Possible decel valve problem, Idle likes to drop to zero after warmed up. Options

[boxsterfan]

1974 2.0L D-Jet

Once the engine gets warmed up, my engine RPM likes to drop down to zero when letting off the gas. This is not just a "gauge" bouncing issue but rather you can feel that the motor has dropped to zero (stalling).

Does this sound like a condition that can occur because of a faulty decel valve? Anyone in San Ramon, CA have a known working spare one i could test with?

Part#: U039133551
Referred to affectionately as a decel valve, this factory installed device lets the idle down slowly to more effectively limit the unburned gas exiting the tail pipe and keep the engine from stalling. mounts on the right side of the firewall just above the pressure sensor and just behind the battery and three vacuum lines attach to it.

[DRPHIL914]

Remove and plug the AAR, decel! and all other vacuum lines! including the vacuum advance to the distributor.
Leave only the MPS plugged into the plenum.

Then troubleshoot.
Do valves, dwell and timing
Check the TPS to verify it's set properly
Check fuel pressure
Check your CHT resistance cold and hot.

Then add only one vacuum system at a time
If the car runs fine without any new vacuum systems, you will find the culprit.

Tune up the car first or you will waste a lot of time.

(IMG:style_emoticons/default/agree.gif)

my 75 used to do this, and one issue was the tps not being set right, and the other was with the MPS, which was not in spec, and was way too rich, and once adjusted is now perfect, 800 should be your warmed up idle, probably should be some where around 1200-1400 cold with the AAR fully open, - maybe your AAR is not opening up properly as well, so i would check that too.

phil

[boxsterfan]

Remove and plug the AAR, decel! and all other vacuum lines! including the vacuum advance to the distributor.
Leave only the MPS plugged into the plenum.

Then troubleshoot.
Do valves, dwell and timing
Check the TPS to verify it's set properly
Check fuel pressure
Check your CHT resistance cold and hot.

Then add only one vacuum system at a time
If the car runs fine without any new vacuum systems, you will find the culprit.

Tune up the car first or you will waste a lot of time.

(IMG:style_emoticons/default/agree.gif)

my 75 used to do this, and one issue was the tps not being set right, and the other was with the MPS, which was not in spec, and was way too rich, and once adjusted is now perfect, 800 should be your warmed up idle, probably should be some where around 1200-1400 cold with the AAR fully open, - maybe your AAR is not opening up properly as well, so i would check that too.

phil

How do you adjust the MPS? I know this part is not made any more and heard that you can send in for a rebuild to board members (Bowlsby) or have AA rebuild. I don't recall seeing any knobs for adjustment on it, but I didn't look that hard.

[worn]

[/quote]

How do you adjust the MPS? I know this part is not made any more and heard that you can send in for a rebuild to board members (Bowlsby) or have AA rebuild. I don't recall seeing any knobs for adjustment on it, but I didn't look that hard.
[/quote]

Do the tests, but the car won't run without the MPS so you cannot just unplug it, like you can with some of the other components. And it may be that the diaphragm in the MPS has cracked. You can test this by pulling vacuum on it.
Do not adjust the MPS. If you find that it has been adjusted (epoxy plug removed), then a previous owner may have tried to adjust it, causing the problems you have now. I do not recommend trying to adjust it back. That is unless you have everything else tested and tuned, you understand how it works, you have installed a wideband air fuel gauge and are ready for a fairly large headache. A swap with a known good MPS would be better, and buying one is better than trying to adjust the one you have. I have collected and rebuilt several of them, and once you get them out of adjustment it is hard to find your way back IME.