Momentary total ignition failures...why?

[Formerly Steve Wilkinson]

During the last couple of months, I have been experiencing, every 50 or 100 miles, brief (one- or two-second) total ignition failures. Car blasts along strongly, then all of a sudden for an instant nothing, then it comes back to life just fine.

Car is a modified '83 SC with twin MSD ignition and it runs splendidly otherwise. And it's not that one bank of plugs (i.e. one MSD box) is cutting out, it's a total quick cutoffof all 12 plugs at moderate speeds; last happened to me this afternoon, going up my driveway.

Alternator is charging adequately and the battery is good. Because I'd been having an alternator issue previously, I carry a multimeter in the car and check the battery before each startup (typically 12.46 volts after being parked idle for a week) and right after each shutdown (typically 12.89 volts when it settles on a steady reading).

Anybody have any ideas what could be causing this? One odd symptom I noted when it happened recently: I was going down a long hill with a small throttle opening at about 2,500 rpm, then I suddenly realized the car was moving along just fine but wasn't actually producing any power. And I noted the tach was indicating zero even though the engine was still turning at 2.500 or so. A second later, the car bucked slightly as the power came back on and everything was fine, including the tach.

[old man neri]

Bad/loose ground? I am only guessing wildly in the dark.

[Formerly Steve Wilkinson]

Unfortunately, in the 911 world "bad ground" is the answer to everything from bubbling paint to windshield crazing, and I've given up on it. Car was totally restored five years ago, and if there's a bad ground somewhere, the thing doesn't deserve to live.

[TRE Cup]

Do you have any relays providing power to the msd's? factory round or square one? If routed thru a factory round relay, we have seen those circuits overheat momentarily and cut power.

USUALLY in cases like this, the relays get really hot. Go for a drive, recreate the situation, and test touch the relay(s) in question. Careful as you can brand yourself !

IF it does get hot, pull it, check the socket and in some cases the socket needs the wiring to it followed. This may mean cutting insulation off

good luck

[Mysterytrain]

A few years ago my SC suffered from this type of cut out. I felt it was electrical. I couldn't find the problem and had a few local porsche mechanics look at it. Replaced the ignition switch, fuel pump relay,rev limiter, sent the tach out for rebuild, new fuel pump...still had the problem. Anyway, I had a distributor failure. The bearing went, seized the shaft which sheared off the drive gear dropped it down in the case where the intermediate gear shoved it out the side of the case....so time for a major rebuild. I get the car back and the problem is gone. The electrical system is the same except for a new distributor. So, I felt that the distributor was in someway the problem. A few months later I'm tweeking the shift coupler and see that someone used too long a screw on the cover sheetmetal and the tip of the screw had dug into the big black starter cable in the tunnel..hmmm, I wonder it that was the problem all along. Ok, I remove the screw and replace with the proper length..size does matter. Then a few weeks after that I'm installing headlight relays. I'm poking around the long fuse block and see that is has a horizontal break along its length. So, this really makes me think that perhaps I was losing a fused connection to one of the ignition circuits and that was what was causing the car to die. Still the problem didn't re appear after the rebuild. Of course, I replaced the fuse block and all the fuses. Hopefully, I have given you a few places to look for something that might be causing your problem.
In my case if I turned the ignition off the car would restart. It seemed to occur when I was getting on it. My big fear was that it would happen when I was hanging my ass out is a turn.

[Mysterytrain]

A few years ago my SC suffered from this type of cut out. I felt it was electrical. I couldn't find the problem and had a few local porsche mechanics look at it. Replaced the ignition switch, fuel pump relay,rev limiter, sent the tach out for rebuild, new fuel pump...still had the problem. Anyway, I had a distributor failure. The bearing went, seized the shaft which sheared off the drive gear dropped it down in the case where the intermediate gear shoved it out the side of the case....so time for a major rebuild. I get the car back and the problem is gone. The electrical system is the same except for a new distributor. So, I felt that the distributor was in someway the problem. A few months later I'm tweeking the shift coupler and see that someone used too long a screw on the cover sheetmetal and the tip of the screw had dug into the big black starter cable in the tunnel..hmmm, I wonder it that was the problem all along. Ok, I remove the screw and replace with the proper length..size does matter. Then a few weeks after that I'm installing headlight relays. I'm poking around the long fuse block and see that is has a horizontal break along its length. So, this really makes me think that perhaps I was losing a fused connection to one of the ignition circuits and that was what was causing the car to die. Still the problem didn't re appear after the rebuild. Of course, I replaced the fuse block and all the fuses. Hopefully, I have given you a few places to look for something that might be causing your problem.
In my case if I turned the ignition off the car would restart. It seemed to occur when I was getting on it. My big fear was that it would happen when I was hanging my ass out in a turn.

[BrokeMyCar]

Do you know where the MSDs are drawing their power from? I think many folks run to the starter motor lead, and maybe along the way that wire is partially sheared or melted (e.g., when going between the bouncing engine and the stable body).

Babak

[TRE Cup]

Mysterytrain brought up an old memory- we had a client with intermittent cut off problem. Car would either start or not and there was no pattern to the failure whatsoever

Turned out one of the bridges on the back of the fuse board was bad- the rivet was loose and whenever enough load caught its fancy to fail, it would !

[stlrj]

What do you think would happen if you returned to the stock CDI? If that resolved the problem, would there be any question then?

[Formerly Steve Wilkinson]

No relays in the car other than horn, headlights and fuel pump.

MSDs get their power directly from the starter, and that heavy line is brand-new--replaced it because I suspected it was a problem.

Can't return to the stock CDI since I'm running twin plugs.

Maybe I'll try replacing the fuse block if nothing else works.

Thanks to all for good leads (no pun)...

[304065]

Stephan,

Please describe your tachometer drive circuit in detail with regard to the following items:

1) What distributor?
2) What is used to trigger the spark? Points? Pertronix? Magnetic trigger? Other?
3) What generates your tachometer signal? MSD boxes have a 1/4" faston on the side that outputs a 20% duty cycle 12V square wave which can be used to drive the tach if the tach is converted to accept that kind of signal. Yet another way to drive the tach is directly off the points terminal on the side of the distributor.
4) Describe the wire that runs forward to the tach. Factory? Or an add-on wire, routed how?

What I'm driving at is that you could be experiencing an ignition short on the power side. If, for example, you were using points to drive the boxes (a common way to drive MSD) and the points wire is also used to drive the tach (as it was with the old-style distributors) then a short in the tacho signal wire to ground will prevent the ignition from firing. Given that the typical installation entails running a single piece of 16 gauge around the various factory clamps with sharp edges, the possibility of a dead short in that wire is nonzero.

Unlikely, particularly for a former Falco builder where things like chafe and the routing and bundling of wires are of paramount importance, but it definitely sounds like something is shorting out between the distributor and the boxes.

Now, if you are using Pertronix ignitors or Mallory XDI optical ignition triggers, may I humbly suggest that these be given the "water test"-- throw them into the nearest convenient lake or stream (bonus points if this can be executed within the plain view of your local Sierra Club chapter) and if they float, they are suitable for use on Porsches. The trouble with electronic ignition triggers is that intermittent failures can have you chasing electrical "gremlins" that are, in the words of most A&P mechanics, "impossible to duplicate on ground." (bold face added in homage to Warren)

Lastly, it is a simple matter to jack and properly support the starboard side of the car with a jack plate and stands, crawl under with a flashlight and a 13mm box end wrench and inspect the stud where the copper ground cable from the transmission is bolted to the car. It was not until the 964 that somebody actually paid attention to things like keeping the water out of electrical terminals, and that stud is right at the center of the Trinity of Water, Dirt and Oil. While you're performing undercar Yoga, check the other end on the gearbox.

Once circulation has returned to your extremities, check the battery connections up front. It's also within the realm of possibilities that either end of the negative cable has come loose, oxidized or otherwise given up the ghost. Unlikely, I agree, but so easy to check that it pays to eliminate it as a possible cause.

Good luck!

[moneymanager]

My 73 T did this for about two years before we finally found the problem. Ignition would just quit, then somehow fix itself. Or the relay wouldn't even kick the starter then, two minutes later, it would be fine. Problem? Bad connection in the 6-8-10? point plug under the dash carrying electricity from front to back of the car. Pull the plug out, clean the contacts carefully and replace. This may or may not be your problem but it will cost you 10 minutes to check.

[dad911]

nevermind. wrong post.

[Formerly Steve Wilkinson]

John, the distributor is stock but running under a 12-plug RSR cap (I believe it is) and teh entire unit was built up for my by Steve Weiner. It has a magnetic pickup. The tachometer is driven in stock fashion and doesn't use the MSDs as a driver, therefore there's no wire from them running forward to the tach.

Battery connections are solid, and a new battery was installed quite recently--an Interstate, after my Optime failed all too quickly.

I can't imagine the main transmission-to-frame ground has gone bad, but I'll check it; everything there was replaced when I rebuilt the car 15,000 miles ago, and it has been checked several times since. Besides, wouldn't everything fail--all the gauges, for example--if that went bad momentarily?

[Superman]

For what it's worth........I wonder about power to the fuel pump. The reason is because you are not reporting a backfire when ignition comes back on. CIS is very different from carbs of course, but turning ignition off and then back on quickly was always a good way to make a loud BANG. If ignition goes off briefly but fuel continues to spray, I would expect the engine to backfire with a fairly loud BANG.

[304065]

Stephan,

I have to agree that the odds of a total ground interruption and reconnection are about the same as winning the New York State Lottery or being attacked by a Nurse Shark on 42nd street. But it reminds me of the old troubleshooting script for major appliances: the first question is always, "Is the unit plugged in?"

Dr. Weiner is a legendary Sasquatch and a fellow Aviator besides-- but methinks something has gone horribly afoul betwixt the distributor and the MSDs. In a normal SC, the tach is driven off the TD pin of a six-pin Bosch CDI, which also provides the spark. There's a coaxial cable between the distributor body that carries signals from the variable reluctance transducer to the six-pin box to fire the spark.

Do you still have a six-pin box AND the MSDs? That would seem like a lot to carry around. Dr. Weiner is an expert at interfacing the SC distributor to the magnetic trigger of the MSD boxes, but I don't know how he drives the tach if not off the boxes-- ordinarily the six-pin box is used to create the tach signal.

Anyway, my bet's on the green coaxial wire from the distributor being bad and the source of your problem, most likely at the bitter end that goes into the distributor.

A little posthumous help from our friend in the thread below.
http://66.236.61.177/showthread.php?t=358877&page=2

[Formerly Steve Wilkinson]

John, funny thing is I just replaced the trigger wire about 200 miles ago, purely as a precautionary move after The Weiner told me they're prone to inexplicable failure, and I cleaned (as well as I could) the receptacle in the distributor. So that's brand-new. And, no, there's no CDI in the car. It's as elemental as a Model A.

Superman, the fuel pump (which is only about 10,000 miles old) is an interesting thought, though I should think with carburetors--six floatbowls--the failure would be somewhat irregular as one and then another emptied. This failure is like throwing a switch. But I will keep your suggestion in mind.

[Steve@Rennsport]

A few thoughts on a Friday,......

The dead tach at failure time would eliminate the fuel pump so I'd look at things like the fuse boxes (front & rear), alternator ground and wiring/connectors, transmission ground strap, etc.

The green coaxial cable and connectors at the distributor are well known trouble spots. Its very difficult to access (wiggle) the connector at the side of the distributor while its running, but that can be done in a distributor machine to see if that connector is intermittent. Sometimes it breaks inside the distributor body that makes it a real challenge without pulling the unit to test it off the engine.


P.S; I have a distributor machine here so I can check that if you want,....

[avendlerdp]

Just try swapping the dist. with a known good unit and see if the issue goes away. I am not sure but is it possible for the advance mechanism in the dist. to move and short a bad wire when the car gets to a certain rpm?

The dead tach says dist. issue to me.

[Superman]

Originally Posted by Formerly Steve Wilkinson

Superman, the fuel pump (which is only about 10,000 miles old) is an interesting thought, though I should think with carburetors--six floatbowls--the failure would be somewhat irregular as one and then another emptied. This failure is like throwing a switch. But I will keep your suggestion in mind.

Let me be more clear. If you have carburetors, then the switching off, and then on, of the ignition system WOULD cause a BANG. With CIS, if power is lost to both ignition and fuel at the same time, perhaps not. When power is lost to the FP in a CIS system the effect is as abrupt as ignition cutoff. The engine dies IMMEDATELY.

Just trying to be helpful. I do not know the cause of your problem, which very well could be ignition. Usually is. But again, under many conditions, a momentary shutoff of the entire ignition system generally loads the cylinders and the exhaust manifold and pipes with an ideal mixture of air and fuel. When ignition returns, all that fuel is ignited.