Results of a lot of Diag on 2000 Jeep Cherokee XJ with codes4.0 rough idle check engine light on p0352 p0300 p0302 p0305 p0456

So, I haven’t spent a lot of time on this until recently. I have a 2000 jeep cherokee xj sport with a 4.0 inline 6.

This is not my daily driver but I do use it quite a bit.

It all started this past winter so it has been around 7 months since the problem began while driving in a abnormal snow storm the jeep started stalling and bucking and lost a lot of power. The check engine light came on while the vehicle was doing this and started flashing. (FYI the reason a check engine light flashes is to indicate that air fuel mixture is so far out of spec that it is damaging the catalitic converter.

I simply slowed down and turned the vehicle off (we were driving on a back raod so there was little traffic but ofcourse it was snowing a lot and I didn’t want to be stuck only 4 miles from my house in the snow.

After stopping I got out and checked over the engine and look for any obvidous sgns like a spark problem or overheating ect.

Nothing was abnormal at all about anything. So I hopped back in the jeep turned it on and to my suprise the vehicle was running like normal again.

I checked over everything again that night at home in the garage. I started with checking for any damaged or lost hoses and found that an evap hose coming from the PCV on the valve cover was leaking so I fixed that with replacing the hose from a local parts store and that fixed my evap codes. This was an easy fix for an evap leak because a lot of times that are nearly impossible with out pressurizing the system and checking for leaks with a gauges and shop smoke or a real good ear. If you would like to understand better how the evap system does and how it works click here for my page about it.

I still couldn’t find anything wrong with the engine so I closed the hood and figured it was just a weird insodent.

around 3 months later the problem came back one day while I was going to pick up a truck from auction towing a car dolly. When I jumped the vehicle off that I was taking home from the auction the jeep started with the same problem again. I turned it off and then restated it remembering what happened last time I was hopeful that this would work again. What do you know but it did work.

While driving home towing the other vehicle the jeep did the same thing numerous times on my hour long drive home. Therefor there was a lot of turning off and restarting it.

This being a problem that seemed like it was no longer nearly as intermitent anymore I decided to diag as soon as I got home.

I started this time by removing the coil and testing it. The coil tested good but just for reassurance I used a known good coil and tried it with the known good coil on and the same codes and problem still happened.

I tested and checked all the spark plugs and found no problems with any of them either. I changed the order of the spark plugs that I took out to make sure that if a plug was causing a misfire that it would not be on the same cylinder or the same companion cylinder this was to find out if there was indeed a bad plug that it would have to change the check engine light code to indicate a miss on another cylinder. (Vehicles have corrospoding cylinders in the engine, this means that basicly that are going up and down at the same time oposite of the other cylinders. (if you want to know more about how engines work from the inside click here to go to my engine knowldge page.
(Also this jeep is a 2000 this there is only one coil pack for all the cylinders. on older jeeps there were multiable coil packs and older than those there were just spark plug wires with a distributor.

To test those simply unplug the coil while the car is running to see if the mis changes. If the miss  changes the coil pack is good. If the miss has no change. That indicates either the coil or plug is bad just swap the coil and repeat this process to find out which coil or plug is causing your miss.

These same steps would be used on the distributor type systems by changing the wires around or unplugging

also if your miss only happens under load then you will have to have someone help you and have them get in the vehicle while the engine is running and the vehicle is in gear apply gas to put the engine under a load. (just like you were doing a brake stand just dont let the tires start spinning)

So now I have both the coil pack and the spark plugs tested.

Now having that eliminated I check the cam sensor and cam sensor possition. (click here for how to test cam sensor)

That checked out fine no problems.

I Checked the crank sensor next. No problem there either. (click here for how to test crank sensor)

Now with all these things eliminated I checked the oxygen sensors good as well

So I now move to checking the continuity in every wire from the ignition system to the computer and the fuel system as well. (click here to see how)

What I found out from that was everytime this problem would happen the computer was taking away ground from the coil b ground control wire.

So I replaced the computer with a known good one.

Problem still happening after only a 10 minute test drive which was pretty much how every diag test drive has been on this problem. Basically no changes after replacing the ecu with a known good one.

So everything including the wiring I have tested up to this point has been good. So what the heck is the problem… I have asked myself that question numerous times while working on the project.

By checking with the cylinder leak down tester (see how here) I found that a had a valve sticking on the 2nd cylinder which intern by the vehicles computer system using conveluted lodgic turned off the power to the coil on the 2nd cylinder and intern gave me a coil b2 bad code in other words 2nd cylinder coil is not firing code. That was also why when I would turn the vehicle off there was no back pressure holding the valve so it could fall back down from being stuck!

So with this new found information I use 3 bottles of top engine clean (see how to use top engine clean) After this was done I took it on another test drive for around 30 minutes and there was no more problems.

About 2 more months have gone by and I still haven’t had time to pull the head and do a top end job on this engine.

month 3 vehicle started having the same symptoms and I used another 2 bottlles of top engine clean and it driving great again but I have parked it until I have the time to fix this the right way.

Hopefully this will save someone alot of time that is having this same problem