Compression test 2f confused

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M

Mntnbikefj40

10 Year
Joined
Nov 16, 2014
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13
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What's the proper way to do a compression test? Reading through forums It's sounds like all the plugs are pulled out. But then the throttle is wide open why? Can a compression test be run on an engine setting on a motor stand buy just turning the starter over? Does elevation have an effect? I live above 5000 feet
 
You'd be better served to google "compression test procedures" and read the excellent information that's on the web.

Read more MUD-specific threads by googling "ih8mud: compression test"

For altitude: google "compression test altitude" - there's a table in one of the results HERE

You can borrow or rent a compression test gauge at most auto parts - rent is usually refunded when you return the tool. Not the best tools, but better than buying your own... unless you are building a collection of tools. The gauge should have instructions with it.

As I recall... these are the procedures for doing dry and wet compression tests:
  • Warm engine to operating temp.
  • Remove small + and - wires from coil... You don't want the engine to start - you are just measuring compression
  • Remove all plugs
  • I don't recall why the throttle has to be open - but it does.
  • The choke must be open (push knob all the way in).
  • Begin Dry Compression Loop
    • Screw gauge fitting into first cylinder
    • Crank engine 8-10 times - or whatever the instructions say
    • Check gauge and log cylinder and psi
    • Repeat for cylinders 2-6
  • End Dry Compression Loop
  • Begin Wet Compression Loop
    • Squirt 2-3 drops of oil into first cylinder
    • Screw gauge fitting into first cylinder
    • Crank engine 8-10 times - or whatever the instructions say
    • Check gauge and log cylinder and psi
    • Repeat for cylinders 2-6
  • End Wet Compression Loop
hth
 
Thanks for the info i tried to google compression test and could not find any good info but i was not using the right word
(procedures) I bought an 2f engine, 4 speed trans and trans fer for $500. to put in a 73 fj40. I want to check the compression before i install it. The compression check i did was 120 to 125 adding the elevation factor in the lowest was 138. But this was done on a cold engine. How accurate would this test be on a cold engine? Does pressure drop or increase as it worms up?
 
Not accurate, but fairly close - that's why you should compression test an engine at it's normal operating temperature. Compression increases when the engine warms up.

It looks like the link I gave you has some misinformation - not in the table, but in the discussion after it... you don't add the elevation factor (for your's - you multiply by it - this explains it:

"A standard compression reading of about 150 psi at sea level in Los Angeles would measure significantly less in the surrounding mountains. For example at an elevation of 6000 feet, the expected reading would be 150 psi x .8359 = 125 psi. The cylinders would be reading low if compared to sea level measurements, yet perfectly fine at this altitude. "​
 
Oh i understand so the reading i am getting of 120 psi to 125 psi at 5500 feet are good and should improve when i get everything installed and running. Had me scared there for a min. I multiply my reading by .8617 got a number down around 100. psi ugly Thanks for the math lesson
 
Ok I will but what you are saying makes sense. If this engine needs to be rebuilt it makes sense to me to do it now while its set on the motor stand not after i after i in stall it. So thought the first thing i would need to know if the compression was good. The guy i bought it from bought it for the chassis and did not know the history of the unit at all. I check all the fluids looking for metal shaving no help there. The fluids were clean like they had all just been changed not something i would expect. so any way it looks to me that the compression is good so i will just change all the gasket and seals. any other suggestions are appreciated. Thanks again for the in put.
 
the carb has to be open so that the butterflies don't restrict the air flow and give you an artificially low reading.

As for calculation for altitude... I can't imagine why you'd worry about this, cam timing, plays a large role as to static compression ratio - thus, unless you know the cam and compression ratio - the numbers are meaningless(ish) (you could use the numbers to determine whether or not you need to run super unleaded)

You want the engine warm because the aluminum pistons have a different expansion rate than the steel liners (or iron block) - thus, when it's cold it would give you a lower reading.

which brings me to... if you're testing to see whether a motor is good - you need to know 2 things: 1) is the compression more than 110 psi. You can make a motor run at 90 psi, but it does not ever run well; and 2) you want the readings to be within 10% of each other. Wide variation or one cylinder (or more) that is substantially lower than the others is a sign all is not well.

In addition to the compression test, you could also do a leak-down test... but that test... frankly, I've never done one where it's told me anything I don't already know...
 
what I did...
engine hanging on hoist w/ flywheel and bellhousing and starter installed
cleaned around head ports and removed manifolds
removed all 6 plugs and valve cover(valve cover removal not necessary...just wanted to watch)
charged 12 volt and some jumper cables with a jumper wire off the hot terminal of the starter to an open stripped end(used to bump solenoid, ala bump switch)
6 revs per cylinder, 3 cycles thru all 6 and record values. reference highest value recorded for each cylinder
I did not do a wet test because I had decent readings which were even better after a hot valve adjustment(not spectacular; like 125-134ish on the cold test and high 120s to mid 130s post adjustment- at 3500' elevation.

in truck;
pull plugs
block open throttle; fully open choke
disable ignition
6 revs per cyclinder; 3 cycles of each; reference highest value...


throttle plate should be open and engine (rings and valves and all should be at terminal expansion) should be hot for enough air to pass thru and compression to build to get an accurate representation of what the running engine is experiencing...) it can be done cold and is still a good enough indicator of what is going on...throttle should be open...
 
remembering now.. valve cover was off cause I wanted to do a cold lash pre to get a good reading...that was my point, I think...
 
Just my 2 cents if you pull all 6 plugs the ignition is disabled. I never mess with the carb or choke because I don't feel it makes any difference on the reading you get. All readings should be very close together and over 100 for a minimum. Motors that have sat idle for long periods sometimes give you false low readings due to slight rust build up in the cylinders, that sometimes clears up after minimal run time. Obviously a warm motor is going to give you the most accurate reading that you are going to get.
 
The rule of thumb is that you lose 3% of compression per 1000' of elevation, so I add 20% to the compression readings I get here at 7000' elevation. I'm ecstatic to get 120 PSI or higher, on a 2F at my elevation.

Also, re: the throttle plate. Never forget that an engine is just a big air pump, so you want the throttle plate open to let your engine 'breathe' during the compression test, as much as possible. I agree that throttle plate position seems to make little difference on a 2F compression test.
 
I would offer this:
The reason you are doing a compression test is to find out if you have one or more 'bad' cylinders.- ones that may be leaking by due to different reasons.
I submit that it makes NO DIFFRENCE WHATSOEVER what your altitude is when you do this test. All cylinders are subjected to the exact same atmospheric conditions-no matter where you are. You are looking for DIFFERENCES--not whether they are all high or low. If your engine is tight, it will show equal compression on all cylinders on the Moon, just as it would on Earth.(pistons and rings don't care about altitude(pressure)--they just compress what they suck in to the discharge)
 
When doing a compression test you are looking for two things:
1. Are there differences between cylinders indicating things like blown head gaskets, etc.
2. Are the rings worn out to the point where the compression is low and the engine will not run well.
Just because all cylinders read the same doesn't mean that the engine is "tight". 90-100 on all cylinders would not be a good result.

As to whether the throttle/choke need to be open, the cylinder gets its air from the intake manifold and through the carb. If the plates are closed, it may restrict the intake. It may or may not affect the result-it has for me on other engines.
it might take an extra 15 seconds to pull the choke and lock the throttle open.
I can't imagine spending a half hour doing an important test and not spending 15 seconds to make sure the result is accurate.
 
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