Weird hanging RPM issue

[Pochie45566]

1978 280z stock EFI 

I'm having a weird issue that when the engine is revved, the rpms do not go down once the gas pedal is released, the RPM's kind of just stay where they are, around 3k. I am posting because I have gone through the forums and checked off a lot of boxes, the BCCD has been unplugged, could it still be leaking air? after testing with a vacuum gauge I think that there isn't a vacuum leak, that would need to be an obvious leak also. The throttle linkage is working smoothly and the throttle plate is closing. Im just trying to think of what else could have this effect? I have an AFR gauge hooked up and it is running at about 15 when at idle and stays around there when the rpms are hanging, which I guess is quite lean. Is it possibly an EFI thing? But it needs the extra air to Rev higher? Im just trying to figure out where that is coming from if the throttle plate is closed. Anyway, I would love some ideas on this issue. Thanks.

[Zed Head]

24 minutes ago, Pochie45566 said:

1978 280z stock EFI 

 the BCCD has been unplugged, could it still be leaking air?

Yes, it's possible.  

How long does it take for the idle speed to drop back down?  Or, what are you doing to make it drop?

[Pochie45566]

16 minutes ago, Zed Head said:

Yes, it's possible.  

How long does it take for the idle speed to drop back down?  Or, what are you doing to make it drop?

since I was driving home when it happened, I just had to keep the car in gear and put my foot on the brake. It was pretty horrible but luckily I was about a quarter mile from home. the idle doesn't drop down by itself at all

[Zed Head]

So, what you're saying is that the engine immediately revs to 3000 RPM when you start it?  Your first post makes it sound like something that happens often, but your last one says it only happened once.  Not sure what's going on.  How do you get the idle to drop down?

Look for a split vacuum hose.

[Pochie45566]

4 minutes ago, Zed Head said:

So, what you're saying is that the engine immediately revs to 3000 RPM when you start it?  Your first post makes it sound like something that happens often, but your last one says it only happened once.  Not sure what's going on.  How do you get the idle to drop down?

Look for a split vacuum hose.

Sorry for the confusing language, im not very good at describing these issues lol. When I start the car the idle is at normal, but once I press the gas pedal the rpm's will not come back down, they will stay up until I turn off the car at a stoplight or something. I dont see any split vacuum hoses and they have all been replaced in the last couple years. 

[Captain Obvious]

When you disconnect the BCDD, it becomes enabled. That means yours is currently enabled full time, even when the car is sitting still. So it's doubtful that is what's causing the issue.

It really sounds like the throttle is hanging up mechanically. You said that the throttle linkage is working smoothly and the throttle plate is closing. How do you know? Have you manually grabbed the throttle linkage right at the throttle body and pulled it up to make double dog sure the plate is closed?  I've had issues with that in the past.

[Pochie45566]

1 minute ago, Captain Obvious said:

When you disconnect the BCDD, it becomes enabled. That means yours is currently enabled full time, even when the car is sitting still. So it's doubtful that is what's causing the issue.

It really sounds like the throttle is hanging up mechanically. You said that the throttle linkage is working smoothly and the throttle plate is closing. How do you know? Have you manually grabbed the throttle linkage right at the throttle body and pulled it up to make double dog sure the plate is closed?  I've had issues with that in the past.

yeah, I made sure that the whole linkage was working, I could take the throttle body out to make sure but the linkage has full range of motion and the spring is working well. It sounds like the linkage is sticking to me also but I don't see how that could be happening. I have tried pushing the throttle linkage back down while the rpm's are hanging and it does nothing so it seems like it isn't throttle related. BCDD developed an internal vacuum leak maybe?

[Zed Head]

CO, didn't you do some thought-work in the past about the pressure differential across the throttle blade and how it tends to open the throttle blade if there's no spring force holding it closed?

Might just be that the main throttle spring broke.  The one in the throttle body itself will close it with no air flow, but it's weak.

[Pochie45566]

2 minutes ago, Zed Head said:

CO, didn't you do some thought-work in the past about the pressure differential across the throttle blade and how it tends to open the throttle blade if there's no spring force holding it closed?

Might just be that the main throttle spring broke.  The one in the throttle body itself will close it with no air flow, but it's weak.

The spring has been replaced in the past and while examining the linkage that is attached to the throttle body I can see the whole assembly moving and closing, when the RPM is hanging the throttle plate is shut. 

[Captain Obvious]

ZH, Yes I did some of that thought work because that's the only thing I could come up with for my sticking throttle plate. I had all the springs in place, but the theory was the worn throttle shaft was sticky and a high pressure differential would pull the plate in tight which would put max pressure on the bearings (and they would stick)

4 minutes ago, Pochie45566 said:

I have tried pushing the throttle linkage back down while the rpm's are hanging and it does nothing so it seems like it isn't throttle related. BCDD developed an internal vacuum leak maybe?

Might just be a different point of reference, but when I picture the throttle body, you push down to OPEN it. Pull up to close it.

While the RPM's are hanging. reach in and pull up on the throttle linkage sticking out the side of the throttle body. Don't just look at it and believe it's closing correctly. Pull up.

What happens if you blip the throttle? Does it ever return to normal?

[Zed Head]

 

30 minutes ago, Pochie45566 said:

 When I start the car the idle is at normal, but once I press the gas pedal the rpm's will not come back down, they will stay up until I turn off the car at a stoplight or something. 

Just quoting this for reference.

Another variation on CO's last point is - pull the  gas pedal up with your toe or hand, in the cabin.  There's really nothing that gets actuated by opening the throttle that should not get de-actuated by forcing it closed.

The only other thing that I can think of that would increase idle speed and might get stuck is the vacuum advance in the distributor.  It uses ported vacuum to move the breaker plate.  But it should lose vacuum when the throttle blade is closed.

The BCDD is a weird thing though, with air passages and diaphragms that might get hung up once a pressure differential is created.  It hurts my head to study it.  All that wizardry just to keep things clean while coasting to s stop.

Not sure how you'd diagnose it.  You could find a way to block the BCDD port.  A piece of clay maybe.  Or a pierce of tape.  Should be able to reach the one in front of the blade, I think.  That's what I'd do if I had your problem and was out of ideas.  Block that BCDD air passage.

Edited September 28, 2020 by Zed Head

[Yarb]

Here’s a diagram CO provided when I had a problem after blocking off the bcdd. Plugged the orifice and solved the high idle problem.