Interesting stall issue

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Originally Posted by NorthernThrux

It’s the system that involves the microswitch, the speed relay in the rear electrical console and the cutoff solenoid at the end of the mfi pump.

On the MFI cars. when you lift off the throttle at higher rpm, the throttle plates slam shut, but the pump keeps spraying fuel at a high rate until the pump catches up and provides only idle fuel flow. That leads to a super rich condition with lots of unburnt fuel and often a backfire or two. To counter this, Porsche installed a circuit that does the following. It has an speed relay (technically engine speed relay but Bosch calls it a speed relay) that always puts out 12V whenever the rpm is between 1300 and 1700 rpm. The speed relay gets its rpm signal from the black/purple wire on the dizzie. This 12V signal from the speed relay is applied to a solenoid on the mfi pump that pushes the rack to its fuel cutoff point. Now obviously, you don’t want the solenoid active when you are on the throttle, so Porsche installed a microswitch that feeds the speed switch 12V signal to the mfi solenoid ONLY when the throttle plates close.

Net result? When you let off the throttle above 1700 rpm, the MFI pump still sends fuel (avoiding a massive lurch) until 1700 rpm where the MFI rack is pushed by the activated solenoid and the fuel cuts off. The fuel comes back on below 1300 rpm so you don’t stall. At that point you are almost at idle and you just have idle air-fuel ratios, so no backfire.

The circuit can be cantankerous. Hence people pointing fingers at it. Speed relays fail due to vibration and corrosion of the leads. Mine has been repaired by Bob Ashlock. Bosch also sells a modern replacement. It uses an integrated circuit. I am using it at the moment. My pump is rebuilt, but it is the original solenoid. Microswitches fail. Mine is new. In failure modes, you just get backfires. But if the solenoid sticks, you get a cutout of fuel.

In my case, any intermittent electrical glitch in the car should also occassionally show up when not at idle. The car has mever missed a beat in 1700 miles. So I discount that. It has to be something that only occurs at idle. Hence my focus on the speed switch circuit, but the solenoid in particular.

Ravi

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Thanks, Ravi, great explanation. I am familiar with this system, just never heard it referred to as "anti-backfire"

Ravi, I'm puzzled now because as far as I know, the speed relay doesn't have a 1700 RPM threshold. It always cuts the fuel above 1300 RPM and there is no upper threshold. When the engine accelerates again, the speed relay engages at 1500 RPM. The hysteresis, the difference between 1300 and 1500 makes the system stable and prevents it from flip-flopping continuously around 1300 RPM.

So when you release the accelerator completely at say 5000 RPM, fuel flow is cut immediately. If the system would work as you describe, it wouldn't really do much since the window between 1700 and 1300 RPM is so small.

You can verify this by manually accelerating the engine (standing at the rear) up to a high RPM, then pressing the microswitch. The revs will cut immediately until 1300 RPM, then go up to 1500, back to 1300, etc. Again, if the system would work as in your description, once you have the RPM up above 1700, the revs would not cut at all. This is not the case.

You are correct Frederik. I actually had it reversed.

If you start with the rpm between idle and 1500 rpm and press the microswitch, nothing happens. The microswitch routes a voltage from Pin 1 of the speed switch to the mfi cutoff solenoid and there is no 12V signal at Pin 1 under these conditions. When you are above the 1500 rpm limit, 12V appears at Pin 1 but does nothing because the microswitch needs to be closed to complete the circuit to the mfi cutoff solenoid. When you close the microswitch, that 12V signal goes to the solenoid and shuts off the fuel and the 12V isn't removed from Pin 1 until the rpm drops to 1300.

So it's really a circuit which produces 12V above 1500 rpm and removes it at 1300 rpm (the hysteresis you mention). Whether that 12V gets applied to the stop solenoid depends on whether the microswitch is closed or not. I think I had 1700 in my head from where my tach was misreading, but it is indeed 1300 and 1500 as the cutoff and cutting points for our cars. http://www.bosch-classic.com/en/inte..._teaser_1.html


Thanks for noticing the error(s).

Ravi

I'm glad we are in agreement. :)

Any chance to have the ending of this issue ?

Time to time, I experience pretty much the same behavior, particularly when the engine is not warmed up and after a long decell foot off the pedal.
Coming to a stop, rpm drops very rapidly when engaging the clutch and it's like the shut off solenoid is not rapid enough to authorize the fuel to come back at 1300 rpm or so.

When I disconnect the transducer, no problem.

It's either the transducer or the solenoid, but difficult to tell because you can't reproduce this scenario when the car is stationary. You'd need to wire a light to the transducer so you can see when it disengages the solenoid, restoring fuel, but you need to be able to see this light from the driver's seat.

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Originally Posted by frederik

It's either the transducer or the solenoid, but difficult to tell because you can't reproduce this scenario when the car is stationary. You'd need to wire a light to the transducer so you can see when it disengages the solenoid, restoring fuel, but you need to be able to see this light from the driver's seat.

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Hi Frederik,

I did my tests with 2 "known to be good" transducers.

The original relay for 911E/S : 0 336 611 001 (ON at 1520rpm and OFF at 1350rpm).

Attachment 607157

With this one I still have some engine stall as described even with the engine fully warmed up.

The substitute relay : 0 336 611 006 (ON at 1610rpm and OFF at 1500rpm).

Attachment 607159

With this one I only have engine stall as described when engine is cold ; with the engine warmed up I still see the rpm dropping very low but it's coming back to idle at 900rpm right away.

So it's seems the substitute transducer improves the behavior because it authorizes the fuel to come back sooner (at 1500 instead of 1350).

I would therefore tend to blame the solenoid (unless an MFI pump too lean could induce this issue but I doubt it) ?