Interesting stall issue

[NorthernThrux]

Here's a riddle for socially distanced problem solving.

My car runs great. Starts in 2 seconds, even at freezing temps. Settles into a fast idle that can be regulated nicely by the hand throttle with no issue. Accelerates hard and cruises perfectly. Downshifts nicely with throttle blipped heel and toe. All good.

On a 100 km drive to Bayfield, north of here yesterday, the car worked perfectly on the drive up, involving about a dozen rural stop signs and the overtaking of a few cars. Starting just fine at 2 degrees C. Sat at a friends house while we went for a long walk and coffees outside in the fall colours and to watch film crews set up for a major picture being filmed in the village. Four hours later, after dark, we left. Car started perfectly and ran fine through several small towns where we stopped at lights or stop signs. Pulled into town and into our neighbourhood and when I stopped to let a car go buy before pulling into our driveway, all I did was push the clutch in and the revs just dropped to zero. Restarted right away and I pulled into the driveway so my wife could get out and open the garage door. Revs dropped to zero as soon as I pushed the clutch in. Lights, heater fan etc. all kept running. Restarted again just fine. Pulled onto my lift. Pushed the clutch in and no issue. Idled at 875 rpm or so. Engine Temp about 170F and oil pressure fine. Voltmeter said 13.7 V for the whole drive, but drops to 12.7 at idle. Will go to 13.7 above 1000 rpm.

MFI speed switch works fine and I have had this happen both with the original Bosch speed transducer and with the modern solid state one (currently in the car). Throttle plate stops etc all perfect. Only thing different that what CMA would prescribe is that Robert set the car up to run at 2 degrees of advance instead of 5. Vacuum lines look ok though some of the cloth is frayed at the ends. On the list to replace.

This has occurred a couple of additional times in the past year, pre and post the engine being rebuilt. Always after a long cruising drive when coming up to a stop and pushing in the clutch.

Counting the 2 times last night, it would be 4-5 times in about 1700 miles of driving. Which is 4-5 times too many for me.

Suggestions? Vacuum advance hanging up or something?

Ravi

P.S. Gas tank always at 1/2 full or higher.

[edmayo]

It's actually a vacuum retard, Porsche called for timing at 5 degrees retard at idle with vacuum hose hooked up. Timing should jump to 5 degrees advance when you pull off the vacuum retard hose.

[Richy]

My money would be on the anti-backfire system , probably the rpm transducer , certainly seems suspicious . Maybe disconnect the MFI pump shut-off solenoid and and see if the problem reoccurs .

[NorthernThrux]

I’ll try that. This has happened with two different rpm sensors, of two different types and so I doubt it is the transducer. But it could be the shutoff solenoid gets hung up once in a blue moon. Maybe its return spring is weak. Worth a try. It could be a year before I know if it worked, given how rare this issue is!

Ed, I was thinking that the centrifugal advance weights in the distributor were hanging up on occasion, I could be stuck with a lot of advance at idle. Dizzie was rebuilt and the curve checked during the restoration and looks great inside. I think the solenoid is more likely the culprit.

[edmayo]

Easy to see if solinoid is the culprit, just unplug it and repeat test.

[tom1394racing]

Perhaps the clutch wasn't completely disengaged and stalled the engine. A clutch adjustment perhaps.

[NorthernThrux]

Easy to see if solinoid is the culprit, just unplug it and repeat test.

Since it happens about twice a year, it's gonna be a while before I report back! it's always hard to test for the negative occurrence.

[NorthernThrux]

Perhaps the clutch wasn't completely disengaged and stalled the engine. A clutch adjustment perhaps.

If it were the clutch you'd be able to modulate the idle rpm with your clutch foot as you vary the drag on the engine. Nothing like that here. Clutch is adjusted perfectly. Disengages well off the clutch stop. This is something that happens twice a year, only after long drives. But not all long drives. I have driven the car 1700 miles since May. Any out of adjustment in any of the systems would have shown up on a more routine basis you would think.

[davep]

So, as I understand the situation, you are slowing to a stop, the revs are dropping to idle, you push in the clutch and the engine dies. No fuss, it just dies. Yet it will restart as though nothing is wrong. So you had two instances in a row, and on the third clutch action it did not repeat. I find it hard to imagine an ignition problem there so it seems to indicate the fuel was shut off. The rarity of this problem occurring certainly makes the diagnosis difficult.

[frederik]

I had a similar problem, solved by slightly enriching the fuel mixture at idle. In my case I could prevent the engine from dying at intersections by applying a little heel and toe when coming to a stop, then let go of the throttle and the engine would idle just fine.