"99" Cherokee Sport 4.0L engine chugging..
 

Hey folks. I'm at a bit of a loss here being new to the Jeep world so here goes my first question. I recently picked up a "99' Cherokee sport w/4.0L. I've been going through all the systems doing maintenance and replacing the worn out or broken parts as needed but I hadn't driven my new XJ more than 5-10 miles at a time. This past weekend I had the opportunity to do some highway driving. After an hour on the road at speed (70 mph) I started to notice a very annoying hesitation/chugging. The symptom was completely random but would occur every few minutes.
I'm not quite sure where to start troubleshooting this problem. Your help and suggestions are greatly appreciated.

What maintenance have you done?

This issue can be tune up related, it could be a fuel delivery problem, could be a clogged cat, could be a bad o2 sensor, etc. the list goes on.

Is it throwing any codes?

1. Scan the computer for "stored codes", even if the check engine light currently isn't on.

2. With a new to you vehicle, unless you have believable documentation showing it was done recently, it should be new Champion copper plugs gapped to .035, new quality plug wires, distributor cap and rotor. Inspect/replace air filter and serpentine belt. Eventually get all fresh fluids in there.....every one of them.

Update: Maintenance performed by me...
 

Hey there again. I've done a full "Tune-up" Plugs, wires, cap & rotor. Fresh fluids and filters throughout, including drivetrain. I installed a new muffler and tailpipe. Basically I went through everything I could get my hands on to insure that everything was connected and not leaking. (I think the only thing the previous owner put in to it was the key.) As far as "Code scanning" goes, No I haven't done that yet. No check engine light or any other indicator as to what it might be. It's just one of those things that only occurs after an hour or so of drive time at freeway speed. Is there a way to display "Stored codes" manually? Without a scanner?

Sorry, but that luxury is not available on a 99. That went away mid-1997.

I would start with some basic testing to see if you can't find the culprit. First thing I would recommend is fuel pressure. For a 99, you should have 49 psi, plus or minus 5 psi. See if you are out of spec, or close to being out of spec. Many parts stores will rent you a fuel pressure gauge inexpensively.

what's fuel pressure doing? if you can monitor it while driving all the better. weak fuel pumps or plugged filters can cause this as you'll have fuel pressure build up but under load it drops off. fuel filters ideally should be changed every 1-2 years and on the XJ it's really easy.

monitor fuel pressure first to see what you find. if that's it great, if not, compression test and vacuum test. if those check you start eyeballing the TPS, crank sensor and map.

just have to start testing and ruling stuff out

With his '99, the fuel filter is inside the fuel tank and is part of the fuel pump assembly. So the fuel filter does not need to be changed unless the fuel pump itself takes a crap.

At any rate, as suggested, verify fuel pressure. You may be looking at a new fuel pump if it is not up to the proper specifications.

Follow-up to the responses posted thus far...
 

Hey folks, I just wanted to say thanks for the advise. I found that one of my friends does have a really nice code reader so, we're gonna check that out this evening. I'm starting to lean toward the problem being with the CAT or the O2 sensors. the symptom only occurs when the engine is hot and during an extended drive at highway speeds (70 mph). It's like the engine momentarily misses. No back fire, no loud noise and no loss of RPM on the tachometer. I will post again this evening once we check for fault codes.

Results of "Code scanning".
 

Hello again folks, I wanted to update this thread with the results from the code scanner. So here we go... The only message that came up was "0000" this is a maintenance code for high mileage vehicles. All other systems were clear and no stored fault codes. This leads me to believe that after replacing the muffler and tailpipe I my need to replace the CAT conv. too. My bet is that it's got some buildup in it and when it heats up I get excessive back pressure.

My 93 xj just started doing the same thing yesterday. Also had an intermittent very low voltage condition during the chugging. What ended up being the cause of the problem in your jeep?