Chasing a misfire

[Markymech]

Okay my Jeep guru friends, I'm at my wits end.
2002 TJ 4.0, 32RH auto, 32" tires, 176,000 miles. Has always ran great. Recently regeared from 3.07's to 4.10's. After the regear, above 60 mph (turning around 2800 rpm @ 65) we start getting a flashing check engine light, then after several seconds the Jeep falls on its face and misses so bad it will barely pull itself. I'm assuming it's in a limp mode because if you shut it off and restart it runs fine. Also, does fine around town, etc; it only seems to act up when driven above 55-60 mph. I changed the coil rail, checked the plugs, fuel pressure (about 46 psi). Finally took to a shop that has a great reputation and they couldn't figure it out either. I found one old thread online somewhere that talked about the valves sticking at higher rpm's making the computer detect a miss and go into limp mode. Any thoughts? I mean besides the obvious - 'keep it below 60'

 

[dbbd1]

Around town, are you taking it above 2800rpm?

 

[Markymech]

Around town, are you taking it above 2800rpm?

No, just normal town driving, 35-40 mph, I'm not sure what the shift rpm's are but I'd guess 2500 or lower?. Even up to 55 it runs fine. Higher rpm driving seems to be the trigger but I just can't figure out the issue.

 

[CodaMan]

What codes is it throwing?

 

[dbbd1]

Try manually shifting, trying to get it above 2800 in town. What happens?

 

[Markymech]

What codes is it throwing?

Multiple cylinder misfire, P0300,0301,0302, 0305....it varies each time I reset it

 

[Markymech]

Try manually shifting, trying to get it above 2800 in town. What happens?

Exact same thing...I tried keeping it in 1st, then 2nd...run it up above 2800 and acts like it did on the highway, flickering check engine light, then drops into a terrible misfire until the key is cycled off / on. Then it runs fine.

 

[Daryl]

I know you said that you checked the plugs but when were they last changed? What is the brand and part number.

 

[Markymech]

I know you said that you checked the plugs but when were they last changed? What is the brand and part number.

They were changed about a year ago, but to be honest I don't remember what I put in; I try to stick with OE equivalent.

 

[Markymech]

They were changed about a year ago, but to be honest I don't remember what I put in; I try to stick with OE equivalent.

Oh, they were actually checked by the shop I took it to. I was burned out with it at the time.

 

[Daryl]

Pull one of the plugs and post the numbers off it.

 

[Markymech]

Pull one of the plugs and post the numbers off it.

Will do; it will prob be tomorrow after work before I can.
Also, thank you all for your input, I appreciate you all.

 

[astjp2]

Fuel pump is running out of fuel could be 1 problem to give those misfires

 

[Markymech]

Fuel pump is running out of fuel could be 1 problem to give those misfires

Fuel pressure seemed good when I checked it, roughly 46 psi, although maybe at highway speed it could drop?

 

[astjp2]

At 3000 rpm’s for 10 minutes under a load?

 

[Markymech]

At 3000 rpm’s for 10 minutes under a load?

True, I only checked it at idle and working the throttle by hand. I may need to rig up a gauge to watch as I drive it.

 

[Markymech]

Pull one of the plugs and post the numbers off it.

NGK ZFR5N

 

[Markymech]

Well for what it's worth I put in a new set of Autolite Iridium XP985 plugs, but same result...runs great below 60 mph all day, starts missing and goes in to limp mode at highway speed; I have noticed it only starts missing on downhill / light load conditions. I'll keep tinkering and post any findings. Below is a link to an old post I found describing the same symptoms. I'm planning to build a 4.6, all the parts are in the garage, so if it comes to a need to start pulling rockers etc., I'll just wait for the new engine.

https://www.jeepz.com/forum/cj-yj-tj-jk/29788-4-0l-multiple-cylinder-misfire.html

 

[Markymech]

A quick follow up regarding the high rpm engine miss...it occurred to me, a few years ago the jeep had broke down right in front of a local shop. I was headed to work, busy busy, so we rolled it in there and said 'fix it'. They replaced the crankshaft position sensor. You experienced folks will see where this is headed...a few days ago I thought 'maybe they used an aftermarket sensor and it's causing an issue.'
About $100 and 30 minutes later, with a Mopar CPS in place, the Jeep runs great at highway speed again.
All those stories saying "only use Mopar sensors in your Jeep" are true.