Hey guys!

So I bought a 1977 280z that has been sitting for some 30 years. Earlier this year I redid the engine and all of its components, and a new fuel system front to back . Upon start up it ran, but wouldn't rev past 1200rpm without holding the throttle down. Checked the AFM and noticed the flap was bobbling and barely moving. When I manually held it open The car ran beautifully and could rev quick and easy. So it got me thinking why the flap wasn't opening. Of course vacuum, and I remembered I am an idiot and never plugged the ports I left open after deleting the EGR/BPT valves (I got long tube headers). 

Anyways, I have searched everywhere, but no one seems to know the plug size. Some say it is BSPT, but it seems to be all speculation. Was going to run a plug with some JB weld or thread sealer so these are plugged forever. Does anyone know the sizes? 

Home depot and ace hardware both were unable to help me solve this problem, so you guys are my last resort!

Provided below are photos of what needs to be plugged (one smaller hole on side of the manifold where EGR used to be, and a large one where the EGR pipe went underneath the manifold). I also attached photos of what used to plug them from the old EGR and BPT system.

Thank you guys in advance!

 

I would have to see the holes under that shiny new block-off plate to be positive, but I believe it is unnecessary to plug those two exhaust holes.*

I think the block-off plate seals off the vacuum side of the intake manifold from both those two holes. If you want confirmation, take that plate off and post up a pic of the top-side of the intake manifold where that plate is bolted on.

So that would be the good news. The bad news is I don't think that has anything to do with the problem you are having with the car. 

 

*other than keeping the mud daubers out of them.

I agree with Captain O.   The block off plate has now isolated the vacuum side intake from those two holes.

When deleted all my egr crap there was one sneaky little hose under the bcdd I didn't plug. I have it on my laptop but I'm away from home, bored and using my phone.

The girl I'm with is getting pissed. Exactly what I wanted.

 

2 hours ago, Captain Obvious said:

I would have to see the holes under that shiny new block-off plate to be positive, but I believe it is unnecessary to plug those two exhaust holes.*

I think the block-off plate seals off the vacuum side of the intake manifold from both those two holes. If you want confirmation, take that plate off and post up a pic of the top-side of the intake manifold where that plate is bolted on.

So that would be the good news. The bad news is I don't think that has anything to do with the problem you are having with the car. 

 

*other than keeping the mud daubers out of them.

Will do, I'll remove it and see what lies below. The thing is that every video I have watched the youtuber has either plugged or welded both those holes, even with the block off plate. So they must do something I would think. Especially for the BPT,  because it is totally isolated from the EGR valve on the intake. 

However, if that logic was true you would think they would sell the plugs with the block off plate. Hmmm.

Man I pray it's a vacuum problem and not some other demon:( Otherwise it's just gonna be me, my wallet, a multimeter, and a whole lot of cursing hahahahaha.

Thanks for the help!

1 hour ago, siteunseen said:

When deleted all my egr crap there was one sneaky little hose under the bcdd I didn't plug. I have it on my laptop but I'm away from home, bored and using my phone.

The girl I'm with is getting pissed. Exactly what I wanted.

 

This made my night haha. You got your priorities straight... respect👍

Yah, I will have to check the BCDD again. I rebuilt the little thing when I did the engine. So many little places to mess up it's hellfire when something is amiss in these systems.

3 hours ago, ckurtz2 said:

Earlier this year I redid the engine and all of its components, and a new fuel system front to back . 

Upon start up it ran, but wouldn't rev past 1200rpm without holding the throttle down. Checked the AFM and noticed the flap was bobbling and barely moving. When I manually held it open The car ran beautifully and could rev quick and easy. So it got me thinking why the flap wasn't opening.

The AFM vane should move free and easy, in concert with the throttle blade.  When you "redid" the engine and components did you modify anything?  PCV tubes, maybe?   All air must pass through the AFM.  The PCV system is connected to the intake system.

1 minute ago, Zed Head said:

The AFM vane should move free and easy, in concert with the throttle blade.  When you "redid" the engine and components did you modify anything?  PCV tubes, maybe?   All air must pass through the AFM.  The PCV system is connected to the intake system.

The vane itself moves freely. There's some pressure, but that is from the cog that holds the spring tension inside. It doesn't bind up at all when moving it. The things I modified was that I removed all of the AC components, EGR delete, and BPT delete. As well as a cold air intake. I have the same PCV hose that the car came with on it. Looked a little crusty, but wasn't cracked, replaced the PCV valve under the intake as well for good measure.

 

        It's the weirdest thing. The car starts up fine. As the car heats up you can watch the AFM begin to bobble. The bobbling then gets worse as the AFM tries to correct whatever the hell goes on, and then finally the car will buck and die. However, I can keep the car running decently smooth if I hold the AFM vane open so it runs just a little richer when it sends the signal to the fuel injectors. If I let go it bucks and dies. I have looked up this problem extensively, but it seems no one else has written about this problem. There is so many problems it could begin I don't even know where to start, so I went to the logical conclusion of bad vacuum not opening up the AFM properly.

I am all ears!

 

Your 77 would have the AFM fuel pump switch.  You might try turning up the idle speed to see if it will stay running once it warms up.  More iar keeping the vane off of the switch.  If it keeps running after warmup with a higher idle speed it will be a clue.

 

Not really sure what this means either.  Kind of sounds like a blockage in the intake system.  Mouse nest, rag, etc.  It should rev just fine even when cold.  So the advice above might be irrelevant.

3 hours ago, ckurtz2 said:

Upon start up it ran, but wouldn't rev past 1200rpm without holding the throttle down

 

41 minutes ago, Zed Head said:

Your 77 would have the AFM fuel pump switch.  You might try turning up the idle speed to see if it will stay running once it warms up.  More iar keeping the vane off of the switch.  If it keeps running after warmup with a higher idle speed it will be a clue.

 

Not really sure what this means either.  Kind of sounds like a blockage in the intake system.  Mouse nest, rag, etc.  It should rev just fine even when cold.  So the advice above might be irrelevant.

 

I pulled apart the intake and everything on it and cleaned out the inside and out. So I am pretty pretty sure that shouldn't be the issue. The fuel pump works, because if I leave the ignition on and move the AFM vane I can hear it turn on and run, as well as it shoots out the injectors. Learned it was shooting out the injectors (rebuilt) because after playing with the AFM with the ignition on I went to start it up and had a backfire as loud a 12 gauge go out the back..... Shame on me lol. Won't make that mistake twice. 

 

When you say change the idle speed do you mean the idle screw on the intake? I have played with that while running, and it does pretty much nothing until it is totally closed and the engine will die. No difference when I loosened it all the way. Another sign it may be a vacuum leak?

Edited March 27, 20213 yr by ckurtz2

9 hours ago, ckurtz2 said:

The thing is that every video I have watched the youtuber has either plugged or welded both those holes, even with the block off plate. So they must do something I would think.

Remember that just because you saw it on youtube* does not make it true. (Even if you saw more than one person suggesting that it was.)  

 

* or facebook, or twitter, or reddit, or parler, or etc

10 hours ago, ckurtz2 said:

I pulled apart the intake and everything on it and cleaned out the inside and out. So I am pretty pretty sure that shouldn't be the issue. The fuel pump works, because if I leave the ignition on and move the AFM vane I can hear it turn on and run, as well as it shoots out the injectors. Learned it was shooting out the injectors (rebuilt) because after playing with the AFM with the ignition on I went to start it up and had a backfire as loud a 12 gauge go out the back..... Shame on me lol. Won't make that mistake twice. 

When you say change the idle speed do you mean the idle screw on the intake? I have played with that while running, and it does pretty much nothing until it is totally closed and the engine will die. No difference when I loosened it all the way. Another sign it may be a vacuum leak?

If air flow drops low enough the fuel pump will shut off.  That's what it is supposed to do.  

Yes, the idle screw on the throttle body.  If idle speed doesn't increase that's not right.  If the engine dies when it's closed that's a sign that there is no vacuum leak.

The backfire makes me think of incorrect timing. 

Any chance that there is a hole in the tube between the AFM and the throttle body?  If the engine is pulling air the AFM vane should move.  The tube between the 
AFM and throttle body needs to be sealed.  

Last thought - has the AFM spring been messed with?  If somebody tightened it it might not move fast enough to let the engine rev.  When you open it by hand you're telling the ECU to give more fuel.

14 hours ago, ckurtz2 said:

Upon start up it ran, but wouldn't rev past 1200rpm without holding the throttle down. Checked the AFM and noticed the flap was bobbling and barely moving. When I manually held it open The car ran beautifully and could rev quick and easy.

 

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.