What is going on with my compression?

[adventureboss]

I have been chasing a P0303 misfire code for quite some time now. I went through everything and determined it is most likely an engine issue. 2006 Jeep Wrangler with the 4.0 and 190k miles. I did do the water cleaning trick where I held the motor at 2000 rpm as I trickled a quart of water into the throttle body. After doing this, I have not got any misfire codes for the past 1000 miles but I will on occasion get a rough idle so I did a compression test on a hot engine, held for 8 cranks, and I recorded the numbers.

Cylinder 1 was 175 psi
Cylinder 2 was 180 psi
Cylinder 3 was 145 psi
Cylinder 4 was 170 psi
Cylinder 5 was 160 psi
Cylinder 6 was 180 psi

Because that third cylinder had the misfire code and had slightly lower compression, I added a teaspoon of oil and retested. Compression went up to almost 160 psi. This is roughly a 10 percent increase, so would this indicate a top end issue with most likely the valves? Or do I have an issue with the piston rings on that cylinder? I would assume some wear on the rings after 190k miles is normal, correct?

I would have a leak down test done but I am having a hard time finding anyone with availability anytime soon or someone who can actually do a leak down test. Should I just jump directly to pulling the head and sending it off for a valve job or is that still a little premature without further testing?

 

[iCurious]

Do a leak down test on cyl 3 and see what happens.

 

[Donald]

Oil in a cylinder only helps rings, not valves.

As a first step repeat everything.

Your engine is worn. Possibly a piston soak might help.

What are your plans for the Jeep? Is it in good shape, rust free?

Not sure the codes are connected to the compression issue.

I would keep driving until it gets a lot worse. Then if you still plan on keeping it, get a reman engine.

 

[Dave9]

Without a leak down test, I might try putting some solvent in the cylinder to soak the rings for hours, crank it with plug out to flush that out (if it hasn't leaked away) and see if you release stuck rings by running it a while.

Otherwise, seems like the head needs to come off to get a better look, since the compression increase could be rings or cylinder wear, leaking head gasket, crack, valve seats (though less likely to keep pressure then, by adding oil). I would not assume yet that it only needs a valve job, but you can inspect that as well with the head off.

 

[PontiacHO]

Autozone here has a leak down tester. I’d do that to confirm the low compression (rings). Has this engine ever been overheated and/or has had spark knock issues? Can the oil pan be removed with engine in the vehicle? If so the pistons can be pushed out and new rings installed after a “dingleberry” hone.

 

[Chris142]

Not saying it can't happen but a burned valve or not sealing valve is very rare on that engine. Pull the valve cover and make sure the intake lobe on #3 is not flat. I would also look at the valve springs real close.

 

[Propflux01]

190k + low compression on one cylinder + brought up by oil addition + previous misfire = worn rings/bore/piston.

How much oil is added between OCIs?

 

[adventureboss]

Oil in a cylinder only helps rings, not valves.

As a first step repeat everything.

Your engine is worn. Possibly a piston soak might help.

What are your plans for the Jeep? Is it in good shape, rust free?

Not sure the codes are connected to the compression issue.

I would keep driving until it gets a lot worse. Then if you still plan on keeping it, get a reman engine.

I'll try doing a piston soak and see if it changes anything. The Jeep is rust free and in great shape. I plan on keeping it forever if I can. Too much fun, too many memories with it and easy to work on.

 

[adventureboss]

Autozone here has a leak down tester. I’d do that to confirm the low compression (rings). Has this engine ever been overheated and/or has had spark knock issues? Can the oil pan be removed with engine in the vehicle? If so the pistons can be pushed out and new rings installed after a “dingleberry” hone.

I don't have an air pump to power to leak down tester unfortunately, but I'm still looking for a shop that can do the test for me. The previous owners did not take care of the Jeep as far as I can tell. The coolant system was sludged up and it did have an overheating issue until I replaced everything and flushed the block, etc.

No spark knock issues though, and the oil pan can be removed with the engine in the car.

 

[adventureboss]

190k + low compression on one cylinder + brought up by oil addition + previous misfire = worn rings/bore/piston.

How much oil is added between OCIs?

Zero, burns no oil. I have pushed 6500 miles on occasion on Rotella T6 and the oil level has stayed the same. That's what throwing me off. I would think on a worn engine that it would burn SOME oil atleast.

 

[boom10ful]

Without a leak down test, I might try putting some solvent in the cylinder to soak the rings for hours, crank it with plug out to flush that out (if it hasn't leaked away) and see if you release stuck rings by running it a while.

Otherwise, seems like the head needs to come off to get a better look, since the compression increase could be rings or cylinder wear, leaking head gasket, crack, valve seats (though less likely to keep pressure then, by adding oil). I would not assume yet that it only needs a valve job, but you can inspect that as well with the head off.

I'd do that and run Valvoline R&P.

 

[burbguy82]

slightly lower compression

that is not slightly, it is a bunch. I have a 4.0 with 110 compression on #6, and it has a bunch of blowby......with 290,k I am going to let it roll.

Possibly a piston soak might help

I think this would be the best thing to do, followed by a short hard drive and an oil change. I recently did this on my 91 YJ, and in gave made a difference in power....but sure got a lot of nasties in the oil

 

[Propflux01]

Zero, burns no oil. I have pushed 6500 miles on occasion on Rotella T6 and the oil level has stayed the same. That's what throwing me off. I would think on a worn engine that it would burn SOME oil atleast.

Get you one of those borescopes you find on Amazon and snake it down the spark plug hole.

 

[Donald]

Recheck first. Go through all the cylinders again.

Your compression rings could be worn but oil control rings not worn. Start the engine and pull the oil fill and see if there is blowback.

One can pull the head, send it out to be redone and then pull each piston and change the rings. But probably not a good move financially unless you can do a lot yourself.

I replaced a 4.0 engine in my 2001 Jeep Grand Cherokee. I think it was $3000 for labor and a reman engine. But several years ago.

 

[adventureboss]

Recheck first. Go through all the cylinders again.

Your compression rings could be worn but oil control rings not worn. Start the engine and pull the oil fill and see if there is blowback.

One can pull the head, send it out to be redone and then pull each piston and change the rings. But probably not a good move financially unless you can do a lot yourself.

I replaced a 4.0 engine in my 2001 Jeep Grand Cherokee. I think it was $3000 for labor and a reman engine. But several years ago.

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.

 

[iCurious]

You go by the manufacturer's specification of allowance. If it's 25% and the result is 38%, it's a Fail.

Carbon buildup does not cause low compression readings, but instead high compression readings.

Any leaks observed by ear - hearing air escaping from the intake, crankcase and etc would be intake valves not sealing.
Leaks observed coming out from exhaust pipe would be exhaust valves not sealing.

Add some oil to #3 cylinder if compression improves, it's the piston rings.

 

[Hohn]

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.

Leakdown tests should always have a time reference as well, not just a pressure loss percentage or PSI value.

Your prior test where you added a teaspoon of oil proves basically nothing. You have no way of knowing if the higher pressure resulted from some oil improving oil seal or if it was improving ring seal or if it had no effect on sealing at all and it was just the effect of replacing some compressible air with incompressible oil.

Assuming they followed the service manual procedure to appropriately apply the service manual spec, then you have a failure for sure.

You can somewhat isolate the source of your compression leak by applying compressed air to the spark plug hole with that cylinder at TDC right before power stroke (valves closed). Leakage should be minimal here, but of course some leakage will occur.

Take a small balloon and put it over your PCV hose that feeds back into the intake. If the compression is leaking out past the rings, it shows up as crankcase pressure and your balloon will inflate at a faster rate than it will if the test is repeated on another cylinder.

If the balloon on the crankcase hose inflates the same on all cylinders when the same pressure is applied, then the compression isn’t leaking into the crankcase (past the rings) it is leaking past the valves and into the intake or exhaust ports. It’s usually the exhaust port/valve that leaks first because it runs so much hotter.

It’s trickier to determine which valves (intake or exhaust) are leaking, but since the exhausts run so much hotter, it’s 9/10 times the exhaust valves that develop the leaks first. You might be able to hear the leakage in the intake tract (throttle body) or exhaust system (tailpipes) with a mechanic’s stethoscope.

 

[adventureboss]

The misfire came back even after the GM top end cleaner, and going through these responses I am hearing a few different things.

One being my piston rings could be the issue. I know I cannot totally rule them out, but if my piston rings were truly the issue, how am I getting high compression numbers on all cylinders including # 3? Factory spec is 120-150 psi, and most cylinders are between 162-178 psi with cylinder 3 coming in at 145 psi. Per my knowledge, if I had worn piston rings, these numbers would be lower, correct? I don't think anyone has addressed this yet, I do not have low compression. If I am misinformed and I can have shot piston rings with high compression numbers someone please correct me.

On top of that, I do not burn any oil, as in zero, and I would think a piston ring issue would be more consistent and not just an intermittent misfire issue at idle. I do not have any misfiring at higher than idle RPMs. Secondly, it seems like some say my compression number on the third cylinder going up 10 psi after putting in a teaspoon of oil in shows ring wear, however, some say that the test does not prove anything. For now, I will disregard this then.

What I know for sure is that cylinder 3 failed the leak down test at 38%, and they did hear leakage from the exhaust and intake valves, so I know it is an issue. I spoke with a local mechanic who is familiar with these engines and I can have a new head on the motor for $1400. I think this is a good deal and will probably go through with it. I know I'm taking somewhat of a risk and there's a million other things I can do to be 1000% sure of what the issue is but honestly the amount of time and money I've spent with just diagnosing is getting to be quite a bit.

It may or may not solve the misfire but I know the head is an issue and will need it sooner or later so I'm going to start with that.

 

[willbur]

Get you one of those borescopes you find on Amazon and snake it down the spark plug hole.

And what will you see?

 

[Donald]

The misfire came back even after the GM top end cleaner, and going through these responses I am hearing a few different things.

One being my piston rings could be the issue. I know I cannot totally rule them out, but if my piston rings were truly the issue, how am I getting high compression numbers on all cylinders including # 3? Factory spec is 120-150 psi, and most cylinders are between 162-178 psi with cylinder 3 coming in at 145 psi. Per my knowledge, if I had worn piston rings, these numbers would be lower, correct? I don't think anyone has addressed this yet, I do not have low compression. If I am misinformed and I can have shot piston rings with high compression numbers someone please correct me.

On top of that, I do not burn any oil, as in zero, and I would think a piston ring issue would be more consistent and not just an intermittent misfire issue at idle. I do not have any misfiring at higher than idle RPMs. Secondly, it seems like some say my compression number on the third cylinder going up 10 psi after putting in a teaspoon of oil in shows ring wear, however, some say that the test does not prove anything. For now, I will disregard this then.

What I know for sure is that cylinder 3 failed the leak down test at 38%, and they did hear leakage from the exhaust and intake valves, so I know it is an issue. I spoke with a local mechanic who is familiar with these engines and I can have a new head on the motor for $1400. I think this is a good deal and will probably go through with it. I know I'm taking somewhat of a risk and there's a million other things I can do to be 1000% sure of what the issue is but honestly the amount of time and money I've spent with just diagnosing is getting to be quite a bit.

It may or may not solve the misfire but I know the head is an issue and will need it sooner or later so I'm going to start with that.

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