Engine jerks when braking to a stop - JeepForum.com
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post #1 of 18 Old 09-19-2017, 10:44 AM Thread Starter
AntBanks78
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Engine jerks when braking to a stop

Hi everyone I just got my first jeep. I've wanted one for a while after helping a buddy swap all his good stuff from wrecked late 80's Cherokee to an 84 I think. With very little mods I was surprised!!
I have a 1996 grand Cherokee Laredo with 5.2l. I tried to search here but no results for anything I type in??!? So sorry if this is discussed already. I'm having problems finding why this thing jerks when I'm braking or idling in gear no brake applied. It feels like a misfire but runs smooth while gas pedal is anywhere but closed. Compression is good on all cyl. It had a low spike on asiliscope for #4 after I changed plugs and wires (jerking was intermittent before sparkplug change) rerouted plug wires and spike is no longer on #4 but #4 plug was white ashy as well as o2 sensors. Replaced up stream o2 no change except in data stream, cat may be bad. Removed and cleaned throttle, reset adaptive fuel and idle, cap and rotor r new, thought about coil but it runs and coil checks out according to identifix specs. Switched #4 injector with used one so now I'm scratching my head and confused. Has anyone ran into something similar with v8 pushrod jeeps? I did read about champion sparkplugs on here, im headed to c what brand I put in.
If anyone has any ideas I could use some jeep experienced advice and would GREATLY appreciate the help!!! Thanks in advance!!!! :-)

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post #2 of 18 Old 09-19-2017, 10:57 AM
BorderWalker
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Try NGK copper. Champion has started going downhill.

How's the engine mounts?

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post #3 of 18 Old 09-19-2017, 03:04 PM Thread Starter
AntBanks78
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The engine mounts look busted from underside but engine doesn't flop up if I throttle down while holding break. I also just replaced fuel pump because check valve wouldn't hold pressure and cranked forever to start. I used acdelco plugs, coppers if I remember right and 2nd time I put scope in ignition the low spike wasn't on just one plug, it was random across board. Is there a common problem with injectors maybe? I have a can of injector cleaner that goes straight into fuel rails but trying not to spend money on parts that don't fix problem.

It feels like a big cam engine that lopes at idle but even those don't jerk the car forward ounce per engine revolution.

Should also say vehicle sat for a long time and seems to have some moisture damage on electrical components in dash like dash light reastat and a/c controls flip to defrost when WOT like passing someone on road. Checked grounds under hood, seem ok pretty low ohms from bat to body and engine. It also has a rust hole in the muffler, my truck seems to vibrate hard with no muffler so only reason I mention. I think I will try some ngk plugs unless advised other wise
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post #4 of 18 Old 09-20-2017, 01:50 PM
Uniblurb
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AC Delco used to make real good parts but not anymore. Agree on getting rid of those AC Delco plugs and going with NGK copper. Hopefully you didn't install a cheap fuel pump module and only go with Bosch/Mopar which are the same thing. Oxygen sensors should only be NGK too or Mopar.

With your engine loping and AC defaulting to defrost it sounds like you have a vacuum leak you need to fix. If a manual control, Lardeo's usually are, check your vacuum lines. Under the battery tray is the vacuum reservoir with hose on the bottom and sometimes this hose rots off. Also make sure the cruise control servo doesn't have a leaking hose and check near firewall where small black plastic line goes through to to the HVAC controls. Brake booster rubber grommet is another place vacuum can leak but on a separate line than the other controls.

The PCM ground on your 96 5.2 is right above/behind the coil and kind of under the alternator edge. It has 2 tan/black wires going to the double wire eyelet and make sure this ground is real clean.

Since sounds like your tune-up did not include new plug/coil wires you may want to consider a new set. Use the old skool method of looking at your engine running in complete darkness to see if you have any wires arcing. This doesn't tell you the wires are good but just if there's any breaks in the wire insulation or other bad connections. Good luck.

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #5 of 18 Old 09-21-2017, 09:09 AM Thread Starter
AntBanks78
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Ok I will check all those! I did replace the bad one way valve for vacuum but forgot to run a/c to c if still switches in wot. I did replace wires, I grabbed the top of the dizzy and got p zapped out of me. O2 is a denso and goes to .6 volts where other did not g that low but old one was sticking or holding at one voltage for a long time so that's y I replaced it. Pump was the most expensive option, didn't c a mopar. Dizzy cap and rotor new too. Anyway I will check the rest. I have a squeak in rear suspension and when I let off brake before hitting throttle it goes squeak , squeak squeak squeak lol pretty anoying:-) irrelivent info just funny. Thank you!!!!!!
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post #6 of 18 Old 09-21-2017, 09:10 AM Thread Starter
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Oh yea I did the plug wire reroute that mopar suggested to, there was an induction problem from factory
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post #7 of 18 Old 09-21-2017, 02:40 PM
jm8881
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My 96 5.2 did the same thing when I bought it. When slowing to a stop it felt like the engine made the whole jeep lunge forward. Turned out to be a way out of adjustment transmission shift cable, a bad 2-3 spring, bad transmission pressure sender, and a bad transmission governor. Replacing these fixed the issue.

After re-reading your post my issues and your issues may not be the same. Although it may just be the way I'm reading it.
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post #8 of 18 Old 09-22-2017, 08:40 AM Thread Starter
AntBanks78
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It does shift quick from 2nd to 3rd then back to 2nd sometimes when taking off from stop light. And it had a shift silinoid dtc when I first got it but hasn't come back as of last week. I'll check again c if any dtc's came back
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post #9 of 18 Old 09-22-2017, 08:50 AM Thread Starter
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I work in family owned auto repair shop and know all about the quality of crap being sold as auto parts. I have to order 3 hub/bearings at a time to find one better than the hub I'm replacing!!! Total garbage!!!! Won't buy electric motors of any kind from napa cuz they draw to much amperage and only last 6 months. Water pumps too, damn sure won't put their water pumps on a timing belt car!!!! Did that 1 time only!!!!! Lucky the pump didn't come completely apart but u could c the ball bearing after I removed thru what was left of seal, less than 6 months old!!!!!!
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post #10 of 18 Old 09-22-2017, 08:54 AM Thread Starter
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Ok forgot to say I haven't checked the stuff on the post suggested above yet, gonna try to today or tomorrow and let u know. Trans and grounds and vacuum stuff
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post #11 of 18 Old 09-22-2017, 05:24 PM
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Interesting you replaced that vacuum line check valve to try and stop it from defaulting to defrost. On OE there was no check valve so did somebody already install one per below TSB fix?

http://dodgeram.info/tsb/1996/24-01-96a.htm

I've seen the 5.2 having the above problem a lot more frequently than the 4.0 which rarely has this issue.

Just happened to think you may want to check your charcoal canister behind far side of driver's side bumper to make sure it's not cracked or hoses have rotted away/come off. Another good place for vacuum leaks.

That doesn't sound right your trans jumps around from 2nd, 3rd, 2nd again when accelerating. Best to check it out and good info jm8881 posted. Good luck.

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #12 of 18 Old 09-26-2017, 10:48 AM Thread Starter
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We may have installed that valve some years back. I bought the jeep from a customer cuz it sat for a long time and needed couple thousand dollars in repair but only cost me parts. That valve I could see threw and after I replaced the valve it no longer switches hvac in wot. I pulled the trans cable backwards but to much. It doesn't do the 2-3-2 shift now but won't shift until 2600 or so rpm light throttle so need to push forward some. Just guessing on that cable tho, I need to look it up in identifix c how to properly adjust. I haven't had time to mess with anything else yet. I should b able to get it in today and check the carbon can and gonna try some ngk sparkplugs if vacuum stuff looks good. Maybe watch trans data stream if I have time. Thinkin bout sending vin to Chrysler see if an ecm reflash is still available. Read about that on a thread here.
Hey thanks for all the info I really do appreciate it!!! I'll keep plugging along and let u guys know
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post #13 of 18 Old 10-09-2017, 09:30 AM Thread Starter
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Checked vac can, no leaks, grounds on front of pass side head are good. I got a crank sensor code that didn't come back but my autel scan tool finds a C3E dtc which is upstream o2 shorted to voltage and a U3F system rich, r-idle adap at lean limit. Can't find a P code to cross and look up tho. I need to take a picture of a eight to ten wire connector that is crumbling, it's between battery and wheel well. I haven't been able to go get another so filled it with silicone grease till I can. Haven't found anything yet but now it vibrates at 70 mph like engine is misfiring hmmmm
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post #14 of 18 Old 10-09-2017, 11:08 AM
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If that crank sensor code was legit it may be worth checking the resistance and test in my signature.

Probably should have used the NTK upstream o2 sensor rather than a Denso which isn't spec for these ZJ's. But still shouldn't give you shorted voltage and would think you'd know if it was against the exhaust since you're the one who changed it.

Would also be interested to see a photo of that crumbling connector along with location and good luck.

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #15 of 18 Old 10-19-2017, 04:06 PM Thread Starter
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Ok the o2 code hasn't came back after switching the rear with old upstream o2. replaced the old rusted muffler with a flow master I had in storage and is easier to get up to highway speeds but still has the jerking forward problem but may be a little lighter. I have a pic of the connector but am not sure how to post it. I'm trying to go over the electrical system cuz the radio volume knob intermittently works along with interior light reastat in light switch. Thinking there may be other electrical issues in other places. Just tried to remove ecm and found lower outer bolt was rusted off and stuck in the boss and outer upper ecm bolt hole was plumb full of white aluminum dust, outer side of ecm is heavily corroded so was going to remove and look inside but that bolt was stuck in ecm housing :-(
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