What is going on with my compression?

[adventureboss]

When did you buy this '06 Jeep with ~110k.?
What are the symptoms which got you to "chase a P0303 code"?
Did you always have some indication of misfire or hiccoughs?

How do various cooling system components look?
Maybe your car had a 'hot lunch' during its previous ownership?
The cam lobe and valve spring inspection mentioned is a good idea as it's so inexpensive to do [on that engine].
The involved valve or valves might not be traveling smoothly.

Also mentioned above was a water treatment (decarbonizing?) which was described as only benefiting the cylinders.
Maybe hit the intake with a solvent spray? Or borescope the intake runners for gunk?
How's the PCV system?

Where's your guy sourcing the head?

I have only had the Jeep for the past year and a halfish, and am at 190k now. Must have bought it June 2023. I drive quite a bit. The symptoms I'm having are the occasional rough idle, and then misfire codes. Other than that, nothing. Only throws the misfire code at idle as well, no issues when driving around. It seems very random. I ruled out everything except the engine.

Funny you mention the cooling system, I'm not sure what the previous owners did but they took zero care of the cooling system. There was so much sludge and the Jeep would overheat or get very close to it (250F+ coolant temp) until I replaced the entire cooling system and flushed the block out dozens of times. I'm sure part of the reason I'm having this issue is because of that. Poor motor did not live an easy life. Who knows how many times the PO overheated it as well.

I think carbon build up may have been an issue with this motor as well. I remember clearly when I first bought it and got on it hard a ton of smoke came out the tailpipe and the cats needed replacing shortly after.

I am very sure it's the valves. I've done seafoam, the sprays, anything someone could think of honestly. I wish I had access to a borescope to check. With almost perfect compression now I don't know what else could be leaking. I don't think there is really a PCV system on these Jeeps, but also I'm not too familiar with how they work so I could be misinformed. As far as I know, there is no PCV valve to replace or clean. I did clean the idle air control valve though just to rule it out.

I believe it is a new head from cylinder head express. I am skeptical because I don't know much about them but there's also not many options. My mechanic said that his machine shop was not willing to rebuild my original head because the exhaust valve seats apparently usually go bad and it's not easy to machine those, or something like that. I'll hang onto it anyways.

 

[nascarnation]

Honestly it's more so that it bothers me. I don't want to feel like I'm driving a POS. It's been a good vehicle over the 80k miles I've put on it so if a head replacement is all it takes for it to run fine then that's okay.

That's all good. I believe some years of the 4.0 have a head cracking problem so you might want to interface with Jeep-savvy folks to make sure your next head is the right choice.

 

[adventureboss]

Replacing the head solved the issue. The exhaust valve on cyl 3 had recessed enough to cause the intermittent miss. After replacing the head, it's running great. Bottom end looked great as well when we pulled the head off.

 

[MaximumMini]

Replacing the head solved the issue. The exhaust valve on cyl 3 had recessed enough to cause the intermittent miss. After replacing the head, it's running great. Bottom end looked great as well when we pulled the head off.

Thank you SO MUCH for coming back and posting the fix.

I'm in the throes of a V6 with one cylinder low compression, but much worse. That one is at 60psi 85% loss on leak down test.
Added tablespoon of oil to the cylinder and compression went from 60psi to 100psi.
It's also a Chrysler product (Caravan 3.3L)
I should have the head off today if I can just stop procrastinating.

Odd thing is that under load it feels smooth and powerful...as if the problem only exists at idle.
P0302 code confirmed and it's the cylinder with the 60psi compression.

So much helpful information in this thread.

Here is a link to my thread about this issue. I'll also post the final outcome.
https://bobistheoilguy.com/forums/t...-3-8l-low-compression-on-one-cylinder.394455/

 

[adventureboss]

Thank you SO MUCH for coming back and posting the fix.

I'm in the throes of a V6 with one cylinder low compression, but much worse. That one is at 60psi 85% loss on leak down test.
Added tablespoon of oil to the cylinder and compression went from 60psi to 100psi.
It's also a Chrysler product (Caravan 3.3L)
I should have the head off today if I can just stop procrastinating.

Odd thing is that under load it feels smooth and powerful...as if the problem only exists at idle.
P0302 code confirmed and it's the cylinder with the 60psi compression.

So much helpful information in this thread.

Here is a link to my thread about this issue. I'll also post the final outcome.
https://bobistheoilguy.com/forums/t...-3-8l-low-compression-on-one-cylinder.394455/

Of course! I absolutely hate when I'm having the same issue as someone else and they don't post the outcome. Good luck with yours! Saw you're trying the MMO piston soak, I'd check compression before and after the soak and see how much of a difference there is.

 

[MaximumMini]

Of course! I absolutely hate when I'm having the same issue as someone else and they don't post the outcome. Good luck with yours! Saw you're trying the MMO piston soak, I'd check compression before and after the soak and see how much of a difference there is.

I tried it for 2 days and NADA difference.
I'm not a patient person.
I coulda had that motor out and half way rebuilt in those 2 days.

Im gonna go ahead and pop the other head off, then try to get the motor out before dinner.
In a few days I'll have all new bearings, new rings, new oil pump, all new seals, 2 rebuilt heads (well, this'll probably slow me down a week to 10 days due to machine shop) , water pump, lifters and gaskets. (and a few other things).
I love this car. well worth it.

REBUILD-A-RAMA !

 

[Chris142]

Replacing the head solved the issue. The exhaust valve on cyl 3 had recessed enough to cause the intermittent miss. After replacing the head, it's running great. Bottom end looked great as well when we pulled the head off.

That is very unusual.

 

[adventureboss]

That is very unusual.

I heard it is, especially at my mileage, but strangely enough a machine shop I talked to were the ones who actually suggested I order a new head because they already expected the exhaust valve to be recessed, said they wouldn't be able to rebuild it most likely. I guess they seem to find it common.

 

[Chris142]

I heard it is, especially at my mileage, but strangely enough a machine shop I talked to were the ones who actually suggested I order a new head because they already expected the exhaust valve to be recessed, said they wouldn't be able to rebuild it most likely. I guess they seem to find it common.

That's odd that they didn't want to install a new seat and guides.