Doesn't sound like you have a problem Day. Don't know how sage it is, but are you using a shroud on radiator? Maybe you're getting more fan flow over the the front left bank with no accessories bolted on.

Yeaah... that's all I've got.
Wish my bad theories could be dismissed as easily. 

I am shooting the top of the header tube after it exits the flange. The headers are old and brown no coating other than rust 😀

It’s the stock truck cam. Didn’t realize they were different. I thought Ford used the same cam for both applications.

You might try swapping injectors and see if the cool cylinders move.  A slightly sticky or leaky injector could cause that I guess.  If you hold it at a fast idle...say 2K for several minutes do they even out?

TKOPerformance wrote:

The two colder cylinders are running a bit rich, which is why they are cooler. 

Exactly, that is why I was using the thermometer as a diagnostic tool to begin with.
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Bullet Bob wrote:

If you hold it at a fast idle...say 2K for several minutes do they even out?

Brilliant idea!!  most of the running was at idle, I will check into that the next time I have a chance to mess with it and get back to you.

Raymond_B wrote:

Do the plugs for those two cylinders look different?

Yes and no.  There is variation in all the plugs because of all the other fueling setups that have been on this engine up to this point.  There is not enough time with any one setup to read the plugs accurately.  Also during a previous test there was an issue with a leaking intake and oil getting in the chambers so some of the plugs are blacker then they would normally be.  I think your idea is also good.  I will go buy a set of cheap plugs and install them that way I can read the plugs after the next running.

Daze wrote:

Here is the specifics:

351W on the run stand
EFI
5.8 lower 5.0 upper
stock roller cam
aftermarket aluminum heads.

Here is the "problem"  (I mentioned it earlier on a different thread but thought it was related to fueling issues) I like to use an inferred thermometer on the header tubes as a way to look for a lean cylinder.  When I was running the carb intake and my TBI EFI I noticed cylinder tubes 5 & 6 were not getting as hot a the other 6.  Figured it was the TBI setup.  Then when I went to the 5.8 lower and 5.0 upper I used my carb to EFI adapter and fired up the engine.  Again tubes 5 and 6 were again cooler.  Now that I am running MPFI header tubes 5 and 6 are still about 200º cooler.  This to me says there is a problem in those cylinders BUT it is not running rough, it's not missing and the engine seams to be running strong.  I did a compression test and all 8 cylinders are around 175psi give or take 5.  I then checked all 8 spark plugs and they are all firing.  All 8 spark plug wires are also working no issues and the distributor is doing its job.  I am not getting black smoke or a rich gas smell in the exhaust .  I have come to the conclusion that there is nothing wrong with the engine but it still bugs me.  Any sage advice or a solution would be welcomed.

 
I had my infrared laser temp gun out tonight because I was checking temperatures at the inlet and outlet on my radiator and decided to check my header tubes too for sh!ts and giggles. My headers are ceramic coated a charcoal gray color. At idle, all  of mine were between 575-600 degrees (measured at the same place you did), but #2 and #6 were 100-150 degrees cooler than the others. Engine runs fine. I have decided I am not going to worry about it. 

We’ve all been there, an d I am 100% sure that 90% of what I know is because I did something wrong the first time. Sticking with it payed off, glad you figured it out. I am yet to attempt EFI.

EFI can lead you down the "garden path".  Twice I have had a significant vacuum leak at the manifold/head gaskets and both times the engine ran just fine.  First time it didn't want to hot-start and the second time it had the "woo-woo" some of you may recall me complaining about.  Butt...I still think I'd rather have EFI than a "dripping pot".

BB1