1999 surging then slowing on highway.

PROBLEM:

My revs on the highway are varying from 2250-2500. Doesn't seem like much variance but if im locked into a speed, it certainly feels like it. This surging only happens on the highway, so its probably engine speed related. Sometimes it feels like the engine might drop revs so much that it might stalll...but that never happened.

OTHER SYMPTOMS:
my cruise control doesnt always work. I can turn it on, but it will not set and lock into a speed. This started happening around the same time I noticed the engine rev problem a few months ago. It might work maybe 1 out of 20 trys. And when it does, the rev problem will kick in, and disable the cruise control.

Yesterday I noticed that my oil pressure gauge is really really high. On the highway its at 80psi, and at a red light its about at 45.

CURRENT STATE:
I havent done a spark plug/cap/rotor/wires change in about 2+ years. Removed a random plug a few weeks ago, and it looked in good condition, just maybe needing a re-gap.

Also just changed the oil. Castrol synthetic blend 10w30, and Purolator Pure One filter. This is the standard oil change usually.

TPS was also replaced 2 months ago, with no change in symptoms.

Does anyone have a thought as to what might be going on here?

Any codes?

Sounds a lot like your intake manifold may have a leak and/or your vacuum system - that would cause the surging and cruise control issues.

The rest, I don't know.

JJacobs, Im going to run to autozone soon and have them run a scan. Ill report back with any codes.

kastein: I know the headers have a leak at the base, I patched it up with some high temp metal putty a few years ago. How would I go about searching for a intake manifold/vacuum system leak?

Easy way of searching for an intake leak is to get a ten dollar propane torch at home depot... set it up, turn it on, do NOT light the flame, just let it leak gas at a fairly decent pace. Now sweep it slowly along all the vacuum lines. When you hear the engine rev up a lot, your leak is nearby and it's sucking the extra fuel in. Goes the same for the intake manifold but be more careful because it's tougher to get to without getting your fingers in fans/serpentine belts/exhaust manifolds/other non-finger-friendly places.

Man that sounds like a disaster waiting to happen for my appendages!

I second a possible vacuum leak. The fact that your cruise is on the fritz as well could be related. A vacuum leak is a very common root cause for a cruise control problem. Inspect all hoses closely and re-do any.....sometimes inspection doesn't do the trick.

As for the high oil pressure, that should not be related to the other symptoms. Oftentimes, it is because of a faulty oil pressure sending unit, a very common failure on the 4.0 engine. If you want to confirm, get your mitts on a mechanical oil pressure gauge and temporarily install it where the sending unit resides.

If you replace, I highly recommend purchasing from a JEEP dealer, aftermarket oil pressure senders have kind of a shaky reputation for accuracy/reliability IMHO.

How about your torque converter locking/unlocking?

Check the brake pedal switch, would also effect cruise control.

X2 with joe. If you are maintaining the same speed, but revs are going up and down, doesn't it have to be slippage in your drivetrain?

Joe
You might be on to something there. I'll take a look at the tranny fluid and see how it looks. It hasnt been changed in a couple years. Other than the fluid, its probably a hardware issue huh? Assuming thats the problem, how much do those run?

Sincerely thank you for all the suggestions guys, what a great place to diagnose a problem!!

Torque convertor locking/unlocking is an electrical thing. I know that a few of the guys have an indicator light to signal when theirs is locked up. If you rigged that, at least you would know if it is electrical or mechanical in the torque convertor itself.

Yeah but that seems kind of roundabout if thats not the problem. Ill have to get a scan and report back with any data.

Try unplugging the TCU and drive it. If the problem goes away then it is an electrical issue, wiring harness, solenoid(s), or the TCU itself.

Did you check the brake pedal switch?

Well I got the ODBII scan, and it was throwing out NO codes. The autozone guy seems to think its either a vacuum leak or a tranny hardware issue.

Which one is the TCU again?
Did not check the brake pedal switch...

The TCU on your 99 is a couple inches away from the steering column near the knee panel. Small rectangular silver box with a long electrical connector.