Good evening gentlemen. I am stumped here. Had multiple mechanics here in Tampa check this truck out and no luck. 2004 F150 Lariat 5.4L 4X4. Has a new ford transmission in it and rebuilt eagle motor with about 15K on it. After the truck is warmed up to full operating temps it does this rpm jump. It’s very light but surges 200-300RPMS when accelerating. If I floor it the surging stops. We’ve replaced the computer, throttle body, MAF, checked all possible misfires, fuel pressure, and the gas pedal and sensor. We tried unhooking the torque converter wire, and it still acts up (thinking it was trying to engage and disengage). Any thoughts on this? I will see if I can get some better videos tomorrow. This video at least you can hear what’s going on.

thank you!

 

how fast were you going in the video, 70mph?
If you are driving at a constant speed and the rpms fluctuate 2-300 rpms then it seems like the issue is in the transmission/torque converter... In other words if the driveshaft is going a constant rpm at a constant speed, but the engine is bouncing, what connects between the two?

 

I was going 45-50MPH. That is my thoughts but it’s weird it only does it after the engine is fully warm and then probably another 10min of driving and it will do it. It doesn’t do it at idle only when moving. Maybe something mechanically has failed in the torque converter? We ruled out that it is being sent a signal to engage and disengage. I will see if I can get a better video tomorrow.

 

After all this time it just now throwing 2 codes. P1702 and P0708. I’m going to try and look into this more. Anyone have experience with these codes?

 

No direct experience with those codes but they both indicate an issue with the transmission range sensor on the side of the trans where the shifter hooks on.

I will say I had a 94 f150 and when that went bad on that truck it would shift into neutral as I was going down the highway and then back into drive kind of like what your truck is doing.
I replaced the switch and the connector on that truck and not more problems.
not sure this newer one works the same way but...
I think if I was you I would at minimum take the connector off and clean inside it to make sure there's no corrosion.

If it's not too pricey might want to just replace that range sensor. Some folks call it the neutral safety switch nBut it serves both functions, tells the pcm what position the shifter is in.. possible yours is shifting between D and N rapidly?

 

Well I swapped the sensor and unfortunately it did not solve the issue. I have it a transmission shop now and they are saying it is not transmission related.

 

Finally! We fixed it. Everything is perfect. Found a shop with a very experienced 5.4L tech. Which I visited most shops that said they had lots of experience and they were stumped since it was throwing no codes. It was the intake valve runner. The tech thought of it soon as he got in the truck.