Hello All. New to this forum and XJs so if this post doesn't belong here I'll be glad to move as necessary. Ok here goes

I recently purchased a 2000 Jeep Cherokee Sport which has had a recent (~2-3 years ago) engine replacement with a west coast engines stroker(?) according to the previous owner. Currently there is Check Engine Light (CIL) showing P0305 which upon evaluation says that there is a misfire in cylinder 5. Not sure if related but more important in my opinion is that when the engine is started cold it runs and throttles up great. However when the engine coolant temp reaches ~180F there is a perceptible rough idle that gets progressively worse as the coolant temp gets to running temps ~210F. When I drive the vehicle around the block in the warm condition there is little to no power, and throttling over more than 1/4 (goosing) causes the engine to fall flat, sputter violently until the throttle is released. Again, when the engine is cold <180F it runs very well.

Here is what I have done so far in an attempt to resolve this issue

• New Park Plugs (champion OEM replacement copper gapped to 0.035 in)
• New Coil Pack
• Oil Change (AMS OIL 10-30)
• Compression tested at cold condition (all results showed 165 to 175 psig)
• Vacuum tested manifold at cold and hot conditions - Results were 15inHg vacuum at idle and increased to ~23 inHg under throttle both hot and cold
• Tested fuel pressure both static (jumping the relay) and while running. r- Results showed 48 psig +/- 3 psig with no significant bleed down after shut off
• Cleaned Inlet Idle Control (IAC) with carb cleaner
• Inspected the Manifold Absolute Pressure (MAP) sensor assembly for damage (none noted)
• Removed the valve cover and inspected number 5 valve set for damage and assembly (torque on rocker bolts)
  • Note I am unsure P0305 is cylinder # 5 or the fifth cylinder in the firing order ( cylinder #2) , the internet failed to decide for me
  • I also turned the engine over with cover removed and noted that all valve rockers seemed to operate (visual)
  • Removed and inspected #5 push rods for bending (none noted)

I am stumped at this point, but this is where my thoughts are...Something in the valve train that gets worse as the coolant temperature rises (possibly a lifter)? Plugged Catalyst?

Thanks for any help you can provide

Might wanna try this:

PO305 is for cylinder # 5 , Have you tried moving the injectors to see if the misfire moves ?

Hi thanks for replying. I should have mentioned that I replaced the cylinder #5 injector with a new one and swapped the cylinder #5 into cylinder #2. The code remains P0305 and even comes back after clearing it with OBDII reader.

Hi thanks for replying

I will try this. I will note that when I previously ran the vacuum test the needle bounced quite a bit unlike in the video where needle was steady. In the video he mentions that if it bounces it could mean there is an issue. Any idea what he may be referring to?

If the vacuum gauge is bouncing then check the valve springs .

There were some years that had weak valve springs. 99 and later?
As a result, carbon got between the valve seat and valve.

New springs didn't always fix it.

Grab a can of BG44K. Add it to a full tank of gas. At half tank, fill up with gas. Drive til nearly empty.
That may get the carbon out.
If not, time to do water decarbonization.

When you say check the valve springs what should I be looking for? Is it visual or is there a procedure I should follow for disassembly and inspection?

If a spring is broken, you'll see it. Can you feel this misfire?

Yes when the vehicle warms up, the misfire is very noticeable. When starting cold the engine there is no misfire noticeable until engine coolant temp gets to ~180F

The other thing you can do is run the engine and loosen the rocker arm so it makes noise and the slowly tighten it bad up and see if it misfire goes away when you loosen the rocker arm .

Not surebif this is related to anything but it needed to be repaired regardless.

For sure it does.

Battling same issue right now with P0305. Jeep idles different after running hard or in sugar sand. Rough idle isn’t super noticeable but it’s there. We live off grid and land is deep with sugar sand during dry season. P0305 check engine also started after we got vapor locked once. Vapor lock happening more often now. We have done everything you have done other then the vacuum test. We are out of the woods and running highways more this weekend due to work. Still runs like a champ but that blasted check engine light haunts me.

Tried the water?