79 sc Stalling and Backfiring

[ntiras]

I am a fairly new owner of a 79 sc targa. For the first few months of using it as my daily driver it gave me no problems at all. Recently however I have been having trouble getting the car to start. I i start it in neutral without any gas it just cranks but will not start. If i put my foot on the gas the engine sounds like it is trying to start and sometimes backfires (I do have a pop-up valve) and about 15% of the time it will kick in, revving rather high and then continue to run.
Once the car is started it runs fine, but normally after about 20-30 minutes of driving the car it stalls, backfires and won't start again.
The strange thing is that sometimes it starts up just fine, it seems to be a random chance of whether it will start or not. This leads me to think that it could possible be a loose connection in a wire because sometimes it starts and sometimes it doesn't.

Please help, I'm not sure where to go from here and could use any input.

[ntiras]

Does anyone have any experience with a problem like this, or might now what it is thats causing the problem?

[mca]

Normally these cars will idle high for a few minutes when started with a cold engine. The idle should then return to normal.

Without giving the car any gas, will it eventually turn over after several attempts?

Does your car always cut off after 20-30 minutes or is that an intermittent event?

You say that it won't restart when it cuts off ... I assume that it eventually restarts or else you would have towed your car a bunch of times. How long does it take to get it started again?

[holtjv]

Major air leak?

At first when I read about it not starting, just cranking, I thought your pop up valve could be stuck open (after a backfire) against the airfilter. If it runs, maybe it closed but because you're lean, backfire again, pop-up stuck, etc.

If this is true, you are either very lean or you have an air leak somewhere--simple trick: get the car started and open your oil filler cap. If the idle drops considerably, your vacuum system is tight. If not, you probably have an air leak somewhere.

If this checks out try enriching your fuel/air valve just a bit and see how that works.

Good luck.

[holtjv]

Oh..don't use any throttle when you start--guaranteed backfire and pop-up valve blow.

It is has been running well until recently, check your vacuum connections.

[boyt911sc]

Nitras,

Your problem is a typical one for CIS cars. If you browse the next 5 pages of this section (Technical Forum for 911), you'll find you're not alone. There are numerous threads about 'car won't start' just for this month alone. Why? The change to cooler weather means that cold start valve is needed; metals contract, rubber hardens, causing more air leaks, etc.

Do you have some reference manuals? If I would do a troubleshooting for your car, I would do the following:
1). Check the fuel pressures (control and system). You need a CIS fuel gauge.
2). Check CSV (cold start valve)
3). Check for air leaks (not easy to locate culprit).
4). Check other CIS components, do this as you get familiarize with the system.

There are lots of help available in this forum. Just ask the right questions and you'll get good replies. Keep us posted.

Tony

[ntiras]

Should I be able to feel a vacuum with my thumb at the distributor hose when cranking the engine?

[ntiras]

OK, I found sticky goo around air flow sensor plate. When ever the car doesn't start, I simply lift the sensor plate, let it come to rest and the car will start. I looks like the plate occasionally doesn't seat. How does one clean around the air sensor plate?