I have been tuning and tuning since I put the 385 in my 86 Vette earlier this year. Started with a modded LT1 intake and then changed to a Holley Stealth Ram in mid tune (probably my first mistake?).

Anyway, the tune was coming along nicely and then I purchased a Wideband O2 (AEM) and placed it into the passenger side collector (same position as the heated O2 in the drives side). I noticed that at idle the WBO2 was always lean in closed loop fluctuating from 15.1 to "---" (greater than 19:1). Low load cruise was also lean. No missfires noted while idling or under load.

I started diagnosis and noticed that if I unplug an injector (any of them) on the passenger side while running it will go to "---" constantly.

If I unplug an injector on the drivers side the AFR on the WBO2 is in the normal 14-15 range (ECM compensating for the lean reading on the NB I assume/confirm with Datqamasters data log)

Checked vacuum - steady at 15".

Purchased an IR temp gun and checked header temps at idle. Initially found that the front four cylinders were in the 550 range and the rear four were in the 650 range. Resealed the plenum of the HSR and rechecked temps. Now all of the cylinders are in the 550 range - EXCEPT #6 still 650.

Pulled the entire intake today and resealed. Swapped injectors on the passenger side around (front to rear). Readjusted all valves (roller cam and 1.6 full roller rockers). Changed spark plugs.

Still a 650 reading on #6. If I unplug #6 injector and watch the temps they rapidly drop to the 500 range.

Closed loop AFR is still lean but I did notice that while in open loop it is actually normal and slightly rich (as it is supposed to be at startup).

My only other thought is that maybe I have a vacuum leak at the screw in rocker stud. For the life of me I cannot remember if I put sealant on the threads. From what I have read some of the threaded holes do penetrate the intake runner. The intake ports valve tops were really clean and I didn't see any evidence of oil being sucked in.

Bad NB O2 throwing everything off? Something wrong in the tune?

Am I looking too hard for an "Issue"?

I need to stop buying tools - everything was great until i put in the WBO2. That led to the IR temp gun. Jut kidding - never enough tools.

The engine seems very strong and runs good - but I am not done tuning yet.

Any Ideas or Theories?

 

Sounds like #6 is still lean for some reason. Try the throttle body cleaner test to see if it's an external leak before you go and take a rocker stud out. What heads are you using? I don't recall ever seeing bolts/studs protrude into an intake runner on any I've ever seen.

Have you tried putting a thin coat of silicon around the runner ports? I know it's not common practice, but it may have a small leak there. Did you have the block or heads decked? That could throw off the intake alignment just enough to develop a small leak.

You did right by asking, that's for sure. You can't be too careful when it comes to getting a fresh motor right.

 

The block was cleaned up but not zero decked.

The heads are ProComp 210 CNC. I know, I know, should use AFR. Didn't have the money and after checking independent sources found that they flow better than you think and some folks are making really good numbers with them.

I did seal the intake ports in the head side - guess I will pull it again and seal both sides.

I know one issue may be trying to mate a HSR to the recommended 1206 gasket. This time I went with a 1266 (twice the thickness). The ports seem to match up pretty well - easy to see with the plenum off. Pulled plug #6 and it looks pretty good with the exception of a portion of the porcelain is black (had a thread about this a while back - I'll see if I can find it). I just put these in yesterday.

Maybe I also need to really check out this used HSR or have the machine shop check it over.

Think that I may be finding my own questions here..... Maybe writing it out and reading it helps?

 

Found and updated pic in post. It's not #6 but it looks like this just not that bad.
Did check #4 today and no more black - maybe another clue to my answer?

http://forums.corvetteforum.com/c4-t...or-o-ring.html

Time to pull the intake again - woohoo. At least it's not a TPI.

Also realized it is easy to check for injector leak down after prime with the plenum off. There are no clips to keep them in the rail so you can't pull the rail and energize the fuel system. Just energize and look in the ports.

 

Decided to buy another tool - a Bore Scope - from HF ($92.00 with tax). Always wanted one so I got it and investigated the intake.

Pic of the intake gasket - nothing telltale but going to change and seal it again.


Pic of #6 intake valve. That is a piece of RTV and a piece of gasket from the plenum removal.


Pic of the runner where the rocker stud penetrates and what appears to be oil?


All of the intake valves look similar and all the runners have rocker stud penetration. Looked back through all my assembly pics and none show me with sealant on the studs.

Hmmmm....The plot thickens......

 

Yes, it does! Cool tool though. I've got an old style with the eye piece, but have been wanting one like this for a while. Didn't realize they had gotten so cheap. Maybe it's time for me to get one too.

Thanks for your update and info/pics on the scope.

 

Have you checked to make sure the valve lash is correct, and not too tight?

 

Been through the valves a couple of times - one cylinder at a time through the firing order method. I find zero lash and give a half turn then lock them down.Going to do it again apparently after I pull the rocker studs to seal (or reseal) them.

Not sure if it is that noticable in the pics but I could actually see and follow the dark oil trail from the stud penetrations to the valve seat. I was shocked to see that buildup on the backsides of the valves.