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Warm start problem but coldstart ok

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Old 02-14-2018, 08:44 PM
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GerritD
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Question Warm start problem but coldstart ok







Hi fellow 928-colleagues,

can perhaps someone point me to the right direction, as in which component could be the culprit for my warm start troubles :
1. I have a 928S euro of 1980, 4.7L 300Hp 3sp AT with K-Jet
2. cold start is no problem. Engine fires up right away without pressing throttle pedal
3. once engine is warm and I shut engine off, I can easily start after 5 to 10min, without pressing throttle pedal
4. however, when engine is warm and I shut it off for more than 30min, I cannot start the car again, not even when pressing pedal to the metal
I can hear engine wanting to start up but no luck. At the same time I can smell fuel at the exhaust.


Here is what I already did :
- check injectors : they are not leaking
- check system and control pressure when engine is cold and is not running : pressure within specifications
- I replaced diaphragm and seals of the WUR
- I replaced my ignition coil
- I replaced distributor rotor
- I replaced ignition leads
- I replaced my spark plugs
- I checked my emission value and it is 2% CO, so within specification of Porsche (between 1,5 and 2,5)

Is a bad WUR the culprit?
Or should I look for other failing components....
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Old 02-14-2018, 08:57 PM
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Here is a picture from one of my spark plugs :

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Old 02-14-2018, 11:14 PM
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Hi neighbour .
If your engin run fine when hot and if you have just problem at hot restart ...
May be you loose pressure to quick .
Check the no return valve in the fuel pump , leak in the pressure accumulator and the O ring of the pressure valve in the fuel distributor .
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Old 02-14-2018, 11:22 PM
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You need a pressure gauge to test the timing of pressure fall .
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Old 02-14-2018, 11:32 PM
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Richard S
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I once had the same symptoms on my '79 928 with CIS injection. In my case the fuel distributor required a rebuild. It would not hold fuel pressure after warm shutdown.

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Old 02-15-2018, 05:18 AM
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The Forgotten On
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Check your fuel accumulator in the rear of the car to see if the diaphragm inside of it has failed. It will have gas leaking out of the top of it.

This is a classic symptom of this part failing.
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Old 02-15-2018, 06:51 AM
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Then there are 2 things I can check:
- return valve fuel pump: easy to check : when engine is warm and before I try to start, I can jumper the fuel pump
to build up pressure again and then try to start. I can also measure pressure fall with a clamp on the fuel line at the
pump side.
- measure control and system pressure when engine is warm
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Old 02-15-2018, 07:01 AM
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Originally Posted by allan29 View Post
You need a pressure gauge to test the timing of pressure fall .
Originally Posted by The Forgotten On View Post
Check your fuel accumulator in the rear of the car to see if the diaphragm inside of it has failed. It will have gas leaking out of the top of it.

This is a classic symptom of this part failing.
That is indeed what I still need to do, I just am afraid of creating fuel leaks during tests with a gauge especially with a hot engine.

Btw I do not have a fuel accumulator, nor on this engine, nor on my old 4.5 240Hp engine
Before I swapped my 4.5l engine with this 4.7l engine,
everything worked. I never had starting issues.
Except from the ignition distributor, all CIS components are the same.
So I reused them on my newly rebuilt 4.7l engine.
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Old 02-15-2018, 07:14 AM
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Originally Posted by allan29 View Post
Hi neighbour .
If your engin run fine when hot and if you have just problem at hot restart ...
May be you loose pressure to quick .
Check the no return valve in the fuel pump , leak in the pressure accumulator and the O ring of the pressure valve in the fuel distributor .
Dear Allan,
when I have a problem with my return valve of the fuel pump, shouldn't this also occur at cold start?
I will try to eliminate the return valve issue first by putting a clamp on the fuel line near my fuel pump
If fuel pressure still drops, the problem is my fuel distributor.

However, I still suspect my WUR as I needed to knock the pin (where bimetallic coil is attached to) further into the WUR as
my control pressure at cold was too high(3,5bar instead of 1,3) and thus I could not start the car as I got too much fuel when starting.

but of course this will also influence my pressure when warm, not?
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Old 02-15-2018, 10:35 AM
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For the residual pressure test , you don't need to have the engine hot !
Disconnect the WUR and the additionnal air valve , put pump on without start the engine during 2 minutes minimum and stop the pump .
Watch the pressure after 10 , 20 and 30 minutes . You can see 2,7 Bar , and 2,6 bar and 2,5 . If less, it's not good .
All CIS system have a pressure accumulator , it's hère to resist to the vapor lock .
If you don't have , you need one !
Before incriminate the fuel distributor , check the pressure valve and piston O ring in !
If you speak French language , email me , I'm better ...
You have a problem with your WUR too .
The high cold pressure is a problem .
Your engine run fine when hot ?
The test was with vacuum ?
Clamp the hose of the pump is a good solution , but you cannot know if the problem is the fuel return valve or the pressure accumulator .
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Old 02-15-2018, 10:50 AM
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Some pictures ...



Watch the last 3 digits of the Bosch référence number for the good diagram .
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Old 02-15-2018, 10:52 AM
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Hère for a 4 cylenders but it's the same ...

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Old 02-15-2018, 10:59 AM
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The little screw for checking if the accumulator leak ...
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Old 02-15-2018, 03:44 PM
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Any K-jet I ever worked on had a fuel accumulator. Porsche,VW,Mercedes etc... It's purpose is to hold residual fuel pressure after pressured up.
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Old 02-15-2018, 05:44 PM
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Originally Posted by allan29 View Post


Hère for a 4 cylenders but it's the same ...

Hi Alan,

now I know what you mean. Is there a rebuilt kit with the O-ring so that I can replace mine just to eliminate faults on the relieve valve?

I speak fluently French As I am working in Brussels, but my technical language in French is not so good.

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