Roblaw

'75 280z #6 cylinder acting weird

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    Been trying to tune the car. Idles really well. Seemed gutless on the road though. Pulled injector connectors one by one and each one made it stumble, sans #6 cylinder. Switched the #5 and # 6 connectors and still with #6 injector disconnected did not make engine stumble. Interestingly, with the #6 connector on #5 injector, it idled fine until disconnected, then stumbled.

    I figured it was the #6 injector (a new to me rebuilt one). Bench tested fine with 9 volt battery, sprayed carb cleaner really well. So, now I think it's cleaned out, reinstall. In my mind the #6 injector connector is good because it fires the #5 injector and the #6 injector obviously sprays when voltage is applied. Put it back together, however, again pull the #6 connector from the #6 injector when running and no stumble. It really does idle nice though. 

    Pulled #6 spark plug and tested. It fires like mad when running outside of cylinder and it's not wet. When I pulled the #6 wire from plug when running, no noticeable stumble in idle. However, when getting wire close to plug, the idle picks up ever so slightly.

    Could this just be a timing issue that renders changes in #6 cylinder relatively inconsequential?

     

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    also a good time for one of those see thru color tune plugs.
    I'd go colourtune too. I'd guess you have an air leak at #6.
    • Like 1

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    smoke it out if you suspect an air leak. I had a missing injector o ring that cause issues, cant recall if it was like this.

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    17 hours ago, Roblaw said:

    Could this just be a timing issue that renders changes in #6 cylinder relatively inconsequential?

    All of your testing (except for my other comments below) indicates that the #6 injector is not opening at idle.  It might just be too sticky to open quickly enough for the very low idle pulse duration.  There's not much to an injector, a spring to close it and a solenoid to open it.  Carb cleaner can't fix rust or corrosion or baked on deposits.  The injectors get really hot after the engine is shut off.  You could swap injectors and see if the problem follows the injector.  Probably should have done that the first time.

    On the other hand, doing a lot of work to try to fix a diagnostic test doesn't make much sense, if the goal is to drive the car.  Most EFI cars have a "missy" kind of idle due to the way they open, in batch mode.  I'd be happy with a smooth idle and well-running engine.  Is the #6 plug the same color as the others?  If it's not opening the plug will be whiter.

    And, on the other other hand - I think that the idle get smoother on the EFI engines when they are running rich.  There might be enough residual fuel floating around that disconnecting an injector doesn't have an effect.  At idle there's a lot of backflow in the manifold.  

    Might be that the real problem is running rich at idle.  Not #6.  

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    It keeps getting stranger. Today I pulled off the VC to check valve lash, all in spec. Put it back together and timed it again. It was right on at 8 degrees advance, 800 rpm. It was purring like a kitten.

    Now the weird part. I ran the same test I did yesterday, pulling off #6 injector connector but this time it stumbled until I plugged it back in. I did nothing to the car but what I already detailed except I did disconnect neg battery terminal overnight last night, which I do often anyway.

    ECU? Any other thoughts?

     

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    What was engine temperature for the first test compared to this recent one.  My other comment had some thought behind it.  These systems are notorious for running rich.

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    Zed, it may be running rich. The plugs are black/sooty but not wet. It was running pig rich or more like not really running at all after warmup until I changed the CTS. So, this is definitely progress. It idles flawlessly, kind of stumbles off idle when driving and is not pulling very hard but will rev out. Gutless is my word to describe it. This is the 1st time I have driven this car but I have had other Zs, so have an idea of what it should feel like.  

    Compression is good in all cylinders, actually better than I expected. Vacuum seems tight and other than leaky PVC (changed out), have found no leaks. I have not pressure checked it though (no gauge). Distributor vacuum diaphragm is intact.

    I'm thinking about advancing my timing. Should I do it with vac advance disconnected? 

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    What is timing set to now?  Are you sure that the vacuum advance is connected properly and works?  Does centrifugal advance work?

    I'd measure timing at idle.  Rev it up and see if timing advances.  Disconnect the vacuum advance hose with the light connected and see if anything happens.  It shouldn't if it's connected properly.vacuum advance is ported vacuum.  Rev the engine with no vacuum advance to be sure centrifugal is working.  Connect your vacuum advance hose to a full time vacuum source and see if timing advances.  Make sure everything is working right, set your timing to a conservative number then go drive it.  See if it responds better to pedal to the floor compared to easing your way up in RPM.

    The distributors on these old cars tend to get gummed up and have various parts break.  You could easily be stuck at a single timing setting.

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    OK. Got some more time under the hood. When I suck real hard on the vacuum advance hose, I can hear the arm actuate in the dizzy and my tongue will be sucked in the hose and stay there until I pull it off. Seems good. With the light hooked up, revving the engine, timing advances from 8 degrees at idle to off the timing scale. Seems good. So, I advance the timing. It maxes out at 15 degrees at idle with dizzy turned to the stops.

    Took it on a run with this timing and no appreciable difference in power. Idle was a little higher but no seat of the pants pull/torque. Revs past 5K without issue but slow getting there. Feels like it's missing cylinder or multi cylinder inputs. 

    Engine wants to stall when oil cap is removed.

    Does any of this spark anyone's brain synapses?

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    I must have heard myself talking when I said "feels like it's missing a cylinder". I pulled plug wires one by one and engine stumbled on #4 cylinder. The wires were all shocking me too, even with rubber gloves on, super fun. So, pulled the plug, burned it with a torch (old dirtbike fouled plug trick) cleaned it up in the plug cleaner, regapped it, shot it with contact cleaner and blew it off. Drum roll please........... the car finally ran like I remember a Z running 😁. So, I cleaned all the other plugs the same way just for good measure. Also going to get new wires, don't really expect that will make it run any better but at least I won't get shocked anymore. 

    Now, on to the next problem. The car idles real high when pulling up to a stop. If I pull the pedal up with my foot, it'll calm down. Doesn't stick like this revving in the driveway.

    Seems like I read some threads that address this problem..

    The 15 degrees max advance at idle still has me puzzled but at least it runs good now.

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    My mistake. When I pulled the plug wires, all cylinders stumbled the engine except #4 cylinder.

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    That would be about a 17% power loss.

    The 15 degree limit, or less, is a fairly common problem.  Somebody in the past probably installed your oil pump incorrectly.  Or you have the wrong base on your distributor.  You don't need more than 15% initial.  You can wait on fixing it.

    Getting these things tuned up right is a long slow process for most, after they've sat.

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    10 hours ago, Roblaw said:

    I must have heard myself talking when I said "feels like it's missing a cylinder". I pulled plug wires one by one and engine stumbled on #4 cylinder. The wires were all shocking me too, even with rubber gloves on, super fun. So, pulled the plug, burned it with a torch (old dirtbike fouled plug trick) cleaned it up in the plug cleaner, regapped it, shot it with contact cleaner and blew it off. Drum roll please........... the car finally ran like I remember a Z running 😁. So, I cleaned all the other plugs the same way just for good measure. Also going to get new wires, don't really expect that will make it run any better but at least I won't get shocked anymore. 

    Now, on to the next problem. The car idles real high when pulling up to a stop. If I pull the pedal up with my foot, it'll calm down. Doesn't stick like this revving in the driveway.

    Seems like I read some threads that address this problem..

    The 15 degrees max advance at idle still has me puzzled but at least it runs good now.

    The idle problem is the BCDD, I think its supposed to act that way. below 10mph it disables (idle drops). Mine does the same, not a big deal.

    the design is trying to avoid high vacuum, lean run, high temp exhaust smog stuff. with out a computer and bunches of sensors it just does some crude things by todays standards to keep that from happing. At least that is my take on it from reading the FSM emission controls section.

    Edited by Dave WM

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    My BCDD had a high idle when I first got the car running decent.  I was able to adjust it now it's fine.  If yours drops when you pull the pedal up that has me thinking your throttle blade is gummed up and sticking.  If it stays high without fooling with the pedal I'd clean the throttle body but if it drops down after a few seconds you need to try and adjust the BCDD.  It's very easy to do just by driving around your neighborhood and turning the nut on bottom.  It'll get better one way and worse the other way.

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    14 hours ago, Roblaw said:

    The car idles real high when pulling up to a stop. If I pull the pedal up with my foot, it'll calm down. Doesn't stick like this revving in the driveway.

    Might be a dirty throttle body and blade.  Nissan changed the PCV system in 77 because the PCV gases were gumming up the throttle.  The fact that you can stop it by moving the throttle back with your foot is a sign.  Mine only did it when the engine got good and warm.

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    If you decide to clean the throttle body take your time with that accordion boot from the AFM to the throttle body.  It'll split if you look at wrong.  I wouldn't even try to get it off without first unsecuring the afm from its base.

    Once you have access to the opening an old toothbrush or 2 and some lacquer thinner will clean it up like new.

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    Thanks to both of you. I'll be cleaning TB in the next couple of days. Zed, funny you mentioned the PCV routing change. When I was putting the engine bits back together, I looked at pictures of 280z engine bays and noticed the PCV arrangement was different then mine.

    So much to learn. At least my other car obsession (Ford Maverick Grabber) is a simple beast.  

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    Finally reinstalled the exhaust heat shields last night after clutch install. Then I started fiddling with the BCDD and found the bolt on the bottom hanging down moves in and out freely about a 1/2 inch, kind of floppy. When pushing in, there is what could be described as soft resistance. However, the bolt can not be twisted with just my grasping fingers. Is this normal or has the bolt been completely backed out which I believe would render BCDD inop?

    When I get more time, i'm going to pull the TB and clean it up.

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