Strange behavior
 

I've had my '75 911s for almost a year, and have had few problems with it. Drive it probably an average of 60-100 miles a week. On two recent occaisions, the following happened. I drove about 25 miles, then almost immediately turned around and drove straight back 25 miles back home (all highway). when I'm almost home, travelling about 60 mph on the highway, the revs go down to zero and I get no engine power, the car starts to coast, and after about 1-2 seconds, the revs go back to normal, and the engine re-engages. This happens quite a few times in quick succession until I get home. last time, it even let out a really loud backfire one time when it disengaged, then came right back online. Any ideas? Could the long highway miles just be too much, or is there something I can check? Other than this recent development, the car usually starts and runs like a dream. tia

Sounds like a heat soak issue in the CDI box. Even more likely if you have a Permatune.

Hi,

I had a similar problem, and it turned out to be a loose connection on the alternator. I believe it was the B+ connection. I also had a bad Perma-tune box years later, but it would kill on me after the car was driven then sat for a few minutes and then was restarted. The bad box did cause backfires, but I can't remember if the loose connection did. Good luck on tracking this down. I started carrying a timing light with me so that when it died I could check to see if I was getting a spark or not.

-Rutager

so, I'll check the alternator connections first. If the problem persists, how can I determine if its the heat soak issue, and if it is, do I just need to replace the box? Thanks.

There is a Pelican that rebuilds the Bosch CDI boxes, if I remember correctly. Ingo?

bad cdi or msd
 

i had a bad msd and had similar problems

Ok...so I checked the distributor, and all looks ok...I'm thinking it must be the CDI...I've got a Bosch...So what would be the consensus recommendation? Have my Bosch rebuilt, purchase a rebuilt one? Buy a new MSD? Advice would be appreciated...Also, is there any way to double check to see if my current CD box is definitely causing this issue? Thanks in advance!

This is a WAG from someone who barely knows anything at all but how old is your fuel filter? This sounds to me like a brief starvation issue.

I'm probably completely off base but it never hurts to start with the simple stuff...

Good luck :)

Does your tachometer go to zero when the engine dies, but is still in gear turning? If so I would look at electrical, CDI, Coil, distributor points cap rotor in good shape?
Does the alternator light come on? Try driving with voltmeter hooked up and see what voltage is doing at time of cut out.
If tach still reads and alternator light is out, then I would look into fuel problem.

Also make sure to check all the wire's going to coil hiding under the little black boots I had that same thing happen to me and it ended up being a broken wire where I couldn't see it till a went to replace the coil and wala there it was, also for repairs of your CDI box try ECU Repairs,Specialized ECU Repair - Porsche DME Repair, Bosch and Motronic Control Units, 1-800-328-1425, { no affiliation } I used them to do my CDI on my SC and after about 5k miles no issues his prices I thought where more then fair and since I'm in south Florida I just took it to him, I hate to ship stuff if I dont know where it's going,but for yourself or anybody else who needs a DME,ECU or CDI box I would recommend them they do other vehciles as well.

Ernie

All good suggestions above but check the points first.

Check the coil. I had this exact problem.

Cool...thanks...Coil and points are first orders of business...if those are not the cause, then it's time to install a new MSD! Thanks everybody!

update....Finally had a chance to take the coil out and check it on a multimeter....primary showing a reading of .5, which I think is ok...the secondary is showing no continuity...So I'm assuming that means that the coil is definitely on its way out, and that's the root of my problem...sound right?

I'd still check my fuel filter and the filter at the bottom of the tank although the coil failures exhibit the same behavior

Thanks...Will-do...but if those look ok, would I be correct to assume that the multimeter readings prove that the coil is most likely the cause of my problem?

to your question, sounds likely, but could you say what connections you tested?
(from/to terminal names)

First, I checked from A to B, then between the center tower on the coil to both a and then B...sound right?

and did you check the points for dwell etc....

I would check the points, coil, all wiring before changing CDI


I had this happen when the points were misadjusted and just not enough open time. I switched to a Pertronix Hall effect ignition instead of the points. Great upgrade.

Are you sure you are making good contact to the high-voltage terminal when trying to measure the secondary coil resistance? I doubt is is open since this would render the coil "kaputt"

The fact that you tachometer goes to zero when you loose power points towards the triggering. The tachometer would register with just the points opening and closing. There is only a pull-up resistor to +12V inside the CDI box that has to be functional. So in other words the tachometer is not registering spark events but trigger events.

Heat soaking of the CDI box affects the SCR and causes it to short the DC/DC converter. With this failure mechanism you typically need to power-cycle the CDI box (turn the ignition off and back on) to get it to work again for a short time.

As a first action clean the contacts of the CDI box, inspect your trigger wire and the points before "throwing parts" at it.

Cheers,
Ingo