1998 XJ sport, 2wd, auto. I bought this Jeep not running with a blown engine, so I don't know anything about its history. Replaced the motor and she fired right up, but I've been chasing a P0121 code and an erratic idle for weeks after installing a replacement 4.0. I've read all the posts on the subject of throttle position sensors. I've tried numerous sensors, checked all the grounds, vacuum lines, wiring, clock spring, fuses, etc.
I'm kind of at my wits end, but had an idea. I previously replaced all the injectors with a 4 hole design from K Suspension and unfortunately threw away my old injectors. If these new injectors were too large and were delivering too much fuel, the IAC would try and compensate by adding more air, but the computer would see that the throttle position was closed. Could this throw a TPS code? I'd love to swap in some stock injectors just to see if it fixes the problem, but I don't want to buy a set that I won't end up using.

 

the IAC will throw a code on its own, the computer knows the TPS is closed. The O2 sensors would trigger a rich/lean code if the injectors were throwing to much fuel that the ecu couldn't compensate for.

An old thread on NAXJA says this code can also be tripped by a bad map sensor or a vacuum leak throwing off the map sensor. I'd check that possibility too.

 

Thanks. Yeah, I’ve replaced the MAP sensor and the IAC sensor with new parts and also swapped on a junkyard throttle body with all factory sensors still attached, but still no change. With key on, engine off the TPS voltage input reads 5.1 volts. The signal wire with a multimeter shows .720 volts with throttle closed and only 2.9 volts at WOT. Seems like it should be 4.0 volts or something. With a scanner attached and at WOT it shows the throttle as being only 60% open.

 

The P0121 code signifies a "Throttle Position Sensor/Switch A Circuit Range/Performance" problem, meaning the PCM is receiving a signal from the TPS that it deems incorrect.

I would check that you have all the vacuum lines connected correctly. That includes where the MAP line connect to the throttle body that you have the line going into the hole that has vacuum.

 

Thank you. I have done that several times. I discovered a slight vacuum leak at the intake manifold by the firewall using propane so I changed the gasket and used the super expensive one that is guaranteed not to leak. However this made no difference. It does behave like it has a vacuum leak, but I can't seem to find it. When I start the car for the first time it races up to 2500 RPM and stays there. If I start it with the TPS unplugged it idles more appropriately.

 

Closed should read 0.25v and open is 4.5v. I think you have a bad clock spring friend.

EDIT : They usually make the voltage higher than normal.. But that reading is wrong so I would start there. .72 volts is why its idling rough. I'd check the TPS signal voltage again while a friend turns the steering wheel. Or just turn the wheel while its idling to see if that changes the idle.

 

I removed the clock spring entirely when installing a replacement turn signal switch and there was no change. I've checked all the grounds, all the wiring and connectors and can find nothing. I've read everything I can find on this forum and others regarding that code and the TPS. So, out of desperation I sent the ECU off to be tested, although I doubt the ECU is the problem. Time will tell. I appreciate all the responses.

 

The possibilities include cam sensor, crank sensor, injectors, and possible wiring/resistance issues.

I would check these issues and fit used parts from a wrecker

imo, poor quality Chinese injectos could case a problem

 

I think you've either got a bad TPS or the throttle isn't actually opening all the way. If you're only measuring 2.9 volts at WOT at the TPS, then the computer saying 60% makes sense.

 

I know, it’s nuts. I have tried four different throttle position sensors. The last one was brand new from Mopar. The throttle is definitely opening all the way. I have tested these sensors off the throttle body by hand as well and the max voltage is under 3.0 volts at WOT. I even swapped on a different throttle body I got from the wrecking yard with all the sensors attached and it behaved the same. Out of desperation I sent the ECU in to be tested/repaired. Time will tell.

 

Assuming you are correct about the throttle not opening all the way, why would the engine race up to 2500 rpm at idle with the throttle closed?

once the engine starts and with the throttle closed my scanner is measuring 2.9 - 3.0 volts from the TPS