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Thread: Hanging RPM

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    Default Hanging RPM

    So, I have a problem that has been plaguing me and have done a fair amount of googling and searching on here and other Z sites and haven't found the bucket to stick my problem into to troubleshoot.

    Please see the video below.



    Here is a description and some details:

    The car's idle speed will "hang" after acceleration. For instance, accelerate through a gear up to 3k,4k, etc rpm. Push in the clutch and you would expect rpms to rather expediently hit idle, but here it drops from the initial high to

    2.5k rpm rather quick then takes 10-20 seconds to cover the next 1600 rpm back to idle.

    1978 280z

    Deleted AAR and the coolant plate under it

    A/C idle increase arm nippled off

    EGR nippled off

    added helper spring to throttle body return with little effect. Helps come down from high speed but will not help under 2.5k rpm

    It seems to only do this when warm.

    Any ideas where to look?

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    Registered User siteunseen's Avatar
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    BCDD under the throttle body. Unplug and see if it quits. Small black wire off the BCDD butt connects into a small white wire going into the wiring harness.
    B boost C controlled D deceleration D device
    If you deleted the EGR and BPT there's a vacuum hose on the side of the BCDD that needs to be plugged with a small bolt, circled in yellow in the picture.
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    Last edited by siteunseen; 04-18-2014 at 03:32 PM.
    1972 240Z #918 New Sight Orange
    1977 280Z #305 Light Blue Metallic
    1972 240Z #110 Persimmons Red

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    I plugged that vac tap that runs up to the BPT. I'll check unplugging it, thanks.

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    Hey I forgot about a possibility. The TPS, little black box above the BCDD attached to the side of the throttle body. You could pop the cover off and see if 1 of the contacts may be stuck or unplug it and see what happens too. I didn't know you could do that until I read Blue suggest that to somebody. Good luck, I'm just throwing stuff out there.
    1972 240Z #918 New Sight Orange
    1977 280Z #305 Light Blue Metallic
    1972 240Z #110 Persimmons Red

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    Do you still have the throttle linkage dash-pot? They get worn and sticky. The throttle blades get sticky in the bore also due to PCV backwash. Carb cleaner around the blade will clean it up.
    1976 280Z, with some minor modifications

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    Update from testing:

    Unplugging BCDD had no effect

    Unplugging TPS caused idle to hang the same throughout RPM range. ie, no hard drop to 2.5k followed by slow descent. Only slow descent the whole way. Car ran like poo with it unplugged, of course.

    I do not have the dashpot in place. That was the first thing I eliminated when I started looking into this.

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    Do you have the round gold colored item above the red circle in siteunseen picture? If you do this could be your problem. It screws out and in to help the throttle return slower when shifting.

    After I cleaned everything on the top of my engine, I had the same problem the rpm would not lower fast enough and that item was the problem, mine was too far down. I loosened the nut and screwed it out and that did the trick.
    Michael 11/75 - 76-280 - HLS30-281,114
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    Mgood, nope its not on the car.

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    Glliw can you post a good picture of the top area of the engine?
    Michael 11/75 - 76-280 - HLS30-281,114
    Web site -Click Here and ORIGINAL OWNERS OF THE 280Z (1975-1976 -1977 - 1978 - ONLY) REGISTRATION[

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    It would help to tell us what car/engine/modifications you have. If you have a carb'd 260Z, there is a dash pot that prevents the throttle linkage from closing too quickly, say when blipping the throttle. If this is hanging up, it could cause your problem.
    1971 240Z HLS30A 17574 L24-021025

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    Quote Originally Posted by mgood View Post
    Glliw can you post a good picture of the top area of the engine?
    I'll take one here in a bit.

    Quote Originally Posted by djwarner View Post
    It would help to tell us what car/engine/modifications you have. If you have a carb'd 260Z, there is a dash pot that prevents the throttle linkage from closing too quickly, say when blipping the throttle. If this is hanging up, it could cause your problem.
    The car is in the first post. There isn't a mods list because there aren't any, except for the vaccum deletes whilst troubleshooting this unless you think the front air dam affected this.

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    Make sure throttle return spring is still attached. Just throwing things out there.

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    Quote Originally Posted by rcb280z View Post
    Make sure throttle return spring is still attached. Just throwing things out there.
    Yep, both the torsion spring and the linear spring are in place. I installed a secondary linear return spring to see if it was a worn spring issue but did not see to help.

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    Here's an engine bay shot.

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    I think you should run a resistance test on your TPS sensor. A throttle position sensor is an electronic device called a potentiometer, its function is to increase in resistance as its dial is turned. Run a resistance test and when turning the dial slowly by hand watch for any erratic behavior as it indicates a bad spot in your pot
    Early 1974 260z
    https://sites.google.com/a/thecomputerrehab.com/260z/

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    I know this sounds simple but have you checked to see if the throttle is actually retuning to closed when you let off the gas?

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    Quote Originally Posted by er34gtt2000jp View Post
    I know this sounds simple but have you checked to see if the throttle is actually retuning to closed when you let off the gas?
    Yep, it is. I pulled the AFM boot off and watched it return. Comes to a shut nicely. When the engine is not fully warmed up, say, around 1/3 temp gauge and below, a throttle blip will result in a quick rev and return to idle. The problem is greatly pronounced once at operating temperature.

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    Quote Originally Posted by Kurbycar32 View Post
    I think you should run a resistance test on your TPS sensor. A throttle position sensor is an electronic device called a potentiometer, its function is to increase in resistance as its dial is turned. Run a resistance test and when turning the dial slowly by hand watch for any erratic behavior as it indicates a bad spot in your pot
    Between left and center pin, resistance is infinite at closed throttle and goes to zero throughout the whole range.

    Between the center and right pins resistance Is zero at closed throttle and goes to infinite slightly off of closed and stays that way throughout the full range.

    Between left and right pins there is infinite resistance throughout the range except for a small window of approximately 5 degrees off of closed.

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    Has it ever run right since you've deleted half of the the emissions and cold start systems? I've never seen anything like what you have there.
    1972 240Z #918 New Sight Orange
    1977 280Z #305 Light Blue Metallic
    1972 240Z #110 Persimmons Red

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    Quote Originally Posted by siteunseen View Post
    Has it ever run right since you've deleted half of the the emissions and cold start systems? I've never seen anything like what you have there.
    It did this before the deleting began.the deletes were part of troubleshooting.

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    Does happen cold and hot?
    Have you checked to make sure the AFM flap is closing?


    Ok I see now, open happens when warm. Seems like a BCDD issue but you said you eliminated that as a problem.

    How long have you had the car? Did it do it when you bought it? etc.
    Last edited by rcb280z; 04-19-2014 at 12:19 PM.

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    Throttle blade hanging due to PCV gunk was an issue documented by Nissan. Addressed with a factory published remedy in a Nissan Technical Bulletin.

    Just to more clearly restate what I posted before. Easy to test. Stick toe under throttle pedal and pull up when RPM are hanging high. Takes a little finesse but you can actually get skilled at it while diagnosing. If RPM drop the problem is between your toe and the throttle blade/throttle bore interface.
    1976 280Z, with some minor modifications

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    Quote Originally Posted by rcb280z View Post
    Does happen cold and hot?
    Have you checked to make sure the AFM flap is closing?


    Ok I see now, open happens when warm. Seems like a BCDD issue but you said you eliminated that as a problem.

    How long have you had the car? Did it do it when you bought it? etc.
    Seems to happen only when warm. I have not checked the AFM.
    Yes it has been an issue since Ive owned the car. I'm complaining now as I only drove it from purchase origin then to body shopwhere it sat all winter.
    Quote Originally Posted by Zed Head View Post
    Throttle blade hanging due to PCV gunk was an issue documented by Nissan. Addressed with a factory published remedy in a Nissan Technical Bulletin.

    Just to more clearly restate what I posted before. Easy to test. Stick toe under throttle pedal and pull up when RPM are hanging high. Takes a little finesse but you can actually get skilled at it while diagnosing. If RPM drop the problem is between your toe and the throttle blade/throttle bore interface.
    That used to help. I cleaned up the space around the throttle plate and added the helper spring which helps it come down quicker initially but still slow at2.5k downward.

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    I know little of the complexties of FI, being an SU guy, but there are basic similarities. I doubt the problem is fuel ratio related. The only way any engine will run at higher RPMs is increased air supply. More fuel is needed to sustain the higher revs but it starts with the air supply. Something is controlling the butterfly and preventing it from closing. Most carbs have a dashpot,(vac. diaph.) that allows the revs to fall normally, until just above idle. Think of it as a rev cushion. Whatever is causing this problem is attached to the throttle shaft or linkage and is temp. related too. Just out of curiousity how do your plugs look? It is possible running a very lean A/F ratio that an increse in fuel will raise the revs.
    Last edited by Mark Maras; 04-19-2014 at 01:27 PM.

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    Quote Originally Posted by Mark Maras View Post
    I know little of the complexties of FI, being an SU guy, but there are basic similarities. I doubt the problem is fuel ratio related. The only way any engine will run at higher RPMs is increased air supply. More fuel is needed to sustain the higher revs but it starts with the air supply. Something is controlling the butterfly and preventing it from closing. Most carbs have a dashpot,(vac. diaph.) that allows the revs to fall normally, until just above idle. Think of it as a rev cushion. Whatever is causing this problem is attached to the throttle shaft or linkage and is temp. related too. Just out of curiousity how do your plugs look? It is possible running a very lean A/F ratio that an increse in fuel will raise the revs.
    Thanks for the thoughts, Mark, as well as everyone else. Though i do not think that is it. While the 280z does come with a "rev cushion", that has long since been removed as that was the first item I thought to look at.

    To back up my statement on how the car performs when cold, please check out this video. The rpms act as one would think they should and return to idle in a reasonable period of time. Once the engine begins warming up, the issue will persist.


    I also popped the cover of the TPS and checked its function. I cleaned the contacts with a electric parts cleaner. Everything looked cherry in there though.

    Do you think one of the temp senders may be playing foul here? I didn't think any interact with the system like this, but with it only happening at warmed up and consistently then I don't know what else to think.

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    Registered User siteunseen's Avatar
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    I think you have a huge vacuum leak, EGR. Do you have a vacuum gauge? It'd be interesting to know what it's pulling.
    What about the A/C idle control? That round thing with the hook on your throttle rod. Take it off and see what happens, 3 bolts. 1 more place to put a return spring on the throttle rod is coming out of the drivers side fire wall on the top "marble" to the underside of the hood latch hole. You might could loosen the locking nut on the A/C idler hook and just turn it up? I don't know for sure. Or I'm thinking the flat piece that the hook catches is held on with an Allen screw, you could maybe take that off?
    Last edited by siteunseen; 04-19-2014 at 03:27 PM.
    1972 240Z #918 New Sight Orange
    1977 280Z #305 Light Blue Metallic
    1972 240Z #110 Persimmons Red

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    Quote Originally Posted by siteunseen View Post
    I think you have a huge vacuum leak, EGR. Do you have a vacuum gauge? It'd be interesting to know what it's pulling.
    What about the A/C idle control? That round thing with the hook on your throttle rod. Take it off and see what happens, 3 bolts. 1 more place to put a return spring on the throttle rod is coming out of the drivers side fire wall on the top "marble" to the underside of the hood latch hole.

    I would think the same except the issue isn't there at cold engine temps. Also, EGR is effectively non-existent since the vacuum source to it is blocked off via the BPT block off, unless a massive leak is at the gasket surface.

    I do not have a vacuum gauge handy.

    The a/c idle control is disconnected and can be seen as such in the pic above as I had the same thought. Doing so had no effect.

    A few days ago I did do the "yogurt cup test" by using a yogurt cup to block off the intake after the AFM, creating a closed system. I pulled off the brake booster hose and pressurized via that. During that, I could pressurize with my mouth to a point and could hear a leak, probably past the valves. It would hold the pressure for 5-10 seconds. I sprayed soapy water at each nipple connection on the intake manifold and did not see any bubbling. The AFM boot does not have any tears in it.

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    Take a hand held mirror and look under the intake manifold, mine had a leak where the EGR tube from the exhaust went into the manifold. I put my valve cover gasket on upside down one time and caused a vacuum leak up front. When I did the yogurt cup test through the AAR hose it parted my hair! It's something simple, I'd bet my house on it. Don't get frustrated, you'll find it, watching the videos you have a good car.
    1972 240Z #918 New Sight Orange
    1977 280Z #305 Light Blue Metallic
    1972 240Z #110 Persimmons Red

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    I saw the word "booster" earlier in the thread and assumed it had been covered, but does the problem happen when your foot is on the brake pedal? A bad booster diaphragm will cause a vacuum leak. Foot-on-brake might be coincidental with high RPM.
    1976 280Z, with some minor modifications

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    Quote Originally Posted by siteunseen View Post
    Take a hand held mirror and look under the intake manifold, mine had a leak where the EGR tube from the exhaust went into the manifold. I put my valve cover gasket on upside down one time and caused a vacuum leak up front. When I did the yogurt cup test through the AAR hose it parted my hair! It's something simple, I'd bet my house on it. Don't get frustrated, you'll find it, watching the videos you have a good car.
    It always is something simple. Only thing is that there are 30 simple things in the system.

    Thanks again for the help, I definitely appreciate it.

    How long roughly should it take the intake to leak down during yogurt cup testing? 5, 10, 30 seconds?

    I'll buy a cheapo vacuum gauge and a mirror here shortly and follow this write-up: http://www.atlanticz.ca/zclub/techtips/vacuum/

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    Quote Originally Posted by Zed Head View Post
    I saw the word "booster" earlier in the thread and assumed it had been covered, but does the problem happen when your foot is on the brake pedal? A bad booster diaphragm will cause a vacuum leak. Foot-on-brake might be coincidental with high RPM.
    It sure does, but it also happens when my foot isn't on the brake so I don't see a correlation there. In the videos above, I am not on the brake at all. Just run it up in 2nd gear and put the clutch in.

    I just did the vacuum test. I tee'd in at the brake booster line, at the firewall mount to try and include as much of the system as possible. From my perspective, I look pretty solid from a vacuum stand point. It hold at -18in.Hq at idle with maybe a 1/4 to 1/2 in. Hg fluctuation towards positive values. When blipping the throttle, it spikes to 0-ish, and returns down to -20 and settles back in at -18, just as it should.

    If there are any vacuum leaks at all, they are very minute and I do not think they are a driving factor here.

    I also inspected the underside of the intake manifold. I did not see anything obvious under there.

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    To siteunseen's point on the EGR- there have been cases where the internal passages of the EGR have rotted out. If the tube at the intake manifold from the exhaust manifold is open, and the EGR passage in the manifold is rotten, it would be a vacuum leak, even if the vacuum nipple is blocked or even if the top of the EGR has a plate sealing it.

    In the same vein but a bigger stretch, the internal-hex head nuts in to the EGR passage could be loose. Very unlikely though since they're really hard to loosen.
    1976 280Z, with some minor modifications

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    Quote Originally Posted by Zed Head View Post
    To siteunseen's point on the EGR- there have been cases where the internal passages of the EGR have rotted out. If the tube at the intake manifold from the exhaust manifold is open, and the EGR passage in the manifold is rotten, it would be a vacuum leak, even if the vacuum nipple is blocked or even if the top of the EGR has a plate sealing it.

    In the same vein but a bigger stretch, the internal-hex head nuts in to the EGR passage could be loose. Very unlikely though since they're really hard to loosen.
    Wouldn't that show as a vacuum issue on the gauge that I just tested with though? I would have to be below -15 in. Hg to begin to think I had a strong enough vacuum leak to create this effect I think. That and the whole works fine at cold temps part pushes me away from thinking vacuum.

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    I'm unclear still if it's vacuum, electrical related, or strictly mechanical...

    Warm it up to the point where it starts hanging. Once it's warm enough and starts exhibiting the problem, open the hood and rev the engine by grabbing the throttle linkage all the way back at the pivot point by the firewall. Then drop the RPM's slowly by releasing the linkage gently. The object is to get it to hang.

    Then once it's hanging, grab the linkage right at the throttle body and manually force the throttle body butterfly closed.

    Does it still hang while you're forcing the throttle body closed, or do the RPM's drop like they should?

    http://www.classiczcars.com/forums/f...ging-idle.html

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    Quote Originally Posted by Captain Obvious View Post
    I'm unclear still if it's vacuum, electrical related, or strictly mechanical...

    Warm it up to the point where it starts hanging. Once it's warm enough and starts exhibiting the problem, open the hood and rev the engine by grabbing the throttle linkage all the way back at the pivot point by the firewall. Then drop the RPM's slowly by releasing the linkage gently. The object is to get it to hang.

    Then once it's hanging, grab the linkage right at the throttle body and manually force the throttle body butterfly closed.

    Does it still hang while you're forcing the throttle body closed, or do the RPM's drop like they should?

    http://www.classiczcars.com/forums/f...ging-idle.html
    I will double verify this tomorrow, but I have done this already with no effect as I recall.

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    Quote Originally Posted by Glliw View Post
    Wouldn't that show as a vacuum issue on the gauge that I just tested with though?
    The reading on the vacuum gauge tells about the state of the various engine components - valve timing, valve condition, ignition timing, valve seals, rings, power balance between cylinders, etc. If the engine is running, it doesn't tell you anything about vacuum leaks in to the manifold. With a vacuum leak, the engine RPM can increase, actually increasing the intake vacuum or keeping it the same.
    1976 280Z, with some minor modifications

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    I was just reviewing this and thought it would be worth mentioning that the EGR system shouldn't engage until the car is up to full temp so its consistent with your symptoms. ZedHead is on to something there worth checking.
    Early 1974 260z
    https://sites.google.com/a/thecomputerrehab.com/260z/

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    That's a good point, Kurby. I think I'll buy an egr block off plate and rule that out entirely then. I haven't had a chance to look at it any this week and won't until the weekend most likely.

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    Just to be clear, you might need to seal the bottom tube also, to be sure. There are two passages in the manifold itself, the valve on top controls the flow between them. The plate will seal the passage but if the wall between has holes, it won't matter. You'll see how it works when you take the valve off the top. Look inside and you might see some damage. Just an improbable possibility.
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    1976 280Z, with some minor modifications

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    Gendt, The Netherlands
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    33
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    You still have a mechanical linkage, it could be sticking somewhere there. Usually the problems I had with the older injection engines was that the throttle plate would not close fully due to gunk, although it looked like it closed fully, you can not see those few mm...
    HLS30 32581, 5/71 Matching numbers

    Jay Leno : You know one week after the Americans have walked on the moon, the Japanese introduced this sports car, and…if you’re a car guy pretty equal. I mean walking on the moon was pretty good, but how many times you’d gonna walk on the moon? But here was an affordable sports car that had real performance and looked like it cost a lot more than it did.

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