chris911spain

Hello everyone,
New to the forum here.
I am trying to sort out a hot start issue on my 965 turbo 3.3 from 1992 (cat).
Starts well cold, starts ok when hot if immediately restarted. But only starts with very long cranking when left about 45 minutes with the engine hot. Then she runs very rough for about 10 minutes before she clears up. Lots of backfiring and misfiring.
I want to start by measuring the fuel pressure drop in the system, however I don't know the factory fuel pressure values.
I have searched for a workshop manual to download but haven't had any success.
I just ordered the Bentley 964 book but it will take 2 weeks to arrive.
If anyone has the information I could get a headstart on my troubleshooting.

Thanks!
Chris

Black_Hat

Black_Hat

chris911spain

Thanks!
Thats very helpful.
Which manual are these pages from?

Chris

Black_Hat

fritz k.

If the holding pressure drops too quickly it is not necessarily a faulty pressure accumulator causing that. More often it is leaky injection valves that flood the cylinders with fuel when the engine is shut down which causes the starting problems.

Fritz

peterpullin

other issue can be the check valve located at the front pump. if this leaks pressure all values drop immediately. with running pumps everything seems fine.

weberlukas74

It could be the fuel pump acting up, especially if it's old or weak. When the engine is hot and needs more fuel, a worn-out pump might struggle to deliver adequate fuel pressure.
Your symptoms might also point to a problem with the fuel pressure regulator. If it's not doing its job properly, the engine might have trouble starting and rough running, especially when it's hot.
Another possibility is a dirty fuel filter, which can mess with fuel flow and pressure.
So, check the fuel pressure both when the engine's off and when it's running.
Faulty fuel injectors can also cause similar symptoms. Check the injectors for leaks.

chris911spain

Ok thanks for all this information and advice!
I will check the pressures and keep you posted.

Chris

chris911spain

So, i checked system pressure between the fuel distributor and Wur. Very low pressure, 3.5 bar. Checked pressure directly from the pump outlet 7 bar. Measured outlet of pressure accumulator and fuel filter: 7 bar. So it seems like i have a restriction in the fuel distributor itself. I am planning on starting with the primary pressure valve. This car has had this problem since I bought it, so I can't assume that the fuel distributor hasn't been tinkered with by a previous owner. Is there a fuel flowchart for this fuel distributor so I can try to deduce where the restriction might be?

peterpullin

are you sure you did it the right way? 3.5 should be the right value for coldstart at about 2x degree.
your car would not run with this value under any circumstances if this was the system value. not eveb the injectors would open..

so please check again with closed valve.

chris911spain

I agree with you. Just tested directly at the outlet of the fuel distributor going to the WUR and got a reading of 5.1BAR. Looks like the 3 way valve between the fuel distributor and the WUR on my tester has a leak. So now the reading makes more sense. Its enough to open injectors and enable the car to run but below the 6.1 to 6.8 bar we are looking for.

chris911spain

UPDATE. I narrowed down my pressure "leak" to the pressure regulator. Ordered a rebuild kit and installed it with the shims in order to get the correct system pressure of 6.4 Bar.
However... now my cold pressure with the WUR is much too high for the ambient temperature at 4,8 Bar. Could it be that the WUR had previously been adjusted to compensate for a low system pressure?
I still haven't received my workshop manual, its supposed to arrive in 10 days... Does anyone have the information for WUR diagnosis including the wiring diagram as well as the procedure for WUR adjustment?
Thank you very much!
Chris

fritz k.

Hi, if there are deviations in the control pressure, the WUR has to be replaced, it is not intended that settings be made. Neither the WSM nor the circuit diagram therefore contains an explanation about this.

However, there is a method developed for this, which can be found under “Adjustable WUR” and can be recognized by changes to the bearing bolt for the bimetal spring. If you are not sure post pics of the WUR.

Fritz

Nyx

1) Is your dial gauge recently professionally calibrated? We are talking about small changes in values. (Depending on scale)

2) Don’t use ambient, use a IR thermometer on the body of the WUR.

3) Run your fuel pumps to normalize temp

I had a gauge which was 10% off before getting it calibrated.