No throttle response
 

Hey guys, I have a 2000 superduty with a 5.4 with 77,000 miles on it. When you floor the pedal, it doesn't do anything while driving at about 20 mph. If you gradually give it throttle it will gain speed but eventually fall down again especially when the outside air temp is hot. I changed the fuel filter, airfilter and cleaned the throttle body with throttle body spray. It idles alot smoother but it still has no throttle response. It had a 0174 bank 2 lean code so I cleared it and it has not come back yet but it is still not running right. I was thinking a throttle position sensor? Any ideas?

TPS should give you some transmission issues too. Have you checked for vacuum leaks? the truck is 11 years old, the rubber is getting old and vacuum leaks are easy and probable. A bad vac leak could even cause an o2 lean code.

I'd also pull the throttle body and wipe down the back side. Carb sprays don't cut all the gunk that builds up behind the butteryfly and cause hesitation and hunting idles.

You are probably going to find a failed TPS. It can be tested with a multi-meter.
Begin your diagnosis there.

I looked at all of the hoses and I couldn't find any leaks. I was told from a dealer to take the throttle body off and clean it like you said. That will be my next step. The transmission does act a little funny when you floor it also, it seems like it is getting "confused" on what to do. I put a fuel pressure guage on it and it is at 45 right where it should be.

Also, the transmission will not shift even when the motor makes it up to 3000-4000 rpm. It will just stay in that gear until you take your foot gradually off the throttle then it will shift and pick up speed.

Besides everything everyone else already said, check the fuel pressure. It should be 28-30 at idle, and 38-40 under load.

The "lean" code is telling you the computer is either not giving enough gas (or unable too because of fuel pressure), or not able to see how much air is getting into the intake.

Check the PCV vacuum plumbing for a cracked elbow.

I could not find any vacuum leaks, pcv valve working, changed the fuel filter, changed the tps which made a big difference, took the throttle body off and cleaned it and it still is not getting enough fuel. I am going to check the fuel pressure one more time just to make sure. When the engine is still cold it runs perfect, but when it warms up it loses the power, if you floor it, it does nothing until you slowly start taking your foot off then it picks up. How about the cylinder head temp sensor?

It sounds like a closed loop issue since it runs nice when cold. ECT, 02, and vacuum leaks still sound like places to work. Try using a can of starting fluid to check for vacuum leaks. With the engine running spray around the hoses, if the engine revs up, then you've sprayed near a leak.

Then I would put it on a scan tool and look for what changes after entering closed loop.

Found the problem. I should have bought a new fuel pressure tester in the first place instead of using an old crappy one. Idle pressure and the "hold" pressure is perfect. When you floor it, it goes up to 40-43 then drops like a rock down to 15-20. As soon as you start taking your foot off the throttle it goes back up to 30-35. I took the vacuum line off and it idles and drives right at 40, doesn't move. When floored it stays at 40 then drops back to 15, same as before with the line attached. So, it's either the regulator or the line itself (I hope), I'm banking on the regulator, what do you guys think?

You definitely have a fuel FLOW problem, either the filter is clogged, the pump is not putting out enough, there isn't enough current available to the pump, or the regulator itself is clogged or not opening all the way. Or, the pickup in the tank is clogged.

If it were me, and I really wanted to see what was up, put a gauge after the fuel pump, but before the fuel filter, and see if it drops the same way under load. KInda hard to do without cutting a line or some other crappy way of going it, and unsafe if you don't know what you are doing. I would consider changing the regulator (AFTER the fuel filter) just for the heck of it before tackling the fuel pump and pickup screen.

A few people have told me they think the fuel pump is junk. I told them with the regulator disconnected it stays at 40 until you floor it and drops right off. They felt it should have flooded the engine, not drop off but increase because the line was off and not regulating the fuel flow. I may try and find the return braided rubber section of the fuel line and gently pinch it off to restrict it back to the tank and see if that makes a difference. Could I take the line off the inlet on the fuel filter and blow some low pressure air back into the tank and see if the screen might be plugged without damaging anything? Can I just replace the pump itself or the hole modular assembly , I think this is going to get expensive.