Yesterday I was driving the Cherokee home from the grocery and when I started to pass a slower vehicle (unusal for me), The engine slowed some when I gave it more gas. I slowed to about 50 and it hesitated several more times. I started to fear I wouldn't make it back without incident and I didn't do anything to try to make it change. I just slowed a little more and kept heading home. I didn't want to get stuck with a bunch of food that needs to be kept cold. After I got home, and offloaded, I checked for trouble codes, nothing, Just to be sure I connected the scanner, no codes stored, all operating readings looked normal, the jeep seemed to run perfectly. I checked the key on fuel pressure and while running and all is normal. The day before, a couple of times when I started the Jeep the idle jumped up to about 2000 rpm and stayed there for about 15 seconds then it slowed to about 1000 and I got tired of waiting for it to slow down and took off. A couple of times while sitting at lights I noticed an occasional very brief stumble in idle speed. So I join the ranks of 4.0 owners with mysterious symptoms and not many clues (WooHoo!). I don't believe it's contaminated gas or fuel pump or filter problems, can't find any vacuum leaks, CPS is less than 3 months old. Any suggestions on where to look next? I'm fixin' to go for a ride to see if I can get it to do it again. <img src="/forums/images/graemlins/confused.gif" alt="" />

I would look at an exhaust restriction next, melted cat or delaminating pipes.

I'll check it out sunder, I canceled the test ride cause the guy I was going to see showed up here(saving gas!). I wouldn't be surprised to find a problem there with 249,000 miles logged on what is probably the original cat. I do have a cracked exhaust manifold, that could be messing with the the O2 sensor. Do you reckon that could make it run rich and clog the cat? I should go ahead and fix that, maybe that would make it a tiny bit more efficient. I'll see what happens the next time I drive it. Thanks!

Pete have you cleaned the iac and the tb?
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

Not yet, but I will. Thanks BJ,! I'm putting the Jeep under the shade tree now! <img src="/forums/images/graemlins/cheers.gif" alt="" /> <img src="/forums/images/graemlins/notooth.gif" alt="" />

Well, I cleaned the IAC, it needed it bad. I believe that was the cause of the idle problem but don't see any way it could affect performance any time the throttle is open. I decided to go ahead and do a tune up. Checking the vacuum hoses with the engine running I noticed the port in the manifold for crankcase ventilation was clogged, I had unclogged the orifice on the valve cover months ago but didn't think to check the manifold port, Must remember that one! I pulled #1 spark plug for inspection, I never checked the plugs since I bought the Jeep cause it ran good before. The color looked good but the electrode was very round and the gap was around .06" Specs are .035. The plug wires look very old so I went and bought new plugs and wires. I had the dist cap off, cleaned and inspected it and the rotor a few days ago(I was comparing the distributor to another I have in stock), so I know they are good. While I had all the plugs out I ran a compression test just for the heck of it, all the plugs looked great except for wear. Highest reading was 165 lowest was 145. I was pleased! After putting it back together with new wires and plugs and a clean IAC I took that ride I meant to take earlier. A 20 mile round trip to retrieve my FSM. I ran the thing as hard as the law allows and then some. It seems to run perfectly now. On starting it goes to 1700 rpm and within 4 seconds settles down to normal idle speed. On the hwy, cruising at 55 I floored it, It downshifted to second gear (AW-4 auto) and when I hit 65 @ 5000 rpm it went back into third. I slowed down at 75, don't need any tickets! I'm thinking about taking this Jeep to the drag races! Why anyone would want more power than a good running 4.0 in a wrangler or an XJ is beyond me. It ran perfectly for the entire 20 mile trip no matter what I did. I reckon it just needed a tune up. Time will tell. Thanks for the suggestions guys! <img src="/forums/images/graemlins/patriot.gif" alt="" /> <img src="/forums/images/graemlins/cheers.gif" alt="" /> <img src="/forums/images/graemlins/cheers.gif" alt="" />