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

[interestingusername]

Hopefully this is the right board for this thread, my apologies if not(also, forgive me for making my first post a "help me" post!)

I have a 1977 Z I have been working on getting running, I'm at the point where my focus is on the engine.

Unfortunately, I've hit a wall because of an issue with a constantly high idle. Sometimes the engine will start at around 2000-2200rpm and slowly climb until it settles at around 2500-2600, other times its immediately at 2600rpm. The AFM and throttle body move smoothly, and nothing is clogged/sticking inside of either. Throttle response is pretty sharp for what its worth, even at this extreme idle.

I have a few thoughts as to what it could be, but I feel like I need some fresh eyes on the problem before proceeding.

I have removed and blocked off the BCDD(it was the smaller, later model one) and this, if anything, made the car idle HIGHER. There is a three way rubber hose going to the BCDD housing (picture: https://i.imgur.com/12SzCHs.jpg) that I didn't remove, would removing this make a difference?

A bizarre issue I've noted is that, sometimes, if I tap on the ECU, the engine will idle down to around 1000RPM, but it will also run SUPER rich. I believe the car is running rich even while idling high, but it becomes very noticeable after tapping the ECU. Could an ECU fault cause the car to idle high?

I haven't had the opportunity to check my cars fuel pressure, could a bad pressure regulator cause a high idle?

One of the biggest issues I'm currently facing is that the engine is from a 280ZX(F54 Block, P79 Head). Unfortunately, I have no knowledge of if this engine ever idled correctly while in the car, and it appears as if the PO just threw the engine into the car without swapping any of the accessories from the original engine, as there are multiple sensors and such with nothing on the wiring harness to connect to them. It's made it difficult for me (at least psychologically) to reference the 280Z service manual due to differences between the two engines.

I also believe that, due to the nature of this engine swap, a few vacuum lines may have been reattached in creative places. I've been having trouble finding a vacuum diagram for the engine to confirm this, however.

To my (inexperienced) eye, many of the vacuum lines check out visually, and I can't hear a vacuum leak, but are there any lines in particular that could cause an idle like this that I should check more in-depth?

For what its worth, if I disconnect/plug this line (picture: https://i.imgur.com/r1ow9Hi.jpg), the car will idle at about 1000rpm. However, I feel like this is probably more indicative of removing a huge source of vacuum from the engine, rather than it being evidence of the problem coming from this area.

I'm really unsure of where to turn next in terms of repairing this idle, so any help is greatly appreciated! Thank you!

[Zed Head]

Engine speed is controlled by the amount of air it can get.  It's always trying to get more.  So if you want to reduce engine speed you have to reduce the amount of air available.

The quantity of fuel available will only control the quality of the engine speed, not the quantity.  The quantity of fuel in a gasoline engine is always match to the amount of air, to control the qulaity of the combustion.

So, your first task should be to  control the quantity of air getting in to the intake manifold.  The throttle blade should control almost all of it, especially since the BCDD is gone.  One area that people tend to miss is the PCV system.  It is connected to the intake manifold and the engine's crankcase.  So any air that gets in to the crankcase, like through the valve cover, can get in to the intake manifold, and cause a high idle.  Normally, extra air would cause some increase but a little more would cause a decrease as the mixture gets too lean.  But, if the ECU or FPR is allowing extra fuel, the extra air can cause the idle to rise a lot since there is extra fuel to go with it.

The PCV diagram in the Emissions chapter is worth studying.  It will give some ideas about where excess air could be getting in and which hoses to plug or connect properly.

[siteunseen]

Two things I can add to Zeds response. Behind the black cover on the afm is a sweeping contact vane. Check that to make sure it's clean and shiny. I had a spider web on mine one time, moisture and carbon buildup can be suspect too.

The other is the pcv dumping oily air in front of the throttle blade and gumming the throttle body up. That could cause the blade to not shut completely letting air past and high rpm. I clean them with an old toothbrush and lacquer thinner.

When I'm on my laptop I will link some info with great pictures from member formally known as "Blue's tech tips" now I guess we call it Philip's tech tips Or 240260280'S Tech Tips. 

You'll have to plug the bcdd vacuum port from inside the throttle body if I remember right. That's another tech tip I'll post for you. You don't want any open hoses or hose bungs to suck in air.

[interestingusername]

Thank you both! I did replace the PCV valve at one point, but haven't looked in-depth at the entire PCV system It's definitely a new place to look, so when I get a chance I'll definitely check them out.

 

1 hour ago, siteunseen said:

Two things I can add to Zeds response. Behind the black cover on the afm is a sweeping contact vane. Check that to make sure it's clean and shiny. I had a spider web on mine one time, moisture and carbon buildup can be suspect too.

The other is the pcv dumping oily air in front of the throttle blade and gumming the throttle body up. That could cause the blade to not shut completely letting air past and high rpm. I clean them with an old toothbrush and lacquer thinner.

When I'm on my laptop I will link some info with great pictures from member formally known as "Blue's tech tips" now I guess we call it Philip's tech tips Or 240260280'S Tech Tips. 

You'll have to plug the bcdd vacuum port from inside the throttle body if I remember right. That's another tech tip I'll post for you. You don't want any open hoses or hose bungs to suck in air.

I checked that part of the AFM out, and (to my eye) its mostly clean inside. If it's of any use, here's a photo of it: https://i.imgur.com/IgkcaZR.jpg
It may be worth noting the silicone around the cover. I assume someone has been in here before me,  though I don't know if they were playing with the fuel mixture or anything else important.

My cars throttle body was definitely dirty, but I'm not sure if it was dirty enough to cause a problem this extreme. I have cleaned it since inspecting it, but haven't had a chance to run the car yet.

With my engine being from a 280ZX, the BCDD was located on the intake manifold, rather than the throttle body. Would there still be an extra port to plug?

I appreciate the help, and look forward to any info/tips you can show me! I'll do what research for them I can, as well.

Edited October 18, 2017 by interestingusername

[Zed Head]

You can check for the stuck throttle blade by pulling up on the pedal with your foot or manipulating it by hand under the hood.  There are two return springs, one on the TB itself and the other attached to the linkage.  On the ZX engines I think that the linkage spring is very visible, right on top.

I snipped ypur picture and drew an arrow to the spring and linkage,  Make sure it's free and working right.  Also arrowed the ZX idle adjustment screw.  It works by letting a small amount of air past the throttle blade. 

If you figure out the excess air problem you'll still have the goofy ECU problem.  That could be a loose connection or a bad ECU.  Is it a ZX ECU or the original Z ECU?  They're different.  Should be next to your left foot, behind a panel.  With a sticker on it that ID's it.  Take a picture of the sticker.  It's probably the Z ECU and they are known for failing like yours is, with the tapping it rich/lean problem.

[Captain Obvious]

I suspect you didn't block off the BCDD correctly. As siteunseen alluded to above, it's not as simple as removing it and slapping a flat plate in it's place.

I've not looked into the details, but I suspect the later ones under the manifold are pretty much the same concept as the earlier ones on the bottom of the throttle body.

Do you have any pics of what you did there?

[Zed Head]

The FSM picture looks pretty clean.  It's probably where the hoses were connected.  But you do also need a good solid seal on whatever blockoff plate you attach also.

[Captain Obvious]

It's not that simple. You can't just remove the guts and slap on a simple plate. When you remove the guts, you remove the valve that seals one side to the other.

I whipped this up to hopefully illustrate the issue:

[Zed Head]

I thought that the plate would go directly on to the manifold with all of the hose connections removed.  But I think I see what you're showing now.  Those hose connection is still there.  Nissan shows an interface.  I wonder if that piece can be removed and plated there.

[Zed Head]

I have a ZX manifold in the garage if anyone needs a BCDD.

[interestingusername]

1 hour ago, Zed Head said:

You can check for the stuck throttle blade by pulling up on the pedal with your foot or manipulating it by hand under the hood.  There are two return springs, one on the TB itself and the other attached to the linkage.  On the ZX engines I think that the linkage spring is very visible, right on top.

I snipped ypur picture and drew an arrow to the spring and linkage,  Make sure it's free and working right.  Also arrowed the ZX idle adjustment screw.  It works by letting a small amount of air past the throttle blade. 

If you figure out the excess air problem you'll still have the goofy ECU problem.  That could be a loose connection or a bad ECU.  Is it a ZX ECU or the original Z ECU?  They're different.  Should be next to your left foot, behind a panel.  With a sticker on it that ID's it.  Take a picture of the sticker.  It's probably the Z ECU and they are known for failing like yours is, with the tapping it rich/lean problem.

Return springs & linkages are free and going back into position the way they should.

I'm fairly certain it's the original Z ECU, here is a photo of it so you can confirm: https://i.imgur.com/S9QH4wL.jpg

 

1 hour ago, Captain Obvious said:

I suspect you didn't block off the BCDD correctly. As siteunseen alluded to above, it's not as simple as removing it and slapping a flat plate in it's place.

I've not looked into the details, but I suspect the later ones under the manifold are pretty much the same concept as the earlier ones on the bottom of the throttle body.

Do you have any pics of what you did there?

I believe you're onto something. I did a simple block-off.

From what I can tell, there's a piece that bolts onto the manifold(this is what that three-way hose is connecting to), and then the BCDD attaches to it. I took some photos of it, but they're not the greatest(please excuse this awful sealer work, this was an attempt to deal with a broken screw):
https://i.imgur.com/6JtdOUY.jpg
https://i.imgur.com/JLtHK4z.jpg
https://i.imgur.com/4JrlKiy.jpg
Worst of all, I managed to discover a hole in this plate I had no idea was there! https://i.imgur.com/Y67nTkt.jpg I must've had this covered from sealer and it failed after running the car some, I can't believe I would've missed this on reinstallation. 

Zed Head also mentioned this, but could you just remove the piece connecting to the manifold and make a block-off plate for that, or would it be more in-depth?

Thank you all for the responses!

 

 

 

 

 

Edited October 19, 2017 by interestingusername

[siteunseen]

When you're bored as hell this thread has some pertinent info for your car, I think. 

http://www.classiczcars.com/topic/50850-no-vacuum-at-throttle-body-ports/