Luis de Prat

Would a bad DME relay make the car run badly from start up, or would it suddenly make the car run bad once it had heated up?

My 951 runs like a top, boosting and accelerating flawlessly until about 45 minutes have passed, when it suddenly turns into a backfiring, black smoke belching mess.

TIA

Matt Sheppard

Hey Luis,

DME relay is digital (either on or off) and so it will either run (power to fuel pump) or not. Intermittent failure means it will run and then die, restart will run then die, ect. It sounds like a temp sensor problem or a spark problem to me from the black smoke you mention.

reno808

did you try testing the O2 sesnor?

Luis de Prat

Come to think of it, I recall cracking something when replacing the ISV. What temperature sensor are you referring to?

Matt Sheppard

There are 2 on the front of the head about cyl. 1 and between cyls. 1 & 2. My understanding is that the forward one is for the gauge and the rear one if for the DME. Check the wiring first.

Bri Bro

If the contacts on the relay are getting resistive, they will slowly generate heat that makes the contact resistance go up, which generate more heat. This will continue until resistance causes enough voltage drop across the contacts to make devices the KME relay services work incorrectly or not at all. This heating action will cause the DME relay to fail at some point in the future causing a no run/no start.

So, the short answer is yes, heat and time can make a DME relay cause problems with the engine. A quick test is simply to touch the DME relay after you start it, and again after it starts to act up. If the relay is really hot, you found the problem. I have seen the DME relay get so hot it starts to melt the plastic next to the pins the relay plugs into.

spanky

The relay will either pull in or or it will not. The DME on the other hand can fail in stages as mine did on my 944 na. I had it stop 5/6 times and recover and operate normally only to have it cut out again later. I replaced it and nothing else and cured the problem

tommye

Luis,
Check the NTC II sensor (blue connector/arrow in the attached pics, near #1 sparkplug). The workshop manual states a resistance of 5 kohms or so when cold and a couple of hundred ohms when warm. Open circuit would cause overly rich condidtion. Another test procedure (ws manual again) is to pull off the connector with warm engine idling - engine should cut off due to over-enrichment. Maybe the connector is already off ?

Good luck,
Tommy

Luis de Prat

Thanks for all the replies. I'll go over the different sensors with the shop on Monday to see if they missed testing any.

As of Friday, however, it's looking like the DME control unit itself is failing.

Luis de Prat

OK, here's one for the archives.

Car is a 1991 Euro spec 951 with 76K miles.

Car would previously lose boost only to recover by shutting down and restarting.
One day, car lost all boost.
Replaced the cycling valve with a new unit from Porsche.

Car then developed a vacuum leak from the ISV and wouldn't hold an idle.
Replaced the ISV.

Car then developed another vacuum leak from a broken check valve behind the intake manifold.
Replaced the check valve.

Car still wouldn't hold an idle.
Replaced TPS and both reference / speed sensors and set idle correctly bypassing the ISV.

Car would drive normally boosting perfectly for about 45 minutes and then suddenly start backfiring, bucking, hesitating and cutting out under acceleration.

All pertinent engine temperature sensors were tested, DME relay checked out okay, distributor cap and rotor replaced, and I'm probably leaving stuff out, but in the end,

it was the Motronic control unit!

Today we replaced my original DME with an identical very rare 951.618.121.10 unit I managed to locate in France, and after 2+ hours the car has held up and boosted perfectly. FINALLY!

The moral of the story is, don't rule out the DME control unit, because it can cause all kinds of symptoms and be very difficult to isolate unless you try swapping out the DME itself. Since my car "only' has 76K miles, the DME unit was frankly the last item I thought to check.

Thanks for all who've helped with suggestions and advice over the past several months!