911 Mystery

pbmitchell · 10-20-2003, 10:30 AM

I have a 1987 911 Carrera Cabriolet - it's immaculate and has covered only 50,000 miles from new. I've owned it since 1989.

The car has never given me a moment's trouble and has always started with the first turn of the key.

Until recently, that is.

A few weeks ago, I was driving along the road when the engine suddenly cut out as if the ignition had been turned off.

I rolled to a halt by the kerb and found that all the electrics appeared to be working - the ignition light and all the dash gauges were working and all the other electrics seemed fine. The starter spun the engine over as fast as ever - but it would NOT start and made no attempt to do so.

I called out the AA (that's the Automobile Association, Britain's largest roadside repair/recovery organisation) but they couldn't find the fault. They towed me to a Porsche dealer where various other things were tried without success and the car was taken in for safe-keeping overnight.

The following morning, as soon as one of the Porsche mechanics turned the key in the ignition the car started immediately!

A few weeks passed and the car went in for its annual service at another Porsche dealership. They collected the car on a trailer (as usual) and returned it the following day - but, when the driver tried to take the car off the trailer, it wouldn't start.

Five minutes later, it started perfectly and was driven from the trailer onto my driveway. Five minutes after that, it wouldn't start. A few minutes later, it started perfectly.

Since then, there has been another similar episode.

It might be relevant that the car is fitted with a Clifford Concept 50 alarm/immobilisor but, at all relevant times, it was in the 'off' position and was not operational. I've contacted the engineer who fitted the immobilisor but he says the symptoms don't suggest a fault with the unit.

Any ideas/suggestions would be very gratefully received.

Par911 · 10-20-2003, 10:36 AM

It sounds like you are having similar problems to mine. Do you hear the fuel pump if you turn the key to position 1? You might want to check the fuel pump relay, ground wires, and the fuse box. The problem can be caused by a number of things, it could be a faulty fuse, a loose wiring connection, or even a bad fuel pump. Get your mechanic to check the fuel pump and fuel accumulator. I hope this helps you solve your mystery.

jluetjen · jluetjen's Garage

I'm guessing that it is the ignition system. I had a similar problem in my old Fiesta and it was the ignition module failing when it got hot.

I know that I've seen similar symptoms described on this BBS before, have you done a search? I'll bet that you'll find something.

SpeedracerIndy · 10-20-2003, 10:49 AM

Could this be related to the DME relay? Sounds similar to other posts about it being faulty.

Jamie79SC · 10-20-2003, 10:55 AM

You can also get a similar symptom from a dying CD box. Is it origional Bosch or something else?

callipygian911 · 10-20-2003, 11:03 AM

i had the same problem, and it turned out to be 1) a faulty flywheel speed sensor along with 2) a loose wire in the ignition system. just some things to check.

Dan Owens · 10-20-2003, 11:42 AM

I'd check the DME relay as they have a habit of cracked solder joints inside the unit itself. When that happens it can be very intermittent. I'd also check the grounds for the engine to engine compartment and trans to chassis.

Milu · 10-20-2003, 11:51 AM

First choice would be DME relay, however I also had a similar problem caused by the electronic key for disabling the alarm/ immobiliser. It may be worth bypassing your system and seeing if the problem persists.

pbmitchell · 10-20-2003, 11:56 AM

I'd never heard of a DME relay before reading the replies on this thread and, having now read a few articles about the relay, it certainly sounds like the likely culprit . . . but . . .when the car was towed in to the first Porsche dealer and I told them the symptoms, a mechanic immediately produced a relay or two from the workshop and swapped them for ones under the passenger seat - and the car still wouldn't start!

On that basis, it seems it's not the DME that's to blame.

304065 · 10-20-2003, 12:03 PM

DME relay or crank-angle sensor.

Eric Coffey · 10-20-2003, 12:17 PM

My guess: Bad head temp. sensor.

Superman · 10-20-2003, 12:39 PM

This sounds like a failure of one of those fancy-dancy newfangled systems. DME Relay, or Speed Sensor, or Cylinder Head Temperature Sensor or Throttle Plate Sensor, or...... In the olden days, this would be a broken wire somewhere. There would be just a few suspects.

Today's cars are (according to what I hear) immensely reliable, fuel efficient, powerful, etc. etc. I wish I could say the cars of yesteryear were better. They're not. Except in one area. Troubleshooting and repair.

pbmitchell · 05-19-2004, 01:54 PM

Until today, the problem hadn't recurred since I last posted here - although I've only covered about 50 miles (the English weather is not good for convertibles!).

I took the car out at the weekend and it ran perfectly well - except that on a single occasion during the drive it cut out (or appeared to cut out) for a fraction of a second under acceleration. It's only ever done that once before and that was a few months ago on a day when it later stopped running altogether due to its mysterious fault

Today, after standing in the garage for 3 days, it refused to start.

I tried it every 5 mins for about half an hour and after several failed attempts . . . I came to the car, turned the key and it started perfectly.

My research into DME relay failure suggests that it tends to occur only when the engine is hot and that the problem disappears once it cools down.

Given that my car hadn't run for 3 days when I tried to start it today, does this rule out a DME relay failure?

If so, do the circumstances give anyone a clue what might be wrong?

kqw · 05-19-2004, 02:02 PM

As suggester here on this thread, the DME relay located under the seat is failing. It can be very intermittnet and heat has nothing to do with it.

Symptoms:

Intermittently cuts out

Will not start, runs perfect for a while then all of a suddent it decides not to start.

Early_S_Man · 05-19-2004, 02:06 PM

DME relay problems seem to be independent of the temperature or how long the car has sit, etc.

Has water ever gathered/collected under the seat ... even if it didn't get up to the relay or DME controller? Corrosion/oxidation on the relay socket contacts are a potential problem. I suggest cleaning socket with pipe cleaners and isopropyl alcohol, then get a new relay, and use the old one as a spare. Work the new relay into and out of the socket several times, and retest starting. After good, 100% reliable starting has been re-established ... coat relay pins with clear silicone grease. Dow Corning 4, 111, or 112 are my preference, but other brands may be more readily available.

GrantA · GrantA's Garage

I'd also go for the DME relay, I've read back thro' most of the threads on this (for interest) and it seems to have intermittent failures before it finally goes duff. Other than that start at basics checking all the wiring connections you can get to from the ignition switch to the distributor.
LOL, Grant

yelcab1 · 05-19-2004, 02:15 PM

Similar symptoms will occur with ... the DME box itself.

cyprusx · 05-19-2004, 04:59 PM

Yup... Sounds like either the relay or one of the sensors. I would probably replace the speed sensor first if the DME relay is good.

rbcsaver · 05-19-2004, 08:59 PM

I vote DME relay. I carry an extra one.

pbmitchell · 05-20-2004, 04:42 AM

Inspired by the forum's confidence behind the DME relay being the culprit and remembering the posting that recommended tapping the relay with a screwdriver when it failed, I decided that if a faulty relay could go from 'not working' to 'working' with a tap from a screwdriver - it seemed likely that a faulty relay could go from 'working' to 'not working' with a similar tap.

To test the theory, I started the car and tapped the relay - and the engine stopped.

I restarted the car and squeezed the relay between thumb and forefinger (squeezing the front of the relay and the rear of the connector) and the engine stopped again.

I then unbolted the relay from the floor and restarted the car. When loose, any movement of the relay would sometimes stop the engine and sometimes not - although I couldn't find a consistent way of stopping it (i.e sometimes a tap a shake or a squeeze would stop the engine and sometimes it wouldn't.).

Seems the fault lies in the relay or the connector or the adjacent wiring.

I've ordered a new relay from my local Porsche dealer.

It'll be here next week, so I'll keep you all informed.