924Board.org

[Brendan]

I'm looking for sounding board help. I’m trying to diagnose a fuel pressure problem on my 77 NA 924. The car starts and runs, but the fuel pressure is erratic, and it drops as the car runs for a minute or so. I have my fuel pressure gauge hooked up with valve open, and the pressure is about 60-65 PSI at startup, and bounces erratically, then it starts to fall down to 40PSI after a minute or so, and the overly rich mixture chokes out the engine and it dies.


Background:

I’ve pulled the fuel distributor apart and cleaned it up. Fuel tank had been full of varnish. I cleaned out about a cup of varnish from the bottom of the tank. Tank is clean now, and flows plenty of fuel. I’ve blown compressed air through the fuel lines. The varnish problem killed the original fuel pump. I found no evidence of the varnish making it to the fuel lines upstream of the fuel pump. Not sure about the state of the accumulators. Fuel filters are new.

Fuel pump is now a new Bosch 044, which is supposed to put out the right pressure for the early CIS (75 PSI). I ordered the wrong adapters for the inlet fitting, so the inlet hose size drops from ½” at the tank to 3/8” at the prefilter and inlet fitting at the pump. I assume this is no big thing, but I could have gone with an AN-8, -10 or -12 fitting at the pump to a ½” barb, which would have removed a restriction on the low pressure side of the pump. I imagine the 3/8” fitting can flow plenty for this little 2.0L.

Fuel accumulators need to be replaced, I assume. I have the double 20ml setup, and I have new hoses on order along with a new single 20ml accumulator. I’d like to go with the 40ml, but my understanding is that the size of the accumulator was increased over the years to solve the hot start problem, but the size of the accumulator has no effect on the function of the system when it’s running. If the single 20ml can’t handle it, I’ll upsize.

Fuel injectors all close, and all spray a nice pattern. I have not yet pulled the CSV to see if it is functioning correctly. I have not checked the function of the thermo time switch.

Analysis:
My assumption is that the fuel pump can put out the 75PSI that is advertised by Bosch, so if I’m seeing only 65 PSI of system pressure at the gauge, then the control pressure regulator (WUR) is the obvious culprit. My theory is that the WUR is not functioning, and that maybe the old rubber seal and spring on the pressure regulator are letting the system pressure run lower than normal. What I don’t get is why the fuel pressure drops after a few seconds.

Action:
My next move is to try my spare WUR, and see what effect that has on the pressure. Accumulator and new flexible fuel lines will be installed when they arrive.

I think I hooked up the gauge backwards, so I don’t have a reading yet on the control pressure. With the valve open or closed, I don’t get much of a difference in the gauge reading. I know that control pressure is not 60-65PSI, because the engine is running rich. High control pressure would make it run lean. Tomorrow, I’m going to switch the gauge around and try to get a reading on control pressure.

I just went through and got all the lights working. Not one pair of lights was working, and every problem was due to really old wire. The wires themselves are starting to fail. I’m finding black corrosion on wires, not just at the old connections, but even quite a ways (3”+) back in from the old factory connections. Some wires are so oxidized that I can’t even solder new connectors onto them. Solder won’t flow onto the old wires. The connections at the fuel pump are new, but I’m questioning the ability of the old wires to carry current enough to feed the pump. I may run all new copper back to the fuel pump.


Questions:

1. How much fluctuation should I see in the needle on the fuel gauge if the system was operating properly?
2. Would a bad accumulator explain the fluctuation?
3. Would a bad accumulator explain the dropping fuel pressure?
4. My ‘Bosch Fuel Injection And Engine Management” book recommends shimming the system pressure regulator in the fuel distributor if the system pressure is low. This seems like a corrective measure to cover for some other problem. Is this a common thing that needs to be done on CIS fuel distributors?
5. Is it conceivable that the restriction on the low pressure side of the pump is causing the fuel pump to cavitate, and that is what is causing low and falling pressure?



Thanks,
Brendan

[Ozzie]

You didnt mention the fuel filter?

Also the pressure regulator in the CIS dissy controls system pressure.
Adjustment shims are available but it wont explain the drop unless the springs and seals are shot.

The WUR adjusts the control pressure.

1x40cc accumulator or 2x20cc. it obviously needs it.

I think the varnish has got into everything but the filter probably caught most of it.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

[Brendan]

Thanks, Ozzie, for the response. Fuel filters are new.

Good point about the control/ system pressure. I'll be back at the car in 4-5 hours and will try to get a real reading on the pressures. I need to do that before I go swapping parts around.

I could see the seal at the pintle on the system pressure regulator failing. The spring seems like an unlikely culprit.

I have not come up with a way to inspect for varnish in the fuel lines or clean out the fuel lines. I have to wonder, if only because it's one of the few things that I have not replaced, if the fuel lines could have a bunch of varnish in them, and friction in the restricted line could be causing the pressure drop. It would be causing a constant drop, though, and would not explain the drop over time as the car is running.

[924RACR]

A build-up of electrical resistance due to heating in corroded circuits could explain the slow loss of fuel supply pressure...
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[Brendan]

I like it. Easy enough to replace the wiring.