924Board.org

jimcarver · Joined: 12 Dec 2006 Posts: 45 Location: London, UK

Hi i have posted before about cold/warm start problems.
I have replaced the cold start valve and auxilary air valve but still have problems.
Now the car starts fine from cold or hot, but when i have driven the car and let stand for about 1-2 hours i have problems. The only way i can get it to start is to leave my foot down and keep the key turned and after about 8-10 seconds of turning over it will start. If its left for a few hours or more it will start fine cos the engine is completely cold again or if i restart when the engine is still hot its fine.

Its about 5 degrees C here at the moment if that helps.

Thanks in advance.

924RACR · Posted: Mon Jan 22, 2007 12:01 am Post subject:

Congratulations; you've fixed the cold start problems, now you have the hot start problem! That's classic.

I believe it's covered in the FAQ, but most likely you're losing pressure in the fuel system as the car sits; either one or more injectors leaks, or the check valve at the fuel pump leaks. Either way, when you crank, there's no pressure to run the car right away, until the fuel pump's run a little bit.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

Try checking the thermal time switch. If shorted to ground it will cause the cold start valve to inject uneeded fuel. Testing is with an ohm meter. first check for a circuit between the two posts. If the engine is warm or hot there should be no circuit. Next check for a circuit between each post and ground. There should be no circuit at any temperature.

Hope this helps.

Dennis
_________________
81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

Paul · Posted: Mon Jan 22, 2007 12:11 am Post subject:

This is a very common problem with our cars, do a bit of searching for "Hot Start".

Basically your fuel system is losing pressure allowing the hot underhood temps to boil the fuel in lines. When you try to start the car, there's no fuel to inject until the lines clear.

3 things to check:

Check valve in the fuel pump
fuel accumulator(s)
Fuel dizzy relief valve

The best way to check this is with a CIS tester. Hook it up, shut off the car, and see how fast the fuel pressure drops.

Your method of clearing the lines is the one recommended in some owners manuals.

I like to turn the ignition switch on long enough for the pump(s) to run and shutdown (without engaging the starter). Then turn the switch off. Repeat about 5-10 times then start the car.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

morghen · Joined: 21 Jan 2005 Posts: 9095 Location: Romania

i dont know about you guys...but i wired the fuel pump relay to go forever and when i start the car i let it pressure up for a second or two...dead cold or red hot...its starts. Tho' when hot i can see that there is a hot start problem because it cranks for 1 second until the vapor goes away and the sistem fills up with gas. But hey...it starts !
_________________
Supercharger and EFI kits
https://www.the924.com

Paul · Posted: Mon Jan 22, 2007 1:12 am Post subject:

It a roll of the dice.....

Running the fuel pump any time the ignition is on, clears the lines quickly.

The concern:

I hope you are never trapped in a crashed car trying to reach the ignition switch to shutoff a ruptured fuel line....
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Min · Posted: Mon Jan 22, 2007 4:35 am Post subject:

jimcarver · Joined: 12 Dec 2006 Posts: 45 Location: London, UK

I see a couple of mentions of the fuel pump valve, theres a buzzing sound coming from the rear of the car which after closer inspection seems to be the fuel pump. Would a faulty valve make this noise?

Could this be a likely cause, i thought if it was faulty it would have affected the car at all temps?

Smoothie · Posted: Mon Jan 22, 2007 5:22 am Post subject:

The fuel pumps themselves are noisey, that's normal. The check valve would be inaudible. Its' purpose is to hold pressure in the system after shutdown. While the pump is running, the check valve is just opened, allowing fuel to flow forward. The noise in the OE Bosch pumps is from metal rollers spinning around inside.
With key in and switched to ignition-on, you may only hear the fuel pumps for 1-2 seconds, but they should run continuously while you have the starter turning. They're generally noisey enough to hear from the drivers' seat.
Holding the pedal down helps to clear vapour lock quicker - if that helps, you probably have the hot start problem caused by a leaky injector, cold-start valve, fuel pressure regulator check valve or fuel pump check valve, or the fuel accumulator's trashed as was mentioned before.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Ozzie · Posted: Mon Jan 22, 2007 6:58 am Post subject:

Mine would start cold but would be a cow when hot if left for longer than a few minutes.
My injectors were at fault and are easy enough to check.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

People keep referring to "leaky" injectors. How can a CIS injector be leaky? When subjected to fuel pressure, they're supposed to spray!
_________________
Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

Lizard · Posted: Mon Jan 22, 2007 9:20 am Post subject:

the CIS injectors are supposed to open at 35PSI of pressure,
when you turn off the car the pressure to the injectors drops below that, if they are leaky then fuel continues to come out of them, draining the residual pressure in the system
_________________
3 928s,

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Well, that explains it. I thought they were always "open" like a nozzle and the dizzy controlled everything.
_________________
Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

Mike924 · Joined: 12 Aug 2004 Posts: 2601 Location: IoW UK

Jim, try changing your fuel accumulator (baked bean can looking thing connected after the pump).

In my experience (limited though it may be), that is the most likely cause of the classic 'hot start' problem. Cured it on my car...
_________________
1985 Porsche 924 'Lux', Kalahari Beige (my ex)
1993 Porsche 968 Coupe, Midnight Blue, 6 spd

'There is no substitute for a little grease under your fingernails.' - Chrenan, 924board.org

jimcarver · Joined: 12 Dec 2006 Posts: 45 Location: London, UK

Thanks for all the advise.
I'll check the injectors.
Is there a simple way of checking the accumulator and fuel pump check valve as i dont wanna keep replacing parts that may not be the issue.
Would installing the cold start switch as mention in the tech section help me, or would this just cover up the problem.
Cheers