Fuel Pump Stalling

[Brewster]

Porsche lines front to back. If you crack the filter outlet when the pump is struggling it suddenly sparks back into life. Two pumps have been tried and both display the same symptoms. It's a real pig of a fault.

It might be worth jumping a direct feed to the pump to prove it's not wiring or a relay getting hot somewhere, but it would be unlikely with the symptoms.

[Strictly]

have you measured your system and control fuel pressure?

The WUR will lean things out quite a bit when it warms up, and thats after about 5 minutes. Try unplugging the WUR, BEFORE you start the car up for the first time that morning (i.e overnight), if that makes a difference, then you need to set up for WUR for cold and warm control pressure.

I can explain how

P.S. If you want to rule out the fuel pump relay (the red one), just run a wire from 30 to 87a, if you look at your relay it should have the numbers. You only actually need 12v power to terminal 30 for the fuel pump to run, the 87a is to feed 12v to the WUR, AAR, a TTV, if you have one fitted. Ignition on of course.

I once bought a a car with no fuel pump relay (hotwired), half the CIS injection electrics cut/not connected, and a faulty WUR, i got it all fixed without spending more than a few quid on wiring =)

[Brewster]

I tried disconnecting it when it was warm and it made no difference, but I'll give it a go cold.

Would a faulty WUR prevent a return flow to the tank? If it was simply a case of the engine spluttering out when warm I'd be convinced it was the WUR. It's the fact it happens at the same time as the pump struggling because it has too much pressure because of a much reduced return flow to the tank.

[Strictly]

Ah intresting, warm it wont becuase the heater has allready done its thing.

Basically the wur starts rich, say 1.8 bar, then the heater goes on, and it leans out to say 3.2 bar. If you disconnect the heater it would stay at 1.8 bar. Now in your case, if you have a faulty wur, it could be going up to 4 to 4.8 bar which is system pressure. When this happened to me, there was no fuel going through the wur, it was forcing in back through the relief valve of the FD and straight to the return on the tank. I have to say my fuel pump sounded normal even with a wur at 4.8 bar

I think you definatly need to buy a CIS tester. I can link you to one. The WUR and FD combined woulld be £1800 and they may never be quite right for your system (i.e there are loads of different variation in WUR's) but you can only buy one.

I'll post a link to a tester. If you own an SC, your going to need to use it sooner or later. Ive used it quite a few times now

[Strictly]

I bought this one. Its okay, my valve leaked out of the box. But i do like the guage with a purge on it for the air.

http://www.ebay.co.uk/itm/NEW-XXR-TOOLS ... 43c41133bd

The main bit you use is the long piece with the valve on it, this allows you to switch between control and system pressure. It comes with a few different connections.

I can explain how to use it. Google testing 911 cold control pressure if you want some background reading. Using this kit, you can diagnoise the fault.

Im by no means an expert, but i have been lucky enough to use it to pull apart a wur and set it up for warm and cold pressures, which sorted my particular problems

[Strictly]

Read the above posts anyway, but to answer your question

Would a faulty WUR prevent a return flow to the tank?

NO, there is a relief valve on the FD, if the were was faulty (4 - 4.8 Bar) and not allowing any fuel through it, the relief valve in the FD would kick in, and the fuel would return along that path.

If it was simply a case of the engine spluttering out when warm I'd be convinced it was the WUR. It's the fact it happens at the same time as the pump struggling because it has too much pressure because of a much reduced return flow to the tank.

Im not sure i understand this bit. How do you know the pump has too much pressure. You can also use the tester to measure the fuel pump pressure - except this is more like 10 - 12 bar, and the fuel gauge of the tester dosent go that high. I think i would do that as part of your tests.

I still think you have to start by measuring pressures so you can rule things out, like the WUR etc. Im not that experienced on the FD side, as i haven't had any problems with one, but at least by testing system pressures (FD), it will allow others to guide you better.

An no i dont sell the testers lol!

[/quote]

[Brewster]

Thanks Strictly, that's some great info. I'm going to bite the bullet and buy one of those kits if the relay swap and wiring frig doesn't work.

[Strictly]

ok, when you do, post, and i'll tell you exactly how to connect it, you'll get there - think of the fortune your saving not having to pay someone else!

[Strictly]

p.s brewster if you can find the same tester - with the black line with the valve on it, but with a gauge that went from 0 - 13 bar you could also test the fuel pump pressure as thats definatly over 10 bar, it shot my guage to max when i tried it.

[crisc204]

After the wiring and actual pump are tested and your happy with the pipe runs I'd move on to fuel pressures...
Sc's are very sensitive to fuel pressure variance

They use huge fuel pressures, you would need a higher rated gauge to go with that kit.

After that if the system is doing what it should, you should propbably move back to start testing the Warm up reg and fuel head and/or send them off to a man that can with all the 80's kit

I quite like Bosch K Ke jet ect its old school but old is gold!

[Brewster]

SUCCESS!!!!!

Both Pierburg pumps we tried had the same symptoms. After a few minutes the car would starve of fuel, cut out and the pump would sound like it was dead-heading. Crack any fuel line in the engine bay and you'd get fuel by the bucketload squirting out everywhere and the pump sounded much better.

This morning we tried an experiment of wiring the pump direct to the battery through steering cover in the boot. This eliminated any wiring issue or relay issue. The problem was still the same, but now we noticed that the pump would start struggling without the car running and when it was cold.

It was then that we decided to swap the Pierburg for a Bosch. A quick trip up to Leeds OPC to get the bits, fitted them and the whole car ran like a dream. Never skipped a beat. It even passed it's MoT this afternoon. I've wracked my brain as to why this might be. The pump would seemingly give excellent pressure and flow whenever and wherever the circuit was split, but both Pierburgs stopped working in exactly the same way.

Anyway, I'm pleased as punch.

Also a massive thank you to Tim (impmad) for his kindness, cups of tea and encouraging words yesterday.

For the Google bot to help others in future;

Pierburg fuel petrol gas pumps do not work on with 911 SC
911 SC Pierburg pump fails does not work

[Strictly]

ah intresting! good to know, glad you got it sorted - good hunch on going for a bosch.

If you ever need to set your wur up for warm and cold control pressure then you know where to find me!

L

[Brewster]

ah intresting! good to know, glad you got it sorted - good hunch on going for a bosch.

If you ever need to set your wur up for warm and cold control pressure then you know where to find me!

L

I might well take you up on that. I took the car out for a spin this afternoon and it bogged down terribly when it was cold. Like a sewing machine when warm though, at speed and at idle.

[crisc204]

ah intresting! good to know, glad you got it sorted - good hunch on going for a bosch.

If you ever need to set your wur up for warm and cold control pressure then you know where to find me!

L

I might well take you up on that. I took the car out for a spin this afternoon and it bogged down terribly when it was cold. Like a sewing machine when warm though, at speed and at idle.

Now you've been playing with it, the car will likely need to have the C0 set up again.
basically change anything on an SC engine and then you check the C0. Modern cars self regulate the mixture all K-jets are set at the fuel head assembley

Just double check around the inlet bits you've had off for air leaks then get someone to set the mix
if you can't get the mix right take yer'man above up on his offer!
hth