I drive a 1999 4.0 automatic as my daily driver and only vehicle.
This week my engine began to run like crap and misfire. The check engine light came on and the codes read for a misfire on cylinders 3 & 4.

So I replaced all 6 plugs, the wires, the distributor cap and rotor, and I went ahead and replaced the ignition coil. I also put in fuel treatment and I am running 93 gas.

The engine still runs terribly, and the misfire codes came back today.
I checked the brand new plugs for 3&4 and they look awful.

There isn't any oil in my coolant or coolant in my oil as far as I can tell, but the plugs don't bode well. I fear I have a cracked head gasket and I am devastated.

Can someone give me some good news, or am I absolutely screwed?

 

That looks like carbon fouling buddy, leaky injectors?

 

Thank you!

So carbon fouling that extensive can happen almost instantly? I put these plugs in on Friday, and have driven 20 miles since.

 

Yeah, carbon fouling. Caused by an overly rich condition. Those plugs may not be firing. Verify spark is going to those two cylinders. Or not.

If coolant is leaking into a cylinder, the plugs will look steam-cleaned because in essence, they are.

 

Dirty/leaking injectors

 

How many miles are on your XJ? Without even considering the condition of the spark plugs, I would have said your number 3 & 4 injectors are bad.

Considering the condition of the plugs, it appears you have a combination of oil getting into the combustion chambers and leaking injectors.

If your engine has a whole lot of miles, it could be a simple case of worn rings, which, beyond a rebuild, you can't do anything about. Is your oil fresh? If not, fresh oil may help. Also, if you're running 10W-30, for example, moving up to 10W-40 could also help a little.

I would suggest starting out with replacing the number 3 & 4 injectors then changing the oil to see how things run. If another check of the plugs shows 3 & 4 burning cleanly while the other cylinders are still black, then replace the other four injectors. That is, unless you can find a good price on a "six pack" of injectors which would save you money over buying them individually. If that's the case, pick up a six pack of injectors.

 

JMO, but if you are going replace two, why not just get a set of 4 hole injectors? If ya gotta replace might as well upgrade.

 

Do a compression check on all cylinders.

If you have the right equipment you should also do a compression loss check. It will tell you if you have bad rings, bad valves, blown head gasket, etc.

Personally, I think you'd be barking up the wrong tree changing injectors.

 

Yes. Isolate the cause before throwing parts and money at it.

 

I plan on doing a compression check.

What is your rationale for thinking that it isn't faulty injectors?

 

Compression test is free - injectors aren't.

If you have good compression then look towards injectors. If you don't have good compression (120-150PSI. Maximum variation between cylinders is 30PSI.) then you need to look towards other things as injectors won't cause low compression.

 

I have a 2000 Jeep Cherokee Sport. I have a severe misfire on cylinders 3 & 4. I have performed a compression test on all 6 cylinders and all read 160 to 175 PSI of compression. The injectors have all been replaced along with the COP coil pack and all 6 spark plugs. The old plugs looked like normal wear with no fouling. I also performed a leakdown test of cylinders 3 & 4 with no negative results. What could my issue be because it only does it when it is warm. I have also reset the system learning several times and it is good for about 3 miles then starts in with misfires again. I apologize but I am more of a VW guy. I'm new to Jeeps.