75 280z rpm issue

[galloguy05]

So I have recently been trying to get a little seat time in my z before I park it for paint, but a weird issue has me confused and I have no clue how to search this in the search criteria without being too vague.

 

The car is a stock 75 280z with only a muffler on it for the sound affect but still running a cat and all stock emission control devices. I also have a cold air intake on it.

 

My issue is, when driving the car, after taking it out of gear and coasting (without the clutch pressed on just coasting) the engine rpm will stay at whatever rpms i was at in gear until the car is at a COMPLETE stop. Then they will drop back down to idle correctly.

 

What may be causing this? Maybe a vacuum leak? I am going to try and check all my hoses today, with the little time I have available being that its fathers day and have family obligations,to look for any obvious hose leaks.

 

What else might cause this though?

 

P.s. Sorry if this has been covered before. I simply don't know what this issue would be called to search for it in the search function.

 

Thanks,

Matt

[RTz]

Sticky throttle linkage?

[jhill]

b.c.d.d. valve?

Edited June 20, 2010 by jhill

[BRAAP]

Matt,

Three things come to mind that could be causing this.

1) Busted Brake Booster diaphragm. IF the idle RPM thing seems to be related to brake pedal actuation, this could be the cause. With the your engine running, car stopped, push in on the brake pedal, slowly, keep pushing harder and harder, your RPM should not rise, or if so, only maybe 100 RPM and then settle back down as your the brake pedal stops moving. (The faster the pedal travels down, that will cause a 100-200 RPM rise then fall, but that is generally it.)

 

2) Throttle sticking.

a) If the pedal feels unusually soft and easy to push down, good chance the throttle return spring on the Throttle body is rusted/busted.

Pivot points and ball/socket joints in the throttle linkage dry, gummed up, etc. Lube all pivots, ball/socket joints etc.

 

3) B.C.D.D., (Boost Controlled Deceleration Device), is supposed to behave in the manner you described your issue, though it is supposed to be subtle, sometimes can be exaggerated as these cars age. The BCDD is the goofy looking protrusion on the bottom of the throttle body. I have not spent any time personally getting them to work properly, only removing them. If you search for BCDD, B.C.D.D., or Boost Controlled Deceleration Device, you quite a bit more info on it, how and why it does what it does, etc.

 

Here are a few specific threads on the topic of the BCDD;

 

http://forums.hybridz.org/index.php?/topic/23280-runaway-idle/

 

http://forums.hybridz.org/index.php?/topic/88866-strange-idle-issue-78-280z/

 

http://forums.hybridz.org/index.php?/topic/70142-bcdd-removed-now-backfires-at-downshift/

 

Hope that helps,

Paul

[galloguy05]

Paul,

Thanks for those links. The second link is exactly what my car is doing. I am going to research on repairng the b.c.d.d. Valve or replacing it. From what that post said, it wasn't getting the 12v needeD when the car was warmed up. So I am going to try and trace that issue down.

[rallyeo]

I don't know if this will help or not but I had the same problem only the idle would not go below 2 - 3K rpms most of the time. I took the BCDD apart, messed with the idle, timing, nothing worked. It turned out to be a bad vacuum line going to the bottom of the distributor - about $5 worth of tubing and 5 minutes later, problem solved.

[galloguy05]

Alright guys here is an update. Actually first I need to add a little more background info. Before I discovered this BCDD issue, my car had an issue with soundig like it was missing ad wanted to die at idle. The timing and everything was correct and it was getting enough fuel and everything. So all I did was unscrew the idle needle and when it idled at about 1,000 rpm it seemed fine, but toned back down to 800 the car seemed to want to die ad the engine would shake as if it was missing on a cylinder. So I just turned the idle screw back and left it at 1,000 and then I noticed this issue.

 

I would like to repeat that I am positive that the timing is not off and it is in fact getting enough fuel.

 

Now to this current issue, I took off the afm to get to the BCDD. I removed the BCDD needle (or whatever it is called that is screwed into the side of te BCDD at the bottom of the throttle body. With it removed I just put the afm back on and started the car to see what would happen. Now when cold the car is not going to act up so I wa by able to see if it changed anythig. I did however notice that it was ifleing higher than what I had it previously set it at. So iturned the idle back down to it idleig at 800 and, without the BCDD Needle in, it idled perfectly at 800. So I was sort of perplexed and put the BCDD back on to see what would happen and it remains to idle perfectly now at about 800 rpm. Now keep in mind ALL I did was remove the BCDD needle. I did not clean it or adjust anything. All I did was remove it and start the car, shut it off, put it back on and restart it.

 

Can anyone tell me why this may have happened or hy it possibly might have fixed the issue? I haven't been able to take the car for a spin yet so I cannot comment on how it performs when warm. Maybe the needle in the BCDD Was simply stuck or something? If anyone can give me any insight on this I'd appreciate it.

 

I know one solution is to plug the BCDD port, but I don't want to keep having a backfire and possibly ruin my cat (or worse stories I read about splitting the exhaust in half at the seam). So how do I adjust this needle on the BCDD? Simply by adding another washer or something?

 

Thanks everyone,

Matt

Edited June 21, 2010 by galloguy05