1980 911 SC troubleshoot starting problem
 

A few weeks ago I was driving & stopped for 5 min, then the car wouldn't start. Cranked fine, but no fire. Couldn't tell if it was fuel or ignition. After some time it fired right up.

Since then it has done this another half-dozen random times. Hot/cold is not a factor. I found that turning the ignition off, then trying again, it would finally fire right up after between 3 and 15 attempts. Starts & runs perfect except for these random times.

Now it won't start at all, even after many attempts. I can smell fuel, which makes me think it is an ignition problem.

Can a marginal battery spin the engine ok but rob CDI of enough voltage to fire? Battery will crank the car 15 tries of 3-5 seconds each without sounding weak. I've charged the battery & tried again with same results.

Troubleshooting is complicated by the aftermarket alarm in this car. I don't have a manual, and have no idea what brand or model.

Any ideas?

What is a good source for an ignition system schematic?

By the way, ignition on I can hear the CDI high-pitched squeal.

First check on CIS is fuel pressure. Got access to CIS gauges?

Hot start cuts out the cold start valve and requires residual fuel pressure. There is a check valve in the neck of the fuel pump that tends to lose it's ability to maintain pressure. Cheap and easy fix.....other parts are more expensive.

But your first check is the fuel pressure before you start throwing parts at it.

Don't have access to CIS gauges...
but this happens hot or cold. (Happened after sitting overnight as often as when warm)
Also, I can smell fuel after several cranking attempts.

Then pull a plug wire and check for spark....

In a situation like this you always want to determine if the problem is fuel or spark related. Its easiest to check for spark. Tests must be done when the engine is in failure mode. Another check you can do if it doesn't start is to lift up on the air flow sensor. You should hear the injectors bleed off. If it starts then you likely have a problem with the fuel accumulator.