RPM surges at highway speed

lfhoward · **Posted:** Mon Jun 18, 2012 9:48 am

Recently my Jeep seems a little bit jittery when holding speed at 50-70 mph, sort of like the engine has trouble settling in at a constant rpm. I used my Scangauge to check the RPMs, gas mileage, etc, and it is not my imagination. Every 5-15 seconds there is a slight surge in RPM of about 100 or so, for example: the engine goes from holding at 2000 to about 2100, then settles back in at around 2000 again. At the same time, there is a corresponding drop in immediate fuel economy, like from 21 to 18 mpg, but then recovers back to 21. It happens both on cruise control and also when keeping the skinny pedal constant manually. I have used my Scangauge for 6 months to keep tabs on gas mileage stats as I drive, so I am pretty sure these rpm fluctuation symptoms are a new development in the last 2000 miles or so. It is most noticeable on the flat, or on a slight incline, on the interstate highway. I don't notice it in town at slower speeds. There are no CEL codes or pending codes, and for now this seems like an annoyance rather than a problem, but my gut tells me it will become worse over time. A quick :google:

brought up the idea that the Throttle Position Sensor (TPS) might be going bad, or possibly the torque converter was disengaging and engaging. However, none of the posts I found were about KK's. Anyone else have this problem, or is there a Jeep mechanic on here who's seen this before? Thanks in advance! --LFH

irollgen4s · **Posted:** Mon Jun 18, 2012 11:41 am

any chance you've been using the AC lately and weren't using it prior to those 2000 miles?

lfhoward · **Posted:** Mon Jun 18, 2012 11:46 am

irollgen4s wrote:

any chance you've been using the AC lately and weren't using it prior to those 2000 miles?

I pretty much use the AC all the time. I haven't changed my AC use or driving behavior in the last 2000 miles. Good to rule it out, though.

Bmxer524 · **Posted:** Mon Jun 18, 2012 1:32 pm

I noticed this around 40-50 mph. Don't know if it always did that or not because I rarely stay at a constant speed that low lol. I hit the tow/haul button the other day when it happened and that stopped it, so I think it might have something to do with overdrive engaging around those speeds.

lfhoward · **Posted:** Mon Jun 18, 2012 10:00 pm

Seeing if I can replicate the problem with the Tow/Haul button engaged is a good idea. It would rule out overdrive as the issue.

On another note, since our KK's are drive by wire with electronic throttle control, do we even have a TPS? The TPS going bad seemed to be the most common cause of surging rpms at highway speed (according to the many posts I read on various forums), but I can't seem to find a Mopar part number for that for an '08 KK.

lfhoward · **Posted:** Wed Jun 20, 2012 9:12 pm

I drove about 3 hours round trip to Ann Arbor today, two-lane county roads one way and I-94 the other, and the engine behaved beautifully. No jumpy rpms. Hmmm. I'll keep an eye on it, but for now it seems to have fixed itself!

Bmxer524 · **Posted:** Thu Jun 21, 2012 1:55 am

Yeah I never got the surge outside of 45ish mph. It wasn't even a surge on mine. I just had the rpms going up and dropping such as a car with a cam would have when idling.

RPM surges at highway speed

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