warm engine will stop and not start again

[felipedelhierro]

I have a 2.7L 1975 911. When cold it will start with no problem, but after 15-20 minutes it will start to backfire and quits, no way to start it up again until is cold... realy cold. I already change fuel pump and check the coil but is still the same problem. Any ideas?

Thanks

[ossiblue]

Originally Posted by felipedelhierro

I have a 2.7L 1975 911. When cold it will start with no problem, but after 15-20 minutes it will start to backfire and quits, no way to start it up again until is cold... realy cold. I already change fuel pump and check the coil but is still the same problem. Any ideas?

Thanks

Spark, fuel pressures, or air leaks--could be any one or combination of all three. Eliminate each, one at a time. Spark could be the easiest to do because when the the symptoms begin, you can simply pull a plug wire and test for spark. Heat will increase resistance in electrical circuits so if your CDI, coil, plug wires, etc. are compromised a failure can show up once the engine compartment gets hot.

Really, the more likely causes are air leaks and/or fuel pressures. Air leaks can happen when split hoses or even a crack in the air box, open under warm conditions causing an overly lean mix. Add to this fuel pressures that are too high (overly lean mix) and any air leak is amplified to the point where the car ceases to run.

If it were my car, I'd begin with a spark check. If that's good, I'd proceed to test the fuel pressures. Once those are spec'd, I'd move to the air leaks. Keep in mind, we don't know the history of your car and it's possible there is gunk in the fuel tank which blocks the flow of fuel. Lots of possibilities, but pick one line of diagnosis and follow it to completion, then move to the next. Eliminate all possible suspect areas one at a time.

Tell us how you plan to begin your investigation, and report back all your results often. Feedback is essential to helping someone from long distance.

[felipedelhierro]

Thanks a lot. I will beging with this check list.
I will let you know

[felipedelhierro]

Nothing yet... I check the fuel preasure from pump to WUR and is ok, and no air leaks.

Can it be the WUR? what is the correct preasure from WUR to fuel distributor?

thanks again

[mreid]

This is a classic coil problem. It works fine when cold, then starts to break down as it heats up and finally stops working until it cools off again.

[ossiblue]

At this early stage, I'd eliminate the ignition as a cause first, as suggested by mreid. Spark is the easiest to determine (you either have a strong spark or you don't), plus the fact that heat increases resistance in the ignition system which tends to make marginal components fail, only to function again when cooled.

[ausgezeignet]

Agree with prior posts...eliminate ignition issue completely.

Then I would check Fuel Accumulator and Check Valve operation (I just replaced mine on an 80SC, it took 3 Accumulators to get one that worked properly...and they getting hard to find and damn expensive). Mine was a classic hot engine/ vapor lock issue. You've already replaced the Fuel Pump (and hopefully checked for contamination from tank to injectors).

Time to state obvious: That leaves air....

Best of Luck...Please let us know the outcome.

Mark

[felipedelhierro]

Thanks you all.
Spark plugs are almost new. I already check them and are fine. I think I will go for the coil problem. I have not up grade my ignition (still use points). Do you sugest that instead of replacing just the coil, i should go for an upgrade such as the Ignitor or Ignitor II Electronic Ignition

[thumbdoctor]

Two quick checks for bad coils:
1) Heat coil with a heat gun while engine's running.
2) Hold a strong magnet near the coil while engine's running.

The second test, you might want to insulate yourself from the magnet in case the high voltage break down is external.

[Jerome74911S]

Sorry still learning - Well, not really sorry. . . What is the object of the magnet near the coil?

[thumbdoctor]

Originally Posted by Jerome74911S

Sorry still learning - Well, not really sorry. . . What is the object of the magnet near the coil?

If the coil has an internal HT (high tension) break down, the magnet flux will interfere with the coil's field collapse and provoke arcing at weak insulation points (an old television CRT fly-back transformer trick).
Par ailleurs Jerome, belle voiture, l'amour de la couleur intérieure.

[Targalid]

Make sure your points are gapped to .014 inch. If they are much less than this, the car will start easily but stumble when hot and be difficult to restart. The point gap closes over time as the wear pad is consumed.