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

 

[blankman]

Gotcha. so I've got the throttle gauges added and the afr. I'm not sure what the MAF count would be under. also as far as fuel trims ive got a lot of options. Short and long, banks 1 through 4.

 

[PoCoBob]

Have you tried the basics first? Run a can of Seafoam in a tank of gas and replace the air filter. Lots of times people tend to over think these things.

 

[blankman]

Filter was replaced in April along with my oil change. Every year I run a bottle of cleaner through the fuel system during first fill up.

It doesnt happen at idle or while holding a constant rev while in park or neutral. This is leading me to believe it is not motor related. Also happens with gas from different stations. i havent tried holding it in gear using sport+ yet to see if it happens but I'll do that in the am on the way home from work.

 

[2021LMP750]

These symptoms kind of sound like intake runner solenoid issues it won’t always throw a code but can still cause intake surging. May not be it but it’s probably worth checking.

 

[blankman]

These symptoms kind of sound like intake runner solenoid issues it won’t always throw a code but can still cause intake surging. May not be it but it’s probably worth checking.

Would it cause intake surging at higher rpms? Im about to leave work so im going to see if happens at a higher rpm while holding a lower gear.

 

[K4fxd]

Need to post a log, what software do you have?

 

[blankman]

Just an obdmx+ adaptor and their app, and forscan on a laptop.

 

[blankman]

Need to post a log, what software do you have?

Just using an obdmx+ adapter, their app and I Do have forscan on a laptop. More than happy to install something to log if you can point me in the right direction.

Edit :: I can also confirm the surge does not happen above 3500k rpm while holding a gear. So I'm not sure if that also points to something. I would have rode it longer but I had to slam my brakes on just to hit something on the road because of cars nearby I couldnt avoid it. Luckily I don't think there's damage as it did sound like a metal impact.

 

[Gregs24]

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.

Auto box?

If so it could be the torque convertor unlocking which does exactly that

 

[Bob Lob Law]

Mine did this a bit when on the stock tune. I haven't had it happen since being tuned.

 

[blankman]

Auto box?

If so it could be the torque convertor unlocking which does exactly that

That is correct. Stock auto 10spd. Thats one of the things I read from Google about the TCC slipping or trying to find the best gear. Thats what advised me to try it in the different modes and sport plus to see if it happens in those as well.

 

[blankman]

Mine did this a bit when on the stock tune. I haven't had it happen since being tuned.

Ive still gotta the 5 year 60k available, so I can always call the dealerships which I may do. I just like to try and figure out whats going on and why it might possibly be happening. The more knowledgeable I am , the better off I am.

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