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Levans · Posted: Thu Mar 11, 2004 1:21 am Post subject: Fuel Pump question ??

My question is : is the fuel pump suppose to come on when the key is set to accessories position? (without cranking the engine over)

i ask this because it seems that i have LOW fuel pressure at start up, warm or cold. (low RPM, engine dies if no accel is used) and around the same time i developed the hot start problem.
fuel pump is only 12 months old.

Just looking over the wiring diagram it looks like the plug that goes to the fuel distributor has a major part in activating the fuel pump.
is this correct?
or maybe the accumulators are now faulty

advice much appriciated!
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78 Dark met Green "THE 924"

924RACR · Posted: Thu Mar 11, 2004 1:44 am Post subject:

I take it you have a relatively early car? They have a connection on the air flow meter, which is a switch that allows the fuel pump to turn on. If it's disconnected, the pump will not turn on.

Depends on what year you have, unless the Aus cars are drastically different. However, I'd still expect it to be a go-no-go situation. Low pressure, unless the pump is bad (or not getting enough power, which can certainly happen), I would expect to more likely be caused by... a clogged fuel filter? When's the last time you changed it?
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Levans · Posted: Thu Mar 11, 2004 1:56 am Post subject:

924RACR, its a 78 and is the same setup as the euro cars.
this problem occured when i changed a bad fuel pump a year ago (making wiered noises)
it would go straight to idle with the bad pump but would not run well when driving. many things have been replaced since like a completly different fuel dizzy, warm up reg and all the rest

the only thing i can think of are
a bad electrical fault (ie connection to the fuel dizzy)
fuel acumulators (not been replaced)

oh and the fuel filter was replaced on my last service (1000k ago)

and when the weather temps get up the fuel pump doesn't seem to like it, it will start making a shorting sort of noise(not good) and along with it is a missing sounding engine (fuel shortage?)

so when you turn your ignition on you can hear the pump turn on?
because mine doesn't
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78 Dark met Green "THE 924"

Finbarr · Joined: 14 Jan 2004 Posts: 85 Location: Worcestershire, UK

The pump should turn on for a second or so after turning the key to the ignition position (i.e. all the gauges come to life) and then shut off until the engine starts turning over - this behaviour is determined by the fuel pump relay. You could check it connecting a small 12V bulb across the fuel pump and watching it while you try to start the car.

It could be that the pumps are clogged or weak and unable to get going and build up sufficient fuel pressure in this time. Has the fuel tank ever been cleaned out? A lot of muck can build up in that many years. It could be corroded connections to the pump - check the earth point inside the luggage compartment just behind the number plate, which is where the fuel pump earths to.
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1980 931 (with the odd electrical quirk...)

Minus000 · Joined: 29 May 2003 Posts: 363 Location: Sidney, B.C.

I have a 1978 and my pump has never started pumping before actually cranking the car. My start problems were lost fuel pressure from a bad WUR and fuel accumulator.

Finbarr · Joined: 14 Jan 2004 Posts: 85 Location: Worcestershire, UK

Interesting - that was my understanding of what happened on my car. I ought to do the same test myself and make sure.

However, the older cars had a slightly different setup to mine, with no coil connection to the fuel pump relay, so maybe their fuel pumps don't run when the key is first turned.
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1980 931 (with the odd electrical quirk...)

Smoothie · Posted: Thu Mar 11, 2004 12:28 pm Post subject:

I don't have any direct experience with the '78 model, but as Levans and 924racr mentioned, there's a switch on the fuel distributor that goes to the fuel pump relay and controls the fuel pump on-off. I believe that with cars that have that switch, the starter has to be engaged, engine has to be turning and sucking in air - which in turn moves the air metering plate and activates the switch, the relay, then the fuel pump.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Levans · Posted: Fri Mar 12, 2004 12:54 am Post subject:

well, today i have gone through and cleaned the ground points at the luggage compartment (especially the thicker brown fuel pump ground)

no change in situation though.

this afternoon i pulled some accumulators(2x 20cc) off a 924 from my wrecker and tomorrow i will fit them. they didn't seem to leak when i undid the screw from the bottom of them. (i read some where that they are bad if they leak when the screw is undone?)

do you think that a bad accumulator(s) would cause the fuel pump to not function properly? (ie, to much back pressure?) and cause the syptoms like

-low rmp at any startup either warm or cold (engine dies when cold, idles low when warm started ~300-600 rpm) and once running for a minute or two will come up to 900rpm

-and also the hotstart situation

maybe my fuel pump is on the way out again after 12 months or maybe it was faulty to start off with

i only say this because i have a huge list of things i have replaced in the fuel sytem. I think the accumulators are the last and only thing i haven't replaced yet.

what do you guys think?
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78 Dark met Green "THE 924"

Finbarr · Joined: 14 Jan 2004 Posts: 85 Location: Worcestershire, UK

Well, your fuel pump *might* have died prematurely. Or it could be a whole load of other things.

However, you could very easily spend a great deal of time and money replacing one part after another in the hope that sooner or later you'll change the right one and fix the problem. I always try to test the parts that are there as much as I can, and as systematically as I can, before resorting to this.

Rule out electrical problems by connecting a multimeter to the terminals on the fuel pump (not the wires, if possible, so that you rule out corroded terminals) and looking at the voltage present while cranking the engine. If that's OK, test the fuel pump itself (go to the 924 Technical Section and look in section 3 (Fuel Emission Exhaust) for a procedure). If the voltage is not OK, work through the electrical system till you find the fault. If you haven't got a mulitimeter get a cheap one - it'll always be coming in handy!

I'd say only once you've eliminated all those things should you start looking further afield, like fuel filters, accumulators, fuel distributors etc.
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1980 931 (with the odd electrical quirk...)

Minus000 · Joined: 29 May 2003 Posts: 363 Location: Sidney, B.C.

be wary of wrecking yard accumulators. I bought and used one for a few weeks but it burst. I think if they sit the rubber dries out and when put back into use the wear will tear the rubber bladder.

Paul · Posted: Fri Mar 12, 2004 9:33 am Post subject:

Time for a CIS tester so you can check your fuel pressures.

D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

Paul's got it. The CIS tester is much cheaper than most of the parts you've mentioned and can rule out or pinpoint which of the parts are not working correctly. Much cheaper (and easier, I might add) than replacing the WUR, accumulators, fuel pump, cold start valve and so on.

Also, if you haven't cleaned the tank and replaced fuel filter, this would be a good time.

Could also be vaccum leaks around injector seals and boots. If those have never been replaced in the life of the car, it's definetly time.

Levans · Posted: Mon Mar 15, 2004 1:44 pm Post subject:

well, i think your right Paul.
I had better get a CIS Gauge and see what is at fault.
most of the stuff i have replaced has been spares that i have in storage so it hasn't cost me too much anyway

I will test the fuel system and post the results and hopefully then i will find the problem.

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78 Dark met Green "THE 924"