IJM

Cliff's Notes: Runs like crap a minute after I start driving. Fine at WOT. No codes. Could it be the TPS? Can't figure out how to test it.

I've been having these symptoms for a few weeks now, and it's finally bugging me enough to post about it. It started out where it would only do it after a hot start, but it did it this morning after sitting for nearly two weeks.

Like clockwork, the engine will bog, stumble, and buck for about 10-15 seconds about about a minute or two after I start it. It's worse on a hot start after it's been sitting for 10+ minutes, but lately it's been doing it on cold starts as well.

It'll run fine when I start it, then it'll act up for 10-15 seconds, and then go back to being fine. I can drive it all day without turning it off and it runs perfectly after that hiccup after startup.

I notice it'll bog down and lose power, and when I give it about 1/2 to 3/4 throttle, it'll start bucking a bit and backfiring out the intake. If I mash the pedal to WOT, it runs fine. I'm not getting any codes, and the transmission shifts fine. It'll happen at idle as well.

I suspected it might be the throttle position sensor, so I went to test it, but I couldn't figure out how to back-probe the connector with my voltmeter like CCKen posted here: https://www.cherokeeforum.com/f2/gre...3/#post3077689. There's some sort of sealant where the wires go into the back of the connector, and I couldn't fit a probe in there.

Any pointers on how to test the TPS or what else it might be? Everything look normal under the hood, and like I say, it'll run fine all day otherwise.

Jeep Driver

Sounds like O2 to me./

Turbo X_J

Paper clip or the like. TPS should sweep clean no flat spots.

BlueRidgeMark

A safety pin works pretty well. Paper clip might be too blunt.

I have a probe that has a sharp point on it, just for such problems.

Oswalla

Add to your list a pressure test on fuel. Fuel pumps failing do strange things, they don't always just quit at once.

IJM

Ah, good idea.

CCKen

Safety pin back probe:





Example of back probing:





It does sound like an upstream Oxygen Sensor causing the problem. Had a similar issue with my Heep that a new O2S fixed. Use only NTK O2S'.

Cummins93

Unplug the upsteam o2 sensor at the connector. Driver side under the motor mount. Without a signal the ecu will run rich in a preset open loop map. You should get a check engine light but if it's the sensor it will run without bucking. No need to mess with the downstream o2 sensor. Test your tps with an analog volt meter, avoid a digital meter if you can. Fuel pressure test will be after these 2 tests simply because it's more testing equipment to buy.

IJM

Why is that? Because it's easier to see if there are irregularities across the sweep?

Cummins93

Yes.

CCKen

TPS Tests. Pass curser over image to read title. Click image to enlarge. Click again to enlarge further.


https://www.cherokeeforum.com/g/album/3307914

BlueRidgeMark

Yes ++.

It's almost impossible with a digital meter.

I keep an El Cheapo analog meter around just for this kind of test. 10 bucks at Lowes.

IJM

OK, backprobing successful (that's what she said).

Here's what I got following the two procedures posted by CCKen 2 posts up.

For the static test, I just wrote my voltages down on the diagram and scanned it. Everything looks normal except for pin #1 which was reading 0.008v and the #3 pin with the ignition off.




For the sweep test, I just used my digital multimeter, as that's all I have right now. I'll see if I can borrow an analog one. The voltage ranged from .76v at idle up to 3.9v at WOT. It looked to be smooth, but it can be difficult to tell with a digital meter.

Cummins93

Ever unplug the front o2 sensor?

CCKen

The .008 volts and .5 volts isn't that far off. I wouldn't sweat it.


"For the sweep test, I just used my digital multimeter, as that's all I have right now. I'll see if I can borrow an analog one. The voltage ranged from .76v at idle up to 3.9v at WOT. It looked to be smooth, but it can be difficult to tell with a digital meter."


Exactly.