Merry Christmas / Seasons Greetings everyone. I have a question for those that might know: I just did a compression test today on my 1999 TJ 4.0L that has about 174,000 miles (280,000 Klm) on it and I got this: #1 - 149 , #2 - 156 , #3 - 163 , #4 - 172 , #5 - 145 , #6 - 145. I'm thinking #3 & 4 might be carboned up. The spark plugs all looked the same so no concern there. My questions are these: Are these higher compressions (163 & 172) high enough to be concerned about? If so and it is carbon build-up what can I use to clear some of it out..Seafoam?? I would appreciate any insight some of you might have. Thanks!!
Engine Compression
- Thread starter MarkAllan
- Start date
What's the cause for a compression check?
Is your gauge accurate and a good source of data?
Do you have access to a bore scope to inspect the compression chamber?
Did you repeat the test to confirm numbers?
I only ask as I spend good money on gauges for checking values. Cheap gauges can give off bad repeated readings.
Compression is not always equal on all cylinders when miles are applied.
Is your gauge accurate and a good source of data?
Do you have access to a bore scope to inspect the compression chamber?
Did you repeat the test to confirm numbers?
I only ask as I spend good money on gauges for checking values. Cheap gauges can give off bad repeated readings.
Compression is not always equal on all cylinders when miles are applied.
Thank you for your reply, excellent questions! I got the TJ about 2 months ago and have been doing a multitude of repairs and upgrades so when I decided to change the sparkplugs I thought I might as well check the compression....It was running fine, smooth idle and no cause to suspect an issue. The gauge is new and I believe accurate...It's not a cheap POS so I'm going to say that is a good source of data. My borescope on the other hand is a POS, too grainy and too narrow a view to be of any use. Yes I ran all 6 twice with only very slight variances.
I would say that as long as all cylinders are with 20% of each other you should have no issue and quick math in my head puts you at around 10% +/- 1%. I have no idea the expected compression of a new 4.0
Might have to look in the Service Manual for a number.
The Service Manual says 120 to 150 which is why 163 & 172 concerned me. Cyl #2 is 156 but that doesn't concern me as much. I always considered the 10 to 20% percent rule more applicable to cylinders that were dropping a bit...but that's by no means a professional opinion. I'm thinking maybe trying a decarbonation treatment but not sure if that stuff is just snake oil.
That's why I questioned the repeatability of the gauges. Sometimes one side of the gauge might not be as accurate as the other. Upper cylinder cleaners are not 100% snake oil but you can only hope they work. If it was me I would run it and see what happens after a few bottles of fuel cleaner and then pull the head to confirm condition. 4.0 heads are an easy pull and a little head work helps a bunch. The last 4.0 I had the head worked on ran great compared to my current rig. Keep in mind I never ran compression numbers as I was just jumping in for other issues.
Water works and won't mess up sensors. Just don't pour enough at once to damage your motor.
I agree with you...I think running a couple of tanks of gas with some fuel treatment in each tank..maybe doing a seafoam treatment and then rerun the compression test...If I'm still getting those larger numbers then pulling the head and cleaning things up that way...I had the head (valve) cover off the other day to change the gasket and took a look at what would be involved with pulling the head and it seemed pretty straight forward. One thing about those compression numbers is it tells me the cylinder walls and piston rings are still in good shape so maybe just pulling the engine in the near future and doing a rebuild/refresh wouldn't be a waste of money. I pulled the oil pan off two days ago to change the rear seal and pan gasket and no glitter or metal in the pan...cam looked good too. Thanks for your advice...it's much appreciated!!
I've heard about people doing that...with mixed results. I've heard that too much water and you can lock it up.
This is legit but I didn't know how to explain it safely. I used to use a squirt bottle on mist to add a little moisture to the mix while running to knock off carbon. But again, I did not want to assume everyone understood the concept.
I'm watching some youtube videos now of people doing it...The science is sound but the application needs to be fairly precise. I can see how the squirt bottle would be a safer application...I just watched one guy pour water into his air intake and another guy suck water in through a vacuum line...both methods seemed a bit haphazard.
Cylinders 3 and 4, I believe, will have higher values simply due to their location. They are the first ones to get oil upon start up and they receive a bit more oil throughout the engines life.
I wouldn't worry at all about the numbers you got. Put it together and have fun. Stop creating a problem when one doesn't exist.
I wouldn't worry at all about the numbers you got. Put it together and have fun. Stop creating a problem when one doesn't exist.
I presume you bought it for fun, not to be a perpetual project. If you want a perpetual project, go work in an auto repair shop.
Fix things as they break...yes they break. Don't fret, you'll get plenty of opportunities for projects, just not self inflicted ones.
My hobby is finding vehicles that are in need of some mechanical attention and spending some time in bringing them back to life. The TJ has gotten a huge shopping list of things done so far...Lift Kit...new suspension components, new seals, gaskets, rad, alt, hoses, all fluids, brakes with rotors and drums, 33's tires and wheels, the list goes on...so far it's one of the best vehicles I've ever worked on..I love the simplicity of it.
Similar threads
