'75 280z #6 cylinder acting weird

[Roblaw]

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?

 

[Dave WM]

compression test on #6?

[Dave WM]

also a good time for one of those see thru color tune plugs.

[Roblaw]

I checked compression months ago. 170 psi #6

[jonbill]

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.

[240260280]

EFI manifolds sometimes leak at EGR due to metal erosion.

[Dave WM]

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.

[Zed Head]

 

 

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.  

[Roblaw]

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?

 

[Zed Head]

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.

[Roblaw]

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? 

[Zed Head]

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.