The RPM in my 95 Sportage engine will quickly drop and surge at highway speeds. Around 55 to 60 mph, the RPM will fall and rise 250 to 500 rpm below and above the normal cruising RPM of about 3000 without any additional input to the throttle. Under acceleration, the engine pulls smooth and steady, and around town this problem doesn't exist. Anyone have any idea what may be causing this? The Sportage has about 290,000 mi., is an automatic, and is bone stock. Thanks for any help.

Wow, I thought I was the high miler at 270k!

Just a hunch but whatever is causing this is probably always there but only noticeable at a consistent cruising speed.
Around town your speed is not as consistent,rpm's are up and down so if you were to set your cruise control at 25mph it might appear?
Not a solution but may add to helping diag the problem.

I researched this issue on the net to no avail. Individuals with the same issues reported having problems in the following areas.

1. Spark Plugs - Need Replacment
2. Vacumm Leak - Identify leak - Replace Hoses
3. Throttle Housing / Intake - Carbon Build Up - Needs Cleaning.
4. Oxygen Sensor - Needs Replacement
5. Air Flow Sensor - Needs Replacement
6. Cylinder Firing Order / Cam Adjustment

* Rent an ECU reader and see if you get codes.

Sorry, Others may have more experience with this...

Is it SOHC or DOHC?

I had this problem with my SOHC. It turned out, the shop that did the tune up before I bought it installed a spark plug into cyl 4 that was too long, piston hit it and cyl 4 no longer fired. I'm not saying this is your problem, but I'd inspect the spark plugs first. I always start with the things that are free.

Good luck
JP

The Sportage is a DOHC. Yeah, I'll start with the cheap and easy stuff. It's not a big deal right now, but it won't get any better either.

Say Hi to My Mom and Dad in WA first! <img src="/forums/images/graemlins/smile.gif" alt="" />

Are you getting any codes????

Cause that would basically rule out lots of things (sensors,plugs etc..)!
.....

Thanks,
Mikey

No, it's not throwing any codes. Your Mom and Dad say Hi! Nice folks. Where'd you go wrong?

<img src="/forums/images/graemlins/lol.gif" alt="" />

No Idea! <img src="/forums/images/graemlins/butwiggle.gif" alt="" /> <img src="/forums/images/graemlins/nana.gif" alt="" />


Seriously though: With no CEL It sure changes perspectives a bit.

I would be having a reasonable guess of the torque converter intermittently locking up when it should be solidly locked at that speed. This would be proven if you could put it into 3rd gear at similar engine revs and it does not surge. (the 250-500 rpm difference is the giveawawy on the converter lock-up)

Try a few of the electrical contacts around the transmission. It may not necessarily show up as a fault by the TCM if it only momentary loses contact.

Other possibility could in fact be the torque converter or TCM or splices in looms etc. (IIRC there is one that does throw a code, Axeman knows about it and where/what to do)

Thanks for the ideas DamKia. I'll try terrorizing the neighborhood in third gear this morning. Come to think of it, I did use the "Hold" button to lock the tranny in third gear while climbing a hill a couple days ago. The RPM did appear to rise and fall steadily. I'll double check, though.