How To Do A Compression Test And Decipher The Results - Tools, Technique And Reading Wet Vs Dry Test
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- Опубликовано: 8 фев 2024
- After we did a snap diagnosis of blown head gasket on our 4.0 Jeep, we decided to go back and give it a proper compression test.
Here are all the things you need to know in order to prepare for the test, what the results mean, and how to use oil (wet test) to determine what kind of damage is killing your cylinder's compression.
#diy #autorepair #engine #cars #classiccar
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1947 Jeep flat head 4, all of the sudden its running on the two outside cylinders. My brother and I figured head gasket, no compression tester. We decided to use a blow gun to squirt air down the plug holes, to verifyour diagnosis. He leans over gives it a shot in the second plug hole, the air coming out of the third plug hole blew his hat off. Yup its leaking between 2 and 3. 😂
New head gasket later, it ran like a worn-out 47 Jeep again.
Do the rings brother. You're already asking that engine to tow, something it's not really gonna love, so show it some love with a ring job. It'll be worth it in the end, and you'll know you gave it every chance at succeeding. Also doing the rings SUCKS so there's that....
Why do rings and not rod bearings. Then why not main bearings. At that point pull the cam out and do those too. Why leave the engine in shoulda just pulled the engine and sent it out for boring align honing and decking too. I mean might was well do the pistons while your at it. Nitrous yeah that will help
😂 very good video.
@@eddiereichel9354 Because the rings are clearly the problem. By your reasoning why touch your engine at all just buy a new car. You have to draw the line somewhere and it's clear the rings/cylinder walls need work, there was no knocking in the video so why touch the rod/pistons/crankshaft
....because for towing, it will need all the torque it can muster and a weak compression cylinder will 'unbalance' the power delivery with the rest of the cylinders and it COULD cause all sorts of mischief that you dont want or need.....THEREFORE, the idea of making sure the engine compression is even and up to spec would be a total no-brainer whereas all the rest of that guff that you spout is just dross.
Capiche there, brotato??? @@eddiereichel9354
@@eddiereichel9354 in all honesty this is always the question but it has no right answer. Fix it and you still have a high mile motor that is getting weak. Do more work and now we invested more money and time into it. Sure we have a great engine after that but what's next...
It has no right answer but it changes on each person's risk level, money, and time...
Choose wisely, my friend!
Oh wow, a way back to basics video from UTG! Badass!
This man is an absolute well of automotive knowledge and he's shining the light onto the next generation of young mechanics coming up. Absolutely love your channel Uncle Tony. Thank you!💗
Amen, second that. Also like to add, this channel is GREATLY APPRECIATED, Thanks again Tony
I've learned heaps from this guy, people like this are why I love engines
Always enjoy these basic mechanic-ing videos. Also rain on the metal roof is relaxing.
Thank you Uncle Tony. These videos really help guys like me who are just starting out with engine work. An engine timing video would be great also! Thanks again from New Orleans.
I like that you even used the same tool most of us would go pick up from harbor freight for our home shops good video
The roof noise sounded like loose change in the dryer😁
Definitely popping some popcorn for watching some RUclips later
This is the basic knowledge of a any
Well rounded mechanic back then
And today so please all you young
People pay attention great video tony
You were right there, with everything set up, all the spark plugs out,....You should check the compression of the remaining cylinders so you'll have more data to decide if you should re-ring or not.
I agree 💯%
So let's say the last two cylinders have good compression. That leaves us with one low and two unknown. Do the rings...
Pretty much, with THAT sort of mileage (264K? Wow...can't say the engine didn't hold up, AFAIC, it went LONGER than Chrysler ever engineered it to!), if the compression is low, and it's not something like a collapsed lifter, or valve lash REALLY "out of whack", or jumped timing chain or broken timing belt (and if it's an "interference" engine, it's likely "fucked" anyway), you WILL have to do at least a "ring job", but most likely, a complete rebuild, replacement rebuilt short block, or even a "factory re-built' engine. The particular route to go depends upon what you want to do with the rebuilt/replacement engine, expected service and service LIFE, and comparative COSTS. In many cases, especially IF you've got a good cylinder head, no broken/burned valve seats, no need to replace valve springs/ rocker shafts, to have it serviced at a machine shop and simply buy a short block, when it's going to be strictly a stock motor, is the most reliable and cost-effective solution. If you want to build a "performance" engine, find a reliable performance shop that has experience with that particular line of engines, i.e., don't have the local guy that builds Chevy LS engines redo your Toyota's 22R engine; if you want it to do a "burn-out" like the "revised" Marty McFly's '86 Toyota SR5 extended cab with the lift kit and extra "toys" could, PAY THE FREIGHT and find a reputable shop that regularly rebuilds Toyota motors!
@@selfdo I put over a million miles on my '88. Had the engine replaced at 599K. It still ran, but was gushing oil out of the rear main seal.
@@8avexp '88..Jeep Cherokee? Would have had that indestructible 4.0 liter inline six, but yes, leaky rear main seals were an endemic problem with that engine.
I would also do a leak down test on each cylinder by blowing regulated air in the spark plug hole with the valves closed and top dead center. Listen for air coming out the exhaust, intake and oil filler hole on any engine.
Great video and explanation. I would just add to pull the fuel pump fuse or lock throttle wide open to engage flooded engine mode so your not washing down the cylinders while your testing. Great practical info.
Finally Back to some real practical stuff
This is my favorite style of utg videos!
Couldn’t hit like fast enough ❤
Much love brother man❤
Ty for your tutelage re wet and dry com p ression checks and also possible blown head gasket issues with side to side cylinders very helpfull.recently worked on a lexus and only one cylinder was down around 30 to 45 lbs compression,borascoped cylinders and no sighn coolant.dropped around 30 ccs of magic marvel mystery oil into cylinder via spark plug hole and retested 2 days later and viola! Back up to comparable compression.ol school trick from Jimmy a d daisy dog mobile mechanic!!
This is exactly why I watch your stuff! Appreciate you helping the beginners
Thanks Uncle Tony!
Thanks for the knowledge! Love the channel. It must be alot of work to put out the amount of awesome content you do. keep it up UTG
We used to use the thread-in compression tester hose, schrader valve core removed, and couple it to a shop air line. Bad intake valve, air coming thru carb. Bad exhaust valve, you could hear air at the tailpipe. Early 351 windsor vale guides were bad, burned exhaust valves were common. Especially after unleaded gas showed up.
Always a fantastic video when it come to UTG
Great video and detailed description. Thanks!
Absolutely love videos like this! Keep it up Boss!
Thank you Tony, all the knowledge you drops really helps.
I really enjoyed this instructional video Uncle Tony! Good work!
This is gold, never thought about the wet and dry test, thanking you!
Yep, you hit the nail on the head. For people who are trying to learn this is top notch info.
My '88 Jeep had the 4.0-liter six. Great engine!
Good stuff Tony and well explained. I 've done this before (years ago) but it doesn't hurt to get a refresh. Keep on smiling.
Thank you Tony that was a great tutorial please keep up the good work. And thank you again.
Thanks for the video and the teaching, can't wait for the video fitting it. Once again thank you
Planes, Trains, Rains, and Automobiles. Love it UT.
Great tech session!
Awesome tips Uncle Tony.. Thanks for sharing..
My 1st gen Taco with 243,000 miles still getting 190-200 psi. Valve lash is still good, so plenty of life in the heads. When I first got the truck, it stank of gas and was hard starting when cold. Plugs were a bit sooty. So, I fired the parts cannon at it with a new set of injectors. Sent the throttle body off to a shop in Florida that mills the throttle bore and custom fits a new throttle plate for it. I smoked out the top of the motor and sorted out a number of vacuum leaks. Now she just purrs like a kitten with tremendous throttle response. Can't go wrong with the tried and true Toyota 5VZ-FE engine, free from all the unnecessary electro-gizmo stuff that is destined to break. And it's a non-interference motor so it's a breeze to change the timing belt and just all around is an easy engine to work on.
thank you sir this is what the people need
Great video! After 200K miles makes sense that it needs new rings. Only suggestion I would have is that we all need to train ourselves to return our tools to the workbench, not on the battery, air cleaner, valve cover, intake, etc. I have quite a collection of really nice stuff that other mechanics have left in my cars' engine compartment over the years!
Thanks for this. I appreciate the schooling.
Awesome facts! And I do watch to actually learn here
Great video Tony.....I rememner back in 90,s at Nissan dealer the Hardbody trucks with the 8 spark plug 4 cylinder Head gasket job would come in......Every single one I started on after removing the head....the block was dipped out between cyl. 2 and 3 and that was it....price a engine and out the door it went! Great demo. showing how to do it.....be surprised at older mechanics that would not crack the throttle like you did. Take care brother.
Thanks love and learn from your videos!
Thankyou Tony
Thanks Unko!
Great video... I like to do compression testing on a warm engine. It's a more accurate reading.
It's toast warm and cold
Thanks. Learned something.
Welcome back, Tony. We missed ya.
Thanks Tony
Kiwi only had 70 PSI on his '59 Plymouth Marge, and that got him across the US!
Only a kiwi can do it too 😂
That thing was running on vibes!😂👍
...and it wasnt TOWING a 5k pound trailer....
Good one, Tony...TY
Thanks for this Tony. My compression test on the ski boat 350 showed zero compression on one cylinder. Blew compressed air into the spark plug hole and listened for where the air was coming out. Was coming out the left exhaust so a bent valve. Must have been when the timing chain broke - I fixed the timing chain but missed the bent valve. All fixed now and the boat runs much better on 8 cylinders rather than 7
The 00-01 4.0 had the dreaded 0331 head that was prone to cracking. Other than that, they're pretty much immortal.
3 and 4 are on the same stroke 180 out, with the spark plug in its going out the exhaust on the other.
This will be a tow vehicle, even though the job will be a pain in the butt, do the ring job. Best in the long run Tony. Love the content and how you try to pass on the knowledge.
Never a better man can say 😊
Awesome!
I think new rings would be prudent on a tow vehicle 🙂
Great information, tips and tricks. Does the oil added to the cylinder get pushed up into your compression gauge and does that affect future readouts ?
For the "great unwashed" of the home mechanical world, you should talk about the process of testing cylinders for spark and performance by "grounding" (I inserrt the spring extentions between the cable boots and plugs and use a basic test light) the plugs to test for performance variances for the purpose of basic troubleshooting. That's just off of a lot of people's radar/experience.
Also, as you mentioned spark plug observation and determining condition, please talk about the use of (solenoid) "noid lights" for determining injector performance. Once very expensive and really only available from the tool trucks, they're also now available from Harbor Freight at a home mechanic price range ($30-ish).
How much is "a good amount" of oil in a cylinder for testing?? About a tablespoon, a little more. That's plenty. 😊
Another test in addition would be running compression VS the static type compression test you performed example: early on in my career I was troubleshooting a Saturn with a cyl 3 misfire everything looked good injector working, spark present, it had compression when cranking, GM tech friend of the shop owner happened to be there and recommended testing compression while running all of a sudden no compression..the hell? pulled the valve cover off and Voilà broken exhaust spring never forgot that
Hi Tony! I'm loving the XJ content as I've been madly in love with the little devils for years now. A quick question, I may have missed it but when doing a compression test, do you let the fuel pump run? Or pull the fuse/relay so no fuel flows?
Maybe a suspended ceiling installation would dumb down the rain noise? Better acoustics for filming?
Just did a comp test on my and my friends 12a’s in our rx7’s
perfect timing
Can't really do it the same way on a rotary, if you don't remove the schrader valve from the tester you basically get the highest of the three rotor faces, not the actual numbers from each chamber. Learned that one a lonnnnng time ago when I was racing RX3's.
@@rennkafer13 definitely gotta pull the check valve first
I pulled the motor last night
Dead corner seal springs and some chrome flake on the front housing hopefully It’ll be into the triple digits once it’s back together
@@sam49921 ya, a lot of people don't know you have to do that. Hope it goes back together smoothly.
@@rennkafer13 how old do you have to be to remember seeing more then one rx3?
I’m 23 been doing the rotary stuff for about 5 years and I’ve only seen one rx3 wagon
Me and my buddy where pretty stoked to see it even in a storage unit surrounded by junk
@@sam49921 ollllllllld... I started autocrossing in an RX3 in 1980 when there were lots around here in WA state.
That old 4.0 stuffed in an XJ will get plenty toasty when you're towing 5k pounds uphill with the AC on when its 100 degrees out. I think you'll want those rings in good shape.
I do what's called relative compression. Use an amp clamp on the battery cable. It draws a waveform on the screen. With an ignition input from a known cylinder there will be humps drawn on oscilloscope for each compression stroke. You can count each hump along with firing order to figure out which cylinder would be low on compression without taking a bolt or spark plug out of the engine. Takes about 10 minutes
i put the compression number on the dirt of the underhood, and if its a valve problem if you have manual ajusting valves that are too closed or a bad hidraulic lifter that may be it, if the oil on the rings doestn changes anything!
cheers from argentina
Uncle Tony, test 5 and 6 to see if you need a ring job! Honestly, #1 low, #2 good, #3&4 unk, #5&6 unknown = ring job.
The other thing you can do is put the cyclinder in question in firing postion both vavles closed piston up shoot compressed air into the spark plug hole
Its comes out dipstick needs rings and bore or hone
It comes out tail pipe it exhaust vavle
You get the idea
In your case it comes out spark plug 3 hole
if you put up a few flags or large cloth logos on the upper walls your sound would lose the hollow sound ....just a thought ....love your show
That’s a nice jeep
I had a 2000 Cherokee with a 4 L, it had bad compression on two cylinders. The camshaft was worn down pretty bad I think because the oil nowadays has less zinc in it I replace a camshaft and it ran perfect.
Sounds like a thousand raccoons fighting over marshmallows on your roof
Might as well go for the full rebuild Uncle Tony, as you will be glad you not only replaced the head gasket, but rings and valves as well. I mean the engine has 200,000 plus miles on it, by doing the rebuild you will gain horse power and a longer life out of a vehicle you plan to tow with.
The 99-2000 and maybe 2001 Tupee heads were known for cracking because of casting issues before the updated the head . You can pull off the oil fill cap of the head and with flashlight look directly into the hole and see which casting you have. Another possibility to alway consider until you check it out out , is the possibility someone installed one of the casting defective heads on a 4.0 from an earlier crack prone head and put it on a Jeep that originally came with the better head but it’s been replaced. On a positive side note I’ve only seen 2 of these crack prone heads actually crack and only when they’re yve been abused and severely neglected with owners who continued to drive them despite the engine overheating. One of my customers got a bright idea when his Jeep would over heat . He would simply find a water supply like a hose and spray the entire engine down with cold water to accelerate the cool down time so he then continue to drive as fast as possible before the head finally cracked
Systematically isolate and troubleshoot defective components, replace and/or repair those components.
Check GM performance for where to stagger the ring gaps. It freaking works!
Mikel
Installing a plug on a suspected adjacent cylinder leak-thru should confirm but if the valves on the adjacent cylinder are open due to cam timing then it won’t build any pressure still. You’d have to see rockers moving to know if that’s happening.
I have a 5:1 compression engine with probably 25psi left in her, and she runs beautifully! Lol
Question: Isn't it better to do a leak-down test? It doesn't require to crank to engine and drain the battery, and it will also give you a immediate diagnosis since you can check where air is leaking through (exhaust, dipstick, throttle valve, etc.).
Video discussing the Banks turbo kit for the 4.0?
I don't think you had a good tight seal to the guage on #1 the first time. IDK, if it was me I'd burn off the oil and recheck it dry.
Ty ty sir you don't realize just how vauluble this video Is ... even if it doesn't blow up the youtube algorithm with Genreative AI SEO marketing blah blah hehe :)
It is interesting seeing your engine get lower in compression over time.
When i bkught my 1989 Jeep Grand Wagoneer with a amc 360. It was 150 psi accross the board with 112k miles. It now has 160k and i checked recently. They were at 135 or so. So it deff has dropped.
Still strong though. It towed 6400lbs (Jeep j10 and trailer)home 6 months ago.
Not sure if a 15 psi drop after 40k miles is normal or not. Or normal for 160k miles on a carbureted amc 360. But i hear those engines get tired at 150-200k
Tony I would only add the if you're trying to gear this lesson towards the real newbies you kinda skipped a a bunch. For example don't stop at the first dead hole and think that's it, there may be more, the test needs to be completed. Of course I realize you're going to pull the head anyway but a novice could miss that point. I hope what I'm getting at makes sense, old pros know what's up and what we're planning on doing and it's easy to skip things when we're explaining.
I'm not cracking on you it was just something that caught my ear!
Wth you talking about lmao
@@steveib724 What the hell *_you_* talking about?
Those AMC sixes always have a little bit of extra wear on cylinder 1 because it runs cold. It can help longevity to make sure that the thermostat is at least the stock temperature of 195° but it may be advisable to run a 205° thermostat to keep #1 up to temperature.
I don’t let my 258s or 4.0s run cold.
Also #6 is likely to be oily due to any bypass and the PVC system. On a carbureted engine the 2&5 usually run lean but 1&6 run rich. With fuel injection six might not benefit from the extra cooling.
Are we doing this on a cold engine? Or warm?
Okay Tony, fantastic vid as usual but, per that, “…I mean, you don’t wanna fill up the entire cylinder with oil”, I kind of thought you might have elaborated a bit more on that point of when someone does a wet test after dry, to avoid any oils being too thick viscosity-wise, as liquids do not compress obviously but as well, every single drop of ANY liquid added within a compression chamber, WILL increase compression tests, due to displacement and, the thicker it is, the more likely that oil will NOT quickly travel to potential worn/damaged leak points, like rings, valve seats, cracks or, bad gaskets.. causing FALSE readings. This is why I’ve opted to just squirt a quick shot of basic ass WD-40 or the like, for wet test, into spark plug hole, as it’s way thin & will more likely quickly make its way to any possible leaking points. Also maybe should’ve discussed in brief, the benefits of folks also performing a “leak-down” test, after logging down dry/wet tests… otherwise, this vid is excellent info! Thank You!
just rebuild it and have a great tow mobile. what's their static comp must be pretty low like 9.5 could do with a little shave and port and valve clean up get to 10:1 or leave it low and bolt a m90 on it haha
Also pull the fuel pump fuse out so you don't keep dumping fuel in on compression test
Why not finish the engine you were doing the series on and use that?
Yep or rounded off cam lobes will do the same thing when you pull the plug wires for the bad cylinder (s)
I would do a inframe on it like its a big truck
Pull the head hone it new pistons vavle stem seals new rod bearings will run like new
Done this on my vw rabbit diesel a couple times😂
You should really check out a channel used to be called Dex Jeeps until Jeep told him he cant use Jeep in his name for some reason it is now called Just Dex It (seeing ow there are plenty others using jeep in their channel names), he is here also in TN has over 700 of those style jeeps. But he worked on a Comanche, took a few videos and the problem was crazy. I think you would like a lot of his channel and if you ever need any parts that's all he does besides play with these off road is he sells parts just for this style of Jeep.
what if you want to test a motor thats been sitting for years do i simply crank the motor for a bit to get the oil on cylinders or should i poor a cap of oil down the spark plug hole and wait a day or will it skew my numbers?I understand the wet test bumps up compression numbers but will it still bump them up if i wait a day? i just dont want to crank a old sitting motor dry
Any idea why my 360FE will only run when dumping fuel in it? Open throttle on idle
Seems to me for a tow vehicle, if there's doubt, there is no doubt, you want full compression to pull and climb causebeing low on compression doing that really sucks. :)
Ya a turbo might fix that....not Evan
Do the stroker amc block Tony..
😊
Tony....think about it....towing at max(likely OVER) for the platform, once all the junk is in the trunk(and on the trailer) you are gonna NEED all the torque that motor can deliver.....it wont deliver it for very long with one weak jug.....for every day?? BAH....gas it and go....but for towing???
Ring it.
I've had mechanics tell me that 90 psi is good enough it should run but to me 90 is low on a typical automotive engine.
I've seen a lawnmower run at 25psi not real good. 90psi is about as low as you can go and run decent.
Do i remove the fuse to the fuel pressure or inertia switch for this ?
Yes. Disable the fuel pump. You don't want the injectors pumping gas in. Pulling the fuse is the quickest way.
Compression should be the very first thing you should test when diagnosing ANY engine problem even if it runs OK. Most people throw money at it and then find out nothing has fixed the problem and no amount of money will fix the problem short of rebuilding the engine or buying a good used engine from a junkyard. See it all the time. A simple Compression test in a few minutes and you know what you have to work with. I Compression test every time I take the sparkplug out of anything. Save time money and frustration.
Wouldn't the liquid oil make the combustion chamber smaller? Naturally raising the compression? At least until it was spit out the exhaust or leaked past the rings?
Just read yr msg.. i too just left a msg, addressing this very thing. I didn’t catch this vid the other day when first uploaded; just watched it. You’re 100% correct about that displacement factor. Why i commented Tony oughta have explained a bit deeper of folks avoiding the use of THICKER oils.. thinner the better and, as little amount possible. I prefer just a quick shot of WD-40 or the like, for this very reason.. also advised he maybe oughta/should’a explained at least the basic advantages of performing a “leak-down” test, after logging down results of dry/wet tests