[rm2001wj]

Check the throttle cable and linkage on the throttle body. Sounds to me like the throttle plate (or butterfly) is not always opening up like it should.

[merlars]

I have the exact same problem, as well, with my '99 4.0 wrangler. It's very frustrating and I can not figure out what's casing it. It does it when the engine cold or semi-cool. The engine idles fine, but as soon as I accelerate I get the hesitation/sluggishness...the engine really "bogs" down. If I maintain that throttle position, it will backfire through the intake...every time. If I punch it to the floor it will pull out of it with no backfire. Once the engine warms up, it runs fine. Once it cools a little...even when only being is a store or something for a few minutes, it will start all over again. An interesting side note with my problem is that if the outside temp is below 30 degrees it doesn't do it.

Any help or suggestions that anyone can provide would be greatly appreciated!

Thank you!

[Just dune it]

Well still doing it! I took it to the stealership yesterday they charged me $80 told me they couldnt find anything wrong with it. They thought I should start by changing the O2 sensors eventhough they check fine when it's running good. But believe it or not the front O2 goes rich when it starts missing and backfiring and the O2 after the converter doesnt. In my humble opion I would have thought the converter was doing it's job! They say both O2's need to read the same all the time?? Anyone have an idea? remember when it backfires shut it off ,start it up= good to go..... or smash it to the floor = good to go???

[Just dune it]

Okay here is an update. 17 miles after I got it back from the dealer the motor blew up. Fresh oil change but flooded crankcase gas was in the oil. I parked it until I just put a yard motor in it. Started it up everything was okay. I started to drive it and the same thing. I replaced all the old sensors with the ones from the yard motor and still same thing. Cut the muffler off put in a straight pipe, samething. Now I'm totally pis*** I figure I don't have anything to loose. I crawled under it on the road unpluged the front o2 and started driving the engine light came on and it runs fine. I replaced it with a new one and lights off and it runs good. HUH!! Who would have thought?

[ratmonkey]

Originally Posted by Just dune it

I replaced it with a new one and lights off and it runs good. HUH!! Who would have thought?

that would have been me, 4 posts in, over a year ago, and 1 motor under your belt.

Originally Posted by ratmonkey

only thing i can think of that would do that with the air filter is if it was nearly completely plugged or waterlogged AND installed improperly.

as for the running rough when cold, probably the upstream o2 sensor.

[fotonm]

After having this same problem on my 94 with a 5.2 I changed EVERYTHING that people have been talking about on here the TPS, New Tranny, New fuel filter, New fuel pressure regulator. Sometimes it would run great then other times just very sluggish and especially between about 1000 to the 2000 rpm mark. It didnt matter if it was hot or cold just very frustrating and intermittent.

Well I started to suspect a vacuum leak. While monkeying with the vacuum lines and wiggling them I got a surge and thought I was on to it. It wasn't the vacuum after all.

It was the wires that go into the TPS and when I would wiggle them it would surge and even cut off the engine. I unplugged it and Ohmed out the wires while my ohm meter was plugged into and wiggled the wires and I didnt get any Resistance drop. I figure out that the pin that comes out from the ground part of the TPS was not making a good contact with the connector. When it was not making a good solid contact I would get the problem.

You can fix this one of two ways.... you can replace the entire plug assembly to one where the female end of the connectors make a better contact or you can take up the slack in the now too loose female end with a small piece of wire inserted into the tooled out female end (lol) and now it wont be like a hotdog in a hallway and will make a good ground. Plugged it all back in and now runs like it should with full power all the way through the spectrum.

This worked for me so you may try this!! Its a connection and grounding problem not an actual prob with any of the devices.

if your grounding and connections in that TPS are not solid it spikes all up and down sending the wrong voltage to the rest of the injection system.