Porsche Repair: 944 running problem, fuel injection system, fuel filter


Question
i have a 1989 944 2.7L  it recently ran out of petrol and since then when i start it it idle's fine but when you rev the car to over 4500 rpm the volt gauge drops below 12 then the car cuts out, then when you try and start it it just fires up then cuts straight out again. however if you leave it overnight it fires up again and then idle's fine untill you go over 4500 rpm again. i've allready changed the fuel filter but still this happens any help on this matter would be appreiciated loads with thanks steve..

Answer
Steve,

Just to remind you, my expertise lies in the older 356 models.  Luckily, many early designs/concepts have lasted since the early 50's and appear in revised forms on 911's, 914's 944's, Boxsters, etc.  Where this happens I'll promise to try to give you the best answer I can.

I'd suggest here that there are two different paths we should follow.

1.  You were smart in changing the fuel filter right away.  One posssible cause of the problem is that insufficient fuel is still not reaching the injectors.  This would assume that either the new fuel filter is already clogged to some point; or, that an impurity (a stubborn piece of crud) is lodged in the fuel injection system after the fuel filter.

I'm not so sure we want to embrace the crud idea so quickly...you'd be disassembling the F.I. system almost as the first step.  Just assuming for the moment there IS crud, you'd wonder why 4500 rpm is so critical.  Let's say enough fuel can get by the crud all the way from idle to 4500, but that at this point the injectors start to gasp for more fuel...you'd assume the engine would sputter and at least be able to resume running somewhere below 4500, or most certainly at idle.  But you said it cuts out, which I'm taking to mean it dies, and needs to be restarted.  This doesn't make too much sense unless...and this may be a long stretch...the computer senses the injectors gasping for fuel and it stops the 12v flow through the system to protect something...maybe the turbocharger, maybe the engine itself?

2.  Your good eyes noticed a voltage drop along with the rpm limit of 4500, which makes me believe the empty fuel tank may not have precipitated this new annoyance at all.  Unless the computer is messing with your electrics as above, it has to be that some important electrical connections are tight enough and clean enough to allow idling and some reving, but that at 4500 the looseness or dirtyness of the connections provides enough resistance to cause havoc.

The connections I'd check are the distributor, the coil, and the alternator.   Your tendency will be to wiggle the wires and if they seem tight, you'll move on.  I'd suggest removing the wires, give the connections a thorough inspection (and cleaning if need be) and securely reattaching them.  That way you KNOW they are good.

So...getting a little greasy working with the wiring is much preferable to getting a little gas-y working on the F.I. system.  That's what I'd do first.

I hope these suggestions work out for you.  If not, please write back and we'll see if there's anything else that could cause this 4500 mystery.  At least then you'll already have eliminated some possiblilites.

Dave