I performed a compression test today on my 2006 Evo ix. One thing I realized after performing the test was that I did not have the throttle wide open. How much of an effect will this have on my compression numbers? The reason I ask is because my numbers were a little low. 90,70,90,90 were my numbers

 

You need to have the motor at operating temp. Then pull all of the plugs out. Make sure you have the throttle WOT when cranking the car.

 

What does the throttle have to do with a compression test? ...... ideally you would want all the valves sealed to measure actual compression in the cylinder, no?

 

I am trying to envision why you would need the throttle wide open. What purpose is this serving? Just curious.

 

^Yeah curious on this myself.

 

Excerpt from http://holdenpaedia.oldholden.com/Compression_Testing


The Throttle Must Be Wide Open:

The reason the throttle must be wide open is to ensure that each cylinder fills completely. If the throttle is partially open, a vacuum will be generated in the inlet manifold and the pressures indicated will be lower than expected.

Valve Overlap:

There is another reason why the throttle must be jammed open and that is Valve Overlap. As each piston reaches the end of its exhaust stroke there is a point where both the inlet and exhaust valves are slightly open. This point is called "Valve Overlap". If a compression test is done without the throttle wide open, Valve Overlap will cause some cylinders to read higher than others because they will draw air back through the exhaust valve and exhaust manifold into the inlet manifold which worsen the compression readings.

 

Ok....I see

 

when i do a comp test i also disconnect the crank sensor and also the ignition. this way you wont get spark or fuel being sprayed.

i do the comp test with the engine hot. basically drive the car around let it get warmed up. IMO its best to drive it then let it idle. once it is hot have your tools ready. get cracking at removing the ignition, plugs ETC. i do each cylinder 1 time. i allow it to crank over 8 times. i take readings from 2,4,6,8 cranks. normally i am at peak compression by the 6th crank. hold the throttle to the floor for each test. its just nice to see how quickly it builds up. over kill maybe but more info is better then less.

 

Thanks for all the input guys. I performed the test again today, this time with the throttle wide open. My numbers were about the same 95,70,95,95 .My 06 Evo has got 135,000 miles on it and I am the original owner. I have taken really good care of my Evo and it's still completely stock. Are these compression numbers average for this amount of miles?

 

Standard value (at engine speed of 250 r/min): 1,000 kPa (145 psi)
Minimum limit (at engine speed of 250 r/min): 637 kPa (92 psi)

The difference between highest and lowest should be no greater than ~14 psi. Something's definitely up with #2. Did you try adding a dab of oil and re-testing?

 

I think keeping the throttle open during cranking will only reduce the amount of rotations needed to build the appropriate pressure. at least that's how I see it!
On a side note, how do cars with drive-by-wire do their compression? I am not sure if they allow throttle control when the engine is off (I am not certain though!).

 

I will recheck the number two cylinder tomorrow. How much oil should I add to the cylinder? I am thinking a teaspoon should be enough, what do you all think?

 

Yes, that's about right.
What made u decide on a compression test to begin with? did you experience any issues?

 

Those numbers are worse than "a little low". They indicate the need for a rebuild of some sort. If you are sure you did the test correctly I would follow up with a leakdown test to figure out where your lost compression is going. You could add oil and retest, but it will only give you and idea if your rings are shot and not sealing. A leakdown test will tell you a lot more. You will be able to pinpoint where your lost compression is going (exhaust/intake valves, rings, head gasket, valve seals, etc).

I am not trying to scare you, but the service limits and specs are posted above. Your motors falls into the fail category. There is something wrong the question now is what. The leakdown test will help you figure out where to go from here.

 

The reason I did the compression test was because I was having a hard time starting my car when the engine was cold. I replaced a few things but I am still having the same issues. Someone mentioned doing a compression test so that's what I did. I will do a leakdown test to see where the leak is coming from. Is there a good write up on this?