It just dies.....then starts
 

I'm in a bit of a quandry here. The car runs great....fires right up and runs like a stock 930 should....EXCEPT FOR.....every once in a while, it will just stop dead.

Tonite I took it out intending to take a drive on the twisties. I got half a block and went around a mild corner, and it just sort of coasted to a stop. No sputtering, backfiring, chugging.....just quietly coasted to a stop.

I checked the fuses, relays etc....took them out and rotated the fuses thiking that maybe one had blown or corroded....didn't see a thing.

So I sat there for about five minutes, then started it and it fired right up like there had never been anything wrong. It drove just fine....but I took it right back home and parked it.

I did notice that when I went to start it after it died, I didn't hear the hum from the fuel pump when I turned on the ignition key....and I'm wondering if that could be a clue. I'm beginning to susupect the front fuel pump. When I peeled back the mat covering in the forward trunk, I noticed that there was a clear plastic container over near the fuse box with what looked like a fuel line running into the top and out of it. The container had no liquid in it...empty....is that supposed to be some sort of fuel container that you can see fuel in? Do you suppose that the fuel pump is acting intermittently? Obviously, this is eating into my sense of reliable driveability for this car. Perhaps a fuel pump is in order/?

chuck

Chuck,

I am having somewhat of the same problem. Read my thread and see if your's acts the same.... ie, hot motor problem only, stalls if stopping or turning. Are you running a Permatune? I was thinking fuel pump also, but now wondering about the ignition?? I am hoping some more experienced tuners will chime in here and advise. -Joel

Hey Chuck, if your car is totally stock, that noise you hear when you switch on the key is more than likely the CDI box. Standard wiring for our cars dictates that the fuel pumps don't run continuously unless the car is actually running. Have you checked the green coaxe wire on the side of the dizzy? What about the actual wiring plug/socket on the bottom of the CDI box (mine worked its way out one time)? The connections on the coil?

I am thinking CDI problem.

Chuck, renew any fuses that might be protecting pumps etc etc. I learnt the hard way with the fuses in my 3.2 carrera. They can look fine, test fine but they are shot. Throw them away and replace with brand new. This lesson cost me a lot of money paying two mechanics to fix my car once... :( They built race cars, did not know jack about 911's.

Sandman: I had to put in a MSD 6AL box sometime back when the B0sche unit died suddenly...so my signature needs to be amended to include "stock except for...." I had an indy do the install and the only trouble I had from it was the rotor on the distributor fried....apparently you can't use the stock rotor because ist has a resistor in the epoxy top part and it will burn up.

I set the rev limiter to 6500 just to be safe, but when it died, I was only going about 30 mph and had just turned a corner and coasted to a stop. I've had it do this before....it just decides to stop then you let it sit for about five minutes or more, and then it starts right up like nothing ever happened and runs great. This makes it unreliable because I never know when it's going to act up.

some thoughts..
 

Thats a tough one without actually experiencing it.

1. What is your idle speed when fully warmed up?
2. Is your AC on when this happens? I have not seen a solenoid operated throttle plate idle bypass system on these cars to raise idle speed a couple hundred rpms when the AC compresser kicks on to compensate for the drag and keep it from stalling like most cars have.
Motronic efi uses a computer controlled idle bypass control valve that does just that. Older L-jetronic efi used a simple AC activated on-off solenoid. CIS could use the solenoid too.
3. Is the idle mixture a little too rich or lean? Does the idle ever oscillate when fully warmed up? If it does it's too rich. If it's too lean it'll hesitate or die when you give it throttle. Maybe.. a CO adjustment or cleaning around and reindexing the sensor plate could fix that.
4. Is the ignition timing correct? especially at idle? If the vacuum retard and advance lines are not connected to the distributer properly, the idle timing may be retarded too far and that can make a weak idle. Or maybe the timing is retarded too far alltogether.


It sounds more fuel related than ignition to me because it only does it when warmed up so having a fuel pressure guage and CO meter hooked up when this happens after checking the ignition timing would help diagnose it.

I'll chime in that it's fuel related. And with all the switches and relays that have to work properly , anything can happen. I know you want to keep it somewhat stock, but I bypassed all the stuff in back and grounded the two red relays up front with a impact switch. Now when I turn the key on, I hear the fuel turn on every time. If there is ever a shunt, the impact switch will cut the ground to the relays and stop the fuel pumps.

I usually hear a hum when I turn on the ignition...and now that you folks have been raising some of the issues, I recall that when I DON"T hear the hum....the car won't start. For some reason, I can let it sit for a while, then give it a try and it fires right up.....then it'll run like it's brand new.....until the next time when it decides to be futzy.

If I've changed out the Bosche CDI for the MSD unit....what is that hum that I hear when I turn the ignition key to on? If I don't have a Bosche CDI....what is it that makes the hum if the fuel pump is not supposed to make a noise?

I did have a fellow look at the car sometimes back....excellent tuner. He got the car running better than it's ever run...did most of his tuning by ear and with an allen, tack hammer, and small screw driver...and he mentioned that the front fuel pump "sounded a bit suspicious"....and that I might consider replacing it. I'm wondering if that's the problem....but the intermittent thing is what gets me. Either it should be totally dead or it should run. I did buy a complete set of fuses and relays...I think I'll go thru and change out everythig. I still have all the original stuff in there....but it all looks brand new. The car sat covered in an environmental room.

JFairman: In answer to your questions....here is what I have observed.

1.Idle speed when fully warmed up is about 800 rpm...it's higher when I first start it then drops down after about 30 seconds when the WUR kicks in?

2. The AC unit has never been on when the car just suddenly dies. The AC unit is fully functional and actually blows cold!! I run it periodically to keep the seals healthy and to check the freeon level.

3. The idle seems to be set about right....it doesn't choke or hesitate when hard acceleration is applied and it doesn't ever hunt throughout the rpm scale. It idles at a steady pace.

4. I have no idea of where the timiing has been set, but it seems to run well with the exception of a slight bit of "chugging" or periodic hesitation when in second at around 3500 rpm if you hold it there. If anything, i'd say was running rich....and was reset by an indy a while back so that the inside of the exhaust doesn't look so sooty. I don't know if this helps focus in on why it just dies...but it does make me a bit nervous since I never know when it's going to crap out on me.

Hey chuck, check this thread...I posted some pictures regarding one of the fuel pump fuses in the luggage compartment. The wiring on mine became old and even though the fuse was fine, the proper contact wasn't being made...just food for thought! This fixed the only fuel pump related probelm that I've had to date (knock on wood):
https://rennlist.com/forums/911-turbo-930-forum/354201-intermittent-start-problem.html

Other obvious things to check:
-the yellow overboost relay on the engine compartment fuse panel (driver's side)
-ensuring that the white wire (I've seen some that were white with brown stripe) is properly connected to the overboost switch on the BOV housing.

As Rocket indicated there are many things that have to work togther to get those damn fuel pumps to run when you need them to!

That yellow overboost relay gave me trouble once (at least I think that was the cause). Drove the car to work one day, and it wouldn't start up when it was time to go home. Just cranked and cranked, but no fire. So I grabbed the nearest relay (the overboost) and unplugged/plugged back in a couple times...and she fired right up. No problem ever since. Of course, I've since cleaned the contacts.

OK...here's a bit more info that might help you to focus in on the problem. It did it again to me today...luckily dying just as I approached the entry to a mini-mall with a Starbuck's coffee shop. I pulled it to the side and immediately went to the front fuse box...the first red relay...which I believe is the one for the fuel pump....was warmer than the others. Behind it (back side of the fuse panel) was a red 10 amp fuse that had blown....I have no idea where that one goes.

I traded red relays with ones from other sockets but it wouldn't start. After I let it set for about 10 minutes, it fired right up like nothing ever happened and I drove it home post-haste.

I'm focused in on possibly a bad front fuel pump that might be heating up then dying? The car does not sputter, choke, or lurch like it might be starving for fuel.....just stops dead and coasts to a stop. It will turn over just fine when I try to restart...healthy starter, but not the slightest hint of wanting to fire up. I find it curious that there are times when I turn the key to the ignition side that I hear a distinct hum just before the starter engages. The hum will remain constant until I start the car....then I can't hear it anymore over the engine. Yet there are other times that I won't hear any hum prior to starting. The times when it dies, I don't hear any hum either.

I have another fuel pump coming...but in the meantime, do any of you folks have any opinions as to a possible cause? How do I test a fuel pump?

You want to jump the yellow relay in the rear fuse compartment. This is the overboost relay and will kill the fuel pumps. They are as bad as the DME relay. I would start here. This is a very known problem.

If your front fuel pump gets real loud it could be the pump itself or the checkvalve might be cloged.just food for thought