XinnKoda

2008 JKU 3.8L with approx 70k miles. Consumes oil but otherwise runs pretty strong.

Some months ago I decided to do an engine compression test to see how healthy everything was looking. I don't remember the exact numbers but it seems like they were all around 150psi and they were all pretty close to each other (within 15 psi). Since my JK has been consuming oil a lot recently, I pulled the spark plugs and put a boresnake camera down there to take a look. Sure enough, there was quite a bit of oil on the tops of the pistons. I bought the SeaFoam intake cleaner (the kind you spray into your intake after your MAF) and my wife and I performed this last night. I was surprised at the amount of smoke it didn't produce (seeing videos on youtube made me think I'd smoke out my entire neighborhood). I saw some smoke exit the exhaust but nothing like I expected. I followed the instructions to the T and even set my timer to make sure I was strictly sticking to the instructions. After it "hot soaked" I drove it aggressively, per the instructions until all the smoke had cleared and then I drove it a bit more. I boresnaked the cylinders again and noticed it had a noticeable difference in the amount of carbon it removed. This morning, I go to take a drive and it won't start. It has a crank but no start condition. I check all the basic stuff out before I decide to move to a compression test. The test shows all cylinders around 55-60psi!!! How can this be? The engine acts like it ALMOST can start but just doesn't have enough to get it going. I verified my compression gauge was working by hooking it up to my compressor and blowing some air through it. Compressor was set to 125 psi and the gauge showed 120psi. Why would I suddenly lose all this compression in all my cylinders overnight?! I didn't do anything to the valves or heads or nothing. If I had skipped a timing tooth wouldn't I have noticed it when I was test driving? When I turned off my Jeep last night, everything was working great. I'm at a loss for what to do. Please give me some tips on what could possibly have happened.

silverbullet88

Shoot I don't know maybe someone with more knowledge can chime in

jtphoto JK

Hmm if it's a loss on all cylinders you have cleaned all the carbon a gunk that has :
- Freed the piston rings which no longer seal as well
- or the valve seats are no longer sealing.
- combination of both ..

Sometimes running cleaning agents in an older motor is not a good thing.

zstairlessone

If you have that bore scope available look into each cylinder and see if the valves have any crud keeping them from closing (may have freed up with the Seafoam treatment. Not too many things would drop compression in all 6 cylinders at once, do they all hold at 60? Do they press up to 150 and bleed down? When you crank it are you getting a crankcase pressure event logged? What codes (if any) are active? Did you gunk up the Cats or the O2 sensors when the engine was de-gunked (is the exhaust plugged up/excessive back pressure)? Did you loose the timing belt?

ronjenx

On this compression test, was the throttle plate held fully opened?
Were all the plugs out?

A leak-down test will tell you where the leak is, if there is one.

XinnKoda

Thank you everyone for the replies! To add a little more detail, I performed a wet/dry compression test (while the engine was cold, due to lack of being able to start it) and there wasn't a difference between wet and dry numbers. This would point to something other than piston rings, correct? I do have a borescope that I have been using but unfortunately I don't think I can get the camera angle to determine if the valves are fully seating. One thing that strikes me odd is that all of the cylinders have lost compression. It seems odd that ALL of the pistons rings could have failed, or ALL of the intake/exhaust valves failed to seat all at once. The engine seemed to be running strong when I drove it last. The logical part of me thinks that the engine skipped timing and the valves are still partially opened during TDC on the compression stroke. This would explain why the cylinders universally are giving me similar numbers, wet or dry. But my gut tells me this is something other than timing chain skipping. My gut is telling me that the piston rings were previously being sealed with gunk and now the gunk is gone and no more compression. Seems not likely that all cylinders would experience this all at the same time (you'd think at least one cylinder would be OK out of the 6). I performed compression test with the throttle wide open. My next steps: Leakdown test. I will performed a leakdown test as soon as I get it in the mail on Tuesday. This will at least tell me where I'm losing compression. Also, in order to verify engine timing, the only way is to pull the timing cover off right? There's no inspection window to see if the teeth are still matched with the appropriate chain link?

On a side note, since we bought the Jeep a year ago, my wife and I were both disappointed in the power of the engine. We figured it was just a weak engine from the factory which is why they replaced it in 2012. After these events, my wife mentioned that over the last few months the power has gone down a lot and she states it's gutless compared to when we bought it. The Jeep is my wife's daily driver during the winter so I haven't been able to notice any trends with loss of power since I hardly ever drive it. Part of me thinks maybe it's been losing compression slowly over this last year from excessive oil consumption and maybe the seafoam kicked it over the edge.

tramp

i agree i find it hard to believe that all the cylinders are down the same amount
you say it has been losing power over time
worn cam?
does it blow smoke?

XinnKoda

Small update! I'm still waiting for my leakdown test to come in the mail, but I was able to perform a little bit of a leakdown test on the number 2 clyinder. I took my compressor air nozzle with a rubber nipple on it and stuck it in the spark plug hole with the cylinder being TDC on the compression stroke. I pressurized it to 100psi and took the oil cap off, radiator cap off, and one of the intake hoses off. As soon as I pressurized it, tons of air started escaping through the oil fill cap. I didn't notice any air coming from the intake, exhaust, or radiator. I was only able to perform this test on one of the six cylinders since my air nozzle wouldn't reach and properly seal the other 5 due to clearance issues. To me, it sounds like I'm looking into putting new piston rings in it. I'm not pulling the trigger yet, I am going to wait until I have my leakdown test and I can properly diagnose all 6 cylinders. My Jeep is a 2008 and I have a suspicion that my Jeep was one of the ones that had the piston rings installed incorrectly. Maybe when it was new, the rings provided enough compression that the engine would still run but allow oil to come up and eventually carbonized. I'm thinking it's possible that over time the piston rings became seized to the piston and then the carbonized oil was the only thing holding it together and sealing it. This is just a theory but if I end up needing new piston rings then I'll be doing the repair myself and I will film the entire thing. I noticed there's very few videos on YouTube of people disassembling the 3.8 and so I figured if I have to do it then I might as well show an in-depth video on it. I'll update more as I find out more.

ronjenx

Leakage through the rings is inevitable. The tester will tell you if it's excessive. The leakage will sound worse the higher the pressure applied. Using the tester may require a lower pressure. Mine is internally regulated to 80 psi.

If you do end up disassembling the engine, pay particular attention to:

-Ring installation; some were said to be installed upside down.
No need to notice ring gap positions. They rotate at random rates when the engine is running.

-Direction of oil squirt holes in the rod big ends. They should point to the right side of the engine. That's the major thrust side between piston and cylinder.
Some were said to be pointing to the left side. (right and left as viewed from the flywheel end)

-Cylinder for being round. Some were said to be out of round from day one.

XinnKoda

Great info! Over the next few days I am going to spend a lot of time on everything that should be upgraded in the motor due to factory defects or errors. This was the first time I heard about the oil squirters being installed towards the wrong side. Is this why spun rod bearings are so common in these engines?