What is going on with my compression?

[Astro14]

Actually, @adventureboss you do have low compression.

When doing a compression test, the absolute value is not what is important. There’s enough variation in people’s test method, the readings on gauges, and other little factors that the absolute value doesn’t really matter that much.

What matters most is the variation between cylinders. And you have some pretty big variation between cylinders, so that means you’ve got low compression on number three.

If your shop thinks they can fix the engine for 1400 bucks by replacing the head, and they believe it’s a valve leak on that number three cylinder it is causing your low compression in that cylinder, then I’d go ahead.

 

[Propflux01]

And what will you see?

The inside of the combustion chamber.

 

[supton]

Pulling the head is easier than dealing with rings, but if adding oil to the cylinder raised compression then I think you have stuck rings or a broken ring. With no oil consumption then I suspect the bore is fine—but if the ring is cracked, then no rebuild in a can will touch that,

But at 145psi I wouldn’t think it would be missing… everything else has been ruled out? plug and wire et al?

 

[AdmiralYoda]

I'm wondering if you have two separate issues. Your compression on #3 is low, but not horrible. The leak-down numbers show a fail but is it bad enough to throw a misfire code? Before you throw good money at it try a few maintenance things.

Every Jeep 4.0L I've owned has had a filthy intake manifold and intake valves. The water trick only helps the inside of the combustion chamber. Run a can or two of Seafoam or B12 through the throttle body. It really helps clean up the intake manifold and top of the intake valves.

Do a prolonged piston soak with B12. For days. If there is gunk on the rings this should help free it up. Run an oil that helps clean like Valvoline Restore and Protect and run it for 3k miles. Check the compression again. Maybe the compression improves and you can rule out piston rings.

If you are still getting a misfire code swap the coil and plug to a different cylinder. If the code follows the coil/wire then there is your answer. If not... time for a some cylinder head work.

 

[willbur]

The inside of the combustion chamber.

And...

 

[PontiacHO]

I had a shop do a leak down test and it came out to about 38% on cylinder 3. To them, this passes, but the service manual says anything above 25% fails. They suspect carbon build up as well because the water cleaning thing helped and sold me a can of GM top end cleaner. They could not find any actual mechanical failures, it sounded like the leak was coming from the piston rings a little bit, but also the valves. No blow by really?

I am going to do the GM top end cleaner as well as a piston soak and see if compression numbers improve and the misfire goes away to start.

I'll retest compression next week.

This should have been the revealing test. Leaking past valves or rings or both? If this shop didn’t give you a definite answer then they really did nothing but take your money. The head will have to come off if it’s not resolved by water, B12, or Seafoam anyway. Can the oil pan be removed with engine in the frame? If so, the pistons-rods can be pushed out through the top to re-ring the engine. If there’s a ridge at the top of the cylinders, it will need to be cut out first. Ridge reamer is a rental. Hit every cylinder with a dingleberry hone (with Dexron on the cylinder walls) and send the head to the machine shop. The hone is a rental. Clean the ring groves with a piston ring groove cleaning tool. It’s a rental tool also. Gasket set, rings, rod bearings, oil, filter, and coolant with the cost of machine work and your set for another 100k.

 

[Propflux01]

And...

What things do you think you'd be looking for with a a boroscope on an engine with low compression?

Bore wear, cracked pistons, burned or leaking valves, etc..

 

[adventureboss]

Actually, @adventureboss you do have low compression.

When doing a compression test, the absolute value is not what is important. There’s enough variation in people’s test method, the readings on gauges, and other little factors that the absolute value doesn’t really matter that much.

What matters most is the variation between cylinders. And you have some pretty big variation between cylinders, so that means you’ve got low compression on number three.

If your shop thinks they can fix the engine for 1400 bucks by replacing the head, and they believe it’s a valve leak on that number three cylinder it is causing your low compression in that cylinder, then I’d go ahead.

That's what they think but it's concerning me that literally none of them seem to be completely confident that it is the issue. I did try with another compression tester though and got the same numbers.

 

[adventureboss]

I'm wondering if you have two separate issues. Your compression on #3 is low, but not horrible. The leak-down numbers show a fail but is it bad enough to throw a misfire code? Before you throw good money at it try a few maintenance things.

Every Jeep 4.0L I've owned has had a filthy intake manifold and intake valves. The water trick only helps the inside of the combustion chamber. Run a can or two of Seafoam or B12 through the throttle body. It really helps clean up the intake manifold and top of the intake valves.

Do a prolonged piston soak with B12. For days. If there is gunk on the rings this should help free it up. Run an oil that helps clean like Valvoline Restore and Protect and run it for 3k miles. Check the compression again. Maybe the compression improves and you can rule out piston rings.

If you are still getting a misfire code swap the coil and plug to a different cylinder. If the code follows the coil/wire then there is your answer. If not... time for a some cylinder head

See I have literally ruled everything else out and done seafoam. B12 I have not tried though. I swapped the plugs and swapped the coil as well too, as well as the injectors, as well as the o2 sensors, as well as the cam sensor, and did a bunch of other things as well like check all the injector grounds, check if they are getting power, etc. etc. None of that was the issue.

What a headache, I can't wait for when this is resolved

 

[adventureboss]

Pulling the head is easier than dealing with rings, but if adding oil to the cylinder raised compression then I think you have stuck rings or a broken ring. With no oil consumption then I suspect the bore is fine—but if the ring is cracked, then no rebuild in a can will touch that,

But at 145psi I wouldn’t think it would be missing… everything else has been ruled out? plug and wire et al?

Literally everything anyone can ever think of I'm pretty sure I've ruled out.

 

[adventureboss]

This should have been the revealing test. Leaking past valves or rings or both? If this shop didn’t give you a definite answer then they really did nothing but take your money. The head will have to come off if it’s not resolved by water, B12, or Seafoam anyway. Can the oil pan be removed with engine in the frame? If so, the pistons-rods can be pushed out through the top to re-ring the engine. If there’s a ridge at the top of the cylinders, it will need to be cut out first. Ridge reamer is a rental. Hit every cylinder with a dingleberry hone (with Dexron on the cylinder walls) and send the head to the machine shop. The hone is a rental. Clean the ring groves with a piston ring groove cleaning tool. It’s a rental tool also. Gasket set, rings, rod bearings, oil, filter, and coolant with the cost of machine work and your set for another 100k.

Yeah unfortunately, that is exactly what happened. None of them can give me a definite answer. I don't understand why it's so difficult for them to do a dry and wet compression test, leak down test, and if it's borderline stick a camera in there and take a look around. I'm not a mechanic though just an occasional DIY guy so maybe I'm oversimplifying things and there's more to it than that so I'm giving them benefit of the doubt.

The oil pan can be removed with the engine in the frame. I was originally going to do this work myself but I'm also a college student and have gotten in trouble quite a few times for working on my Jeep in the parking garage at my apartment (parents who have a garage live out of state) so unfortunately with the rings and bearing roll, it is kind of dependent on what my mechanic is comfortable with. I asked for his thoughts though and he said he'd ask around and see what the best move is. Told me he's never done it that way but he'll call me back in a little bit and tell me what he thinks.

 

[Chris142]

Make sure new head is not a #0331. They can crack. But that may be older 4.0 Jeep engine.

That was 99-01 only. They continued to make the 0331 heads until 06 but from a different foundry. The newer ones are 0331tupy heads.

 

[Donald]

That was 99-01 only. They continued to make the 0331 heads until 06 but from a different foundry. The newer ones are 0331tupy heads.

The 01 - if the cracked head did not get you the cracked piston skirt would. Yet people loved their Jeeps even with a 4.0 engine.

And the engine needed a guard on the exhaust manifold else there was a place for chipmunks to store a few acorns on top of exhaust manifold and they could smolder, maybe catch fire.

 

[PontiacHO]

Yeah unfortunately, that is exactly what happened. None of them can give me a definite answer. I don't understand why it's so difficult for them to do a dry and wet compression test, leak down test, and if it's borderline stick a camera in there and take a look around. I'm not a mechanic though just an occasional DIY guy so maybe I'm oversimplifying things and there's more to it than that so I'm giving them benefit of the doubt.

The oil pan can be removed with the engine in the frame. I was originally going to do this work myself but I'm also a college student and have gotten in trouble quite a few times for working on my Jeep in the parking garage at my apartment (parents who have a garage live out of state) so unfortunately with the rings and bearing roll, it is kind of dependent on what my mechanic is comfortable with. I asked for his thoughts though and he said he'd ask around and see what the best move is. Told me he's never done it that way but he'll call me back in a little bit and tell me what he thinks.

Current technicians don’t do too many old fashioned re-ring jobs. Especially with the engine still in the frame. Since the head has to be removed anyway…..

 

[nascarnation]

Unless you're in an inspection state and the code is problematic for registration renewal, my advice is just drive it. There's many millions of cars on the road with way worse than one cylinder at 145 compression.

 

[adventureboss]

Current technicians don’t do too many old fashioned re-ring jobs. Especially with the engine still in the frame. Since the head has to be removed anyway…..

Yup he said he doesn't think it's a good idea, would apparently add another 8 hours of labor? I guess the oil pan adds three hours... I'm confused as to how, it's a few bolts looks like a 30 minute job but what do I know.

 

[adventureboss]

Unless you're in an inspection state and the code is problematic for registration renewal, my advice is just drive it. There's many millions of cars on the road with way worse than one cylinder at 145 compression.

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.

 

[adventureboss]

So I redid my compression test after all the cleaning I did with water, seafoam, GM top end cleaner, etc. Cylinder 3 is upto 158 psi from 145 originally. The rest of the cylinders stayed about the same. I also re did the wet compression test and this time did it on all cylinders. Added a teaspoon in. Compression only went up 5-7 psi on each cylinder. Looks like carbon was part of the issue on mine?

I think I can safely rule out piston rings at this point, I'm going to continue on with the head replacement because of the 38% leak the shop found on the third cylinder.

 

[Kira]

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?

 

[Chris142]

Yup he said he doesn't think it's a good idea, would apparently add another 8 hours of labor? I guess the oil pan adds three hours... I'm confused as to how, it's a few bolts looks like a 30 minute job but what do I know.

The exhaust with the pre cats is in the way of getting the pan out. In order to get that out the belly pan has to be dropped.