I've got a 2000 xj sport with a 4.0 auto trans. It either wont idle, or it idles at about 200rpm. As long as my foot is on the gas it runs fine. I know it's going to be asensor, I just dont know which one. This started overnight, it was working great two days ago. I plan on cleaning the MAS tonight as well as the TPS. Any and all ideas are appreciated, I just don't want to start replacing sensors without knowing for sure which one it is.
CF Veteran
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The TPS can be easily tested if you have a multimeter. It's a sealed unit, so if it's bad, it's bad.Originally Posted by DigginDeep13
I've got a 2000 xj sport with a 4.0 auto trans. It either wont idle, or it idles at about 200rpm. As long as my foot is on the gas it runs fine. I know it's going to be asensor, I just dont know which one. This started overnight, it was working great two days ago. I plan on cleaning the MAS tonight as well as the TPS. Any and all ideas are appreciated, I just don't want to start replacing sensors without knowing for sure which one it is.
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The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad
You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.
The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad
You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.
Member
Don't know about the newer ones, but the old ones have an idle air stepper motor or IAC. This sounds a bit typical of an IAC failure.
No codes yet, I haven't made it over to AutoZone for their reader. I replaced the IAC about 5 months ago.
This morning as I pulled into work it idled properly for about 4 seconds at 750 rpm, then it cut out and dropped back down to 200. When it drops down to 200, the amperage from the battery drops down as well, but as soon as I give it gas it comes back to normal. It doesn't fluctuate on the road, and the headlights don't fluctuate when revving up, so I'm pretty sure that eliminates the alternator.
Is there a way to test the iac? I did make the mistake of buying it from AutoZone..
Seasoned Member
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I don't know why people are always saying autozone parts are junk. Originally Posted by DigginDeep13
Is there a way to test the iac? I did make the mistake of buying it from AutoZone..
their "duralast" brand electronics are a mix of wells and OEM.
when i worked there i pulled a coil for a guy's saab, checked the coil before i gave it to the guy and it had a "saab" jewel logo on the top.
either way, you'll be okay, if the part fails its not as if it will prevent you from making it home. it's not hard to change, and if you have to change it, it's lifetime warranty.
Seasoned Member
well it certainly wouldn't be your TPS. when that thing goes... well haha it goes. but I would certainly clean out all the passages in the iac as well as a good throttle body cleaning and go from there.
Either your TPS is giving a faulty reading or your IAC is stuck. tssguy123's post has a nice little writeup on testing the TPS which can be done with a multimeter. If you find it's dead avoid BWD branded TPSes. NAPA Echlin is pretty good, Mopar of course being preferred but admittedly expensive.
Spontaneously dead electronics from Autozone are pretty common. Would not surprise me if it's failed. However, there could simply be a problem with the wiring. Corroded connectors, chaffed wires, and bad grounds could interfere with the signal to the stepper motor. I think the IAC is bundled into the same harness as the fuel injectors, so go take a look.
Unfortunately the FSM does not describe how to test the IAC. One thing you can try is just disconnecting the battery to dump the PCM's short-term memory. If it performs normally you've found a flaky motor... no reason it should lose position like that. If it happens again I'd replace the IAC. If it still doesn't idle at more than 200 RPM you can check airflow through the IAC passage and through the CCV. Take the intake hose off of the throttle body and start the engine. If you have no air flowing through the IAC passage it obviously isn't opening. Rear CCV line can be checked too - yank it off of either the manifold-side or the valve cover-side and plug the hole. If the engine stalls you've confirmed a bad IAC that is not opening.
Spontaneously dead electronics from Autozone are pretty common. Would not surprise me if it's failed. However, there could simply be a problem with the wiring. Corroded connectors, chaffed wires, and bad grounds could interfere with the signal to the stepper motor. I think the IAC is bundled into the same harness as the fuel injectors, so go take a look.
Unfortunately the FSM does not describe how to test the IAC. One thing you can try is just disconnecting the battery to dump the PCM's short-term memory. If it performs normally you've found a flaky motor... no reason it should lose position like that. If it happens again I'd replace the IAC. If it still doesn't idle at more than 200 RPM you can check airflow through the IAC passage and through the CCV. Take the intake hose off of the throttle body and start the engine. If you have no air flowing through the IAC passage it obviously isn't opening. Rear CCV line can be checked too - yank it off of either the manifold-side or the valve cover-side and plug the hole. If the engine stalls you've confirmed a bad IAC that is not opening.
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Absolutely not true. The TPS is electrically identical to a volume ****, failure modes vary wildly. Testing is essential to finding a problem.Originally Posted by 4.slow95
well it certainly wouldn't be your TPS. when that thing goes... well haha it goes.