Not sure where to begin
 

I have a 1983 911sc. It runs really well about 80 percent of time but it surges on occasion & you can be running down the highway at 70 or whatever & it will die. When this happens you can push in clutch & release & it will take off like nothing happened. It will also act up on occasion when stopped at a red light. The idle will start moving up & down and sometimes will die but will start up immediately & usually run fine but occasionally it will struggle to run leaving a light for a few seconds then clear up & runs fine. This is all very intermittent. I have no idea where to begin.

Check your green wire. Does it ever happen when your car is still cold or only when warmed up?

Yep - green wire, coil, fuel filter.
Alan

It's the mixture, too rich if its surging. I suppose there are more faults. First and foremost check for vacuum leaks, these might be the cause that somebody richened the mixture by manipulating the CO screw.
Use a smoke generator as discussed in other recent threads here like https://forums.pelicanparts.com/porsche-911-technical-forum/1121020-proper-vacuum-leak-test.html and
https://forums.pelicanparts.com/porsche-911-technical-forum/1069752-cis-fuel-pressure-test-results-scrutiny.html

Then check the control pressure and the spry pattern and amount on the injector valves and fuel distributor. Ignition and dizzy should also be checked, but this is not the root cause for your problem.

Thomas

On mine - it was the coil.
I did a lot of switching various parts until swapping the coil finally did the trick.
Hard to diagnose because it happens sporadically,
Bill K

Thanks for replies. I had a friend check fuel pump pressure & he thought it was low. I changed fuel pump & filter but that did not help. I will change the coil& see what that does. The coil I have is only a couple years old but from my reading it sounds like coils are a constant issue with the currently available coils. Hopefully it's something that easy.

Thanks again

Keith

It doesn't seem to matter but may get a little worse as it warms up. It's very sporadic

What am I looking for with the green wire?

Base of distributor. In a plastic housing. Takes a hard turn towards the CDI unit. Often the cause of intermittent issues. Engine idling, try jiggling the base of the wire.
Yes - several threads on suspect replacement coils. Did you replace like for like?
I think they are matched to the HT lead types.
Alan

Where to start looking........
 

husky,

You’ve been given sage advises where to look for the culprit/s.

• Green wire from dizzy to CDI.
• Ignition coil.

Do you have a picture of the ignition coil? Hope you don’t have the silver Bosch ignition coil made in Brazil. This particular coil is notorious for failure.

Tony

I doubt if this is the problem. But it's only one minute to check it.

I had not seated the distributor cap properly on my SC. There is a plastic cover to stop crud falling down the shaft and it was misaligned, resulting in the distributor cap being on crooked. The car ran fine when cold but would start to miss at certain revs when hot.

Just another thing on the list, but do you still have Lambda. Also cracked box at the seam, not emmediately noticeable. Bob

There is a resistance in the rotor - on some models. And the coil needs to be matched to that.
So if any old coil was replaced rather than the proper one, that would do it.
Apart from the fact many replacement parts these days, incl coils, seem to be duds.
Alan

I currently have the silver Bosch made in Brazil coil. I will order new coil & look at other suggestions.
Thanks

Keith

MSD 8222 is a great replacement for the failure prone Bosch units.

Do a bit of a search on here on Coils. There are a few recent threads.
But underneath your rotor there should be a resistance number stamped in the body. The coil needs to match that.
Plenty of Brazil Bosch coils causing issues. You may have found your problem.
Alan

Fuel pump relay can cause an intermittent fault; I had this happen a while back. Engine would cut out, and if I was lucky restart. Local Porsche specialist identified the relay as the culprit. Now I always carry a spare.

Problem ended up being the CDI. Thanks for responses

My 83 had a coil issue with symptoms like yours when I first got it I switched to and MSD blaster and have never had an issue with it in 15 years.

Good to know. I will get one & keep it on the shelf