Acceleration Hesitation/Bucking when Floored

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I'm having a problem with my 95 Grand Cherokee 6 cyl (125,XXX). The symptoms only really occur once the vehicle warms up and has reached normal operating temperature. When I push the gas pedal to the floor, the engine will wind up to about 4000 rpms, but the Jeep will buck like it's not getting fuel or something. The rpms will remain constant, but it just bucks and jerks but won't accelerate.

I'm not sure how to explain it, but the longer I drive it, the weaker the Jeep seems to get. If I hop in it and floor it while the Jeep is cold, it gets on it pretty good. However, I can tell that its hesitating ever so slightly. If I drive for 15 minutes, it will buck/jerk very strong and I can let off the gas to about 3/4 and it will accelerate mildly. But, on a recent 3 hour trip, towards the end of the trip the Jeep seemed VERY weak, jerking only mildly on full acceleration, but I had to let off the accelerator about 1/2 way to get it to accelerate at all (and this was very slowly).

Here are some things that I've done/had looked at:

The dealership couldn't get any codes, I can't get any codes with onboard diagnostic, and the check engine light is not/has not ever came on.

I had a muffler shop pull and examine the cat but they said it looked fine.

The fuel filter has been replaced

My exhaust manifold had a crack in it that I had welded about a month ago.

I'm hoping that this is not tranny related, but since the problem only really manifests when the engine gets hot, I don't know how it really could be a transmission problem. I'm wondering if the Cat could really be causing the problems once getting warmed up. Any suggestions would be greatly appreciated.

posted by  wadger

Man that sounds like my 95 grand cherokee, I needed a new tranny cause mine was getting to be a pain in the :cussing:

posted by  Voda48

I've talked with a couple of people that mentioned that this could be a bad coil or perhaps bad oxygen sensors. Any thoughts?

posted by  wadger

is your exhaust blowwing?

posted by  cinqyg

yes, the exhaust blows out

posted by  wadger

My first guess would be what you are suggesting.

Something to do with the oxygen sensor or catalytic converter. The heat is causing it to plug up and/or malfunction. That does sound logical. Those two problems being related as the oxygen sensor is before and after the catalytic converter.
However, as you said no error code. That actually eliminates that possibility.

I cannot think of why it would be the coil, that would seem to cause malfunction regardless of the engine heat. That is easy to test with a voltmeter right on your car. It doesnt go bad often and it doesnt go bad like that.

This is very hard to figure out through a message board.

I would do a fuel pressure test possibly. Perhaps the fuel pump has intermittent gum on it and it slows it down at high rpm? Not too good a solution.

You say you have a tach and it hits at 4000 rpm? That can actually be the transmission. Is it just as it is shifting gears? clutch plate slippage possibly? so it does this problem at a certain rpm regardless of speed correct? like from 1st to 2nd and 2nd to 3rd possibly?

Hmmm well following that possibility, pull the cooling lines out of the transmission ( go ahead let it leak out thats what your checking for) to make sure the transmission is getting coolant. It can plug up and it gives you no warning. what is the color of the auto trans fluid? ( Im assuming this is automatic if its not ignore this. In a manual i suppose this problem could be the clutch slipping but that seems very unlikely) Two thin metal lines usually go from the bottom of the radiator to the transmission. You didnt put radiator gunk in ur radiator did u?

change the whole auto transmission fluid with a syphon, and put in the chrysler auto transmission fluid. It has anti slip additives.... not dexron 3.

posted by  stamar

Had more problems today....................

I'm not sure but, it seems like the problem gets worse the warmer the vehicle gets, AS WELL as the lower the fuel level in the tank gets. Took her down the road about 30 miles and it was acting up at about 1/2 tank left. Just for grins, I pulled in and filled the tank up. Having a full tank reduced the severity of the symptoms - however it would still buck and hesitate when flooring it to a lesser degree than with a 1/2 tank. Would the pump have problems with less fuel in the tank? Is it maybe getting too hot with only a 1/2 a tank of gas to cool it?!

Of course, right before I got home, I drove my wife to the store and waited on her in the lot. I left the jeep running and after about 5 minutes, it started acting like it was going to stall - like it wasn't getting fuel. My wife came out a few minutes later and we pulled out. I didn't think I was going to make it three blocks home - I couldn't push on the gas more than about 1/4 of the way - it just hesitated and bucked. I was able to make it in my driveway.

Just performed fuel pressure test - 39 psi while idling - according to haynes it should be 37-41, so it looks good.

I still want to try the backpressure on the exhaust - I'm gonna try to borrow a guage today.

posted by  wadger

first off, there should only be one O2 sensor because it's a '95(OBDI not OBDII) but then again it's jeep, who knows.

second, the last thing you want is "coolant" in your tranmission. The tranmission cooler is built into the radiator but is seperate from the cooling system...which brings me to my next point...
putting any kind of coolant system sealant in the radiator will NOT affect the trans cooler. even if the lines to the trans cooler were plugged it wouldn't cause such a problem.

a syphon is not an effective way to drain your transmission...the proper way is to loosen the bolts on the transmission drain pan until fluid starts seeping out...if the pan doesn't drop tap it with a rubber mallet....then you can slowly start working around the pan loosening the bolts more and more...allowing more fluid to drain. then you can support the pan with a jackstand or something and remove all the bolts...carefully take the pan down(as it will still have fluid in it) and clean the pan thoroughly...preferably with "brakleen" as it leaves no residue and dries quickly. then you SHOULD change the transmission filter, install a new pan gasket and torque the pan down.

posted by  Sick88Tbird

sounds like your fuel pump is ok...did you pressure test with the engine under a a powerbrake until it bucks and hesitates? if not, i'd recommend doing that...your pump may be on it's way out. also, on jeeps...if there is less than 12 volts supplied to the pump or ignition system neiter will operate.....a buddy of mine had only 7.2V supplied(intermittenly) to his fuel pump on his '87 comanche. turns out there is a TSB out for the ECM...i don't know if it applies to your year...but i'd check fuel pressure under a load...while it's bucking and hesitating if you haven't already.

posted by  Sick88Tbird

I am exhibiting similar behaviour on my Jeep and I was wondering if you figured what was wrong with yours.

posted by  marcopolo

It could be your piston rings. If they arent sealing it properly air/gas could be leaking during compression, which would explain why its making so many RPMs but not going. Or your spark plug.

posted by  Oomba

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