycle that shuts down the pumps a split second before it kills the ignition. 76 ...930 motor in an 80sc .. if i watch the gauge as soon as i shut the car off it will drop to about 30 and slowly go up a few psi. for a minute or two before it starts to drop again i think that's due to the hot engine.

even with the engine pressure in the high 20s after 30 minutes i sometimes have an issue starting hot . and other times no issues starting hot.. frustrating, scared to shut it off when i go out .

[Bucketlist]

I know you have been fighting this for a while so I'll just ask a few basic questions.
When trying to start hot do you hear the fuel pump cycle when you turn the key on?
Do you see the control pressure increase on the gauge?
If it starts does it seem rich, blow any smoke?
Or is it lean, do you have to cycle the key more than once to see an increase in control pressure?
Are you sure it is a fuel problem since it is intermittent?
Good luck!

[gorskined]

Originally Posted by Bucketlist

I know you have been fighting this for a while so I'll just ask a few basic questions.
When trying to start hot do you hear the fuel pump cycle when you turn the key on?
Do you see the control pressure increase on the gauge?
If it starts does it seem rich, blow any smoke?
Or is it lean, do you have to cycle the key more than once to see an increase in control pressure?
Are you sure it is a fuel problem since it is intermittent?
Good luck!

Steve

Funny you should mention are you sure it's fuel.. my fuel issue has been haunting me for quite some time .. I'm convinced I have it dialed in perfectly system and control and afr all spot on cra runs amazing .. I now see why they call them the widow maker.... Super tight system very little drip back .30 psi after 30 minutes 24 psi after 1 hour and 12 psi after 2 hours... I was convinced it was a an intermittent fuel problem . Because the car runs flawlessly. Only time there is an issue is some times after I shut it down for 5 minutes will not start .... Well it did it again yesterday. Super hot day 92 deg ran amazing but after shutting down for 5 minutes no start... I pulled the #4 plug wire had a friend crank the car over I could not see a spark. Stuck a nail in the keeper around my left hand and held the nail I wanted to be 100 percent sure there was no spark. He cranked it over nothing.... I wet a rag and cooled off the coil started right up swapped out the coil this morning another super hot day been driving all day stop and go no issues.... I think I may actually have a drivable car.....

[gorskined]

I think what was happening is once the air stops blowing across the coil the residue heat from the motor gets hot enough to break down the coil.giving me the impression it was the hot start fuel issue . I'm thinking the times I depressed the sensor plate and it started was just a coincidence. Because sometimes it would work and other times it didn't...