Intermittent problems - our favourite kind right?
 

I posted the other day about my 87 930 not running correctly. It backfired and would not allow me to give it any gas without it backfiring. The car "fixed" itself and I drove a couple of hours home.

I have driven the car a couple of times since then without problems; however, today while idling at a stop light the car acted up again. The first indication is the tach will go all the way over to the right (the engine sputters but does not actually change RPM's). I can baby the car at first, but any attempt to open up the throttle is met with backfiring. I was limping home but the problem got worse to where I was finally stopped on the side of the street. It would idle (poorly), but I could not give it any gas.

A strange thing occurred when I applied the brake pedal. The tach needle came part way back and I could give the car throttle without is backfiring. As soon as I took my foot off the brake the tach needle swings all the way to the right and the car will not let you give it any throttle.

I turned the car off, started making arrangements to tow it home. On a whim, I restated the car (it had sat for +/- 10 minutes). It ran fine and I blasted home where it sits now awaiting somewhere to start on the diagnosis? Electrical possibly since the brake light switch is the only thing that is electrical that could affect the other electronics? Any help out there? It is only a five hour drive from here to the closest Porsche shop (RUF in Dallas - my choice) and if maybe someone has seen a electrical gremlin that makes the tach flip out and the car not run?

Believe it or not, when my battery was bad these exact symptoms occured..wild tach and all....I know it doesn't seem to make sense, but a new battery did solve my prob....check and clean your terminals and/or swap it out....

Yep, sounds like an electrical malfunction in the battery/charging circuit. How many miles you got on the alternator?

Battery corrosion.
Clean the terminals with baking soda/water or replace if old. Check the possitive lead at both ends for the nasty blue/green crystals. A weak battery will excellerate corrosion. Keep an eye on it afterward as something has caused this to occur.

Hopefully it is not the alternator. Fingers crossed.

Checked the battery connections - all are good and tight with no corrosion. The battery is fairly new and I keep a battery tender on it. I will need to check the alternator, voltage regulator, etc. Could it be the CD box (the car has a factory unit - wow are they proud of those)?

Sorry to hear that. Battery is an easy (and more common) fix.
Sounds like you know what to do next. Let us know what you find.

When this did me in, it was the alternator. Tach was jumping all over the place.

Thanks all. I am going to search threads for "checking alternator" I have a good voltmeter and I bet this question has been asked a few times before. I will keep you all informed. Thanks again for the help.

Finally did a little checking on voltage "thing". Unplugged the battery tender, voltage at the battery was 12.8 volts, checked it again the next day and the voltage at the battery was 12.6 volts. Started the car, let it warm up and checked the battery with the car running at 13.8 volts. Got a friend to give the car some gas while I checked the voltage. No increase in voltage (so the voltage reg is OK, right)?
Could not get the car to act up again. Something else I remember from the other day was that the speedo is now off calibration from before, could that be related?
Could I have a problem with the CD box (hope not, the factory ones are REALLY high). If it is bad, can I use a Msd 6al 6462 to perform the same duties?
Still wondering what to do to fix the car (if it acts up I will start checking voltage with it acting up). I did check the battery cables (should probably take them off and clean them first) and I looked at the 4th fuse from the right. It looks good, but....
Thanks

Hmm,
I don't think this could affect your speedo. I would check all the grounds you can find, especially the strap at the starter and the one up around the back of the alternator that connects to the crankcase. I don't see how the CD box could make your tach go crazy either.

I had my tach start to fluxuate on another car I had. Turned out to be the coil.

Interesting... It seems that if someone Farts in the back seat, the Tach fluctuates!

Nobody in the back seat farting, that would have been an easy fix.
Had the tach fluctuate/coil problem on two different cars. Driving down the road, tach starts swinging back and forth, motor cutting out, loss of power under acceleration, etc. All this being intermittent just to keep it interesting. Thought I would throw in it with all the other .02's.

Interesting... It seems that if someone Farts in the back seat, the Tach fluctuates!

Voltage regulator...
 

I just wanna say I think it's your voltage regulator. Sounds like you've got it ironed out, once you check it with it acting up.

It's not a hard fix. You can change the regulator WAY eaisier than pulling the whole alternator off for a rebuild. The reg. seems to be the only weak link anyway.

Remember you can always hook up to the cigarette lighter w/ your meter and go drive.

I'm guessing the extra current draw from your brake lights may have made it behave momentarily.

Pull the intercooler, loosen the fan housing, lay the fan down to get to the back, and there are two bolts that hold the regulator on. The rear fan shroud (white thing w/ two fins) should be held on by three of the six studs that hold the alt. in. They are 8mm nylock nuts, an 8mm "gearwrench" works well for these. You can turn the alt/fan assembly nearly 180 degrees to reach all three nuts. They should have washers underthem as well, keep a magnet handy and you'll be fine