'78 SC dies randomly but immediately re-starts
 

Title says it all. The car will randomly lose power without sputter, surge, anything weird other than just no power. It has backfired once right before the RMP's fell completely dead when this happened. Turn the key off and immediately try to re-start and she fires right up and runs fine again for 10minutes.....or 40 minutes... or ??? before it dies again. The problem feels more like a loss of spark rather than fuel. I haven't found any flaky wire connections to the CDI or distributor and the green wire is new. The CDI was rebuilt at some point before my purchase but I don't remember who's sticker is on the back. Wondering if maybe I have a coil on the way out?

Thanks in advance.

Verify the wiring at your fuse panel is secure... sometimes the screws work loose and wires start to lose good contact. This just happened to a friend's car yesterday... first blame was on the CDI, but after substitution and the problem repeated, inspection of the fuse panel revealed the problem, and some gentle tightening fixed it for good. If not, pull the CDI and send it to me to check out. (Email me for address) bob@ashlocktech.com

Remove the CDI and provide the "rebuilt" source. Some "rebuilders" use a completely re-designed PCB, and others use their so-called "better parts" which potentially reduce the OEM Bosch CDI's reliability.
Once you remove the CDi to determine the "rebuilder", remove the CDI back cover and post a picture of the PCB. The very likely marginal component causing your problem can then be indicated.

Your description does not sound like a failing coil, but that's easy to test if you can buy or borrow another coil and swap. A usable coil for CDI systems that is available at your FLAPS is the MSD 8222.

Check the green wire to the distributor. My car had those symptoms, and the white plastic was disintegrated on the distributor side of the wire. I had replaced that wire before, so I didn’t suspect it at first, but they don’t last very long. I may have gotten 15 years from that one, which is long for regular cars, but not for a 911 with 3x that lifespan. The randomness of your symptoms seems consistent with random shorting of an electrical signal.

The green wire is new from the distributor all the way to the plug for the CDI.

Bob, you have PM.

Unless it's the silver one, and not the original small black coil.

Coil is a black Bosch OE unit.

Then it's the CDI. Again, open the CDI and post an image of the circuit board and find out what components typically cause the problem you have.
Hopefully, it's not a Perma-Tune, and the repaired CDI has an unlimited warranty like some have.

I’m not sure if a ‘78 still has points, but on my ‘77, I had a similar issue and it turned out that the inch long wire from points to distributor body had an internal break. Took awhile to find that one.

The '78 SC dizzy does not have points. He has the SC dizzy with the green wire to the CDI.

I had a friend have this happen to him while we were caravanning home from a rally in Utah. We'd be on the freeway and he'd lose power. Re-keying the ignition would fire the engine immediately and be fine for another 10 minutes to an hour. We initially thought spark (installed spare green wire and coil) but no joy. We then jumpered the fuel pump and it was fine all the way home.

It turned out he had a loose connection to the fuel pump relay. He recently installed spade fuses...

Thanks. I tightened all the fuel panel terminals. I'm waiting on a new fuel pump relay socket with pigtails from Timmy2.

Based on this, it's highly unlikely it's a fuel problem! Loss of fuel pressure requires cranking time to again build fuel pressure.

CIS pump is high pressure - it builds pretty quick.

Until the OP makes a jumper to bypass the fuel pump relay, I wouldn't say fuel is unlikely.

Doesn't eliminate an intermittent/bad FP. For a possible fuel problem, he needs a fuel pressure gauge and not a shotgun troubleshooting approach!

The failure occurs intermittently under load. Why not jump the relay and take the car for a drive? Don't get me wrong, I'm quick to hook up a pressure gage, but what do gages tell you when the car is stationary (not under load)? It might idle for two hours without stalling if the failing system is not disturbed.

Entire fuel system is new/rebuilt in the last year as part of getting the car back on the road after a 12 year sleep by the prior owner. Fuel pump relay/relay plug are both suspects. Waiting on a replacement relay socket from Timmy2 to replace the one butchered hy a prior alarm "installer."

Check the sensor plate height and perform a basic setup of CIS, I posted it here several times.

E.g. https://forums.pelicanparts.com/porsche-911-technical-forum/1164792-very-low-idle-200-rpm-sputter-after-long-sit-4.html#post12301399

Thomas

Thanks for the link Thomas. I've gone through those instructions while getting the car back running and believe I have the CIS system properly set up. When the car loses power it is at part throttle well off idle so I don't believe the sensor plate is dropping to the safety contacts and shutting off the fuel pump.