99 TJ CEL issue - help

[benmack1]

99 TJ, 2.5L, 93K. The jeep had been idling odd the last few months (driven very seldom unfortunately), sort of stumbling every once in awhile while idling. It didn't sound like a regular miss but what the hell do I know. Didn't seem to matter warm or cold. I did a tune up a couple months ago (plugs, wires, coil, dist cap, rotor) and some fresh fuel, seemed better maybe but honestly it was hard to tell, probably no real change and the symptoms were never terrible (never quit or anything and revved up just fine, power was fine for what it is - 4cyl old jeep). Fast forward to this morning. I drove about 20 miles on the freeway, no problem. Got off, had a couple stop lights so I was shifting up through the gears and I could feel it stumbling and the CEL came on, It ran real rough in 3rd gear revving up for maybe 15 seconds. I then stopped to turn back and it was not stumbling anymore the rest of the way home. However, when I stopped at intersections it was idling about 1200 or so very high (usually around 600-800?). Drove home 20 miles. Ran fine. When I got home it's running/idling normal. No more stumbling other than when it tossed the CEL on.

Pulled the Codes p0121 and p0551 (TPS and pwr steering pressure sensor). I can replace both those sensors of course but wonder if either is the problem or what might be going on here? Power steering fluid seems on the low side but defintiely not below the add line while hot.

 

[Fabrik8]

Throttle position sensor will definitely cause those problems. Throttle position is one of the primary inputs into the fueling and ignition calculations, and is basically what all of the desired torqued calculations start with, as that is the communication between your right foot and the ECU (on a non drive-by-wire engine).

The high idle is most likely a safe mode with table lookup values if it's not getting valid throttle position data. Throttle position sensors do wear out, and they do get bad spots within their rotary travel range, so I'd replace it. There are sliding wipers and contact strips that just mechanically wear through or get corrosion spots, which is why a lot of engines have non-contact throttle position sensors that work with rotating magnets instead of sliding mechanical wipers.

You may also want to look into whether there is a throttle position adapt procedure after replacing, where the ECU will learn the sensor voltage at the fully open and fully closed positions. I don't know enough about the specifics of Jeeps to know whether that is necessary or if there is a procedure to do so.

It's very good that you got an actual CEL for the TPS, because sometimes they take some thinking to differentiate from other problems with similar symptoms, like MAP/MAF problems or a vacuum leak. Replacement is usually very quick and easy, and make sure to get a high quality part.