Excellent information as always, gents.  I'll investigate.  Thanks.

Looks like I'll be going through the BCDD checks.  I was hoping for the more simple cleaning of the throttle body.

Just turn the nut on the bottom a half a turn either way and drive.  It's been a few years but I'm thinking counter clockwise brings the high rpms down. Google it, "BCDD adjust 280z"

So, I'm guessing from your implication of the above... You pulled up on the gas pedal with your toes and the hanging RPM's did not drop off?

It's not usually necessary to adjust the BCDD.  Just saying.  Here's one last thing to check before you get in to that.

You said the dashpot seems slow.  I'd mark where the dashpot is set now and screw it back out to where it has little effect.  If the BCDD is bad, the problem will still be there.  If not, the problem will go away and you might have a new one.  The dashpots I've had have been creaky and slow and worn and sticky.  They did need adjustment.

Correct, the gas pedal returns fully and pulling up with the toes doesn't move the pedal or drop the RPM.  I'll do the FSM checks on it and also try Zed's suggestion on the dashpot.  Wonder why it only hangs when driving, not when sitting in the garage revving?

On a good note, the rest of the system is checking out well.  I'm taking the advice to clean and check electrical and vacuum connections.  It's going slowly because I'm wimpy and don't like the cold garage.  While in the vicinity I checked the cold start valve, coolant temp and thermotime switches and they pass the tests.  I will probably pull the throttle body to clean it regardless of the result of BCDD checks.

This sludge that comes from the tailpipe upon initial startup is perplexing.  I've got a pretty serious oil burning issue too which I'm thinking is the cause of this.  See my garage door from the one time I had it down when starting.

Edited January 22, 20178 yr by mentalite

That looks like moisture splattering out the tailpipe.  Mine do that too, my garage doors look the same.  Maybe the cold exhaust pipes and the hot exhaust going through them create the moisture.

Rub your finger across it and see if it's oily feeling.

A potential answer as to why it hangs only while driving is that the BCDD is not active when the car is sitting still. There is a speed sensing defeat mechanism built into the speedometer that disables the BCDD below 10 mph (or in P or N for the auto). When the car is sitting still the BCDD should be disabled and have no effect. But when you're out on the road above 10 mph, and the BCDD is allowed to operate, the engine speed can hang.

Take a look at the BCDD operation starting on page EC-5 of the 77 FSM.

There is a single wire leading to the BCDD... Power on that wire defeats the BCDD. No power on that wire allows the BCDD to operate. As a crude test, you can pull that wire off (which will enable the BCDD) and see if the idle hangs even when sitting still.

As for the black water coming out of the tailpipe... Water is a normal product of combustion. I wouldn't worry about that now. Probably completely normal.

Thanks Captain, I was wondering that myself.  Mine would rev and then fall back down nicely in the garage.  When I went for my first drive and came up on the first STOP sign I pushed the clutch in, it stayed at 1,500RPM for quite awhile it seemed to me but I was scared sh*tless when it happened and turned the key off immediately.  Then I read about the BCDD and that's the moment I said   "We gotta dig up DOM." then adjust the BCDD.

My past problem only happened after the engine got warm.  Apparently there was enough clearance for the blade to close until things got hot.  Took me quite a while, messing with springs and poking at the linkage while it was cold (because working on a hot engine in the summer heat sucks), to realize that it was a hot engine problem, that had nothing to do with controls that should be affected by heat.

Just wanted to repeat that.  Finding repeatable problem conditions can be hard but finding them makes the solution easier.  Beside that, working on the BCDD is a pain, from the FSM description.  You're supposed to get the car up on blocks and run the car in gear and measure things.  Better to have a friend ride in the engine bay.

16 minutes ago, Zed Head said:

My past problem only happened after the engine got warm.  Apparently there was enough clearance for the blade to close until things got hot.  

Do you think the blades stick worse when the carbon build up is hot and stickier?  But if stuff expands when it gets hot that would explain your theory. 

My BCDD was easy to adjust, half a turn one way made it worse at a stop sign.  Got out and turned it a whole turn the opposite way and it was fine.  I get lucky sometimes. 

We need a Mechanic Monkey to ride under the hood, like Cartman's " Hooked on Monkey Fonics". 

5 hours ago, Zed Head said:

You're supposed to get the car up on blocks and run the car in gear and measure things.  Better to have a friend ride in the engine bay.

My wife volunteered.  The hard part was sticking my head out the window to see around the hood.