MoMcC

Driving down the road my old 911 dies unexpectedly. The engine stops running.
I can turn it off, and it will start right up.
It starts immediately before it even stops rolling. Then runs like a top again.
Any ideas?

theiceman

you lie .. SCs NEVER break down .... lol ... mine has never broken down in 25 years ..

ok as you know it could be just about anything.

everything from bad ignition switch to bad coil ..
is it a silver coil ?

MoMcC

Ice,
Thanks. I have loved these old machines since the 70s. Finally joined the tribe last year. Didn't get any documentation or history with the sale. But, have loved the initial sorting and repair/replacement process.
The coil is black...with lots of German words on it. The ignition switch seems to work well. I will find the right relay and grounds...
What else comes to mind.

theiceman

if it just cuts out it is likley electrical , usually gas it will struggle for a bit. being this intermittent its tough. really no idea.

MoMcC

Thanks. I'll try the grounds and relays first....

theiceman

if you had a buddy with one you could swap the CDI box see if the problem moves but this is going to be tough.

MoMcC

It looks like the DCI box is available. https://www.partsklassik.com/Porsche...6-pin-new.aspx

Would changing it out be a reasonable place (albeit expensive) to start the hunt for a needle in a haystack?

Edward

What ICE said about cutting out immediately vs stuttering out is an important differentiator between elec and fuel related. From what you describe, it sounds like electrical, esp since your engine fires right back up. Check all wires to/from coil, dizzy, and plugs: wiggle and tug on everything. Also, check the fusebox and every fuse's connection. Wiggle and twist stuff as surface corrosion/loose connections/terminals are typical ...do this while car is idling and see if you can induce it to cut out.

Edward

MoMcC

Edward,
Thank you! That it is the kind of coaching I need....I'm a novice tinkerer.
Mike

Dr. 914

usually when a cd box fails it backfires before giving up

MoMcC

Thanks. It did not backfire. If a CDI fails, will it still run? Mine runs great about 99.9% of the time....

theiceman

yeah this statements are true usually the cdi cuts out and back in is the failure mode before it dies, when it cuts back in it ignites unburned fuel and backfires.. This is an odd one for sure .. when the car cuts out do the light on dash com eon such as oil pressure etc ? does it feel like complete powere failure or just the engine ?? i would get under and check connections to ignition etc as they are just blade connectors.

systemsc

There are other CDI sources, e.g. https://www.systemsc.com/products.htm, for less than half that price.
Besides, many of the so-called "new CDI" units are not Bosch and are not reliable!

pbeezy

I had this problem. It was the CDI. The way I proved it was as follows: I put a timing gun on a spark plug wire and ran the readout it to the cabin. While driving, when I would intermittently lose power I would look at the readout. If the rpm’s went from 5000 to zero, I knew it was the cdi and not the fuel pump. I got a new cdi and it fixed everything

MoMcC

Thank you! Next step install new Bosch CDI.