Jeep p0123 - Hesitation

2001 4.0 manual XJ

It (finally) rained here in Phoenix on Wednesday and I was out playing around running through puddles (not deep, just big puddles). I went through one of them and my tires flung a wash up over the hood (35" MTs with cut fenders). Not long after that, I started getting hesitation on acceleration and a P0123 code.

- The vehicle cranks, idles and drives under steady state cruse just fine (no miss or bucking).
- If I slam it WOT in every gear, it does fine.
- If I drive "normally", there's a point in the throttle travel (around 1/4 or 1/3 throttle) that it hesitates and bucks really badly

I've done the following testing:
- Installed a new TPS sensor = no change
- Installed a new MAP sensor (just to be sure) = no change
- Tested the voltage at the TPS (vehicle on and turn throttle shaft while observing voltage change) = 0.8v idle and smooth growth to 4.1v at WOT (both sensors show the same values)

Since both TPS sensors show the same (good) values, I'm starting to suspect the ECU got water in it and shorted something out. The ECU is in the factory location on the driver's side front inner fender and I'm sure it would have gotten "wet" (again, vehicle was not submerged in any way), but I'm surprised water splashing on it would short it out. I don't see it being the coils/a spark issue since cranking, idle, steady state cruise and WOT are all fine. It's just that low throttle transition (there must be a transition point in the fuel/air table) that has issues and again, if I just slam WOT in every gear, no hesitation (because I pass the "dead spot" in the table).

Any guidance on what to try next? Buying a $250 ECU is going to hurt if that's not the problem.

Thanks,
Chris

The TPS should show .25 volts at minimum throttle opening (idle), to 4.8 volts at WOT wide open throttle.

I would suspect water in one or more wire plugs under the hood, and I always suspect cheap crappy Chinese clone TPS as being out of specification.

That would be my suspicion also, check all terminations and lube them.

Oddly enough, the vehicle sat all day in the parking lot at work (warm sunny day) and now the problem seems to be gone. I'm still getting the code, but the hesitation is gone.

I'm going to pull all the ECU plugs, blow them out and grease them this weekend.

Tim_MN said:

The TPS should show .25 volts at minimum throttle opening (idle), to 4.8 volts at WOT wide open throttle.

I would suspect water in one or more wire plugs under the hood, and I always suspect cheap crappy Chinese clone TPS as being out of specification.

Click to expand...

Original TPS was OEM Jeep and it's voltage values matched those of the O'Reilly unit.