1996 Ford F-250 4x4 ext cab longbox

460 automatic

75,000 actual miles


Vehicle came in with a "falls flat on your face" condition when under a load.

I did the following: new plugs, wires, cap, rotor, fuel filter (old ones were all original)

Scanned vehicle .... No codes present

Fuel Pressure is consistant at 30-32 psi.

Doesn't actually miss, but it feels underpowered under a load.

Upon higher rpm, is feels just fine.

Any ideas??? Thanks in advance.

"Vehicle came in with a "falls flat on your face" condition when under a load.
Doesn't actually miss, but it feels underpowered under a load."

"Falls flat on your face" and "underpowered under load" sound like two very different issues.

If the owner couldn't be bothered to change a spark plug in 14 years, why figure he did any maintenance at all? Do a compression check.

catalytic converter clogged maybe?

catalytic converter clogged maybe?

I agree.... also look at the air filter (maybe its original too!) and O2 sensors

yup check the air filter and all of the ait intake
for being a 96 and only having 75k on it maybe it has a nest clogging up the air intake or wires chewed or something.

My compression reads approx 150 across all 8 cylinders with very little fluxuation.

Revised Symptom:

The vehicle accelerates and shifts normally for about the first 5 minutes of driving, then upon light to medium acceleration, is bogs, when stepped to the floor it bogs for a split second more then accelerates. The loss of power is most noticible when going up an incline, or towing even a light trailer.

There is no check engine light, or stored codes.

The TPS was replaced.

The ignition coil was replaced.

catalytic converter was unhooked for test purposes, still no change.

Thank your for your help with any recommendations. Please don't use sarcasm, it's unproductive, and nobody appreciates it.

Also, the air cleaner looks like new. intake are passages are clear. There is no evident places of chewed wires, or hoses.

check fuel psi when it is warm pump could be on its way out ,fuel filter also

"Please don't use sarcasm, it's unproductive, and nobody appreciates it."

Don't speak for everybody. Certainly I appreciate sarcasm, it's usually funny and anything that produces a laugh is not unproductive.

Is your request a pre-emptive strike? 'Cos I don't see any sarcasm in this thread, it's all serious suggestions. That's too bad - I don't mind spending 10 seconds reading something that gives me a laugh. In fact, I quite enjoy it. It's a welcome break while I'm trying to solve other people's problems that are described over the internet.

I think a performance problem appearing after only 5 minutes of operation is more. likely fuel than ignition related.

Just to follow up, problem solved.

O2 sensors fluxuated rich to lean when in closed loop. As soon as loop closed sensor reading would stick at .500 to .650 volts (rich condition). By reading on scan tool.

Tried new Bosch 02 sensor, same result.

Tried genuine Motorcraft (bosch built) 02 sensor, same again.

Finally I pin tested 02 wires at Engine computer. They read .075 to .900 volts, while scan tool said 02 was stuck at .650.

Verified no voltage feedback on computer grounds (all less than .05 volts with vehicle running)

Installed used computer. Scan tool read 02 volts exactly as back-probed meter.
Vehicle road tested A-OK with no more problem condition.

Thanks for the follow up. I was beginning to think "compleated threads" where just some vicious rumor. In regard to "Cheap sarcasm". Besides being "Cheap" (Expensive is when you can't duck in time???), It's legal, non-controlled & as of this date, non-taxable. Wait a minute.............. Your a Democrat!! It's O.K.(I'm pretty sure), I'm polish & they still let me keep po................................................ .................................................. .................................................. .................................................. .................................................. .....