When I reassembled my 280z, the egr system fell apart so instead of replacing the broken components, I opted to just remove it. Now that I’ve been driving the car, I noticed it holds revs and I’m thinking it is because I don’t have the vacuum line to the bcdd connected. Looking back at my photos before disassembly, looks like it was connected to the egr diaphragm for vacuum.

I also removed the charcoal canister so I’m wondering if I can just pull vacuum from that connection on my manifold? Anyone have any thoughts or suggestions?

I’ve tried taking apart the throttle linkage, cleaning everything, and lubricating before reassembly and it still hangs so I’m leaning towards the bcdd.

Cap the bccd port. That’s a direct path that bypasses the afm. IIRC

26 minutes ago, dar5052 said:

I noticed it holds revs and I’m thinking it is because I don’t have the vacuum line to the bcdd connected.

The hanging RPM's might be because of the BCDD, but it has nothing to do with that vacuum line. In fact, that vacuum connection has nothing to do with the BCDD. It LOOKS like it has something to do with the BCDD because of it's close proximity, but inside, it makes absolutely no connection to the inner workings of the BCDD at all.

There are two vacuum ports on the underside of the throttle body:
One of them is a ported vacuum source that supplies vacuum advance to the distributor under light load conditions.
The other is a ported vacuum source used to actuate the EGR system under light load conditions.

If you aren't using the EGR system, you can cap off that ported vacuum nipple, but it won't (shouldn't?) do anything to change your hanging RPMs. But if it's hanging open, you should cap it off.

I remember removing the valve and installing the plate from ZCD. No instructions of course and the idle was hanging around 1800 rpm. I think it was @siteunseen that posted there was a port inside the afm that needed to be plugged. I plugged it and it solved my issue with the plate. Of course that may not be the case here but my bcdd valve was apparently stuck open in some form or fashion.

Yeah, the BCDD can be troublesome. First thing to try would be to confirm that's the issue. If it's not all messed up inside, you can hopefully adjust to solve the problem.

A test, if you can time it right, is to open the hood and press the throttle blade closed while the idle is hanging. Even if you've cleaned everything, it's a test worth doing. Narrows the possibilities down.

Also not clear if the idle slowly drops to normal or if it's just high since you put things back together. The second might just be a vacuum leak.

I had two cases of hanging high but don't want to complicate the discussion. Yet.

@Captain Obvious wouldn't I need to pull a vacuum on the BCDD for it to function properly? At this point, I just have the vacuum line plugged off so I assumed it’s not opening/closing properly. I’m going to look in the FSM to see if there is more info on how the thing operates. I have a couple spare intake manifold laying around so I’ll take a look at those for the ports you mentioned an ensure I have the egr capped off. I do have my vacuum advance hooked up to the distributor.

@Yarb I might just do a BCDD delete as you described but I’d like to keep it if I can. I don’t have an explanation as to why….

@Zed Head i did do that… I could recreate the high idle by just pushing on the throttle linkage under the hood. When I let go, it would hold a high idle. Then I would literally pull back on the linkage ever so slightly and the rpm’s would drop. One of @Captain Obvious previous posts led me down the path of cleaning/greasing all the linkages but it didn’t really do much. That’s what lead me to the BCDD… also, if I’m driving and I notice the idle holding, I pull in the clutch, blip the throttle, and the high idle goes away… until I apply throttle again… then it’s the same thing over and over. One thing I thought about was the return spring on the throttle assembly… could that have relaxed over the years and not be applying as much force as required to bring the throttle back to zero?

A couple pics of the car so far… still have some work to do but I figured if I don’t start getting it out of the garage, it’ll never come out. Probably not everyone’s style, but it’s mine…

March 27, 2021

I finally found a picture of the hose that has to plugged when deleting the EGR and warming plate, thermal switch or something like that.

It's the yellow circle in my picture and from what I remember it's ported vacuum, before the butterfly. Anyhow just an FYI thing I said I'd do so here it is.

Sounds like a sticky throttle blade to me? Try adjusting the bcdd by turning the nut on bottom clockwise. Its got a funky clip covering the nut that just pops off.

Where are you at? Looks like TN, Ky, VA...

12 minutes ago, Patcon said:

Where are you at? Looks like TN, Ky, VA...

Yes! Beatiful land. I'll live in that barn and be a full time Z mechanic. I'm also pretty good with anything else mechanical or at least I can figure it out with the Google and occasionally youtube.

@Patcon were you asking me where I’m at? If so, it’s PA, one hour west of Philadelphia.