1980 SC starts, revs, pops, dies - worked fine on the drive TO work

[356graham]

Any help diagnosing the next place to check would be appreciated.

Here's the facts
1980 911SC bone stock
Drove to work in the morning, no issues. Parked in the work garage.
After work (car sat about 9 hours) tried to start and here's what happens
- Turns over
- Starts
- Backfires (I do have a pop off valve)
- Dies

Ambient temp. about 58 deg F.

Here's what I've been able to check with the tools I have at work and info on the site so far:
- Pop off valve is properly seated and sealed
- All fuses check OK (looking at the O2 sensor / interior light fuse blown issue)
- Warm up regulator heating element checks OK at 26 Ohms
- Removing the fuel pump relay allows the engine to turn over, but no starting or backfiring - so I'm assuming that the pump is pumping. I'm aslo assuming that it is providing the same fuel pressure that it provided 9 hours ago (perhaps a wrong assumption - what do you think?)

So, please check my thoughts and provide any guidance;
1. Something failed while the car was parked for 9 hours - what fails while sitting?
2. Something related to cold start failed on the drive in to work, but only became apparent on the next cold start.
3. Something was marginally working on the way in that allowed it to start and drive, but now won't allow it to run.

I'm limited on tools here, so any help with things to check so I can get it home would be great. Else, its a tow home.

Thanks.
Jeff

[_deek1954]

check survailance camera / glemlins/carma I feel your pain

[Craig Stevens]

check the cdi box. if you can find a spare just plug in to test.
Good luck, Craig

[mca]

Give the 02 sensor relay a try. Square relay under the passenger seat.

Although mine didn't die, it was running lean - popping, low idle. Read about it here: Suddenly Lean - Left Stranded

[356graham]

>> Give the 02 sensor relay a try. Square relay under the passenger seat.


Thanks for the suggestion. The relay checks out OK. I pulled it, jumpered the coil to 12V and was able to verify continuity between the normally open contacts. So....

Still a couple hours of daylight here.

>> check the cdi box.

Don't have a spare, so that will have to be a test for when I get the car home - one way or another.

Thanks for the help!

[ossiblue]

Don't know much about the O2 sensor except that it can cause start problems, but here's something you can try in the garage, now.

Because it does start and backfire, you do have spark. What you don't have is enough fuel delivery. Try turning the ignition to the run position, lifting the plunger in the throttle body and listening for the injectors to squeal. If they don't--fuel delivery problem. If they do, go into the car and try to start it. Let us know what you find.

[Joe Bob]

Even with a pop off, the air box can develop a leak....

[356graham]

I've now got the car home. Feeling better now about the AAA Plus membership - free towing up to 100 mi!

Checks I've been able to make so far, and still the same issue. Catches, revs, backfires and dies.

- I've checked the airbox and pop-off valve all the way round and found no leaks.

- With the ignition on, fuel pump pumping, I'm able to lift the fuel bar in the air box and hear fuel flowing. Makes somewhat of a groaning / squealing noise. Can't tell if there is *enough* fuel pressure, but there is pressure.

- Checked the AAR valve (cold) and saw the half-moon shape one is supposed to see.

Still very interested in hearing thoughts on what to check next. In the CIS Primer troubleshooting section there is a problem identified as "14. Engine starts but immediately dies ..........." - sound exactly like my problem and the list of possible causes are;

Electric fuel pump not operating <- I know mine is operating.

Excessive fuel delivery to control circuit <- how would I check that?

"Warm" control pressure too high <- can I eliminate because this is a cold start problem?

"Warm" control pressure too low <- can I eliminate because this is a cold start problem?

Primary system pressure outside of tolerance <- going to need to get fuel injection kit to measure this one.

Thanks for any help.

Regards,

[psalt]

Jeff,


First start with the basics, fuel and spark. Forget the O2 sensor, it has nothing to do with your problem. How much fuel is in the tank? The gauge has fooled many and if it is below 1/2 put some more fuel in the tank. Your post implies that you can hear the fuel pump running with the key ON, if so, your fuel pump wiring is incorrect. When you crank the engine, the CSV supplies the extra fuel to fire the engine. The TTS and CSV can fail at any time. It take many more cranks to build up enough fuel to fire the engine without the CSV and it is easy to test at the TTS. Once the engine fires, the lower control pressure gives the rich mixture to make the transition to cold running. The WUR can fail at any time and you can't tell unless you check the cold control pressure. You do not say how many times you have attempted to start, but my advice would be to pull a spark plug and take a look. If it is wet, test the spark with a new plug, then put in a set of new or known good plugs. Once they are fouled, you can spend hours chasing your tail. Then disconnect and plug the grey vacuum retard line at the rear of the distributor (this will give you a much stronger cold idle) and disconnect and ground the red/black wire at the TTS. Crank the engine and keep you foot off the pedal. Do not crank the engine more than 8-10 second with the TTS hot wired.

[mca]

I'd still try the O2 sensor relay. If yours is the original, it can't hurt to replace it or at least pick up a spare. I know you said that you tested it but mine failed slowly - so it could have checked out ok but still been bad. Regardless, it won't hurt to get one. Again, my car didn't die but it was barely hanging on.

You will need the pressure tester to test all pressures. JC Whitney has a superb tester and it is VERY affordable.