HELP PLEASE? FUELLING PROBLEM ON '75 TURBO.

[1975turbo]

We are trying to sort out a problem that is puzzling my highly experienced Porsche specialist who has known my car since new. The car has the 930/50 type engine, with the earlier type Bosch fuel distributor 013 and no fuel accumulator.

The problem we have is overfuelling on initial start up, resulting in having to crank the engine over for 7-8 seconds with the throttle fully open, before it clears and the engine starts. This is a new problem as previously it was an instant starter. He thinks that the fuel distributor piston doesn't appear to stay in the down (no fuel delivery) position after an extended stop. As soon as the fuel pumps are triggered, fuel is injected until the piston returns to its correct position. This only takes 1 or 2 seconds but temporarily floods the engine, giving excessive fuel "wash" and then takes 7-8 seconds to clear and start.

Bosch have overhauled the fuel distributor and warm up regulator and new injectors have been fitted. Fuel delivery system pressure is to spec, as is fuel control pressure. The sensor plate within the metering unit is correctly positioned. Fuel pressure retention is also to spec. The problem is present whether the electric starting valve on the metering unit is connected or not. It has only shown itself since fitment of the new injectors, which are now keeping the injection lines filled with fuel where the previous worn injectors were "dribbling" fuel.

Any help on this problem will be much appreciated. Would lack of a fuel accumulator have any bearing on the problem, such that fitting one would be worthwhile? Would it actually be detrimental to fit one with the type 013 fuel distributor, as they were only factory fitted with the later 016 type?

Photo attached of the car when new, showing (non factory approved) alternative use of the rear spoiler

 

[John_Ward]

Only time I had a fuelling problem on my 1975 930 it was down to a one way valve in the system failing and allowing the fuel pressure to drop - which clearly isn't the problem in this case! Probably worth a call to Bosch Technical Services; I've found them remarkably helpful in the past, and more than happy to tackle something that wasn't current technology.

 

[vitesse]

Well the fuel distributor does require the residual differential pressure difference over the piston to maintain it down in contact with the sensor plate arm & that was one reason for fitting fuel accumulators in addition to the one way valve on the fuel pump output.

Also the piston could well be sticking despite the overhaul until the starting sequence is begun-I suggest the people who overhauled it-whether Bosch or not.

 

[1975turbo]

Hi vitesse, it is interesting that fuel accumulators were fitted to maintain pressure. That would indicate it to be a sensible upgrade, as long as the existing fuel pumps have one-way valves on the output. It doesn't explain why we are only now having this problem, so maybe there is a problem introduced by the overhaul of the fuel distributor? The piston is not sticking, in fact if anything it is perhaps a little loose, though the tolerances involved are very fine indeed. System pressure is also being maintained so it is a puzzle as to why it is not operating correctly.
Thanks for your valuable input and I will report back on developments.
Kind regards,
Chris

 

[1975turbo]

Another complication though is would fitting a fuel accumulator be compatible with the existing fuel distributor? Porsche appear to have changed to a different fuel distributor, or at least different part number, and maybe this was when they started fitting accumulators?

 

[vitesse]

As this was the early days of mechanical automobile petrol fuel injection,I guess they were learning so fast that changes were introduced at frequent intervals,particularly the problems with hot starting,so fuel pumps were upgraded,accumulators went from 1 at 20cc to 2 at 20cc to 1 at 40cc & a solenoid valve was added to the fuel distributor,injector pipes from braided to steel & although I have a copy of the Bosch technical manual,it doesn't have a section on "what can go with what"
To be honest my current understanding generally relates to the 924 systems but the same principles apply to all engines -at start the piston has to held down onto the lifting arm by control pressure helped by the small differential pressure over the 2 sides of the centre steel shim gasket in the distributor