small rpm surge while driving

[blankman]

So I wasn’t sure where to post this. The past couple weeks I have been having an issue where, while driving the rpm will surge maybe 100 to 300 rpm. This happens at any constant speed, with or without cruise control, in regular drive mode and even in sport mode. It will happen at low speeds, like 30mph between 70mph and happens regardless of what gear Im in at the same speeds.

No codes are thrown while this happens. I'm curious if anyone has any insight to the problem. Motor is stock plus a upr catch can. Car is lowered with frtp suspension, and 20 inch tires.

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[sk47]

Hello; If the car was from the era of carbs & a distributor I would have a suggestion. i chased a similar problem for over a year. Turned out to be the vacuum advance on the distributor. More specifically the diaphragm inside the vacuum motor. The vacuum motor would not hold steady, likely had a hole in the diaphragm. This allowed the timing to fluctuate when attempting to drive at a steady pace. A replacement vacuum motor cured the rpm hunt.
That said I know the new engines do not have an old type vacuum advance. The equivalent would have to be in the mechanism which controls timing.
I know of the cam phasers in the cylinder heads. Not clear to me what would make them operate outside of a normal sequence. They are both computers controlled and operated mechanically with engine oil pressure.

The only other clue might be the change in wheel size in your post. I wonder if a slightly different wheel & tire diameter from stock can generate out of phase information to some system. I helped my brother change the front rotors on his F-150. We made sure to count the notches of the replacement rotors tone rings to be sure they matched the factory originals. Point being can there be conflicting data input to the various computers.

Just crude guesses.

 

[robvas]

Data log it and watch things like:

Throttle actual vs commanded
AFR
MAF counts
Fuel trims

I had a bad throttle body that would surge like that at cruise

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