Porsche Enthusiasts Club Forum

Guys

1985 square dash 944. One day last week, it wouldn't start-no fuel pump noise so I immediately suspected DME relay. Took it out, put in my (Untested) spare-still nothing.

Made up jumper wire, fuel pump ran fine, started normally. Resoldered both relays with no improvement. Bought a new relay (Genuine)-still no fuel pump.

Looked closer at the relay board with a view to wiring tests and realised that the earth terminal was missing from the relay holder (Terminal 86, IIRC). Took the fuse panel out and the connector had pushed through the back. Refitted it, fitted relay and the car started fine. Tested the other two relays and again, all started fine but now a new problem....

Car starts and idles fine, but as soon as I give it throttle it becomes lumpy and almost backfires. It is drivable, but not easily-coughing and spluttering at times.

Question-could this be caused by a poor earth to the DME relay? I noticed whilst I was refitting the connector that the wire has been cut and respliced poorly in the past and the terminals are looking past their best. I'm going to replace it anyway, but I'm interested to know if my car is displaying typical symptoms of this problem, or whether it's likely to be something else?

It seems a major coincidence that I have never had any running problems before this occurred.

Thanks for any advice!

Are you sure there is no old alarm or immobiliser in the car?

Could be crank sensors, but I have also seen a bad coil do the same thing, as it upsets the electrical balance in the ECU and DME relay and coil.

It could also be a bad fusebox, the right hand drive ones suffer from tiny pin holes in the battery tray seam sealants and the occasional drip of water, which washes acid from the battery, seep into the fusebox and corrode the internal layers of the fusebox which cause all kinds of gremlins.. I used to hunt down left hand drive 924's and 944's to get their fuse boxes as they are always mint.. But since we stopped seeing as many 924's and early 944's, we do not have the demand for fuse boxes anymore and I guess that there are a lot of 924's and early 944's with odd electrical gremlins out there.

You might even have an ECU/DME going bad in its circuit which converts pulses from the crank sensors into a digital numeric value read by the processor, and that will give you a situation where the crank sensing is poor (poor running) and loss of DME relay switching.

You have a bit of a tricky one, which will be hard to diagnose remotely, but easier to diagnose in person.

No-no alarm or immobiliser fitted. Seems far too much of a coincidence that it only started after the relay failure. I'm going to replace the earth first to the relay and see if that helps. If it doesn't, I'll be looking for other diagnostic tips!

Oh I agree, I think the two problems are connected..

One other thing, if the fuel pressure regulator has failed and is causing high fuel pressure, the amount of amps being drawn through the dme relay will go up, due to the extra work of the fuel pump.. So it may be worth checking the fuel pressure.

Does the fuel pump sound any louder than it once did?

Thanks. No, the fuel pump sounds the same. I did notice that the DME relay was pretty warm after running though-is that normal?
If sorting the wiring doesn't sort it, would the fpr be the next likely culprit? I don't have the kit to test fuel pressure so would only be able to test by swapping for a known good one.

It may be easiest to get a local Bosch service centre or Porsche specialist to test your fuel pressure, it should take them only half an hour, so the cost should be quite low.

When did you last have an MOT? Do you have the emissions sheet from it? If so, if you have anyone with a gas analyser, compare the CO and HC figures, if these have significantly risen, then it would indicate the fuel pressure has risen. If you have a friendly local MOT tester he might check it for you for very little cost.

Fuel pressure test is the surefire check though.

One other thing you can do is take the vacuum hose off the fuel pressure regulator, and sniff it, it is not conclusive, but in 50% of fuel pressure regulator failures, the diaphragm ruptures and causes fuel vapour or raw fuel to enter this hose.

The DME relay should run reasonably cool, so if it is getting warm, then it would indicate a problem.. I know of one Porsche specialist in california who uses a thermal imaging camera to track down short circuits and bad relays.. I want one!

I will report back once I've replaced this earth wire.
One thing suggested by another source is to fit a three way jumper wire in place of the DME relay (Which makes the pump run continuously) and if the symptoms then go, that would narrow it down to wiring or DME fault-does that sound plausible?
(Thanks for your advice, by the way.....!)

A way jumper lead, bridged to join all positions in the fuse box that the large terminals on a DME relay would use, will bypass it 100% providing power to the fuel pump, injectors and engine management unit (DME or AKA ECU)

But I thought you had already done that, sorry I misread your original posts, give it a whirl.

John, the FPR problem that Jon has referred to above is that to which the Clark's Garage diagnosis that I linked to on the PCGB site, refers.

Problems like this usually end up like a birthday for the car, because there are so many things which end up being changed attempting to cure them...

Thanks. Well, today is the first chance I've had to have a look at the car (It's kept on the road, and the weather's been awful!)

Anyway, I bypassed the DME relay with the jumper and the poor running is still there. To recap, I've tried three relays and replaced the dodgy earth wire to the relay. The car starts and idles ok (Apart from the idle maybe being a little low ?) but when trying to rev, it hesistates, splutters and backfires quite regularly.

There was a car parked behind and it seemed to have rather more soot on the front of it afterwards than usual.....

I tried the vacuum hose on the FPR but I can't smell or see fuel.

My problem is that I can't take it anywhere to get it tested because sods law dictates that the MOT expires on Friday. Because it's kept on the road, I have to find somewhere to put it by then.

Are there other tests I can do to see if it's definitely the FPR at fault? They're quite pricy I think, otherwise I'd change it anyway, but I can't afford to just throw parts at it at the moment....

Many thanks for your advice.

Hi

When were the dizzy cap / rotor arm and plugs last CHANGED ...not checked but CHANGED ???

They are the commonest reason for bad running, particularly on revving up....

All the best Brett

Well Brett, I owe you a pint!!

Cap and rotor about 2 years old, plugs 8 months old. But I had a brand new spare set of NGK plugs on the shelf. Nothing to lose by changing them, I suppose......
And the problem's cured! Not sure why. Quality Bosch plugs, nothing visually wrong with them but clearly faulty somewhere.

SO pleased!
Thanks Brett and everyone else who advised.

That is quite lucky truth be told! It is surprising that the plugs failed within a year mind you.

Can you post a close up picture of the plugs? In particular the ends that sit in the combustion chamber, ideally a side on view of the side electrode?

Unfortunately not-they're long gone, but I did notice that the bent metal bit (Not sure of the correct term) seemed to be eroded down to about 2/3 its original thickness....