Kjet Idle Stall

I have been having this problem with my 81 B21f since I bought it about a year ago. When I first start her up, she usually idles pretty low and rough and I have to keep on the throttle some to keep her alive. After warming up for a bit drives perfectly fine. There have been a few times I have been sitting at a stoplight and suddenly start to stall and I have to give her some gas, but this is not consistent.

Usually it goes something like this: I turn the key, she cranks a little long but not too much. She starts up strong, idles well for maybe 3-5 seconds before suddenly dropping way down. She will idle low and lope unless I give her gas, sometimes for a minute, sometimes for 2 or 3. Eventually the loping idle ends, the idle rises back up and she idles strong after that. If I jam the throttle while sitting there at any point during this process it almost guarantees either a near or total stall.

I checked the fuel pressure today and everything looked good. About 50 psi control pressure on a 93* night and around 72 or so system pressure. I have changed the terrible wiring harness out for a DB one last year.

Does this ring any bells for anyone? Is it probably just a vacuum leak I am somehow still missing? Or something else? I'm at a loss.

I would say yes vacuum leaks are a concern. Put a smoke machine on it

Sounds similar to what I was experiencing in my 76. I would start to stall at red lights, but only after it had warmed up. I ending up having a couple vacuum lines routed wrong, one off of the distributor and another off the intake. Fixed the lines and it fixed the stalling issue and now idles much better. I would check all of your vacuum lines against a diagram and make sure everything is in good shape and where it is supposed to be.

I should add I also had other vacuum leaks, like the little rubber o ring on the black turn knob on the intake, manifold gasket etc.. replacing these things and fixing my vacuum lines seemed to resolve my issue.

I was having the same issue, I recently removed my engine and found the intake gasket to be junk, haven?t reinstalled the engine to see if that fixed it but it?s the only possible thing in my case. Check your vacuum lines and all as well. The vacuum line for the vent control can sometimes break right at the firewall pass through but won?t fall

In addition to the vacuum leak checks and hose routing. You also want to check the function of the aux air valve on the valve cover. It's what gives you the cold high idle then the valve closes and your idle falls back to normal.

When you checked your control pressure was the engine cold? If you had 50psi cold control pressure. That is much too high and your engine is leaned out when cold. With a cold engine your control pressure should be around 29psi for example and while watching the engine warm up it should slowly rise. With a fully warmed up engine 50psi is ok as control pressure.

Think of it as a choke that gradually opens up as the engine warms to temperature. That's why it's also called the warmup regulator. It's a very important function. If it's not low on startup the unit could be clogged.

dl242gt said:

In addition to the vacuum leak checks and hose routing. You also want to check the function of the aux air valve on the valve cover. It's what gives you the cold high idle then the valve closes and your idle falls back to normal.

When you checked your control pressure was the engine cold? If you had 50psi cold control pressure. That is much too high and your engine is leaned out when cold. With a cold engine your control pressure should be around 29psi for example and while watching the engine warm up it should slowly rise. With a fully warmed up engine 50psi is ok as control pressure.

Think of it as a choke that gradually opens up as the engine warms to temperature. That's why it's also called the warmup regulator. It's a very important function. If it's not low on startup the unit could be clogged.

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So I did clean out the AA Valve last summer, so that ought to be fine unless they are known to get clogged up quickly. When I first started the car the fuel pressure started around 29psi like you said and it rose to 50 wear it stayed over 2 or 3 minutes, so that seems like it is working correctly. I will try to identify any wrong hose routing. I checked the stuff around the fuel pressure regulator because I saw a couple of posts where that stuff was incorrectly routed and it all looked correct to me. I am trying to track down a smoke test at the moment and will report back if I don't end up finding any obvious vacuum leaks.

Radtap said:

I was having the same issue, I recently removed my engine and found the intake gasket to be junk, haven?t reinstalled the engine to see if that fixed it but it?s the only possible thing in my case. Check your vacuum lines and all as well. The vacuum line for the vent control can sometimes break right at the firewall pass through but won?t fall

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I replaced my intake gasket shortly after getting the car when I did the wiring harness, also did a new temp sensor and cleaned the flame trap at the same time since all that stuff was easy to get to with the intake removed.

rearwheelpeel said:

Sounds similar to what I was experiencing in my 76. I would start to stall at red lights, but only after it had warmed up. I ending up having a couple vacuum lines routed wrong, one off of the distributor and another off the intake. Fixed the lines and it fixed the stalling issue and now idles much better. I would check all of your vacuum lines against a diagram and make sure everything is in good shape and where it is supposed to be.

I should add I also had other vacuum leaks, like the little rubber o ring on the black turn knob on the intake, manifold gasket etc.. replacing these things and fixing my vacuum lines seemed to resolve my issue.

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Did you just use the vacuum diagrams available on DB's website or did you find some more helpful ones elsewhere? I hadn't thought of a gasket on that screw, that's a good idea for me to check. Somebody had taped that thing up and perhaps that was to account for a leak there. I have fiddled with that thing in the past so maybe that allowed a leak to form.

Yes, I agree your control pressure is working correctly. If all else fails and you can't get the idle higher. You may need to reset the throttle plate and then set the idle speed with the thumbscrew. Vacuum leaks usually raise the idle unless they are really massive leaks which usually affects the engine more than just a low idle.

Edit:A sneaky vacuum leak can be the boot between the throttle body and the air flow sensor. It can curl up and not seal and I've had the rubber kind of warp and take a bad shape from being old which caused a massive air leak.

Injector seals, intake gasket, vacuum hoses not leaking and correctly routed usually makes these smooth old engines.

dl242gt said:

Yes, I agree your control pressure is working correctly. If all else fails and you can't get the idle higher. You may need to reset the throttle plate and then set the idle speed with the thumbscrew. Vacuum leaks usually raise the idle unless they are really massive leaks which usually affects the engine more than just a low idle.

Edit:A sneaky vacuum leak can be the boot between the throttle body and the air flow sensor. It can curl up and not seal and I've had the rubber kind of warp and take a bad shape from being old which caused a massive air leak.

Injector seals, intake gasket, vacuum hoses not leaking and correctly routed usually makes these smooth old engines.

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That boot was actually the source of my first gigantic vacuum leak. The rubber was old and rigid and had cracked down the side. After replacing that I wondered if maybe a new leak had formed now that the one letting out all the pressure had been sealed. Might be time to take a look at my injectors, as that is one thing I haven't touched.

QisofrKuin said:

Did you just use the vacuum diagrams available on DB's website or did you find some more helpful ones elsewhere? I hadn't thought of a gasket on that screw, that's a good idea for me to check. Somebody had taped that thing up and perhaps that was to account for a leak there. I have fiddled with that thing in the past so maybe that allowed a leak to form.

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Yes I used the diagrams on Dave?s website, they are the same as in the greenbooks I have. The o ring gasket for the black plastic screw is the same o ring used on the oil dipstick. When I removed my old (original) o ring it crumbled apart like clay.