Throttle Acting Up

[tabrown11]

I successfully completed this job: coolant-leak-crossover-pipe-gasket-t600711.html for my '01 QX4; however, when I went to test-drive the car afterwards, my throttle is acting up when the car shifts from 1st to 2nd and from 2nd to 3rd gear.

When it shifts from 1st to 2nd, the throttle "flutters" [that's the only way I can think to explain it] just a couple of times, causing the car to slightly jolt forwards and backwards; but then the car behaves normally after it shifts. And when it shifts from 2nd to 3rd, the RPMs jump from approx 1500 to 3000 for just a second, then it behaves normally. I test-drove it up to approx 60mph, and there were no other inconsistencies seen/felt.

I'm trying to narrow down what would cause this. What do you think could be the culprit?

Thank you all for the help!

[rgk]

Make sure you connected everything properly after performing the coolant crossover gasket fix. Check the throttle position sensor.

[4xq]

I think I agree with RGK. Also, check vacuum hoses for leaks.

There is a TPS adjustment procedure at EC-109. You need a feeler gauge and a voltmeter. Not sure if that will help, but now you know!

[atraudes]

That TPS adjustment procedure is to ensure the TPS is properly communicating the throttle being fully closed and as soon as it's being opened. Unless you're having issues while idling or taking off, that shouldn't be needed.

The TPS is definitely suspect, though. The transmission makes shifting decisions based on its potentiometer output.

[donald]

I apologize for detracting a bit - but were you able to remove the crossover tube completely to replace the gaskets?

[tabrown11]

I apologize for detracting a bit - but were you able to remove the crossover tube completely to replace the gaskets?

Yes I was... it wasn't easy, but I did the passenger side from above and the driver side from both above and below [It is easier to access the bottom stud from below]. They just barely slid on.

[tabrown11]

That TPS adjustment procedure is to ensure the TPS is properly communicating the throttle being fully closed and as soon as it's being opened. Unless you're having issues while idling or taking off, that shouldn't be needed.

The TPS is definitely suspect, though. The transmission makes shifting decisions based on its potentiometer output.

If setting the TPS isn't the answer, what do you think would be the culprit? A bad TPS?

[atraudes]

Barring shorted wiring, yeah, I would say a bad TPS. Start with the simple stuff first though, like replugging all of the connections on the throttle body and MAF sensor, clean the MAF sensor (with appropriate cleaner), and check for frayed/cracked wires. If you end up replacing the TPS you will need to do the adjustment procedure.

Before you buy a new unit, the FSM (location cited by 4xq) also has some procedures on checking continuity on the potentiometer side of the TPS to ensure the resistance changes steadily as the throttle is opened and doesn't jump around. While you're there, you can also check the other side of it and make sure you have continuity when it's fully closed and none as soon as its open.

I was reading through your original question again and a few more questions came to mind. Do the RPMs jump up and down more than once between shifts, or just once? Does it only happen when it's cold and go away after the engine's warmed up? I just wanted to make sure we weren't talking about transmission flares which would be a transmission problem. It couldn't hurt to check the fluid level and quality.