RPM drops while driving?????

RPM drop while driving
I recently did some work on my truck, new fuel filter, oil, oil filter, valve cover gaskets, spark plug tube gaskets, intake mainfold gaskets, MAF sensor cleaned, throttle body cleaned, and cleaned some dust and dirt off the air filter. About a week ago it started to drop rpm while I drive once i get to speed and the rpms come to the normal 1500-1600. They drop down to 1000-1100 and i can feel it aswell as watch the rpm gauge drop. I am getting no DTC codes or check engine light. I am stuck and have no idea what to check next. Maybe PCV valve? Thanks for any help

 

No CEL or lost power just the RPM dropping to 1000 - 1100 when cruising? Does it run and drive normally?

 

It idles fine and runs fine over 2000 rpm. It only starts to drop rpm to 1000 and jump back up to 1500 once i am up to speed (45 mph) and cruise at that speed. It seems to do it more when the car is cold and does it less once it is warm

 

I'd first suspect the throttle position sensor on the accelerator pedal. Sort of like the wiper on a fuel level sending unit in a car can have bad spots where the wires wear through if the tank is always kept at the same level of fuel. Nothing else comes to mind when you have no check engine light and no stored trouble codes and the engine runs fine at other rpm levels...

 

I was wondering if what you are seeing is the transmission hunting between lockup and not locked. Which will show up as the RPMs change.

 

Like @Jimmyh suggests, this was a very common issue back when lockup converters came into play in the '80's. Made some city driving pretty annoying.

Experiment. Try driving just a bit slower, and just a bit faster, and see if the effect stops.

 

Being that you are experiencing the problem at around 45 mph, I agree with Jimmyh, as that's the speed that the torque converter locks.

 

Only fly in the ointment is that MY 4.0 does not drop 500 rpm when the torque converter locks up. That indicates a TON of slip which seems unreasonable. One missing piece of data is "does the speed drop when the RPM drops?" I had assumed "yes" but if that is wrong, then the TC (or a transmission that is improperly shifting up and down) might be more likely.

 

2nd gen?

 

Found out it was my intake manifold gaskets! Changed them today and the issue was gone. Ill be changing the pcv valve this coming week just for a piece of mind its changed out. Thanks for all the help!!

 

In your OP you said you had changed them already...

Unaccounted air getting in after the MAF causing a lean fuel mixture.

Good you found it.

 

I did change them but not with new ones just ones in "better" shape. I overnighted some felpro ones from Amazon and the issue seems to have cleared up. Huge head ache gone now!