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Old November 3rd, 2019, 14:50
tballer4596's Avatar
tballer4596 tballer4596 is offline
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Bad Misfires, no codes

Hey all,

1999 Cherokee Sport

I know theres many threads about no code misfires but I havent found one that matches my exact issue.

My jeep is running crappy. At completely random times, it will begin to misfire and buck hard. It doesnt matter if the engine is warm or cold, if i turn it on and off, low speed or highway, it just comes and goes as it pleases. This has made troubleshooting very difficult. The only pattern I have noticed is that when it starts, it continues to misfire on and off pretty consistently until it goes away like it never happened. Even between rides and warm up/cool downs. When it comes it comes, when it goes it goes. On top of this, the jeep is behaving weird RPM wise. For example, on the highway while maintaining speed, lets say 50mpgh, if I give it some gas, the RPMs will shoot up to where you would expect them to be, maybe by about 1000 or so, and then drop and settle down by 500. So it constantly sounds like the jeep is ready to get up and go but then it pulls back. It doesnt quite feel like its bogging down, and this doesnt occur at all if I floor it. The same goes for when I let OFF the gas after maintaining speed, except it works the opposite. The RPMs will fall, and then all of a sudden raise up slightly again even though I have not given it any gas.

As mentioned, no check engine light for anything. No pending codes either. Engine can buck super hard with many misfires after one another but no CEL. Light it not out.

I have recently changed my TPS, CPS and upstream o2. I unplugged downstream to see if it was the issue and it made no difference. I did buy cheap CPS and TPS but the RPM issue has been present since before I replaced them. (The misfire has not.)

Also, my fuel trims are in the negative, roughly -2 to -4 long term. I dont know if this is a lot or a little.
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Old November 3rd, 2019, 17:31
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Tim_MN Tim_MN is offline
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Re: Bad Misfires, no codes

Quote:
Originally Posted by tballer4596 View Post
I have recently changed my TPS, CPS and upstream o2. I unplugged downstream to see if it was the issue and it made no difference. I did buy cheap CPS and TPS but the RPM issue has been present since before I replaced them. (The misfire has not.)

Also, my fuel trims are in the negative, roughly -2 to -4 long term. I dont know if this is a lot or a little.

Did you buy a Bosch O2 sensor or NTK ? It is well known that Bosch O2 sensors and the Jeep 4.0L do not play well together.

Have you checked the Cam Pickup Sensor synchronization, and confirmed that the little magnet is not loose ? The idle issue is very likely a dirty or failing Idle Air Controller.



Only buy genuine Jeep engine sensors.

Most auto parts stores sell cheap crappy Chinese made parts, some that even come with a "Lifetime Warranty". These parts are poorly manufactured and/or made from inferior materials. They are often out of specification, or even failed, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts. Cheap parts are cheap for a reason.
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