77 930 Randomly quits running
 

I have a 1977 930 that will randomly shutdown, then backfire, then restart all on its own. This seems to happen only while driving (no load). The engine has been completely rebuilt within the last couple of years and has not had this problem until these last six months. The tach will just go to zero, then the car will backfire (very loud), then it will restart immediately on its own and run normally. This all happens within ten seconds.
My shop and I have replaced:
The engine harness (Kroons)
Rebuilt the CDI (AshlockTECH)
Replaced the green wire and Coil (black) both from Parts Klassik.
New fuel pumps and relays (with bypass to battery up front)
Alternator replaced during rebuild.
Distributor being rebuilt now (Parts Klassik)
Spark plug wires replaced during rebuild.
All wire connections double check again.

Note that we tested the car after each item above was replaced or fixed but still had the same problem after each one.

Also with the temporary distributor installed , it still has the exact same problem.

Any ideas?

Thanks
Lordy

I just realized the Sticky “my car doesn’t start/run” may have what I was looking for.
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Lordy

It could possibly be the ignition switch have you checked that?

RAHL

No, but that’s what I’ll check next!
Hopefully it’s the problem because this is bugging the heck out of me.

From your description, sounds ignition related. If you lose spark for a second but the fuel pumps are still running, that may explain the loud backfire when ignition is restored. Otherwise I would point you to other culprits that can shut down the pumps, like a faulty connection to the over boost switch/sensor, or the infamous rectangular yellow relay ('77 may use a standard round relay, back at the left rear relay panel). I would also point you to under the drivers seat to the overspeed relay but not sure if the '77 had those. I've cut my teeth only on 1987's.

sounds like your "mechanic" is just playing swaparoni without doing any real diagnostics

Don't know if there;s an overboost switch on 77 but if so it can cut the ignition and produce backfire. Sometimes a random no start. Same with the 14pi connector. check all your grounds, almost certainly an electrical issue.

Just checked my receipts and he changed the ignition cutout relay, red relay and ground strap six months ago trying to track down this issue.
Still awaiting for the distributor to be rebuilt by Parts Klassic.
I am going to talk to him about what you have all suggested.
See what happens.

As Mark suggested, check the over-boost switch screwed into the intake manifold. It's a ground point for the fuel pumps. When the pumps get old they draw more current which leads to that ground point getting hot. Eventually the insulation within the switch will fail causing the switch to loose ground.

Tinker

Overboost suggestion is top of the list.

Thanks everyone!

The over boost switch cuts off the fuel pumps.

Simple test would be to unplug the wire and ground it elsewhere. Then go for a drive to see if problem persists.
I think my OB pressure switch is starting to fail. Lately when on full throttle boost, going like a bat outa hell, I'll get a momentary split second stutter...not the classic "face in dash", just a brief hiccup. Boost gauge says I'm not overboosting.
I need to follow my own advice and ground that wire to take the switch out of the equation. It's always something.....

i had an similar intermittent issue with my 76 die backfire then catch itself and keep running .. eventually it turned into, it would not restart for 15 or 20 minutes after i shut it down hot.
turned out to be the coil, when it first stared to fail it would fail then catch itself causing the back fire. Eventually the residual heat from the motor and the i/c with out air movement would cause the coil to over heat and open.. once cooled it would fire right up .
swapped out the coil hasn't happened since. 76 and 77 are simple compared to the later 930. if you dont mind me asking what the going price on the distributor rebuild i have a spare im considering getting rebuilt.

The tach signal loss usually points to the CDI box or one of its inputs or outputs in my experience. You could try checking the green wire from the distributor. That is pretty easy to do.