1977 280Z with 81-83 engine, persistent 2000-2500rpm high idle

[siteunseen]

I know how I am but not too sure about these other jokers but I won't click on a link to a photo. Please don't take that wrong but I think you'd be better off using the forums tools to upload photos or copy and paste them. When you reply look at the bottom left for the paper clip and CLICK TO CHOOSE FILES. it's pretty easy and you'll get more views when the pics are on the post instead of clicking off somewhere else. Just a friendly fyi. 

I spend way too much time on this forum.

I really like the folks on here and they know EVERYTHING. 

[interestingusername]

Sure thing! Uploading images elsewhere is just a habit I've grown used to, but I'll start attaching the images from now on.

I'll definitely give that thread a read, thank you!

[siteunseen]

I just watched this and it has some good info on different things on the throttle body.  Your dashpot plunger might be holding the throttle at a high RPM, I forgot about that thing.  It's for manual transmission cars only and keeps the throttle from dropping too much between shifts.  Mine works good after I adjusted it but I don't remember how I did that. 

The hole I was thinking about that is associated with the BCDD is the vacuum advance.  The video shows the slit in the throat of the throttle body but also shows how he connected the vacuum advance to another ported source for timing advance.

Here are those tech tips that have helped so many of us, especially me cause I'ma sorta stupid.   If you'll click on the EFI and FUEL section there's the gold mine.

http://atlanticz.ca/zclub/techtips/

I'm asking you to click on a link now!  Trust me though, this is a good one.

Cliff

 

[Captain Obvious]

Yup, I think some (or most) of the high idle issue is due to the BCDD blockoff method.

I've never messed with the ZX under the manifold style, but as Zed Head mentioned, you might be able to take off that last other plate that's under there and then maybe use a simple plate to block off the holes. I don't know what (if anything) else that remaining slab does, but you would definitely have to cap the hose connections that go to it if you removed it.

That option doesn't exist on the earlier design with the BCDD on the underside of the throttle body. There's no way to ever use just a simple plate because the vacuum ports and valve seats are integral to the throttle body casting itself. You might have a simpler option with the later version you have.

Good luck. And here's to hoping solving the problem is as easy as it seems at this point! 

[Zed Head]

The big hose is T'ed with the idle speed control air bypass. So a simple plug at the end of the hose or at the mechanism itself should work .  Looks like the bypass feeds directly in to the intake manifold.  Follow the air flow path from one side of the TB, through the screw controlled idle controller, and to the intake manifold to be sure.  You need to be aware though, that removing the BCDD will bring back the original problem it was meant to fix.  You'll have a gassy smelling car if you coast to a stop while in gear.  You might end up wishing you had fixed it.

The 81-83 ZX's have a mass of vacuum controlled devices that can leak.  Here's another.  Which brings up another reason for idle speed to increase - ignition timing.  You might have a sticky vacuum advance mechanism n your distributor.  Pretty common.

There are are also thermally controlled vacuum valves placed around the engine.  Can't remember how many.

 

[interestingusername]

12 hours ago, siteunseen said:

I just watched this and it has some good info on different things on the throttle body.  Your dashpot plunger might be holding the throttle at a high RPM, I forgot about that thing.  It's for manual transmission cars only and keeps the throttle from dropping too much between shifts.  Mine works good after I adjusted it but I don't remember how I did that. 

The hole I was thinking about that is associated with the BCDD is the vacuum advance.  The video shows the slit in the throat of the throttle body but also shows how he connected the vacuum advance to another ported source for timing advance.

Here are those tech tips that have helped so many of us, especially me cause I'ma sorta stupid.   If you'll click on the EFI and FUEL section there's the gold mine.

http://atlanticz.ca/zclub/techtips/

I'm asking you to click on a link now!  Trust me though, this is a good one.

Cliff

The engine in my car may have been from an automatic, or the dashpot has been removed, because it doesn't appear I have one. If the engine was for an automatic, would the throttle body have provisions to add one? I'll give the video a look, thank you!

And Atlanticz has actually been really helpful for me while I've been working on this car! Quite a few things on there have been lifesavers for me.

10 hours ago, Zed Head said:

The big hose is T'ed with the idle speed control air bypass. So a simple plug at the end of the hose or at the mechanism itself should work .  Looks like the bypass feeds directly in to the intake manifold.  Follow the air flow path from one side of the TB, through the screw controlled idle controller, and to the intake manifold to be sure.  You need to be aware though, that removing the BCDD will bring back the original problem it was meant to fix.  You'll have a gassy smelling car if you coast to a stop while in gear.  You might end up wishing you had fixed it.

The 81-83 ZX's have a mass of vacuum controlled devices that can leak.  Here's another.  Which brings up another reason for idle speed to increase - ignition timing.  You might have a sticky vacuum advance mechanism n your distributor.  Pretty common.

There are are also thermally controlled vacuum valves placed around the engine.  Can't remember how many.

If I have my way, I'll eventually re-attach the BCDD. I think the one I removed may be able to be cleaned up, even. It just seemed like removing it was the easiest place to start with eliminating my idle issue(go figure that removing it the way I did may have exasperated it, but you learn!)

I believe I know where that particular vacuum device is, I'll give it and the vacuum advance a check next time I'm by the car. Thank you for the suggestions!
 

11 hours ago, Captain Obvious said:

Yup, I think some (or most) of the high idle issue is due to the BCDD blockoff method.

I've never messed with the ZX under the manifold style, but as Zed Head mentioned, you might be able to take off that last other plate that's under there and then maybe use a simple plate to block off the holes. I don't know what (if anything) else that remaining slab does, but you would definitely have to cap the hose connections that go to it if you removed it.

That option doesn't exist on the earlier design with the BCDD on the underside of the throttle body. There's no way to ever use just a simple plate because the vacuum ports and valve seats are integral to the throttle body casting itself. You might have a simpler option with the later version you have.

Good luck. And here's to hoping solving the problem is as easy as it seems at this point! 

Just for the sake of reference, I removed & shot some photos of the piece that connects to the manifold, and where it connects. There's also a gasket shaped like the manifold-side of this assembly. I think I'm going to try making a blockoff plate when I can(hopefully soon) for this piece, capping the leftover hose connection and seeing where that gets me. I'll give ya'll an update when I do so.

Thank you guys for the help!

Edited October 20, 2017 by interestingusername

[Captain Obvious]

Yup. The way you had it (not) blocked off before was definitely a problem. If you were to blow into the nipple on that last block you took off, that air would come right out the center hole back atcha. It's not blocked off.

And yes, now that you have taken that other block piece off, you should be able to slap a simple plate across the hole on the underside of the intake manifold to block the flow. Remove the hose and cap the nipple that used to feed the BCDD and you should be on to the next issue.

If you are thinking you want to put the BCDD back on at some point, here's a recent thread that talked a bunch about the guts inside. Might help with getting yours back into usable condition:

http://www.classiczcars.com/topic/58748-info-on-bcdd-boost-controlled-deceleration-device/

I've never messed with one, but I'm assuming the basic design for the newer one you have is very similar to the older ones pictured in that thread. I suspect yours is a little simpler and doesn't have an altitude corrector bellows on the underside (unless your car came from CA).

Good luck with the rest of the project!  

[interestingusername]

I got the chance to make a blockoff plate for this other piece of the BCDD, and the idle problem has been mostly resolved! With the air regulator disabled(I'll be reconnecting this when I button the car back up), the car starts cold at about 900-1000RPM. The car still has a little bit of a high idle when its warmed up, it sits at around 1300-1400RPM, but I'm hoping this can be lowered with a bit of tuning up. There's still the ECU issue, I haven't had the opportunity to check the timing and forgot to test out the vacuum advance like a genius.

Of course, after messing around in that area so much, the rubber coolant hose going to the air regulator has sprung a leak, but that's how these things go.

Thank you all again for the help!

Edited October 22, 2017 by interestingusername

[Captain Obvious]

Good news on the extremely high idle situation.

So, now as for the current idle behavior where it starts out cold at 900-1000RPM and increases as it warms up to 1300-1400RPM? That is to be expected. Engines don't ever really like to idle, and cold engines like it even less. It's normal for the idle RPM to increase a couple hundred from stone cold to full warm, and without additional devices (like the air regulator), it's impossible to simply "tune" out. The engine will always idle somewhat higher once warm.

In fact, it's the air regulators job to mitigate this situation. It's job is to apply a compensation bump to the cold idle to force it up a couple hundred RPM. The air regulator then gradually reduces this compensation bump as the engine warms up. In theory, once the engine is warm and the air regulator is no longer adding any compensation bump, the idle speed should be where you want it.

So starting backwards... If your warm idle is where you want it to be,
and you remove the air regulator.
your cold idle will be too low.

and in converse... If your cold idle is where you want it to be,
and you have removed the air regulator,
your warm idle will be too high.

Haha!! Does that make any sense at all? 

[interestingusername]

Nah, that makes sense! I'm so used to modern ECU controlled idles that when my cars idle speed started going up, I thought it was unusual, so I suppose that's up to ignorance on my part. I'm glad to hear that's normal, though!

[Captain Obvious]

Yeah, that's what I figured. You need to remember (or be told about) the "good old days" back when cars didn't do that compensation for you. The 280's were in a "mid-period" where they actually tried to mitigate that situation for you through the use of the auxiliary air regulator and take some of the manual drudgery out of your hands. Before that (with the 240 and 260), you manually used the choke lever yourself to modulate the cold idle speed.

Consider it one of the endearing features of driving such a cool car.   

Now, you just jump in your Civic or your Sentra, turn the key, and pull out of the driveway before the starter has even wound down to a complete stop. It's all computer controlled.

Thank you Moore's Law!!

[siteunseen]

On 10/22/2017 at 2:04 PM, interestingusername said:

Of course, after messing around in that area so much, the rubber coolant hose going to the air regulator has sprung a leak, but that's how these things go.

I was thinking you yanked that coolant line?  The one that goes around the front of the head and to the thermostat housing?  Maybe that was someone else.

It goes to a warming plate under the AAR.  I took mine off, along with the thermal vacuum switch that's connected to it.  That is where the vacuum line goes coming off the BCDD if I remember right.  But then you open another can of worms with the BPT and EGR.