I just got my 383 back together and I have a stumble which I believe is from the number 2&4 cylinders running so lean. I've measured the exhaust temps on both of those cyls. at over 200 degrees hotter than the others. And the longer I let the motor run the worse it gets.

So far I've checked for vacuum leaks and couldn't find any, swapped injectors with no change, double checked the wires, re-gapped and swapped the plugs, and re-adjusted the valves.

Is it possible that I have a vac. leak in the lifter valley? Wouldn't I be sucking oil in the cyl. and be able to see that on the plug? As of right now the plugs all look good except for those two which are a little white.

Any suggestions would be appreciated, thanks.

 

The center exhaust ports always run hotter on an SBC because they are so close to each other. 200*F delta sounds excessive though. When you pulled the plugs did they verify those cylinders are running lean?

 

Yes, the plugs on those two cylinders are the only ones that are discolored. I also realize that a little difference in temp is normal. But if I take the readings when I have the rpm's hovering around 1800-2000, the lean two holes will be reading 700*+ while all the others will be at 450* or so. It's got me a little confused.

 

Is it possible the opti is on its way out is making the car misfire on just those 2 cylinders?

 

i think a spark missfire would make it run cooler would it not?

 

I agree, I don't think it's ignition related. It runs hotter because it's lean (Too much air, not enough fuel). Of course, I could be completely wrong. That's why I'm open to any suggestions.

 

I just thought of something else. Could the injector wiring harness be screwed up, causing the injectors to fire sporadicly? Wouldn't that throw a code tho?

 

If you had dirt in the injectors, they would cause it to run lean, and that would not set a code. But, when two cylinders run lean, the O2 sensor on that side will pick up the excess air in the exhaust, and the PCM will start to add fuel. Now you will have the two healthy cylinders running rich, and the two lean cylinders running not quite as lean.

Swap the injectors to other cylinders, and see if the problem follows the injectors.

 

When does the car stumble? Can you datalog?

My car at idle (see below) runs hot on 3 and very cold on 7 making for over a 200 F difference but my BLM's are perfect and the car runs smooth so I don't worry about it anymore.

further, when I pulled the heads after 500 miles (thought I had a leak and was not the heads), the combustion chambers were showing no unusual signs. All valves were very similar as were the plugs.

 

Already tried that and it didn't make a difference.

 

I can hear the stumble at idle and when I give it a slow rev. It gets worse with any load.
I was just assuming the stumble was from running lean, but I guess it could be a separate issue.
I also do have freescan so I can log it, but I'm a newb when dealing with anything computer related. I'm learning tho.

 

I was wondering about the programming...but I don't think there is anything you can adjust to bias fuel in each cylinder.

Something could be goofy with the harness. It could be some kind of issue with the drivers for the injectors. They may not be pulsing as long as they should. You would need to see what the commanded injector pulse width is and then put an oscilloscope on the injectors.

Do you have a spare computer to swap into the car? That would check to see if the computer hardware is an issue.

You could grab and mechanics stethoscope and listen to the injectors. You may be able to hear if the pulses are shorter at idle. Best to use an oscilloscope though......I suppose a noid light might reveal something too.

Other things to check could be valve adjustment and possible intake leaks between the intake manifold and head.

 

Are your heads ported? If so by whom? The reason why I ask is that if they were ported, those two ports could be better flowing causing it to run leaner. Just a thought.

 

Individual cylinder fuel trims in tunercat will do exactly that, whatever percent you want. They do need to be manipulated for a cammed engine.

Lean without a misfire will raise egt's, so will too rich or a weak/late spark on those cylinders, same goes for getting plug wires on wrong.

 

So the programming could be suspect. He needs to be sure that a good tuner was used to program it.