My idle drops

[UR2H]

I have a 77 280z with an unmodified F54 block and N47 head. I get right around 17 Hg vacuum at idle (~800 RPM) and remains relatively steady (tested the vacuum at the intake manifold at the brake booster hose). There is very slight fluctuation in the vacuum between 1 maybe 2 units but it seems to follow the idle, the idle isn't perfectly smooth but not that bad.

So my problem is, after I rev the engine up, the RPM's when returning to idle, will drop down to ~500, stutter for a second then climb back up to ~800 RPM. It does this after I drop the clutch when I'm in gear driving as well, like when I'm coming to a stop. The engine has never stalled but it does feels like it's right on the verge every time.

Anyone have an idea why it does this?

[Willoughby Z]

Could be the BCDD. An emissions device, it would hold the RPMs on throttle-off, then drop the idle.

[rcb280z]

What Willoughby said....make sure you have good vacuum to it and a clean/tight bullet connector. I'm assuming this just started happening and the car ran fine before?

[UR2H]

Sorry but what's a BCDD?

I know I replaced my air regulator not too long ago because my start up idle was really low.

[UR2H]

Ahhh the BCDD (Boost Controlled Deceleration Device) is the anti stall dash pot right?

[Willoughby Z]

Boost controlled deceleration device. It's located under the throttle body. It can be cleaned internally with carb cleaner.

As Robert asked, was there anything else done, other than the AAR being replaced? The AAR throws a little air into the throttle body ( 10 seconds?) when the Cold Start Valve injects fuel- ambient temperature dependent. When it's cold outside, determined by the coolant temp sensor, the CSV injects fuel to enrichen the mixture and the AAR adds extra air for a higher idle. Once the engine/coolant warms up, the idle settles down to the Idle Screw setting.

[Willoughby Z]

Ahhh the BCDD (Boost Controlled Deceleration Device) is the anti stall dash pot right?

No. The dash pot has a similar cushioning effect on the throttle linkage but more so to prevent the throttle from slamming shut.

[Willoughby Z]

Another thought. The 77 has a fuel pump switch in the AFM that maybe is just getting opened when the counter-weight comes towards the home/rest position on deceleration, momentarily cutting power to the fuel pump. Perhaps not long enough to starve the engine and cause a stall but long enough to lower the RPMs. Easy enough to check... carefully pop the black plastic cover off the AFM and watch the actions of the counter-weight, the rod underneath and the way it interacts with the actuator/ contacts of the fuel switch when revving/idle return.

Edited April 6, 2012 by Willoughby Z

[UR2H]

Boost controlled deceleration device. It's located under the throttle body. It can be cleaned internally with carb cleaner.

As Robert asked, was there anything else done, other than the AAR being replaced? The AAR throws a little air into the throttle body ( 10 seconds?) when the Cold Start Valve injects fuel- ambient temperature dependent. When it's cold outside, determined by the coolant temp sensor, the CSV injects fuel to enrichen the mixture and the AAR adds extra air for a higher idle. Once the engine/coolant warms up, the idle settles down to the Idle Screw setting.

I have replaced all the vacuum hoses and fuel hoses.

How can I clean it internally? Spray directly into vacuum inlet?

Can brake cleaner be used as carb cleaner?

[UR2H]

Another thought. The 77 has a fuel pump switch in the AFM that maybe is just getting opened when the counter-weight comes towards the home/rest position on deceleration, momentarily cutting power to the fuel pump. Perhaps not long enough to starve the engine and cause a stall but long enough to lower the RPMs.

I had my AFM adjusted by a Z mechanic in my area. He does excellent work, so I'm guessing/hoping my AFM is set correctly. I have seen a couple of times, mainly for emergency problems otherwise I can't really afford him.

[Willoughby Z]

I had my AFM adjusted by a Z mechanic in my area. He does excellent work, so I'm guessing/hoping my AFM is set correctly. I have seen a couple of times, mainly for emergency problems otherwise I can't really afford him.

Was the work performed because of this problem? Did it occur after any "adjusting"?

I'd still check the AFM. Like I said, it will easy to see if that's the problem. If it isn't , you can move on.

I have replaced all the vacuum hoses and fuel hoses.

How can I clean it internally? Spray directly into vacuum inlet?

Can brake cleaner be used as carb cleaner?

No. Carb leaner for fuel systems and brake cleaner for brakes, IMO. Use the FSM to locate the the BCDD and find the hose underneath .

[UR2H]

I just went out to check the BCDD.

Bullet connector was free of corrosion and Vacuum line is new.

I'll get some carb cleaner and see if that works. Thanks for the advice.

It's been quite a while since it's been like this. I can't recall for sure if it was doing this before the AFM was adjusted.

Edited April 6, 2012 by UR2H