2002 Wrangler 4.0 Misfire

[Bublius]

I have a 02 Wrangler with a 4.0 and 3 speed auto. About a year ago I started having some running issues and have since replaced the cam and crank sensors, spark plugs, all 4 O2 sensors, and coil pack. However after all this I am still having a misfire issue that jumps around. Any suggestions on what might be the source or other things to try? Any input is greatly appreciated, thanks!

 

[Jerry Bransford]

When do you get misfires? Could they occur only when the engine/conditions are hot? Also, exactly (!) what brand and model spark plugs are installed? Some spark plugs, even though some lists show them as appropriate, can cause misfires in later TJs with the coil rail ignition system.

 

[mrblaine]

I have a 02 Wrangler with a 4.0 and 3 speed auto. About a year ago I started having some running issues and have since replaced the cam and crank sensors, spark plugs, all 4 O2 sensors, and coil pack. However after all this I am still having a misfire issue that jumps around. Any suggestions on what might be the source or other things to try? Any input is greatly
appreciated, thanks?

Flatlander or hilly country? If hilly, does it misfire going up and not down the hills or the opposite?

 

[Bublius]

When do you get misfires? Could they occur only when the engine/conditions are hot? Also, exactly (!) what brand and model spark plugs are installed? Some spark plugs, even though some lists show them as appropriate, can cause misfires in later TJs with the coil rail ignition system.

I seem to get misfire under heavy load once the engine is to operating temperature. The spark plugs are Bosch fine wire double Iridium plugs.

 

[Jerry Bransford]

Bosch plugs are notorious for not running well in our Jeep engines. It has to do with Bosch's more limited range of availabile heat ranges when compared to other manufacturers like Autolite, Champion, or NGK. No guarantees but I'd try an Autolite XP985 (iridium good for 200k miles), Champion 7412 or Autolite APP985 (good for 100k miles) which are both double tipped platinums. All three of those are great choices the 4.0 runs well on. Avoid Autolite AP985 or Champion 3034 which don't run well in later models with coil rail ignition systems due to them being single tipped platinums.

 

[Bublius]

Bosch plugs are notorious for not running well in our Jeep engines. It has to do with Bosch's more limited range of availabile heat ranges when compared to other manufacturers like Autolite, Champion, or NGK. No guarantees but I'd try an Autolite XP985 (iridium good for 200k miles), Champion 7412 or Autolite APP985 (good for 100k miles) which are both double tipped platinums. All three of those are great choices the 4.0 runs well on. Avoid Autolite AP985 or Champion 3034 which don't run well in later models with coil rail ignition systems due to them being single tipped platinums.

Thanks for the advice I will certainly give that a shot!

 

[801 Driver]

I had a real oddity one time causing an intermittant misfire. HV wire off the coil had a straight connector and was rubbing against the distributer housing making the bend and arking under the boot. Be sure your coil wire has an angle connector on both ends on a 4.0.

 

[PatMc]

I seem to get misfire under heavy load once the engine is to operating temperature. The spark plugs are Bosch fine wire double Iridium plugs.

More problems have been solved be removing Bosch whizbang plugs than by installing them. Get the OE type plugs in it.

 

[Jerry Bransford]

More problems have been solved be removing Bosch whizbang plugs than by installing them.

X2.

 

[Bublius]

Well I went and got Autolite APP985 plugs and swapped them over, ran good for about 15 miles then the misfire returned. It’s consistently throwing codes p0300, p0302, and p0305.

 

[PatMc]

The ECU determines a misfire event by monitoring the crankshaft acceleration each time a cylinder is supposed to fire. If a cylinder or cylinders fail to accelerate the crankshaft at the appropriate rate, it will register a misfire code after a certain number of these events. I mentioned that to illustrate that a misfire has nothing to do with spark or ignition necessarily, but with the entire combustion process. It could be anything from low compression to a fouled plug...anything that will make that cylinder less effective than the rest of them. My first test in a misfire code situation is simply using my ears and listening to the cranking cadence. More well equipped shops will put an amp clamp on the battery cable and watch a scope pattern of the voltage fluctuations while cranking...looking for humps in the pattern that are lower, which would indicate a cylinder that requires less energy to run the piston up through the compression stroke. In my experience...one can typically perform this test simply by listening. If the cranking cadence sounds even..rrr.rrr.rrr.rrr.rrr.rrr, then we have a mechanically sound engine for the purpose of this test. If you hear more of a rrr.rrr.r.r.rrr.rrr then we have a mechanical issue. If it sounds good cranking, I would swap the #2 and #5 injectors to different cylinders (mark them first so you don't lose track), clear the codes, and see if the misfire returns and stays on #2 and #5 or follows the injectors.

If you had access to a scope, you could scope these 2 injectors as well as a couple others and probably get a good idea as to whether or not they were acting the same, but the swap method is probably almost as quick and will give you a solid determination.

 

[Bublius]

I’ve spent the last two days tinkering, checking the timing, cleaning the throttle body and IAC, and swapping the injectors around to see if it was fuel related. As of now I can make it about 10 miles before it gets unhappy and throws codes p0300, p0301, p0302, and p0305. Any input on the matter would be appreciated greatly. Thanks!

 

[PatMc]

I’ve spent the last two days tinkering, checking the timing, cleaning the throttle body and IAC, and swapping the injectors around to see if it was fuel related. As of now I can make it about 10 miles before it gets unhappy and throws codes p0300, p0301, p0302, and p0305. Any input on the matter would be appreciated greatly. Thanks!

Does you scanner record the freeze frame data when the code was set? If so, post it up...specifically, fuel trims

 

[Bublius]

Does you scanner record the freeze frame data when the code was set? If so, post it up...specifically, fuel trims

My scanner is unfortunately a pretty basic one that doesn’t have that capability.

 

[RobertJ]

Might be time to invest in one with more features. Misfires can be a real PITA without the proper diagnostic tools. Good luck and please report back.

 

[PatMc]

My scanner is unfortunately a pretty basic one that doesn’t have that capability.

What scanner do you have...is it just a code reader? Any other available functions?

 

[astjp2]

have you check compressions?

 

[RonT]

@Bublius, I hope you solved your CEL issue. Look at my post RonT post#16. CEL could be coming on for almost any reason. Good luck.