Hanging RPM

[Glliw]

So, I have a problem that has been plaguing me and have done a fair amount of googling and searching on here and other Z sites and haven't found the bucket to stick my problem into to troubleshoot.

Please see the video below.

Here is a description and some details:

The car's idle speed will "hang" after acceleration. For instance, accelerate through a gear up to 3k,4k, etc rpm. Push in the clutch and you would expect rpms to rather expediently hit idle, but here it drops from the initial high to

2.5k rpm rather quick then takes 10-20 seconds to cover the next 1600 rpm back to idle.

1978 280z

Deleted AAR and the coolant plate under it

A/C idle increase arm nippled off

EGR nippled off

added helper spring to throttle body return with little effect. Helps come down from high speed but will not help under 2.5k rpm

It seems to only do this when warm.

Any ideas where to look?

[siteunseen]

BCDD under the throttle body. Unplug and see if it quits. Small black wire off the BCDD butt connects into a small white wire going into the wiring harness.

B boost C controlled D deceleration D device

If you deleted the EGR and BPT there's a vacuum hose on the side of the BCDD that needs to be plugged with a small bolt, circled in yellow in the picture.

Edited April 18, 2014 by siteunseen

[Glliw]

I plugged that vac tap that runs up to the BPT. I'll check unplugging it, thanks.

[siteunseen]

Hey I forgot about a possibility. The TPS, little black box above the BCDD attached to the side of the throttle body. You could pop the cover off and see if 1 of the contacts may be stuck or unplug it and see what happens too. I didn't know you could do that until I read Blue suggest that to somebody. Good luck, I'm just throwing stuff out there.

[Zed Head]

Do you still have the throttle linkage dash-pot? They get worn and sticky. The throttle blades get sticky in the bore also due to PCV backwash. Carb cleaner around the blade will clean it up.

[Glliw]

Update from testing:

Unplugging BCDD had no effect

Unplugging TPS caused idle to hang the same throughout RPM range. ie, no hard drop to 2.5k followed by slow descent. Only slow descent the whole way. Car ran like poo with it unplugged, of course.

I do not have the dashpot in place. That was the first thing I eliminated when I started looking into this.

[mgood]

Do you have the round gold colored item above the red circle in siteunseen picture? If you do this could be your problem. It screws out and in to help the throttle return slower when shifting.

After I cleaned everything on the top of my engine, I had the same problem the rpm would not lower fast enough and that item was the problem, mine was too far down. I loosened the nut and screwed it out and that did the trick.

[Glliw]

Mgood, nope its not on the car.

[mgood]

Glliw can you post a good picture of the top area of the engine?

[djwarner]

It would help to tell us what car/engine/modifications you have. If you have a carb'd 260Z, there is a dash pot that prevents the throttle linkage from closing too quickly, say when blipping the throttle. If this is hanging up, it could cause your problem.

[Glliw]

Glliw can you post a good picture of the top area of the engine?

I'll take one here in a bit.

It would help to tell us what car/engine/modifications you have. If you have a carb'd 260Z, there is a dash pot that prevents the throttle linkage from closing too quickly, say when blipping the throttle. If this is hanging up, it could cause your problem.

The car is in the first post. There isn't a mods list because there aren't any, except for the vaccum deletes whilst troubleshooting this unless you think the front air dam affected this.

[rcb280z]

Make sure throttle return spring is still attached. Just throwing things out there.