jschimenti Posted January 12, 2008 Share Posted January 12, 2008 Hi, I am new to this blogging stuff so pardon me if I don't do something correctly. I have a 1976 280z with the standard L6 EFI motor that I have just finished restoring. The entire engine was totally rebuilt and is up to new specs. The car runs fine except for one annoying problem. When I take my foot off the gas pedal, the engine RPM's hang at about 2,000 or sometimes even 2,500 for a few seconds before returning to idle speed of 800. Does anyone know what causes this problem? Thanks, John Quote Link to comment Share on other sites More sharing options...
Administrators RTz Posted January 13, 2008 Administrators Share Posted January 13, 2008 The most common problem is friction in the throttle linkage. Clean/lube the ball joints and bushings. Also, make sure the throttle return spring is doing its job. Quote Link to comment Share on other sites More sharing options...
Evan Purple240zt Posted January 13, 2008 Share Posted January 13, 2008 My memory is foggy, but isn't there some sort of vac. activated solenoid that keeps the idle from dropping too fast when letting off the throttle? Evan EDIT; Sorry I was hardly clear there, maybe this device is defective. Quote Link to comment Share on other sites More sharing options...
Administrators RTz Posted January 13, 2008 Administrators Share Posted January 13, 2008 My memory is foggy, but isn't there some sort of vac. activated solenoid that keeps the idle from dropping too fast when letting off the throttle? Yes. I have yet to see one create a few second, 2000 rpm lag, though. One way to isolate it would be to prop the hood open and rev the motor, if it hangs, try to close the throttle by hand (at the throttle shaft). If it closes easily, its likely friction. Quote Link to comment Share on other sites More sharing options...
MJLamberson Posted January 13, 2008 Share Posted January 13, 2008 are we talking about the boost controlled deceleration device? I have a friend with an s30, 76, his wasnt working, he couldnt go below a 2500, he simply took it off and is now fine, he gets an occasional backfire because of this though, try replacing it, Im pretty sure its called a boost controlled deceleration device... Quote Link to comment Share on other sites More sharing options...
jschimenti Posted January 13, 2008 Author Share Posted January 13, 2008 I have done some reading in the tune up section of the Factory Workshop Manual and found that when you take your foot off the gas pedal, the manifold pressure surges for a while. This causes the RPM's to stay high. To deal with this, the factory installed a "Boost Controlled Decelearation Device" or BCDD for short. I am going to check this out to see if this deals with the problem. Quote Link to comment Share on other sites More sharing options...
kastmaster Posted January 15, 2008 Share Posted January 15, 2008 I had the same problem as described in the first post. My solution was to lubricate the throttle linkage. Now it falls down to idle nice and smooth and quick. Quote Link to comment Share on other sites More sharing options...
Careless Posted January 15, 2008 Share Posted January 15, 2008 same here. same car, same year, same engine. the first time i started it i was so happy that I didn't notice it was just pegged at 3000 for a minute and a half at 10:30 at night with no exhaust. later in the next day I got WD40, sprayed that on and let it penetrate. Then I sprayed off the WD40 after it did it's work for about 5 hours, and sprayed some FLUID FILM (in my opinion, one of the best lubricants and rust-inhibitors that can be removed easily because it's made of wax). Now it's free moving and snaps back almost instantly, save for the weak pedal spring. Quote Link to comment Share on other sites More sharing options...
Evan Purple240zt Posted January 15, 2008 Share Posted January 15, 2008 Careless, best lubricant I have used to date is Kroil. Major penetrating action there. Stuff you mention does sound interesting though! Ill look into it. Evan Quote Link to comment Share on other sites More sharing options...
Careless Posted January 16, 2008 Share Posted January 16, 2008 Careless, best lubricant I have used to date is Kroil. Major penetrating action there. Stuff you mention does sound interesting though! Ill look into it. Evan Kroil is indeed one of the best I've used too, Evan. I didn't have any around and I only had just enough to squeek by at the register with Liquid Wrench Super Lubricant with Cerflon, actually. So it wasn't WD40, my mistake. However, the Fluid Film is not a penetrating lubricant. It's just a lubricant used to protect surfaces from the elements and it's a pretty good sealer. It's moderately easy to whipe off completely, and it takes little effort to apply well enough to protect. It's also thick enough to be used upside down, and it can actually be used as an autobody Undercoating if you really want to. It doesn't harm paint either. Problem is, it's a tad brown. If it was clear I would spray it everywhere. Raff Quote Link to comment Share on other sites More sharing options...
jschimenti Posted January 16, 2008 Author Share Posted January 16, 2008 Thanks to all who took the time to post responses. I do wish that this problem were so simple to diagnose and to fix, but this is not the case. The problem seems to be electro-mechanical. I am trying some new parts and hope that this will fix the problem. Thanks again, John Schimenti Quote Link to comment Share on other sites More sharing options...
Careless Posted January 17, 2008 Share Posted January 17, 2008 Thanks to all who took the time to post responses. I do wish that this problem were so simple to diagnose and to fix, but this is not the case. The problem seems to be electro-mechanical. I am trying some new parts and hope that this will fix the problem. Thanks again, John Schimenti In that case, I'm willing to bet it's the AFM contact points being sticky. I can't remember how many contacts there are but I think there's one for no-load, load, and wide-open throttle. It says there are no serviceable parts inside, but you can open it and clean out the 30 year old die-electric grease and put a new packet in there (you can buy small packets of the grease at any place that sells bulbs for exterior car lighting). Chances are it's taking a bit of effort to fully release the contact so it snaps out of the "load" contact point. It could also be that your idle set screw is set to idle too high, but you have a vacuum leak that causes it to keep a high idle until the vacuum dies down again due to the leak. But I think the contacts would be the one to worry about. My advice would be to buy another one and test before you open the one you need in order to drive the car. Quote Link to comment Share on other sites More sharing options...
OR-JO Posted March 27, 2008 Share Posted March 27, 2008 Anyone have any more SPECIFIC info on changing/altering/bypassing the BCCD? Spoke to a datsun mechanic today and he said if it's just the BCCD its about a $20 fix. Possible instructions to "fix" on my own? Quote Link to comment Share on other sites More sharing options...
OR-JO Posted March 27, 2008 Share Posted March 27, 2008 Anyone have any more SPECIFIC info on changing/altering/bypassing the BCCD? Spoke to a datsun mechanic today and he said if it's just the BCCD its about a $20 fix. Possible instructions to "fix" on my own? Quote Link to comment Share on other sites More sharing options...
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