98 Grand Cherokee 4.0 Bucking/Jerking Issue - Jeep Garage - Jeep Forum

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Old 09-06-2016, 04:58 PM
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98 Grand Cherokee 4.0 Bucking/Jerking Issue

I'm going to apologize ahead of time,for the length of this thread. But, I want to give as much details as possible.

The issue started a couple weeks ago on my work/runnin' around car. The car was jerking extremely badly. It felt like it was misfiring. Which didn't make sense because I just put new plugs in 3 weeks earlier....

So I put new plugs in (again), new wires, new distributor. It seemed to have corrected any issues. But, only lasted a couple of days.

After the issue reappeared I tested the Idol Air valve and the TPS (which sent a code that same day). So, I replaced both....

It continued to go up and down on the RPMs (but not as violently). So, I started to look for any potential air leaks... I found a couple 90's that were in pretty bad shape. One right by the manifold and throttle cables and the second one right by the driver side headlight. I replaced both. It seemed to have work... initially... But, the next evening heading home it started to go up and down again. It kinda felt like an O2 sensor going from open to close. At a lower gear there was an issue but when I went full throttle it was fine... That went away too... And there weren't any error codes, which light up right away.

The next day (yesterday) was on the highway for about 10 min and it started to jerk violently again, for about 1 min. Then it settled down and drove fine... The same thing happened again on the way home.

I figured because the issue was going in and out, it had to be a electronic issue not a mechanical issue. While driving today it kept bucking/jerking for an extended period of time. While it was bucking sometimes the RPMs would fluctuate other times nothing. The issue seemed to have started in a "hilly" area. But, the inclines and declines weren't to steep and continued on a long stretch of flat road... It doesn't feel like the catalytic converter (just because there was still power while going up hill). But, I could be wrong...

Based on a suggestion from a friend who's a mechanic I replaced the MAP sensor. I've driven it 25 miles since then with no problems. But, based upon the last couple weeks that doesn't mean much...

If anyone has had this issue or has any suggestions, any help would be appreciated. Thanks in advance!!!

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  #2  
Old 09-06-2016, 05:01 PM
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Re: 98 Grand Cherokee 4.0 Bucking/Jerking Issue

How many miles on it? Have you replaced the fuel pump assembly? Crankshaft sensor?
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Old 09-06-2016, 05:12 PM
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Re: 98 Grand Cherokee 4.0 Bucking/Jerking Issue

Hi thanks for getting back to my post.

165k miles.

I did the crank shaft sensor a couple years ago. When that went I couldn't even get the car to move... Would that cause a jerking issue?

I haven't lowered the tank to check the fuel pump assembly. Truthfully I was hoping to avoid that unless I was sure that's what it was or I had to...
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Old 09-06-2016, 05:40 PM
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Re: 98 Grand Cherokee 4.0 Bucking/Jerking Issue

I'm somewhat guessing but the in tank pumps are rated for about 100k and ive seen them go sooner and also later then that but at 165k id say that the pump losing fuel pressure under load is a good place to start.....I don't blame you for not wanting to drop the tank......
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Old 09-06-2016, 05:43 PM
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Re: 98 Grand Cherokee 4.0 Bucking/Jerking Issue

Usually when the crankshaft sensors go the vehicle will not start but i was just wondering if you had changed it.....i guess under certain conditions it could cause a rough running condition.....
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Old 09-06-2016, 06:05 PM
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Re: 98 Grand Cherokee 4.0 Bucking/Jerking Issue

Quote:
Originally Posted by Grimmjpr View Post
I'm somewhat guessing but the in tank pumps are rated for about 100k and ive seen them go sooner and also later then that but at 165k id say that the pump losing fuel pressure under load is a good place to start.....I don't blame you for not wanting to drop the tank......

Sadly that was my original thought... But, I've been in denial... I'm hoping for a miracle and the MAP sensor fixes it... Just because its so intermittent. The pump is going to be my last resort I think.
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Old 09-06-2016, 06:09 PM
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Re: 98 Grand Cherokee 4.0 Bucking/Jerking Issue

Well good luck......Hopefully if needed the pump replacement won't be too terrible.
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Old 09-06-2016, 06:26 PM
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Re: 98 Grand Cherokee 4.0 Bucking/Jerking Issue

Quote:
Originally Posted by Grimmjpr View Post
I'm somewhat guessing but the in tank pumps are rated for about 100k and ive seen them go sooner and also later then that but at 165k id say that the pump losing fuel pressure under load is a good place to start.....I don't blame you for not wanting to drop the tank......
You can check the pressure at the fuel rail--with appropriate safeguards and equipment. There was another issue that caused pressure to bleed down, but that never caused bucking.

I would check the O2 sensors [get an FSM, it will pay for itself] and unfortunately some gas in some areas can plug the cat, but that tends to not cause that much bucking even it pretty well shot.
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Old 09-06-2016, 09:53 PM
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Re: 98 Grand Cherokee 4.0 Bucking/Jerking Issue

I was talking to a couple of mechanics while picking up dinner. They said to replace the ignition coil based upon what's going on. Any thoughts?
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Old 09-07-2016, 11:48 AM
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Re: 98 Grand Cherokee 4.0 Bucking/Jerking Issue

Its easier and cheaper to replace then the fuel pump, so I guess its worth a shot....though i'm still leaning toward fuel pump issue.
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