lawt

I have a very unusual problem that has me baffled….. any ideas welcome.

I’m trying to recommission a 1983 930 turbo flat nose after a few years in storage and have encountered a problem which has me foxed……. From cold the engine starts fairly well and quickly settles on a good high speed idle, within a minute the idle speed starts to reduce exactly as I would expect, still running very nicely on all 6 not missing a beat. Then - after about 3-4 minutes - things go down hill, initially it’s a little flutter, an occasional misfire, this then rapidly progresses to a heavy misfire and the engine slows and stops. I can keep the engine running a little longer by tickling the throttle but it’s a losing battle and eventually the engine stops. At this point it will not restart, I have to leave it half an hour and then the cycle begins again.

Things I’ve tried…….
Drained the old fuel and added some fresh.
Checked the timing - spot on
Disconnected the WUR and aux air valve - no change
Checked the fuel pressure and control pressure using test gauges - stable and exactly as described on various CIS tuning pages.
Changed the coil
Checked valve clearances
Checked compression on 4,5&6 as these are easier to get at……. Not good, 5&6 are 115psi and 125psi but 4 is down at 85psi - cold.

With the air filter removed I can see the large diameter disk of the air meter for the CIS, when the engine is first started this disk is nice and stable, but as the misfire starts I observe the air meter starts to bounce about quite a lot and occasionally slams shut. I can stabilise the engine as it starts to falter by steadying the air meter by applying light pressure with my finger, but this does not help for long and the engine will eventually stop.

So I’m stuck - I would expect the low compression on no4 to present itself as a misfire however it initially runs very nicely when cold and I can’t think why it would prevent the engine for running or restarting when it gets warm.

Has anyone seen anything like this????

Many thanks
Tim

fritz k.

Hi,
you've got an interesting problem, the cause could be much to lean or to rich mixture or missing sparks. If i should have a guess i would say it's a fuel problem, but I wouldn't rule out the ignition. Clarifying this question will facilitate further troubleshooting.

To get a clue I would try the following:
Warm up the engine until it dies. Remove a spark plug cap, insert another plug, connect it to ground and check whether a spark occurs when you try to start the engine.
If there is a spark, I would remove 2 or 3 plugs and see if they are wet. Maybe there is a fuel smell when starting?

What were the measured values for system and control pressure cold and warm? Has a measurement of the flow rate been taken?

Cheers,
Fritz

lawt

Hi Fritz,

I will try the spark plug test next time I see the car later this week.

System pressure was 5.5 bar, control pressure was 1.5 bar cold, this increased to about 2.5 bar by the time the engine stopped. One additional detail, the WUR version code stamped is 112 - this I believe is the correct version for a 3.3 turbo for 1983.

Regards, Tim.

lawt

I’ve just had another thought that I will try. I only measured the system pressure before starting the engine when cold. I should have measured the system pressure again when the engine has stopped and refuses to restart - maybe, just maybe it has nothing to do with engine temperature and it is a problem with fuel supply losing pressure due to a restriction/blockage.

Tim.

fritz k.

Hi
- ...112 is the correct WUR, used since 8/82
- Control pressure cold between 1,2 and 1,6 bar at 20 degrees
- Control pressure warm 3.65 bar +- 0.2
- System pressure 6.0 to 6.7 bar

The warm control pressure is therefore too low, but this could be due to the fact that it is not fully regulated after 3-4 minutes.

I would look for the reason for the low system pressure. If both fuel pumps are running, the inlet could be blocked somewhere, e.g. at the fuel filter.
The flow rate for both pumps must be at least 1.5 litres in 30 seconds.

Fritz

lawt

Thank you so much for your support on this Fritz, I will be looking at the car again tomorrow.

I wasn’t aware that there are two pumps, I see what I believe to be a filter, a pump and an accumulator in the front corner of the engine bay - am I mistaken?

Best regards
Tim

fritz k.

In the left corner of the engine compartment you will find the fuel filter and the fuel pressure accumulator, thats right.

The first fuel pump is located on the front underbody under the tank behind a cover, the second on the underbody on the left side of the rear wheel housing.

Fritz

lawt

Hi Fritz,
I think we have a smoking gun!!! The front fuel pump is seized solid, replacement on the way arriving this morning. All the evidence lines up with this fault, the fuel pressure is low (5-5.5 bar vs 6.5 bar), this lack of pressure means the regulator valve will not open so fuel will not circulate back to the tank. Without the fuel recirculating the one working pump will not be cooled by fresh fuel from the tank and get warm leading to a vapour lock and a further loss of fuel pressure. The engine stops and won’t restart until the fuel pump has cooled down!!
I shall let you know how it goes today!!
Many thanks
Tim

lawt

Hi Fritz, close but no cigar!!! Both pumps now working, didn’t measure the volume pumped but they half filled a gallon can in under a minute and when briefly dead headed to my gauge the pressure shot over 10 bar and held their after I stopped the pumps. So now happy we have good supply pressure and volume. When I managed to get the engine running the system pressure was about 6.2 bar and the control pressure started at 1.5 bar and increased to 3.5 bar - so again looking good.
The problem now is the engine will not run unless I manually hold the air meter disk open by hand. When the control pressure is at 3.5 bar the counter force from the fuel distributor is so great that it fully closes the disk cutting fuel off to the injectors and stopping the fuel pumps. I can only think of two explanations for this - a significant air leak in the induction system leaves insufficient air through the meter disk to hold it open , or, some internal blockage or fault in the air meter/fuel distributor unit that is presenting a higher force than it should to oppose the force from the air flow. Again, I’m stuck. I’ve put the thing back together and pushed it back in the garage in disgrace. Any further thoughts appreciated.
Best regards, Tim

fritz k.

Hi,

the K Jetronic is a simple and reliable injection system, but it does not like long downtimes because of the high-precision precision mechanics in the mixture unit.

We have the fact that the air pressure on the meter plate is not sufficient to lift the control piston against the control pressure and release the control slots so that fuel can flow to the injectors.

That the pressure on the meter disk is insufficient due to air leak can be excluded, because a possible leak would be some where in front of the pressure plate with no effect.

I would first make sure that the engine has compression and that the valve clearance adjustment is correct. Then check the position of the meter plate in the funnel in the zero position with the engine stopped. It must be centered in the funnel and have an even 1 - 2 mm clearance all around. The position of the meter plate determines the position of the control piston and thus the fuel flow from idle to full load.

If no improvement can be achieved with this, the reason may be corrosion in the mixture unit. Next would be have the unit checked or replaced.

Fritz