Technical analysis - Why did the engine stall?

[Eddyvdw]

Hello,
I have a 911E MY 1972.
Car runs great (after 18 years of sleep and months of rebuilding). Starts easy, warms up quickly and runs strong, a pleasure to drive. Still something strange happened today.

I took the car (with my 2 sons) for a nice Sunday drive. We drove for 30 minutes enjoying the nice weather. After pushing her tot the limit (6000 rpm) on a nice dual carriage road, I had to stop for a traffic light. Then she stalled...

I tried to restart right away but even after a minute of cracking no life. It was on a very busy road and fortunately some people helped me to push her aside. My 10 yr old son behind the wheel and me pushing was a bit too much for my fellow motorists.

After that I decided to push my by-pass solenoid switch for a few seconds and that brought her back to life. Drove home with no issue at all.

What I try to figure out is what happened? Did the drop from 6000rpm to idle cause the engine to stall? Was it bad luck and 'normal' with a 40 yr old engine? RPM transducer tested and ok.

Any thoughts?

Thanks,
Ed

[Flieger]

What do you mean by "by-pass solenoid"? Is that the cold start solenoid for squirting fuel into the stacks? Some people make that a manual switch.

It almost sounds like heat soak causing fuel to vaporize in the lines or pump.

Did you hear fuel pump whine while cranking?

[Eddyvdw]

Correct, I mean the by pass switch for the cold start solenoid. Installed it myself and use it while starting the car when cold. I think I did hear the fuel pump although it was a bit noisy on the side of the road.

[Flieger]

Well, based on that, it sounds like the engine was just tooo lean at that moment to start. I am going to make a guess that the the spirited drive put some heat into the engine. When you stopped, it stalled for whatever reason. It would not start right away because of heat soak. The MFI fuel pump pumps fuel continuously through the MFI injection pump to help cool it (along with oil circulation). The hot fuel would be less dense and so the car was too lean to start without the extra squirt of cold-start fuel.

What caused the stall? Well, how are your throttle bodies? Mine are worn around the shafts and allow more air into the engine than they should. This causes a high idle and poor low rpm manners. If yours are also worn, they would only exacerbate the lean condition. Also, high-overlap cams on early 911s can cause reversion, making low-rpm work kind of lumpy.

Sounds normal, just remember to keep a little pressure on the throttle pedal as you clutch-in to a stop so that the idle is high enough not to stall. Let the pressure off gently when stopped and it should not stall.



How about a photo?

[docrodg]

Agree with Flieger, probably just too lean to restart and the idle stall could have been the cam profile or the lean mix, even more likely when the engine comes down rapidly from a high RPM - like when you have it at 6k and suddenly clutch and let off the throttle at the same time, if the timing is just right your engine drops so fast to idle that the mixture goes off for just a split second and causes aggravation (rich mix) as the fuel injection is not responding as fast as the throttle plate closes.

[Eddyvdw]

Thanks, at least now I know what happened!

To Flieger response, the idle is set at around 1000rpm. Can't get it any lower so I do expect a worn throttle body. The engine runs strong (no cylinder leakage, hardly any oil spills, new calibrated MFI injection pump, carrera tensioners, new brakes, etc etc) so that has to wait until I'll rebuild the engine.

I'm almost done restoring the interior back to original. This week I'll see a painter so hopefully he's able to repaint her Irish Green (original is metallic green but prefer Irish Green...). See picture how she looks today.

Kind regards from Prague.
Ed

[Flieger]

1000rpm is pretty good. I like mine a little higher: 1500rpm or so hot, just under the threshold for the RPM transducer (I tweaked mine to have a little higher lower limit). I had tuned the pump/throttles to idle alright but that made it rich in the mid-high range. My mechanic leaned it so it runs great when warm and with some spirited revving. The downside to the worn throttle bodies is that the idle is lumpy and higher. Now when the weather is on the warmer side, the idle will creep up when the car is up to operating temperature. This brings the speed switch into play and creates an annoying occilation in the revs. If this was not just a weekend play car, that could get frustating very easily. Luckily, it does not matter when I am actually driving spiritedly, so I will accept the trade off of good mid-high revs performance for a bad idle manners. The car is 90% racecar already so it would not make any difference in comfort.

Just make sure the timing and mixture are good for the mid-high revs and drive spiritedly- you will have more fun and get there quicker.

Irish green sounds nice.

[Eddyvdw]

Already figured out my problem, stupid, stupid, stupid.

Closed the breather hose of the fuel tank because of fume smells in the garage. After my spirited drive I guess I created a vacuum in the fuel tank.....

Will fit the charcoal canister this weekend!

Ed

PS: How can you tell if you lose one cylinder?

[kidrock]

you can tell if you lose a cylinder by pulling one plug wire at a time while the engine is running. If there is no difference in idle after pulling a wire, you have found a problem.