Jeep hesitates at freeway speeds

Ok, let me begin by saying I've done several searches on this and haven't yet been able to simulate my problem. I have already changed cap, rotor, plugs, IAC, TPS, and cleaned the intake with seafoam. I've done a search for vacuum leaks with carb cleaner to no avail. Here's the dilemma...

I have a 1994 Jeep Cherokee Country with a 4.0L HO engine and AW4 tranny. ATM, there are no mods to the Jeep other than the aforementioned replacement parts. Everything else is OEM....and I believe it to be original equipment, though I'm the third owner and no service records. When cold, the Jeep runs fantastic. When it warms up, the idle begins to run a little rough but nothing too undesireable. My problem really seems to be isolated to something I just haven't figured out yet.

When driving at freeway speeds the Jeep seems to hesitate or cut out like its starving for fuel. It only happens briefly then resumes normal driving. Sometimes the issue becomes repetetive, but ultimately I can step on the gas or the breaks and the issue goes away. I've tried warming it up and running it high rev in the driveway, of course, the issue never reappears to even begin to diagnose where it's coming from.

But here's the other kicker....it seems to happen only when I go over a bump. Not a big bump mind you, just an unevenness in the freeway or even crossing the lane dividers often causes it.

It also seems that occasionally, at idle, the rpm's will drop near stalling, recoup and continue to pur. The engine doesn't shake violently or sputter. There's no backfiring. The hesitation does not occur under acceleration or deceleration. I replaced the IAC and and TPS last. Consequently when I checked the error codes I got codes 34 and 24, which, if I'm reading correctly means incorrect voltage at TPS and IAC....just now cleared the codes again to see if they reappear, but after idling for 15min... no codes yet.

Anywho, any suggestions or advice you guys got would be sincerly appreciated....this is really starting to irk me. I just don't want whatever is going bad to finally go bad when I'm out in BFE.

Thanx heaps!

Michael

I am thinking CPS. Take a look at the wiring and see if it is fried. It lives in a very hot environment right next to the exhaust manifold. Sometimes, the casing on the wire will melt and the wire will short out on the manifold. This could mean cutting out when you hit a bump. Mine has done that a few times. With the right tools, it isn't a bad job.

Check your fuel pressure. Had the same problems, checked every sensor and it turned out to be the rubber hose on the fuel pump (internally) was loose and I only had 20 lbs at the fuel rail. Solved a weeks worth of searching and testing. Until I narrowed it down to the fuel pump. Hope it helps

Hmm....think you may have got it...I just turned the Jeep to the on position and depressed the fuel spout (since I dont have an actual fuel pressure guage I have to do it the white trash trailer park way)...not nearly the spurt I was expecting...fluid barely shot onto the valve cover....on my YJ it would shoot all the way across the engine compartment.

Gonna try and find a cheap guage to verify it. But just for reference should the pressure be checked while the vehicle is running or off with key in the on position? Or does it even matter?

Thanx heaps for the suggestions!

Michael

Has to be checked with it running. On my 89 it is suppose to have 31 lbs psi. Not sure what it is on a 94. Auto zone sells the gauge for $39.00 or so. Or try to find someone to borrow. Good Luck.

ok, so I've dealt with the problem for months now because it just isn't that bad...but now it's getting worse so I figured I'd delve back into it.

Checked the fuel pressure last night, 30psi with the vacuum hose, 38psi without the vacuum hose as the FSM states so I'm guessing it isn't a fuel issue. I've also now noticed that it isn't actually downshifting...it is the torque converter lock up. Torque converter lockup in 3rd and 4th both do the same thing. I'm going to tackle the TCM today...but I had a couple questions...how will this affect the torque converter lockup? Will it force the torque converter to lock up or will it force it to remain unlocked? I'm pretty sure its the torque converter solenoid that's taking a crap, but I'd like to isolate it before I drop $120 on a solenoid. I know you can test the solenoids with an ohm meter, but can you test the resistence from the tcm plug or do you actually have to crack the cover off to test them?

Any suggestions are welcome

Michael

Alrighty then!!!!

Seems I found the problem for anyone with similar troubles....

I finally figured I'd give it a go with unplugging the computer, and when I did, I found that a couple of the wire connectors were loose in the plug housing. I blew out the connector and the computer with canned air, applied some dialectric grease to the female plug and plugged it back in...(btw, I ran it without the comp and it ran perfectly for the day I ran without it)...when I plugged it back it, i made sure all the wires were securly located on the pins of the computer by not only inserting the plug but allso pressing each and every wire all the way in.

I just made a 700 mile round trip adventure, not a single hesitation...me thinks I'm cured!!!arty:

wait??? you drove with the computer unplugged??? do you have a carb???

I'd say the CPS as well... mine was doing the same thing, replaced the CPS, no more problem

I don't think you unplugged the comp for the engine will not start or run without it. You may have unplugged the Transmission comp. where was the comp located?

Pioneer84 said:

I don't think you unplugged the comp for the engine will not start or run without it. You may have unplugged the Transmission comp. where was the comp located?

Click to expand...

Hehe, sorry for not being specific enough....but yeah, was the Tranny comp located under the glovebox/behind the dash panel.

Bad news is, the problem is back. I guess I'll try the CPS and perhaps a Mopar TPS as well...this is really getting irritating tho =(

Check the EVAP canister.

JeepDawg said:

Check the EVAP canister.

Click to expand...

That the big black bubble thing mounted to the bumper?

It's just peachy....it's all nice and clean in the back of the Jeep atm =) But I thought that just controlled the vacuum to cruise control and the fan vents.

I might also mention I changed the fluid in the tranny 3 months ago for the first time since I owned it and it was almost black. I changed it again last night and it again was black. I'm not sure if my tranny just has a ton of gunk in it from never being maintained or if its overheating and burning the fluid....gonna try for a road trip to see if the new fluid helps any...if it does, may just try that Seafoam TransTune to see if it helps any.

Not the big black thing in the RH side of the bumper...that is the vacuum canister for AC/Heat flaps and stuff.

The EVAP cannister is a plastic coffee can with a lotta hoses on it....see the chart under the hood.

Back to you problem...CPS would be first on my list...then maybe Egr valve (see chart)...and lastly TPS (test it).