I am working on a f150 4.2 manual trans. the mil & gas cap light came on. The mil code was an evap code with the gas cap as one of the probable causes so replaced gas cap & the light hasn't been back. Both before & after the gas cap change the engine has no power at low end in any gear. In first it feels like you are starting off in 3rd or 4th. I did some fuel tests & ended up replacing the fuel filter, fuel pump, and pressure regulator. No change. After that I heard crackling from the engine plug wires so I replced plugs & wires. No change. Hope somebody has answer I'm am ready to shoot it like a horse.

 

How many miles on it?
Will it rev freely if you put it in neutral?
Does it idle smoothly?
When accelerating - or trying to - does it run smoothly, or does it stumble/miss?

 

160,000 mi.
Yes it runs smooth at idle & when reved in neutral & when trying to accelerate.

 

Is it possible your catalytic convertor is clogged?
Sounds like excessive exhaust gas backpressure, possibly, maybe.
I duuno ??

You positive you have the plug wires hooked up properly?

 

I drill holes in the exhaust before the cats & checked the back pressure & it was less than 1lb. The engine runs to smooth to have crossed wires. If it wasn't for the lack of power you would think the engine was running perfectly normal. I should mention I am a fleet mechanic for a company with about 60 F150's with the 4.2 in them. I've had some stumpers before but this one is kicking my butt.

 

AH, so you know what you are doing, eh?
Cool. This is an interesting problem.
I do want to follow along and see what you find.

How many miles on it?

Is it JUST starting off? What if you are cruising along at 30-40 mph and nail the gas (and down shift), will it move on good or does it labor to accelerate?

Based on what you posted, I'd guessing you checked fuel pressure and it is OK.
Any chance the injectors are clogged?

Is there an RPM range when it seems to catch up with itself (so to speak)? That is to say, is there an RPM where it comes alive, or does it continue to just build power slowly until it gets to where it needs to be?

I was thinking maybe something related to timing and/or cam position, but I'd expect a OBD II code if there was a problem in that area.

Any chance the air intake is obstructed. Seems like long ago (years) I remember some guy having a problem like this and he found a shop rag he had left in the air intake tube, or some place like that to block air intake.

You are correct, this is a goodie. Please post back. I really want to know what you find.

I have faith: you'll figure it out.

 

It has about 160,000 mi.
When it hits around 1300-1400 RPM it has normal power & accelerates fine. It's just when you start off at the beginning of every gear. It's real slow to gain RPM's when you climb hills. Sometimes you have to down shift when going up a grade to keep the RPM's up so you can climb the hill. When on flat it will accelerate just fine as long as the RPM's are there. I've checked the intake, no obstructions. My next step is running injector cleaner through the fuel rail, after that not sure.

 

Now I'm a little confused.
1300 -1400 rpm is just off idle.
Much below that and seems like it'd be about to die even if all was normal.
Maybe I'm missing something here.

 

Idle on the truck is in the 800's & when you release the clutch the Rpm's drop below idle a bit then it struggles to get up to the 1300 to 1400 RPM then takes off. As I said though if you are on a grade it may never get to 1300 depending on how steep it is.

 

Is it possible that the TPS is messed up? Or something else that is just not taking the requested input from your foot and getting that request all the way to the fuel management system.
Maybe the fuel pressure is good, but the electronics are messed up and just not getting fuel into the cylinders.

I am guessing you have tried, but will ask anyways....
Have you disconnected the battery and caused the ECU to reset to defaults?

You know you can press the odometer button when you turn the key on and hold it in to get to test mode and see lots of data?

Stay in touch.....

Thanks!

 

Sorry its been a while, but I had other trucks that needed my attention. Now I'm back on it and I performed a fuel rail cleaning with 3M cleaner that didn't help. I disconnected the battery. same. I pressed the odometer and the test mode came up but didn't show anything but it did flag a MIL that was a missfire on #2 cyl. thats when I did the fuel rail cleaning. My next move is doing a compression check. I'm thinking maybe cam sensor or if lack of compression a valve, but I would think it would run rough. The bad thing is that the engine runs smooth.

 

Might want to check for a worn out clutch.Ford uses a hhydralic clutch release system you dont know the clutch is worn out until this happens.

 

Brian, I gave up looking for the problem and called a company called Identifix. I talked to a technician and he recomended checking the harmonic balancer to see if the key-way had slipped. Didn't think this was possible, but sure enough the key-way wore a larger groove in the balancer and changed the timing just enough to make it lack power at low end under load. I ordered a new balancer, it will be here tuesday and hope that should do the trick.

Thanks for the input and if you don't hear from me the problem is solved.

 

Thanks for the update.

That has been troubleshot and diagnosed a few times here in the past.

Sorry I didn't think to mention it.