964 loses power while driving, HELP

the 964 Kid · 08-28-2003, 02:43 PM

Just yesterday my 1990 C4 started doing something very strange. I was driving about 45mph, hit a bump in the road which causes the car to stall. I was able to downshift, and basically 'roll start' the car again, but I could feel that the car was stalling still. It felt like it was running out of gas in a way. I pulled over and the car immediately stalled again. It would let me turn it over a few times, but each time stalling. I was only a mile from my house, so I thought, what the heck, let me see if I can make it! I turned it on again, gave it a lot of gas, and then pulled back out into traffic, sticking with the low gears. I noticed almost immediately the problem was gone, and the car behaved normally. Later on I took it out again to see if I could get it to do the same thing, and it did, but this time I was able to 'fight' thru any actual stalling out. And today it did it once more. Each time it happens after hitting a bit of bumpy road, but appears to be fine on smooth surfaces. The only factors I could think of are that we had a bunch of bad rain recently (and the car did get rained on), and also about 2 weeks ago I put a new alternator in it, which has been working fine. It sorta seems like some kinda vacuum problem or lose wire, something of that nature. It could be many things, but I'm wondering if these symtoms sound familair to anybody, would be great if I could figure it out, otherwise I'll have to take it in. Any help or suggestions would be greatly appreciated.

Ken.

ekessel · 08-28-2003, 05:31 PM

If you skip the going over a bump part, it sounds like a DME relay problem (it kills the fuel pump). If you open your front fuse box, and check the DME relay, if it's a 944 part number, you should replace it.
-Eric

91C2wrencher · 91C2wrencher's Garage

Ken, the DME relay is good place to start. FWIW you may also want to open the front fuse box, with car running, exposing the realy/fuse board and gently at first tap the board or press on it in the corners. I had a strange intermitant cut out like that a few years ago and after much snooping I decided it was the actual fuse panel assembly that was bad, I could tap/press on it and duplicate the problem, yet futzing with all the fuses and relays didn't cure it. So in when a new board (about $150) and the problem is gone.

the 964 Kid · 08-29-2003, 09:45 AM

thanks alot 91C2wrencher, I'm going to check out that possibility today, and see what happens! Would be happy if thats all it was!

Ken.

the 964 Kid · 08-29-2003, 10:22 AM

thanks also ekessel, i checked it out, and the relay is a 944 part number. Looks like the replacement part is 993 615 227 00, does that sound right? I'm gonna put one in and see what happens.

ekessel · 08-29-2003, 10:40 AM

Yes, the 993 part is the updated relay. This relay controls the fuel pump, and when it dies, you coast to a stop....... and it does age, so replace it first. Let us know how it goes.
-Eric

the 964 Kid · 08-29-2003, 12:10 PM

Eric,

Well i replaced relay, and unfortunately that didnt solve the problem. Any other suggestions??? I would have been so glad to have that little $20 part take care of it!

ekessel · 08-29-2003, 12:34 PM

There was a technical service bulletin about the connector on the top of the air flow meter causing running problems. If you take off the air filter cover, and reach up behind the airbox, at the top you should be able to feel the connector on top of the air flow meter. If the cable still has the wire coming directly out of the hood, and doesn't have a tie wrap pulling the wire to the side of the connector, you might want to look into this service bulletin.
-Eric

(Maybe someone has the whole service bulletin....... )

the 964 Kid · 08-29-2003, 12:55 PM

Thanks Eric, I'll check it out, where would be a good place to find that bulletin? Would that be on a Porsche site? Anybody who has an idea please forward it to me, thanks again.

Ken.

expat · 08-29-2003, 08:09 PM

Loose wire on fuel pump?

Are you sure it only happens on bumps? Never happens when cruising on a nice flat road? How much of a bump initiates the cutout? Is it like a bone jarring bump or just a small irregularity in the road?

alibaba · 08-30-2003, 06:54 AM

964 kid,
It does indeed sound like an electrical problem if it happens only when going over bumps, and in particularly sounds like a bad earthing point since it happened after heavy rain.
I would suggest;
1. Check all earthing points, clean and tighten.
2. Remove and clean all motronic related connectors and the motronic ECU plug.
3. Also check resistance of the TDC sensor, as a failing sensor can also be vibration sensitive. Failing that replace the sensor and see (arguably should be replaced every few years any way, as it is bound to fail with age and is one of the most common cause of poor running or no starting).
p.s. vaccum leak can also produce similar symptoms but not only when going over bumps.
Good luck

alibaba · 08-30-2003, 06:59 AM

964 kid,
It does indeed sound like an electrical problem if it happens only when going over bumps, and in particularly sounds like a bad earthing point since it happened after heavy rain.
I would suggest;
1. Check all earthing points, clean and tighten.
2. Remove and clean all motronic related connectors and the motronic ECU plug.
3. Also check resistance of the TDC sensor, as a failing sensor can also be vibration sensitive. Failing that replace the sensor and see (arguably should be replaced every few years any way, as it is bound to fail with age and is one of the most common cause of poor running or no starting).
p.s. vaccum leak can also produce similar symptoms but not only when going over bumps.
Good luck

the 964 Kid · 08-30-2003, 08:23 AM

Expathk,

yesterday it did happen on a flat road for the first time, but previously the few times before were just after 'bone jarring' bumps in the road. Any thoughts?

the 964 Kid · 08-30-2003, 08:24 AM

Alibaba,

that all makes sense, I'm gonna start checking those out this weekend, and hopefully get to the bottom of this. Thanks.

the 964 Kid · 09-13-2003, 10:20 AM

Well, I took it to my local Porsche specialist, and, after tightening every possible connection he could find, after thing has been running fine. The most likely suspect for the problem was the connector behind the airbox. Looks like Ekessel was right on the money - thanks for your advice.