Wavey

Hi All,
I picked up an 81 928S Auto 4.7L K-jet last year and have been tinkering and making small improvements here and there to improve the engine reliability. A European car.
I have been through all the usual checks listed here and in the WSM surrounding ignition and fuel delivery.
WUR calibration, CIS fuel pressures, plugs, coil, leads etc.
However it has one quirk that I cannot resolve.....
The car will start with the very first turn even after left sitting for 2 weeks. It will run smoothly immediately without any need to give it throttle.
But if I stop the car immediately and try to start again, it will need a very long crank (5-10 secs)and some throttle before it fires up.
This is the case whether the car is cold, warm or hot.
It will always start eventually but does not give confidence in the car.

Can anybody help point me in the right direction?
I have been through all checks in the Bosch K-jet trouble shooting manual also.

Many thanks!

Robert.

Mrmerlin

Replace these in this order,
fuel pump check valve,
the fuel accumulator,
the seal on the pressure spring screw on left side of the fuel distributor

Wavey

Thanks for the reply Merlin.
Fuel draining back to the tank after a period was one of my first theories but I ruled it out due to the fact that the car can start on first turn when stone cold.
You think the parts below could still cause this issue?
Is the fuel dist pressure screw the one shown in the photo below?

fule dist check valve

Mrmerlin

yes thats the screw and O ring seal from the side of the fuel distributor.

IIRC it should have a thinner black O ring on it.

NOTE for hot no starts,
Please follow my suggestions in the previous post.

NOTE I have fixed 4 early cars with this issue and this was how I did it,
by following this checklist

GregBBRD

As long as you started here:
There's another piston an the other end of the spring. It has an "O" ring which can fail and cause your problem. (Not as common as the fuel pump check valve.)
Sometimes this piston will "pop" out of the fuel distributor and get lost. Sometimes it is slightly stuck and you need to use a magnet to get it out.

Once you determine if the O-ring on this pistons is still OK (no chunks missing), you can put this assembly back together.
That "green" O-ring" is the latest version...made from Viton.
Sometimes this O-ring gets cut when reassembled....which will also cause your problem.
Liberally lubricate this green O-ring" when you reassemble, so this does not occur.

Replace the fuel pump check valve, like Stan says. (This is the most common problem.)

Wavey

Evening Guys.

I had the whole pressure regulator out last night and both O-rings are in their correct positions and in very good condition so I think I can rule that out.

I assume a test of the system pressure and of the pressure drop after 30mins would rule out the check valve and accumulator as culprits? I can easily get a gauge on the return line before it heads back to the tank.

I did try forcing on the pump relay for 5 seconds right before trying a "warm" start but it did not help.

Thanks,
Robert.

Mrmerlin

NOTE the parts that fail after the fuel is hot is in my first post.

Wavey

So the check valve an accumulator. But cant these be checked with a pressure test instead of replacing them?
Or is there a scenario where the pressure test would pass but these could still be the cause of the hard start?

Thanks,

Robert.

Wavey

Update....

I fitted a pressure gauge on the fuel feed line from the pump to the FD and I achieved 6 bar bridging the pump relay which fell to 4bar after 10mins and was at 3.5 bar 2 hours later.
That is within spec according to the WSM.
Do you guys think that rules out the check valve and accumulator as possible culprits?
Or it still work replacing the check valve?

Thanks,

Robert.

Mrmerlin

was the system at temperature?
whats happening is that fuel is boiling off in the lines in the engine compartment,a small pressure drop will let this happen.

NOTE this creates a vapor lock if you were to spray some either into the intake then crank it after its hot it will usually start
this happens when the any of the three things I mentioned are not working perfectly

Wavey

Hi Merlin,
This was with a cold system.
I have ordered a new check valve anyways so I'll see how that helps.

Wavey

Just to wrap this thread up to help anyone with this issue..
I replaced the check valve, FD regulator seals, rebuilt WUR and then finally dialed in the control pressures with the help of an A/F sensor.
The WUR & control pressures were what finally fixed this long crank issue.