I dont get it. Please explain this to me.

[NGABaldEagle]

I did my first compression test today.

I tested each cylider with the plugs in the other holes.
readings were.
1-57
2-75
3-75
4-90

The mechanic said ALL the plugs should be out to run the compression test and warm the engine first.

readings
1-91
2-93
3-94
4-94

Since they are seperate combustions chambers on their own, why would it make a difference?

Which way was right?

Why the difference if they are seperate?

I dont get it.
Please help me understand.

 

[studlymandingo]

Re: I dont get it. Please explain this to me.

All out or one out; shouldn't be a difference. Your compression readings will be higher when the motor is warm because of thermal expansion. Your numbers are a little on the low side; you should do a thorough decarb with Sea Foam and then check them again.

 

[JB]

Re: I dont get it. Please explain this to me.

Taking all the plugs out allows the starter to turn the engine faster and at the same speed on all cylinders.

Leaving the plugs in loads the starter and runs the battery down. There is risk that the #4 reading would be taken at a lower rpm than #1.

Do not fret about the absolute psi readings. Differences in guages and technique can make as much as 20psi differences in readings.

Your numbers are excellent for most 4 cylinder engines. Very close to equal, which is what really matters. If there was a problem the likelihood that it would affect all 4 equally is just about zero.

 

[tashasdaddy]

Re: I dont get it. Please explain this to me.

when you post please state year model hp. if this was a 2004 engine i'd be worried. a 1965, not bad.

 

[NGABaldEagle]

Re: I dont get it. Please explain this to me.

The engine in question is a 1987 Mercury Classic 50.
Its a 4 cyl.

Im aware that near equal is good.
Im aware that 90-115 is good.

I still dont understand why it would make a diference.
THE READINGS CLEARLY SHOW THAT IT DOES.

Thats the part I desire to undersand.

Who knows why?

Thanks in advance....

 

[NGABaldEagle]

Re: I dont get it. Please explain this to me.

Also.
A mechanic told me that the PSI could be low now because it has been sitting for a while. He gave me
several reasons why this would happen.

I plan to clean out the carbs and run her for a while and then check her compression again. It should go up.

115 psi is like new.

90 is good but I would like for my range to be 100-105 on all cylinders.

Based on the 91-94 psi readings and the mechanics advice, Im assuming my engine is good.

 

[Silvertip]

Re: I dont get it. Please explain this to me.

You asked again about why the difference. Your question was answered -- "warming the engine makes a difference." I'll too suspect that if you did a thorough decarb on this engine your readings would increase even more.

 

[JB]

Re: I dont get it. Please explain this to me.

Again, Eagle, don't fret about the absolute readings.

Change guages, batteries, warm the engine, open the throttles, pull the spark plugs, increase the number of strokes you measure. . . All make a difference.

Your first set of readings were bad. Could be a lot of things, but most likely is that as you did each cylinder it was better lubricated than the one before it. Starter was cranking too slow because of the load with the plugs in.

The second set of readings were good. Engine is warm and well lubricated, plugs are out allowing the starter to turn faster and at a more consistent speed.

I frankly don't believe that a decarb would improve them, but it certainly couldn't do any harm.

You keep asking why "it" makes a difference. You don't explain what "it" is. If the above doesn't explain it, please explain what "it" you are worried about.

 

[studlymandingo]

Re: I dont get it. Please explain this to me.

Cleaning out the carbs won't affect the compression; doing a decarb which equates to dosing the motor with Sea Foam, may allow for more expansion of the rings which would give a more favorable compression reading. My outboard did benefit from the procedure. Good Luck and post the results.

 

[Frank Acampora]

Re: I dont get it. Please explain this to me.

If the "it" you are asking about is the difference with all plugs out, then the reason is because you have a cold, non-running engine. At this stage, you get leakage past the rings. when you take out all the plugs, the engine cranks faster and there is less time for leakage past the rings thus: higher compression readings. Note that as stated above, the absolute value is not too critical as long as it is not too low and all cylinders are close to equal.

 

[Bill73x]

Re: I dont get it. Please explain this to me.

I've never done a compression test. Can I find a "tester" at an auto parts store or do you need something for outboards?

 

[tmcalavy]

Re: I dont get it. Please explain this to me.

One from the auto parts store or Harbor Freight store will work just fine.

 

[NGABaldEagle]

Re: I dont get it. Please explain this to me.

Thanks oh salty ones for your help.

I appreciate it very much.

 

[F_R]

Re: I dont get it. Please explain this to me.

It sort of sounds like you know what you are doing, but I'll ask this anyway. Are you cranking it till you get maximum reading on the guage? Also, I do feel the answer to "it" is probably related to lubrication. That is, assuming there was fuel in the carbs to lubricate the cylinders as it was cranked. The more you cranked (or ran it) the more it got lubricated.