Greetings... again... I can't seem to keep this thing running for longer than about a week.

For some time now, when I go out in the morning and start the car, it runs at fast idle (around 1500 RPM) just fine, for 30 seconds, or so. Then it drops to "mid idle" of about 800 - 900 RPM At that point the engine would start to "shake" for lack of a better term, and feel like it was misfiring, although it is not. Normally I'd let it idle a couple more minutes and the shaking would abate somewhat. Then I'd drive off, and once the car became totally warm the idle would drop to the normal 600 - 700, and all would be well.

A few days ago I pulled out of a parking spot, turned right, and the engine started running rough, like in the mornings, but worse. And the failsafe message came on, along with ASC not available. I stopped, turned the car off for about 10 seconds, restarted, and all was good. I pulled codes as soon as I could. P0121 Throttle position sensor A circuit range/performance problem.

This happened again for the next couple of days, only in the morning, and only once. Today, it happened a total of three times, even when the engine was fully warm. Same code.

Almost every time, it happens at just off of idle, like when gently pulling away from a stoplight, or when turning a corner. It doesn't happen at idle, and it doesn't happen at, say, 1/4 throttle and above.

I've ordered some contact cleaner and will clean contacts, but I suspect that's whistling past the graveyard. I suspect a bad TPS. When i got the car the throttle body had been replaced. The PO mentioned it, but I have no paperwork. The TB has markings as a salvage yard would use. If it was the same quality as the trans I pulled out of it, then the quality and mileage of that used TB is suspect.

I have searched the forums but not really found my answer. If this system is anything like my Mustang, and it is, when you replace the TB and TPS you have to futz with the TPS a bit in order to get the correct voltage at idle. So, my questions:

1. What should the TPS voltage be at idle?
2. Does anybody have the pinout of the TPS? That will save a bit of time testing it.
3. I see TPS units all over the joint for around $40. My impression is that Jag does not sell them as a separate part. Anybody have any experience with a particular aftermarket TPS?

It seems the few posts I've seen where the owner replaced his own TPS it was a matter of trial and error, futzing with the TPS until a "satisfactory" idle was achieved. But I strongly suspect there's a more accurate method. Gurus, any suggestions?

TIA!

 

OK. I decided to research this and post the results for posterity.

The TPS is located on the RH side of the throttle body. There are 2 TPS sensors, and I believe that sensor #2 is the one visible.

There is a 4 pin connector on the TPS. The pins are numbered from bottom to top, 1 - 4. Bottom meaning the end closest to the engine.

Pin 1 is the ground. It serves as the ground for both sensors 1 and 2.
Pin 2 is the signal wire for sensor 2.
Pin 3 is the signal wire for sensor 1
Pin 4 is +5v power for both sensors 1 and 2

I measured the voltage of sensor 1 with ignition on, car cold and not running, at 1.52v
With car running in "high idle" I measured 1.64 volts
With car running in "mid idle" I measured between 1.49 and 1.52 volts
With car warm I measured 1.46v.
When increasing the engine speed the number goes up steadily.

I can find no information on whether or not these numbers are correct. But the car idles fine after warmup.

Before testing I got a can of electrical contact cleaner and gave both halves of the connection two good sprays. Also the Throttle Motor connections, for good measure. After letting them dry out for about 15 minutes I reassembled.

After a few hours I pulled the car out and ran around the general neighborhood, going fast, slow, turning corners slowly, stop and go. The trip was 20-ish minutes and no sign of a fault code or failsafe mode.

I'll try again maybe later today, or when I get back from Seattle on Monday.