Using a Compression Tester
Turbo Mini Forums

Hi,

Bought a compression tester and just want to check how to do it.

1. Remove all spark plugs.
2. (I have megajolt) so disconnect the lead going to the coil pack. so the plugs do not spark.
3. Connect the meter to each plug hole in turn.
4. Turn the engine over for about 5 secs.
5. Read off the pressure value.

Is this it, i was expecting it to be harder than this?

Cheers

Jon

i just remove one plug at a time and crank the engine

Personally I always do it with the throttle held wide open, otherwise the reading may take longer to reach its maximum.

Schrödinger's cat - so which one am I ???

Just remove each plug in turn, open the throttle and turn the engine over untill you hear the engine cranking as an even note, about 5 to 10 second should be enough

I have found compression testingto be a bit hit and miss, although it is still a usefull tool.

Just be aware that if you find one cylinder down on compression, yet the engine runs without any issue, take it for a good run out and thrash the balls off it, then re do the test. The actual readings will have changed, but if there is still a decent difference on the cylinder that was reading low initialy, there is a problem with cylinder seal.

Use it as part of your arsenal of tools to help you diagnose any issues, but do not rely on it telling you that there is something wrong. The reason I say this is because I recently checked my engine and it showed number 3 cylinder at 8 bar with all the others at 12bar!!! Worrying!!! However, the engine ran very well and did not lack power. I took it out and gave it a good old thrash, then re did the test to find that all cylinders, hot engine, now showed 10 bar. Only last weekend i checked again, cold engine, all cyclinders 11bar.

Edited by Sprocket on 28th Jul, 2009.

That is wierd sprox... Did you do a leakdown?

Nope no leak down.

Its not the first time I have or others experienced this. I spoke to my man and he put it down to the minute chance of all the ring gaps lining up, remembering that for that to happen on my engine with the gapless second ring, is pretty much impossaible

I agree that it is bizzar, however, it is clear the engine is not suffering as after that low 8bar reading, I ran my best times at Avon compared to York at Easter. perhaps that was the weather and the sticky track, I do not know.

Edited by Sprocket on 28th Jul, 2009.

I always been told to put the throttle wide open too when cranking the engine, Can't remember why but this was from my tuner.

Lightweight racer coming soon.

It is so as that you cause as little of a restriction in the flow path of the engine as possible.

It can affect the reading quite bad on some engines.

Maybe the readings are hit and miss because he does not open the throttle or remove all 4 plugs?? Thats how I was taught. Also the only reliable readings are from a hot engine.. not always possible I know..

Terry

website:- http://www.terryhunt.co.uk

Readings will be different when cold, but they should all be still very similar to each other.

Throttles are open to saturate the cylinder.

all other plugs are in as that will replicate the true cranking pressures. leaving the other three plugs out will ease the strain on the starter and battery.

Throttles are open to allow air in surely??

A cold engine will not have proper clearances so is liable to give funky readings, IMO only "reliable" readings are from a warmed up engine.

Plugs all have to come out anyway? why not reduce the strain on your battery?

website:- http://www.terryhunt.co.uk

Bit confused!

Do I need all 4 plugs out at once or one at a time?

Cheers

Jon

imo take all the plugs out, thats what i do. or just do what you feel is right to get the reading, imo i don't think it makes a difference just people have their own way

Just different ways of saying the same thing....

Wide open throttle means the cylinder(s) can get the air in quicker so you will achieve the maximum reading quicker (which will then be held by the non return vale in the tester).

All plugs out means less load on the starter and the engine will spin slightly faster so, again, you will get the reading quicker.

If the engine is in good condition, the end result will be the same however, if the valves or rings are leaking badly you will possibly get a slightly lower reading with the engine turning slower as there is more physical time on each compression stroke for the air to leak past the valve or rings.

Everyone has their own preference (wide open throttle, all plugs out for me) but so long as you do the same every time you have valid comparisons.

And unless the readings are drastically low you are looking to compare each cylinder to the others.

Schrödinger's cat - so which one am I ???