I have a '97 Ford F250HD 7.5 with performance loss. CEL came on and set codes PO171/174. Had a hard time finding why but I did eventually find a vacuum leak behind the throttle body. No more CEL. But during the days of cleaning or replacing parts I noticed performance loss. It was minor at first but each day got worse. I think I noticed it the day after I changed the Fuel Filter and removed the MAF sensor to clean it, but didn't look dirty so I put it back w/o cleaning.
I have also done a Compression check, cleaned Bosch plugs and checked wires.
Replaced Cap and Rotor.
Replaced 2 of the 3 O2 Sensors, have B1S1 left.
Replaced Fuel pump back in August.
Air filter is good. Vacuum is good according to vac gauge. But when I remove oil filler cap for paper test I thought it should have been stronger.
Had a new crate motor from Ford installed in June 2014, had 379,828 miles on last motor. Been running good up until now, even ran better with the hole in the vacuum line than it does now.
I have a half way descent code reader with live data but I don't know what I'm looking for exactly. Pretty sure that the STFT B1S2 shouldn't be flat lined at 99.2% if anyone could help me with this that would be nice. I've been researching other trucks with the same problems but have found that only a small % post what actually fixed the problem. Been guilty of that myself before but never realized how much it could help somebody else until now.

 

Any loose connections?
What's your fuel psi?

 

Short Term: Should never really be too much more than a few %.
Long Term: Generally anything +/- 30% or so is technically "fine", but I like to see long term under 10%.

Start by checking fuel pressure, next I'd try simply unplugging the MAF. It will go into a default mode where it should more or less run fine.

 

Thanks for the replies. I'm trying to get hold of a tester that will go on the fuel rail. But in the meantime I will do the MAF thing.
I should mention that when it is in park and running most of the time it idles fine and responds quickly when throttling. Just hates being under a load.
What is up with my year truck not having a Timing Pointer/Indicator off the block? I've seen on a engine diagram that one mounts on top of the misfire sensor/crankshaft positioning sensor. I don't have one on my truck, so how can I check the timing?

 

Fuel pressure results are KOEO 35lbs, KOER 35lbs.
Didn't find any loose connections and going to drive it with the MAF disconnected next.

 

Unplugging the MAF sensor made it a little bit worse. Going tomorrow to see if I can't find a Timing pointer for it. Does anyone know what other years or models also used that crankshaft positioning sensor set up?

 

1996 and later trucks with Cali. Emissions.

 

Fuel pressure should be 40 PSI when you cycle the key with the engine off, and ~32-34 PSI at idle, and increase to 40 PSI when you snap the throttle.

 

I found out what the problem was, Catalytic converter was clogged up.
Removed it and runs like a 460 should. Nice to my old truck back again. Thanks for everybody's help.