924Board.org

[OldBrownShoe]

Hi everyone!

I'm new to this forum, this is my first post
My car is a 1978 924 2.0L Euro Left Hand Drive.
I've experiencing the famous hot start problem. I've changed the fuel pump and accumulator, WUR, CSV, dist cap, rotor arm, plugs, leads with no success. It starts fine when cold, but whenever you run and stop let's say for more than 15-20 minutes, it won't start again.

I've installed the hot start push button kit (as in the tech section) and it didn't make any difference.

Upon inspection on the electrics, I've found out that my relay was probably not suitable for my year (part number 433 096 059). I decided to replace it for a brand new one as suggested in the technical section for a 1978 (p/n 477 096 059) - the car simply didn't start at all and there was a horrible intermitent clicking sound during the start up attempts. I've switched back to the former one and it started normally (cold) with no odd noises.

I am suspecting on the injectors, but I've been running the car with some of those fuel inj cleaners (redex, stp, lucas, etc.) so I suppose they shouldn't be that bad (?)

I've read somewhere that whenever you have a defective fuel pump check valve you are likely to experience these problems, but I assume mine must be brand new since the pump is also brand new (a pierburg one, by the way).

My mechanic has ran a diagnostic test which accused that the engine is running rich, and he would need to do a complete inspection to diagnostic the cause. I don't know if it has anything to do with the hot start problem, but I simply couldn't afford to pay the labour, which is a small fortune per hour...

Does any of you have any guesses on what could be the issue??

Thanks!
_________________
924 2.0L 1978 Euro LHD

[Smoothie]

You get the hot start problem when the fuel system loses pressure on shutdown anywhere between the fuel pump check valve and the pressure regulator that's built into the fuel distributor. If pressure there is lost, engine heat will cause vapo[u]r lock to form in the fuel injector lines. Possible places to lose pressure are the fp check valve (I think the Pierburg (as well as the OE Bosch pump) only have an overpressure relief valve built-in, and the check valve is another part, so you need to have a good one in place at the outlet of the main pump). Leaky fuel injectors can be another cause - they're supposed to close and seal tight at something around 25psi - otherwise they'll leak fuel into the cylinders until pressure drops to 0. The frequency valve is another possible cause in cars that have it, but I'm about 100% sure that your UK car doesn't have it. CSV - your good with since it's new. WUR as well - it's new and should be good. Last place and cheapest-easiest to fix is the check valve in the fuel distributor. This is also known as the primary (or "system") fuel pressure regulator. There are either one or two small o-rings in it that seal the fuel return to the tank to keep the system pressurized at shutdown to prevent vapo[u]r lock. -Can't be just any olde O-rings of the correct size - they have to be made of a fuel resistant material such as Viton.

Another thing to try in the mean time before it's fixed is to hold the throttle pedal to the floor while operating the starter until it starts. This helps to clear the vapo[u]r lock faster.
_________________
"..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]

Nailed them all Smoothie. My bet is the injectors or the 'O' ring in the regulator.
Check the fuel pump has a non return valve
The "O" ring is the cheapest, do that first and then do the injectors.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

[Paul]

Can also be a bad fuel accumulator.

You can also turn the key on (not to the start position) and let the fuel pumps run until they stop.

Turn the key off.

Repeat 5 to 10 times to clear the fuel vapor and to refill the system with fuel.

Start the car.

Or just install a push button that runs the pumps when pushed.
_________________
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.

[Ozzie]

[Paul]

Agreed, I was just completing Smoothies all possible issues list.
_________________
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.

[dpw928]

Another possible is a defective thermotime switch which can activate the cold start valve when the engine is warm.

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

[Ozzie]

That's an easy check too- good thinking.
Put a meter on the CSV and make sure it switches off after 10 seconds or less.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

[Mike924]

Needs to be 10 seconds of cranking though...
_________________
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

[OldBrownShoe]

Hi Folks,

Many thanks for your contributions!
Some complementary information:
1) The pump, accumulator, plugs, distr cap and rotor are brand new
2) the WUR and CSV are used but taken out from a running vehicle. I am not sure about the condition of the CSV, but the WUR measured 19 ohm - as described in the Haynes manual it's ok I suppose (?)
3) the car is a german left-hand drive and not an UK one
4) I have installed the hot-start push button

Now the questions:

1) Where do I look to find the check valve? How does it look like?
2) Do I have a frequency valve in my vehicle since it is a German and not an UK model?
3) Does the push button, in a way, substitute the thermo time switch during the start up and therefore can I discard the possibility of a defective switch?
4) the bizarre thing is that if I put my feet all the way through the floor the car is less likely to start than if I don't step in it - does this have anything to do with the fuel distributor?
5) if the o ring is the problem, wouldn't it be difficult to start when cold as well?
6) Any other way to test the WUR?

Thanks!
_________________
924 2.0L 1978 Euro LHD

[Smoothie]

1) System pressure regulating valve - in the US Haynes appears as #13 in Fig. 3.1 on pg 73, and in fig. 3.5 on page 76.
2) Frequency valve - it's part of the lambda (o2 sensor) system. You'd only have it if the car has the lambda parts and a catalytic converter. The lambda fuel distributor looks like this - aluminum color, and lines out to, and return from a frequency valve -

3) No. -But the fact that it made no difference suggests a possible problem with CSV activation. I'd check the wiring and test to be sure the CSV is getting 12V when you press the button.
4) Again, suggests the CSV may not be getting activated. (Holding the pedal down with a non-functioning CSV would leave it too lean to start.)
5) No
6) Thee way to test the WUR, as well as the possibility of a leak somewhere is with a CIS fuel pressure tester. They're available from JC Whitney here at about $55. I hear they can be delivered to the UK, and the cost is still much less than sourcing it locally in the UK. There's a topic somewhere here by someone in the UK that ordered one from them recently. The WUR is both electrical and mechanical. The mechanical part (the valve) can get clogged or rusted.
_________________
"..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

[Mike924]

My tuppence-worth:

1) I think the check-valve being referred to is the one on the output of the main fuel-pump. Often, a new pump won't include a new valve, so this could still be the problem.

2) I think the O2 sensor system was only used in US models - or at least nowhere in Europe.

3) The CSV is not activated during hot-starts as the thermo time switch (TTS) is already open.

4) Doesn't this suggest a vacuum leak...?

5) What Smoothie said. When the fuel fully cools (after about 5 - 6 hours!) the vapour finally condenses.

6) Ditto. Even if the WUR checks out electrically, it can still be blocked hydraulically (and yes it basically serves a hydraulic function). It was Juno who recently ordered the JC Whitney tester (not sure ifiit's arrived yet). Tiny (in the UK) also had one brought over from the States.
_________________
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

[dpw928]

Mike924,

I've seen the TTS burn out and short to ground which will cause the CSV to activate during hot start cranking. An easy test for this is to check each connector for a path to ground. They should be open. This problem caused my 78 928 euro to hydrolock after repeated hot start attempts.

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

[OldBrownShoe]

Dear Gurus,

Thanks for sharing your astonishing knowledge! I can say I'm a relatively good driver but a horrible mechanic, totally newbie since I've got this car quite recently and this is my first experience into the 'classic car world'.

More comments/doubts:

1) My car system fuel dizzy is definately different from the picture, so now I am sure I haven't got a frequency valve (one thing less to investigate)
2) I wasn't able to find the check valve... could it be possible that the guy installed the pump without fitting it in??
3) There are two CIS testers in the JC Whitney website, one standard ($39) and other for K-jectronic systems ($59). I suppose I have to go for the more expensive one, right? The problem is that I possibly wouldn't know to operate it as I am a complete ignorant in mechanics (laughs...). 4) Is this device useful to test other components apart from the WUR?
5) I have unplugged the CSV to see what happens and I suspect Smoothie is right about the activation, since it made no difference. I will try to grab a multimeter to test asap.... by the way, how do I test it? Shall I simply disconect the plug and attach the multimeter tips on each pole while cranking and pressing the push-button?
6) If I understand right what Mike suggested, that the CSV is not activated during a hot start and since I've unplugged it with no effect, can the problem the thermo switch then?
7) Has the aux air valve anything to do with this?
I've noticed the 'giant' rubber hose is a bit loose on the distributor, occasioning vaccum leak (??)

Sorry for posting too many questions and thanks for the help!

Andre
_________________
924 2.0L 1978 Euro LHD

[dpw928]

Andre,

The expensive K-Jet tester is what you need. To test the electical portion of the CSV use a multimeter to make sure there is a path between both connectors. To test the TTS is a little more involved. If the engine temp is below 100F, there should be a path between the two connectors. This should switch off (no path) if the temp is above 100F. Also check for a path between each connector and ground. If you find a path on either connector, the heating coil has burnt though and is causing a short to ground. Unplugging the CSV should rule out a bad TTS unless the engine was already flooded.

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