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Acceleration Hesitation/Bucking when Floored


wadger's Avatar
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07-06-04, 08:26 AM   #1  
wadger
Acceleration Hesitation/Bucking when Floored

I'm having a problem with my 95 Jeep 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.

 
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07-06-04, 12:49 PM   #2  
Corleone
Hey bro, try not to floor the car all the way down when the engine is not warm, that will do more damage than any good specially when the oil hasn't lubricated the whole engine yet.

Have you checked your spark plugs or done a compression test ?? another thing you need to look is at that throttle body, has it been cleaned? is the engine stalling all of sudden after you floor it ?? if the throttle body is dirty it willl cause a lot of hesitation and stutters, same goes for the spark plugs.

 
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07-06-04, 01:19 PM   #3  
wadger
I cleaned the throttle body thoroughly when I took the exhaust manifold out. Does this sound like a throttle position sensor (tps) or oxygen sensor to anyone?

 
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07-06-04, 02:12 PM   #4  
Posted By: Corleone if the throttle body is dirty it willl cause a lot of hesitation and stutters.
Not really

If it's bucking and jerking like you say, then you can eliminate the converter as a possibility. It won't act that way. You need to check your fuel pressure to eliminate that possibility but I really doubt that's giving you the problem either. My first suspect would be the coil. It will give you symptoms just like that if it's breaking down internally or cracked. Ignition wires, especially the coil wire will do that also. Pull the coil wire off and check for corrosion on the coil tower or the dist cap tower. If you can get your hands on another coil without having to buy it, that would be the best way to test it. If the wires have over 50K on them, then replace them anyway.

 
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07-06-04, 02:22 PM   #5  
Running fine cold (computer open loop) and badly warm (closed loop) is a good sign a sensor is wigging out somewhere
If it's the original O2 sensor you should probably change it anyway
Does this vehicle have a Mass Air Flow sensor?
If the MAF is crudded up it could behave like that-it's an easy clean
It really should have thrown some codes

 
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07-06-04, 04:22 PM   #6  
wadger
Desi and Slick -thanks a bunch for the input. I've posted this question on a couple of boards and talked to several people and 3 have said check the coil and 4 have said the O2 sensors could be culprits. Given these suggestions, I think I'm going to start with the oxygen sensors and coil and see if either clears it up. I put new wires and plugs in about 15K ago, but didn't replace the coil.

 
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07-06-04, 04:25 PM   #7  
Start with the coil. The O/2's can't really do that by themselves although fresh ones are better for gas mileage..

 
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07-06-04, 05:06 PM   #8  
I would suggest checking fuel pressure to see if its losing pressure under acceleration, and to have a backpressure test done on the exhaust system to see if it is restricted which any exhuast shop should be capable of doing.
you may want to take it to a shop to have it diagnosed instead of throwing parts at the problem it will probably be cheaper in the long run.

 
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07-08-04, 03:44 AM   #9  
Posted By: bejay and to have a backpressure test done on the exhaust system to see if it is restricted which any exhuast shop should be capable of doing.
you may want to take it to a shop to have it diagnosed instead of throwing parts at the problem it will probably be cheaper in the long run.
Restricted exhaust wouldn't give him the bucking and jerking he claims. I did suggest checking the fuel pressure earlier.

 
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07-08-04, 08:57 AM   #10  
wadger
Is it the ignition or pickup coil that I should be concerned with?

 
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07-08-04, 03:40 PM   #11  
Posted By: wadger Is it the ignition or pickup coil that I should be concerned with?
No,No Regular ignition coil is what I'm talking about.

 
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07-08-04, 04:50 PM   #12  
jerking with a restricted exhuast is not all that uncommon if it is bad enough, while it wouldnt be as pronounced as a vehicle losing fuel pressure it is noticable.
the fact that it doesnt lose rpm at full throttle, wich it would if it was losing fuel pressure
another possibility is if the vehicle is equiped with a rev limiter it may be kicking in as 4000 rpm seems kind of high as most automatics will shift well before that rpm even at full throttle but it could be a manual transmission.

 
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07-08-04, 04:59 PM   #13  
My money's on the coil.

 
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07-09-04, 08:15 AM   #14  
wadger
Ok, replaced the ignition coil, dist cap, and rotor yesterday. Haven't had any problems with it for a coupla days - hopefully the coil will take care of it.

Desi, how exactly do the coils go bad? I don't exactly know what they are composed of or what they do. I had someone mention that coils can act real funny sometimes especially when they get hot. Is it normal for them to go out like this?

Sorry for so many questions, I just like to understand things that I work on.

Thanks for all the input!

 
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07-09-04, 03:28 PM   #15  
Coils have 2 sides, a primary winding for low voltage and a secondary winding to produce high voltage. The more load you put on an engine, the more KV the coil will need for the spark to successfully make it back to ground.That coil was very weak and couldn't produce sufficient KV to overcome the resistance created by loading the engine. Simple terms is you just exceeded it's capability. It got weak.

 
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07-10-04, 03:54 PM   #16  
wadger
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. Started it up a minute ago and its acting fine.

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.

 
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07-10-04, 05:19 PM   #17  
Posted By: wadger 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.
We suggested to check the fuel pressure first. You need to somehow drive with that gauge attached so you can see what the pressure is when the problem occurs. It is possible the full tank of fuel kept the pump cooler. You'll never know the real answer until you check the pressure while the problem is present. You could be having what is called a "lean misfire" if the pressure is dropping too low when the pump gets overheated.
As far as the exhaust, you can just drop the pipe at the manifold to run it and see if it changes anything. Every exhaust restriction I've ever seen was a smooth drop in power and never any bucking or harshness.

 
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07-10-04, 06:22 PM   #18  
wadger
Yes, duly noted that fuel pressure was mentioned first - I'm not trying to place blame on anyone when I state that I have had more problems. I just didn't have access to a fuel gauge until today. Hmmm, I guess I'll have to catch it berserking and see if I can get the gauge on and drive it - that should be kind of interesting.

The reason the exhaust is in the back of my mind is because my father and brother in laws have 95 and 93 grands as well - both of them suffered similar bucking symptoms and the culprits were bad cats. Also, over on some jeep forums, many members have suggested this as the problem as well. I think I've heard that there was a design flaw in the cats for some jeep's cats and they crap out on the regular.

But........I also am hard pressed to believe it is the cat because I had a exhaust guy take it off and examine, plus it doesn't make any noise when I tap on it - according to people I've spoke with that had cats go on em, it sounded like a coffee can full of marbles when they hit it. I guess I could be getting exhaust restriction somewhere else, but that's probably unlikely I guess. Anyway, thanks for the input - much appreciated as usual.

 
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