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It starts...and stops.

T

tref

PCGB Member
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I know I have come across this before, and now I can't remember what causes it.

924 Turbo, starts, runs for about 3 seconds, then stops and won't start again for... an hour? when it will do exactly the same thing.

Does this mean it is just running on the cold start injector and not on the main injectors or something? is it a lack of signal from the flywheel sensor? I know I am going to kick myself when I find it!
 
K jet does not get a signal from the d-itc. Sounds like it is running on the 5th injector and then the thermo-time switch is cutting out. Unscrew (in the case of the turbo) the injectors put them in a suitable container, and crank it to see if any jungle juice comes out of the injectors.

Check to see if the moving plate in the metering unit is free and has not siezed up
 
Ah... so a large airleak could cause that as well yes? I think that may be where I have come across the problem before.

Thanks for that.
 
Any airleak will spoil the fuel air ratio. As the plate is a down moving plate, try fixing a cable tie to the beam stop to hold the plate down further. Obviously dont wedge a washer in there as it will get sucked in if the old girl coughs. Connect all the ducting back up, right up to the turbo charger. Have you connected the breather hose on to the stub pipe underneath the metering unit.
 
No work has been conducted in the engine bay, I was fiddling around inside... Changing the front carpet, so centre consol out... and whilst pretty sure I didn't disturb anything around the brain, there is always that nagging doubt.

The other possible related problem... which actually... could be significant... is that I took out a wire by-passing the fuel pump relay... i.e., a P.O. had the fuel pump running all the time the ignition was on... It worked perfectly without it after a fuse-box clean up. I wonder if this is the problem that caused the P.O. to put that wire in resurfacing?

 
That proves the12 v power circuit ( slot 30 to slot 87) for the pump works. Check slot 31 for earth VIA THE MANIFOLD CUT-OFF SWITCH. Check 12v at slot 15 ign on and continuity to terminal one of the coil from slot 1. I can't prove it but I firmly believe that to replace the rather less reliable earth contact on the body of the metering unit Porsche introduced a detection circuit which somehow detects the back emf when the L.T. circuit collapses, and this signal comes from T1 of the coil. Did you put anything quaint like a fuel pump relay back in place of the shunt?
 
Thanks... I'll have to work through that lot with teh car in front of me... which I fear isn't going to happen until next weekend.

When your refer to "shunt" I assume you are talking about what I am refering to as "P.O. Bypass". The "P.O. Bypass" was a piece of wire from the "Ignition on" connection on the back of the fuel pump relay to the fuel-pump itself. Everything else left in place. I disconnected that, and reconnected the feed to the fuel pump relay, and it worked. Apparently intermitently!
 
Well that was conected up wrongly in the first place. To bypass the relay and all safety circuits for the purposes of testing, put a male 15 amp spade onto each end of a short lenth of 15 amp wire and connect 30 to 87 the pump will run all the time, or temporarily bridge 15 to 87 in the same way and it will be wired through the ignition switch, which is a passable sloution for testing but not recommended for road use due to having no "zero volt off " to shut the pump off if the engine stalls, with the igntion on, in the event of an accident. Last thing you want is to heat treat your new paint by feeding any fire with fuel at 3 bar.
 
Unfortunately, a PO had removed the connectors from the fuel pump relay connector block, and run a secondary wire directly to the pump, not bridged the relay as you suggest.
For testing, I disconnected everything, and run wires from a seperate battery directly to the pump!
The problem now is I suspect the problem the PO was trying to overcome... So far, all I have done is put it back as I believe the factory intended - and that involved putting the correct colour wires back in the fuel pump relay connector block.
You are absolutely right... I'd much rather spend a little bit of time now overcoming a problem now than have the worry of the fuel pump running irrespective of whether the engine is turning!
 
Well I checked power either side of the fuel pump relay, and that is fine (both terminals 87, 30, and also at fuse 2 on the auxiliary fuse board), and that is ok... I checked it with teh relay in place, all connected up in other words, and it is working with the engine spinning over... so I don't necessarily think it is a fuel pump issue. Reason for this is that still, when it is cold, it fires up for a second or two. If there was no fuel presure, it wouldn't do that so reliably.

So... whilst I should be going back to basics, check spark, check fuel etc, I have been some-what distracted by the rats-nest of wiring under the dash... So far I have removed one starter circuit disablement unit, one alarm system, which was also cut into the starter motor circuit, and now onto removing a third alarm/imobiliser unit... this one has been installed with more than insulation tape and twisted wires, so is a bit more troublesome to remove...

Another distraction was windscreen washers - thanks for the tip about quickly flashing the fuel pump to run backwards to unsieze itself - it didn't work on the fuel system, but it has on the windscreen washer pump!!!
 
Is it reaally nearly a year?? Aparently so...

Anyway... whisper it... it runs[:)]

OK... So the problem would appear to have been... having cleaned out the tank when I first got the thing, I assumed the tank to be clear... Never assume... after the first pump not working, the second fitted assumed to have a fault, proven by the third running for a short while, and thinking it must be anything but the pump... I took another look inside the tank... and found what looks like the remains of a scouring pad!

So... I am sure that is not the only fault... but now, having substituted a 944 pump, I got as far as fuel up to the metering head, and pressure at all four injector ports on said metering head. It still wouldn't run.

As a result of pump problems, I now have the fuel pump relay wired through a switch, fuse, and a test lamp (Thanks for the idea Brightside!). Today, I primed the fuel system, until I just heard the injectors squeal (at least I assume that is what I can hear up front), then stopped the pump. Turned it over, and it kicked into life. I could then keep it running, for a couple of minutes, by flicking the pump on and off each time the engine started to die. Hardly the most effective means of controling fuel, but I am a firm believer that the best way of getting an engine running, is to get it used to running!

So... I reckon my problem now is too high a fuel pressure... my question is, is the 944 pump really delivering too much that the pressure relief system cannot cope, or, is the pressure regulator not working, or, could it still be the metering head? Somewhere I have a pressure gauge and hose suitable for measuring fuel pressure on a K-Jet system... I just need to find it... Meanwhile... any comment welcome.

 
  • 5th injector opens electrically and virtually zero pressure needed to get fuel into the manifold.
  • check valve in injector requires about 2 bar to open, prevents fuel weep when engine off and system pressurised.
  • 3 bar +/- operating fuel pressure
  • any system that does not use a passive method of disabling the fuel pump is dangerous.
  • does the rev counter work? (same signal for relay generates pulse for rev counter)
 
I've by-passed the fuel pump relay at the moment to eliminate that as a possible cause of problem - once I have overcome the other problems, it will be going back as Porsche intended. The last thing I would want is a fuel pump system capable of pumping the contents of the fuel tank into an engine bay fire for example. I believe I have everything working as it should there... the problem is that it won't run with the fuel pump runnng continually - yes, the return line is clear - that has been blown through.

I think I really need to get a pressure gauage on it...

Your comment about the cold start injector does kind of fit with what I had earlier experienced - it starting everytime it was cold for a couple of seconds, then that being it. Given that the rubbish in the tank probably restricted fuel pressure the rest of the time.
 
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