Update for you - the pcv valve was fine when he tested it per the fsm but the hose was still split which has since been fixed.

The noise is still there but with a new coolant temp sensor and pcv hose, the idle dropped from 2500 to 1300 with the thottlebody screw placed all the way in.  Changing topics from the noise to the idle speed, we've found all the vacuum leaks with the car dying at this point when you take the oil cap off.  With the idle screw all the way in on the throttlebody, could this be an ecu issue causing it to run so fast?

If we pull the temp sensor connector, the car's idle will drop drastically (maybe down to 400 or so) but that's simply because the car is running in max rich mode.  Any ideas besides vacuum leaks that could cause such a high idle?  There really can't be any leak areas left especially after smoking the engine and using a good chunk of starter fluid blasting the manifold...

The BCDD is a known idle speed screwer-upper.  It's on the bottom of the throttle body.  It lets air past the throttle blade under certain conditions.  You might try one of the other TB's.

Just now, Zed Head said:

The BCDD is a known idle speed screwer-upper.  It's on the bottom of the throttle body.  It lets air past the throttle blade under certain conditions.  You might try one of the other TB's.

The TB he's using currently has a bcdd blockoff plate - we also sprayed that badboy with starter fluid and it seems sealed though

Pretty sure that CO has posted that you can't just put a plate on the BCDD mounting area.  There are passages that will still be open, that you have to block.  Pretty, pretty, pretty,sure.  But not positive.  Search around on Captain Obvious and BCDD.

Just now, Zed Head said:

Pretty sure that CO has posted that you can't just put a plate on the BCDD mounting area.  There are passages that will still be open, that you have to block.  Pretty, pretty, pretty,sure.  But not positive.  Search around on Captain Obvious and BCDD.

Didn't know that - will do.  I did remember there is some strange sensor on his engine (75) which isn't on mine (78) and is not plugged in.  It sits right next to the fuel rail/thermostat housing and has 2 bullet connectors and has a vacuum port on it - is this part of the coolant temperature system for that earlier 280?  I looked online but I can't find any pictures of it or a name...

It's probably the solenoid valve for the vacuum advance supply hose to the distributor.  For some reason Nissan only wanted the early years to have vacuum advance in 4th gear.  There's a switch on the transmission.

Edited August 16, 20222 yr by Zed Head

1 minute ago, Zed Head said:

It's probably the switch for the solenoid to the vacuum advance supply hose to the distributor.  For some reason Nissan only wanted the early years to have vacuum advance in 4th gear.

Wow that's odd, ok then

43 minutes ago, chaseincats said:

Update for you - the pcv valve was fine when he tested it per the fsm but the hose was still split which has since been fixed.

The noise is still there but with a new coolant temp sensor and pcv hose, the idle dropped from 2500 to 1300 with the thottlebody screw placed all the way in.  Changing topics from the noise to the idle speed, we've found all the vacuum leaks with the car dying at this point when you take the oil cap off.  With the idle screw all the way in on the throttlebody, could this be an ecu issue causing it to run so fast?

If we pull the temp sensor connector, the car's idle will drop drastically (maybe down to 400 or so) but that's simply because the car is running in max rich mode.  Any ideas besides vacuum leaks that could cause such a high idle?  There really can't be any leak areas left especially after smoking the engine and using a good chunk of starter fluid blasting the manifold...

The car wanting to die when the oil cap is removed indicates a malfunction with the crankcase ventilation system. Removing the cap is creating a huge vacuum leak, which shouldn't be there. 

This car should also have an evaporative emissions recovery system. Engine vacuum is used to pull evaporative emissions, gas vapor from the fuel system and tank, crankcase gases, and into the intake air stream to be consumed in the combustion chamber.

 

 

10 hours ago, Zed Head said:

Pretty sure that CO has posted that you can't just put a plate on the BCDD mounting area.  There are passages that will still be open, that you have to block.

chaseincats, This is correct. On the Z throttle bodies*, you have to be careful about how you block off the BCDD. Search around the forum a little and you'll find details. If you can't find them, let me know and I'll see if I can dig them up.

 

* ZX is different.

1 hour ago, Captain Obvious said:

chaseincats, This is correct. On the Z throttle bodies*, you have to be careful about how you block off the BCDD. Search around the forum a little and you'll find details. If you can't find them, let me know and I'll see if I can dig them up.

 

* ZX is different.

I looked yesterday for bcdd threads with you in it but couldn't find any with sage advice on the process...

Edited August 16, 20222 yr by chaseincats

Found a few. 

https://www.classiczcars.com/search/?q=bcdd block plate&quick=1

 

 

8 hours ago, Zed Head said:

Found a few. 

https://www.classiczcars.com/search/?q=bcdd block plate&quick=1

 

 

Thanks for this, we got the idle down to where it should be and the sound is gone.  The culprit was indeed the bcdd blockoff plate - had no idea you were supposed to silicone the holes inside the throttlebody shut so air doesn't pass through.  These blockoff plates really should come with instructions to give the purchaser a heads up that needs to be done.

What was most likely happening is the intake noise was the sound of air whistling through that passageway in the throttlebody and then echoing down the intake piping.

This is the image that really made it all click from one of Captain Obvious's threads:

 

Edited August 17, 20222 yr by chaseincats