P0175 error. #5,6,7, & 8 cylinders not happy - HELP

Hello to all!
I’m new to the Forum but have been reading posts for a while trying to get this truck up to speed. Bought it at Auction and got it for a song, I think I know why now... One problem I can't tackle, if you can help please?
 
PROBLEM:
It's an '03 Super Crew w/ 4.6l and 121k. 'Service Engine Soon' light is on. Runs rough at Idle. If I rev the engine, it tries to conk-out when the RPM's drop down. Runs fine when accelerating and traveling. Gets only about 7mpg. Exhaust smells very rich.
 
SO FAR:
New Fuel Filter, New Air Filter, New Plugs, New O2 sensors, but No changes.
 
I TRIED THIS:
I took it to my buddy at work who has the $10k Ford diagnostic touch-pad laptop (supposedly the primo one Ford uses, he's at a County Fleet Management Office)... It gave a P0175 error code and passed ALL the diagnostic tests, everything reading fine including the O2 sensors. The compression test showed between 1-4% low on cylinders 5, 6, 7, 8. and in the real-time ignition chart showed that cylinders 5,6,7, & 8 were all misfiring (Mainly 7 and 6). We changed the Plugs and swapped out some Coils but same problem, no improvements or changes. When bringing up the RPM's above idle, the ignitions even-out and do not misfire. The mechanic is baffled about the fact that only the left bank is affected. Any ideas? I’m thinking the left side air intakes may be obstructed or clogged? Are there any mechanical devices that would just affect the 5,6,7 and 8 cylinders and make them run rich?  My buddy says worst-case is replace the head???
 
Enlighten me, someone, please..........?
 
Matt.

 

Could timing chain is off on that bank. Did you hook a vacuum guage to it?

This one sounds like that bank is one tooth out of time.  That would show compression low on that bank and misfires on that bank.  A vacuum gauge probably won't show much because of the new intake design on these motors.  Closely inspect it to see if the valve covers or front chain cover have been removed.  It would be unusual for one complete bank to have cylinder problems, but not for the cam to be out of time after being worked on.  These are not a lot of fun to install the timing chains on and you can easily miss this if your not careful.

Ok, I will have the mechanic check the timing chain on that side, thanks for the info..
Will I have to remove the covers to adjust/check that timing?
 
I also got a suggestion that it was a vacuum leak, and to carefully spray short blasts of carb cleaner on the junctions of hoses and see if there was any change in the RPM's indicating the cleaner was sucked in the leaking hole... no results, just a cleaner engine..
 
Was going to go buy a brand new coil and start swapping to see if there was any change, i read somewhere a bad coil may cause RFI and glitch the opnes close to it?
 
Thanks. Matt, still hunting...

Vacuum leaks don't generally cause rich codes.  RFI may be a cause,  just haven't seen it cause this particular issue, but can't rule it out.