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.

[Java2570]

I think typically the decel valve starts leaking and won't hold vacuum. You can test them with a Mity Vac and see if it leaks or not.....
Lots of folks don't even run the decel valves on their cars; why not just unhook it, plug the lines and see if the issue is the same?

[Java2570]

I'd look at fuel pressure, possible fuel delivery issues, or misadjusted TPS.....

This is a great resource: http://members.rennlist.com/pbanders/djetparts.htm

[TheCabinetmaker]

I agree. Remove the hoses and plug them. Take the valve out of the equation.

[76-914]

What's your idle rpm when it's cold and before it warms to stall condition?

[r_towle]

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.

[boxsterfan]

Thanks everyone. I will troubleshoot this weekend. (IMG:style_emoticons/default/smash.gif)

[boxsterfan]

What's your idle rpm when it's cold and before it warms to stall condition?

Idle RPM (eyeballing it) is 800-900 at cold but it is rough (drops down a little and then back to 800-900.

After warm-up, seems to idle very well.

Then after a while (and getting warmed up even further?), I then experience these issues where the car is stalling (Example: Driving 45 Mph, approaching stop-sign, let off gas, depress clutch. Engine drops to zero (and not just a bouncy gauge). Idle can be rough like when cold (or worse) and it will stall if you don't give more than usual gas when taking off again.

[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

[76-914]

(IMG:style_emoticons/default/agree.gif)

[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.

[DRPHIL914]

[quote name='worn' date='Oct 10 2013, 01:33 PM' post='1938655']
[/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.
[/quote]
i agree with most of what was just said above, and i would look closer at the things that we just discussed first, but like he said you can take a known working MPS from someone else, plug it in and see it there is any difference. - my last MPS that crapped out ran rich which is what their default is, but i did have one that had not been messed with that was way lean too. PB Anders explains how these things work and how they can be adjusted. but to set them to the factory setting after rebuilding one , you need the proper meter - i know several people that have these that can callibrate them to the factory setting. And yes, if you have a wideband A/F meter you can dial them in for your car but don't go there now.
I guess i like to tinker with these things, and now that i have some rebuild kits from Chris at Tangerine, i will be rebuilding a few more and dialing them in.
have fun! part of the fun of having these cars i think is having figuring this stuff out, its why i bought my 914,
- keep us posted on what you find.

[worn]

I guess i like to tinker with these things, and now that i have some rebuild kits from Chris at Tangerine, i will be rebuilding a few more and dialing them in.
have fun! part of the fun of having these cars i think is having figuring this stuff out, its why i bought my 914,
- keep us posted on what you find.

I do too, and someday I am going to experiment with a type 4 VW MPS I bought, but I cannot recommend MPS adjustment for someone at the beginning end of the D-Jet adventure. Again, the epoxy plug should be intact and it should hold vacuum from a mighty vac. Sucking can prove a bad one is bad if there is a big crack, but if the crack is more subtle, you won't be able to tell.

[DRPHIL914]

I guess i like to tinker with these things, and now that i have some rebuild kits from Chris at Tangerine, i will be rebuilding a few more and dialing them in.
have fun! part of the fun of having these cars i think is having figuring this stuff out, its why i bought my 914,
- keep us posted on what you find.

I do too, and someday I am going to experiment with a type 4 VW MPS I bought, but I cannot recommend MPS adjustment for someone at the beginning end of the D-Jet adventure. Again, the epoxy plug should be intact and it should hold vacuum from a mighty vac. Sucking can prove a bad one is bad if there is a big crack, but if the crack is more subtle, you won't be able to tell.

(IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/beerchug.gif)

[rick 918-S]

I just fired up a rebuilt 2.0 I took it for a drive and it was doing the exact same thing plus a rich condition. Everything is new. Petronix, cap, rotor, wires, vacuum hoses, etc. I tested the MAPS with my mity vac and it tested good. Next I installed a vacuum gauge. The vacuum was low. Like 5 psi. I loosed the dizzy and turned it until the vacuum rose to 15 ish and stayed steady. The engine now runs nice.

Be sure all your tune up parts are good first then do the basics.

[r_towle]

I just fired up a rebuilt 2.0 I took it for a drive and it was doing the exact same thing plus a rich condition. Everything is new. Petronix, cap, rotor, wires, vacuum hoses, etc. I tested the MAPS with my mity vac and it tested good. Next I installed a vacuum gauge. The vacuum was low. Like 5 psi. I loosed the dizzy and turned it until the vacuum rose to 15 ish and stayed steady. The engine now runs nice.

Be sure all your tune up parts are good first then do the basics.

Did you set the dwell with a dwell meter and the timing with a timing light?
And then changed it with the vacuum gauge?

If so, what is your timing set at now?

[boxsterfan]

I just fired up a rebuilt 2.0 I took it for a drive and it was doing the exact same thing plus a rich condition. Everything is new. Petronix, cap, rotor, wires, vacuum hoses, etc. I tested the MAPS with my mity vac and it tested good. Next I installed a vacuum gauge. The vacuum was low. Like 5 psi. I loosed the dizzy and turned it until the vacuum rose to 15 ish and stayed steady. The engine now runs nice.

Be sure all your tune up parts are good first then do the basics.

Where do you plugin for the vacuum when doing that adjustment?