75 280 Idle issues

[240Adam]

I have a 75 280 and when I am coming up to a stop and I push in the clutch or throw it in neutral the idle stays at 1500 to 2000 untill I am almost stopped then it drops down to normal idle and it only does it after it warms up. I know I dont have a vaccum leak, I checked with starting fluid. I'm thinking its the valve on top of the intake manifold with the big vaccum line going to it, I don't know what its called. Any suggestions, thanks Adam

[Hrududu]

My 77 was doing something similar before i changed out the head gasket. After putting everything back together, the problem was gone. I'm not sure, but I think the problem had to do with this tiny wire than connects down near the PCV valve hose connects on the front of the intake manifold. Mine wasn't conneted before and I made sure it was after I put everything back. I have a picture of it here, check to see if yours is connected.

[240Adam]

Thanks Ill check that out, I know I have two wires down there one is connected and I dont know where the other goes.

[sblake01]

If your car is a California car you might want to check out your altitude sensor. I had the same problem with mine until I replaced it a few years ago.

[Jeff G 78]

It sounds like the BCDD. It might just need an adjustment. The BCDD is supposed to hold the revs on decel until the car gets under 10 mph and then slowly drop back to idle. Mine was doing the same thing as yours, but hanging at a bit higher rpm. There is a screw on the bottom of the BCDD to adjust it, but I didn't have any luck moving mine. I decided to disable it and the car now runs much better.

If you decide to disable it, remove the rubber intake hose between the AFM and the throttle body. You will see two holes at the bottom of the throttle body. One hole goes to the BCDD and the other goes to the distributor if I remember correctly. If you plug the hole that goes to the BCDD, it will disable it and make your car run better (assuming you can't get the BCDD adjusted properly). In my case, I used a piece of QUALITY tape to cover the BCDD feed hole TEMPORARILY so I could determine for sure that the BCDD was the culprit. As a precaution, I cut the strip of tape long enough to extend upstream and wrap around the opening of the throttle body so that the rubber intake hose would help secure it in case it came unstuck. I didn't want to risk sucking it into the intake. Make sure the tape covers the BCDD hole only (the larger one) and does NOT obstruct the butterfly valve. After replacing the intake hose, take the car for a quick drive to see if the problem is gone. If so, the BCDD was the problem and you can either replace it or disable it by plugging the air hole with some RTV. On my car, the tape trick worked, but at the same time the idle was much lower and the car would stall when coming to a stop when the car was cold. This told me that the AAR was also not working. I replaced the AAR and the car now runs great cold and warm.

[240Adam]

Hey thanks alot guys for the info, turns out it was the BCDD I gave it a turn one way and it got worse so I turned it the other way and problem solved. Thanks alot

[dillie327]

question, could anyone show some pictures of the holes to plug and not to plug? becasue I know there're 2 right inside the TB, big and little, plug the big, but there's also a hole on the other side of the butterfly? and if you plug the BCDD, should you hotwire it with a 12v? or unplug it? and if you hotwire then you're just running a hot 12v to the single wire connection correct?

and here's a stupid question... AAR? is that the air regulator that helps with cold start on top of the intake?

Edited December 31, 2009 by dillie327

[ronroyboy]

I had the same problem on my 77 280z. it was the bccd

[esmit208]

Adam

If none of the other suggestions work try replacing your thermal timing sensor if your vehicle is so equipped. If it is it is located right next to the sensor for the water temperature. NISSAN/DATSUN installed these on early models to integrate with the cold start valve. Good Luck

2MUCHZ

[Pomorza]

question, could anyone show some pictures of the holes to plug and not to plug? becasue I know there're 2 right inside the TB, big and little, plug the big, but there's also a hole on the other side of the butterfly? and if you plug the BCDD, should you hotwire it with a 12v? or unplug it? and if you hotwire then you're just running a hot 12v to the single wire connection correct?

and here's a stupid question... AAR? is that the air regulator that helps with cold start on top of the intake?

Can't really help you on the first question but as for the AAR.

AAR I believe would be the Auxiliary Air Regulator. And yes it allows more air to enter the engine when the engine is cold. Basically its a giant vacuum leak that closes over time. It sits on top of the intake and has two hoses going to it and an electrical connector.

Jan

[dillie327]

alright, thanks. that's what I thought it was, but being new to these cars all these abbreviations are killing me hah!