My dad is only the 2nd owner of this truck which has less than 100k on it.

The problem it has is this: When accelerating under load the engine will sometimes bog after an initial hard pull and act almost as if it is running out of fuel. Then all of a sudden it'll clear up and start pulling hard again. It may not bog anymore and sometimes it will switch between bogging and pulling hard. The problem is most persistent when the engine is at operating temp and not when it is cold and still in open loop operation. There is no check engine light.

He purchased the truck recently from the original owner in barely driveable condition due to the fuel system going bad from sitting. Only one of the two in-tank pumps worked and neither fuel gauge worked. Since he acquired the truck he has replaced one fuel tank and both sending units and both pumps. A new fuel filter was also installed and the fuel system was thoroughly flushed. The engine runs smooth, starts easy and has no other issues. It just won't perform well under load. He has also replaced all the spark plugs, wires, cap, rotor, etc and also installed a new TPS on the throttle body. None of this changed anything.

The only remaining possibility that we have not investigated further yet is an exhaust leak at the manifold flange on the passenger's side (donut gasket). I am wondering if a bad enough air-in leak in the exhaust stream could create a cold enough EGT condition that might cause the o2 circuit to malfunction. Anyone have any input on this? Any help is much appreciated.

Edit: The fuel pressure regulator was also replaced.

 

Try another new fuel filter. It's not unusual to have to replace two or three before the old, varnished gas is out of the system.

Last year I bought a Park Ave that was parked for four years… cleaned the tank and two filters later it runs great. I had my Peterbilt parked for 30 months when fuel hit $4.00/gallon in '06/'07… despite using an anti-bacterial treatment I replaced more than 10 fuel filters in three weeks before the truck ran without fault.

 

We did consider that, however only one tank was 'bad' and had not been in use in many years since the pump failed in it. The other tank was still nice and clean inside (and still had a working fuel pump), it just had a bad fuel level sending unit. I forgot to mention that the performance problem is the same running on either tank now - both with all new parts in them. The fuel system was flushed thoroughly before placing it back in operation with the new filter. I would be very, very surprised if there was enough trash in the short section of fuel line between the tanks and filter to cause any blockage in the filter. The front tank was pristine and the rear tank is brand new.

I am really doubting a mechanical fuel problem. It acts to me like the computer is just pulling the injector pulse way back for some reason. Its not like it sputters or misses, it just reduces power - by a LOT.

 

Next step would be to scan for codes...

 

You sure it's not the trans? You have the trans with the push button OD? Pulling a heavy load, you have that button pushed? If it's trying to shift into OD and jumping back out again it can make it feel like the engine is stuttering. It's just a thought.

 

Nope, that's not it. It'll do it in any gear under load. Something is pulling the injector pulse back...or maybe the timing. And it's intermittent. It's probably going to end up being the ECM.