How To Check Your Engine's Compression

[Dizmo]

This is not my own work, but i thought i would post it to help people out.

For your own safety, it is advised that you purchase and follow the correct auto repair literature before attempting this process. The following text only describes general information and is not vehicle specific. This text does not express or imply any vehicle specific directions.

Q. What is compression in an engine, and how does it work?

A. Compression is a process in which air or an air/fuel mixture is confined and pressed into a smaller volume within the area of an engine's cylinder. This process forces all of the molecules to be "pressed" together under high pressure.

In the case of a gasoline engine, moderate compression is required, 140 to 160 pounds per square inch (psi). Some engines may require as much as 220 psi depending on their size and application; the manufacturer gives specific compression specifications. If the compression in a gasoline engine is too high, it can cause a problem known as preignition or detonation. This can be very destructive, causing damage to the internal parts of the engine. However, a diesel engine requires very high compression, usually 350 psi or more because it relies on this compression process to ignite the diesel oil. Diesel engines are much heavier and louder compared to the gasoline engine. This compression process, combined with an air/fuel mixture and a source of ignition, is what produces the necessary power to operate vehicles of all types.

Q. When should the compression be checked on a vehicle?

A. Generally speaking, the compression should be checked in any instance when an engine is running roughly or is lacking power. Most manufacturers and technicians believe it is important to perform a compression test every time a tune-up is done as part of preventative maintenance; consult your vehicle owner's manual for tune-up intervals. By performing a compression test, internal engine malfunctions, such as bad valves, piston rings or excessive carbon buildup, can be detected before they cause irreparable damage. It benefits the owner to be aware of these problems so they can make an informed decision whether to invest in repairs or sell the vehicle.

Q. How is the engine compression checked?

A. Engine compression is checked in different ways for different vehicles. In this section,

Compression on a gasoline engine can be tested in two ways. The first method involves using a manual, handheld compression gauge.

• Make sure the engine has been warmed up before beginning the test, to ensure that the oil has been warmed up. A cold engine will not test correctly.
  
• Disable the ignition module or coil.
  
• Insert the compression tester into one cylinder spark plug hole at a time.
  
• Hold the throttle to full open position to ensure the engine gets adequate air intake.
  
• Crank the engine continually for at least five to 10 full revolutions to obtain an accurate reading on the compression tester.
  
• Record the reading for each cylinder. If any of them vary 10 percent or more from each other a problem may exist in one or more cylinders. If the variance is greater than 10 percent, specialized testing equipment may be required to fully diagnose the problem.
  
• If all cylinder readings are within 10 percent of each other, no further testing is required and compression is considered optimal.
  

The second method of testing involves the use of an electronic engine analyzer. The analyzer 'shorts' one cylinder at a time with the engine running and calculates the rpm drop. Once all cylinders are measured, a reading is given that will show which cylinders are working the hardest (have the most compression) and which are working the least (have the least compression). For the do-it-yourselfers, the easiest method is the manual compression test.

Q. What if thecompression is too low or too high?

A. If the compression results are too high or too low, you might consider consulting a professional technician. Modern vehicles are complicated to test and I have witnessed disastrous outcomes when a Do-It-Yourselfer has attempted this test. If however, you are fearless in your endeavor to diagnose the problem, then use the following techniques.

Consecutive low compression in all cylinders could mean that the problem of fuel washed cylinders exists. This means that the engine has had too much fuel introduced into it and all of the oil has been washed off the cylinder walls. The oil creates a sealing effect between the piston and ring assemblies and the cylinder walls of the engine block. Without this thin layer of oil, the engine compression would be allowed to escape into the crankcase. This is common with an engine that has a 'flooding' problem.

If the engine seems to run normally but is weak and puffs a small amount of bluish smoke, it could be an indicator of worn piston rings and cylinder walls. In either of these events, use a small oil can and squirt a little oil into each cylinder, then repeat the compression test. If the compression dramatically increases then you have found the problem(s). If the compression readings do not change, then it would indicate a timing problem between the camshaft(s) and the crankshaft of the engine. The timing chain or belt would need to be checked for proper timing.

If you find the compression reading is very low or zero in one cylinder, it is highly probable that internal engine damage exists such as:

The piston could have a broken connecting rod or a hole in it.

A valve could be stuck or leaking.

There could be a broken valve spring or a bent push rod.

The camshaft has excessive wear and is not opening the valve(s).

If the compression is low or zero on two adjacent cylinders, it would indicate:

There is a 'blown' or weak sealing surface at the head to block mounting area, which basically means a bad head gasket.

The camshaft is broken in an area that operates valves for two adjacent cylinders.

When the compression is found to be too high in one or more cylinders, this would be an indication of excessive carbon buildup in the engine. It can only be corrected by performing a chemical de-carbonizing process on the engine or by removing the cylinder head(s) and physically removing the carbon that is attached to the cylinder portion of the head(s) and the tops of the pistons.

Hope this helps a few people.

Ryan

[khunjeng]

lol I read this one 2 weeks ago its got all the good info tho nice1.

[cowie165]

Please excuse the stupid question, but the instructions say to:

"Disable ignition module or coil"

As I understand it, you remove the coil and sparkplug for the cylinder to be tested, but you also need to unplug the ignitor to prevent engine start.

Is this right?

Thanks

[Duncan]

yes exactly, just unplug the loom into the ignitor pack

[cowie165]

Thanks Duncan.

If I'm only testing compression twice a year plus a few family cars, can I get away with a SuperCrap Auto one? I imagine a cheap one will not come with a calibration certificate - is it worth it or just take the car to the mechanic?

[Duncan]

hahaha good question...the gauge I use came from Supercheap lol.

Look, the main thing you are looking for is consistent compression since that avoids all the issues you might have with making mistakes in the process (eg warm motor, charged battery etc etc), so I reckon any gauge is fine.

*except* don't get one of the ones with the sloped rubber head you have to hold in....it is impossible to get a consistent reading. Make sure you get the type that screws into the spark plug socket

[cowie165]

Thanks Duncan (again!)

[cowie165]

Is this what the 'right' type of compression gauge looks like?

http://cgi.ebay.com.au/ws/eBayISAPI.dll?Vi...Y_BIN_Stores_IT

I'm not sure how you screw the adaptor in? Especially for motors that require a sparkplug extension for deep cylinders (eg Toyota 4cyls)... hrmmm

[Duncan]

yep exactly. the thread on the end of the hose either screws directly to the spark plug thread, or you screw it into the adapter which screws into the plug thread. depends on which size is which.

But anyway, the nice long hose means you can read it a fair way from the spark plug hold unlike the crap "try and hold it" ones.

[Cubes]

Duncan,

I tend not to disable the ignition coil but disable the fuel pump so fuel doesn't enter the combustion chamber.

I warm the car up then disable the fuel pump allowing the car to stall.

I have a nice handly little switch as I've ran a direct volt feed to the fuel pump so it saves me removing a couple of screws and unplugging the fuel pumps plug.

Unsure if the small amount of fuel that enters the combustion chamber will skew the result or not IF the fuel pump is left enabled.

[cowie165]

Thanks Duncan, thanks Cubes.

Cubes you sure are the man to talk to for auto-elec advice! You've inspired me to have a crack at a headlight switch relay and a high-beam+dipped-beam relay. !

[cowie165]

Dudes, my eBay compression tester arrived today

Weird thing is, the instructions say to warm engine, then remove ALL spark plugs, labelling as you go.

Is this right? I understand that you are only testing one cylinder at a time however don't you need all the plugs seated to ensure a true reading?

It also said to remove the intake snorkel. I would like the car to inhale as much air as possible however I'm not real keen on having WOT and no air filter...

Any pointers would be great please!

[Cubes]

Thats correct.

The motor will spin over at the correct rpm, place less strain on your starter and battery.

[cowie165]

Wow, that was fast. Thanks Joel

[MoogLe]

sorry for the newbie question but i'm new to skylines,

how do i disable the fuel pump and the starter?

in my old car i used to just pull the fuse for the injectors and unplug the coil packs

[MoogLe]

anyone?

[Duncan]

Just pull the fuse for the fuel pump, try to start it a time or 3 until all the fuel is gone

[Adriano]

I just unplug the cas, as this immobilises the spark and the fuel in one go.

[MoogLe]

so, where's the cas?

[Keeper]

im going to give the electronic method a go tonight,

disable idle adjustment from engine and check the drop in rpm in each cylinder, measure it and see what makes the least difference.

in theory this should tell me if i have 1 cylinder that is crap right?