engine and drivetrain Warm/cold Engine For Compression Test Question

[RAS-2002]

Here's the conundrum:

 

To do a compression test, everything I've read says "the engine should be warm".  But (especially with an aluminum head) everything I've read says to only remove the plugs when the engine is cold. 

 

So how do you first remove the plugs from a cold engine, then perform the compression test on a warm engine with no plugs?  It's been driving me batty....

 

Or is the answer so logical and simple that I can't see it?  Or am I complicating it?  Help!

 

Thanks,

Bob

[Rocan]

Theres no reason you can't remove plugs from a warm engine... Who told you that?

[M3M3M3]

Theres no reason you can't remove plugs from a warm engine... Who told you that?

+1

[RAS-2002]

Who told me that?  Well, I looked up some notes I had written to myself, and found out exactly who told me that.

 

Oh man, did I screw up!  Turns out, nobody told me that.  I remembered my *&%!@# notes all wrong!

It's "check the valves when the engine is cold", not "don't remove the plugs when engine is warm". 

 

Bass-ackwards thought process / memory.  Been a long day.  I'm heading off to bed....

 

Later, (much later)

Thanks again, guys.

Bob

[Guest_anonymous]

phew, glad we got that straight. thought I needed to tear down the engine again

[FB73tii]

You mean I didn't need to build that giant oven? Well, at least now I can powder coat my entire car

--Fred

[RAS-2002]

Maybe I didn't have a complete brain-fart after all. 

 

I found this article from CD, and it explains it all very nicely.

http://www.bmw2002faq.com/_/technical-articles/engine-and-drivetrain/compression-test-101-r20

 

Original Author: c.d.iesel   Jan 03 2013 06:02 PM
How to perform a compression test?

• ALUMINUM HEADS? loosen the spark plugs when cold, and snug up to warm up the motor
• remove all the plugs
• battery must be fully charged and starter motor spinning like mad
• disconnect your ignition primary - even more important if you have a HugeBox-o-Transistor mega-spark system
• also for all of you with powered fuel pumps and injection systems - de-power yer pumps so the possibility of FIRE is eliminated!!
• hold the throttle fully open during cranking
• valve clearance should have been adjusted first- COLD - only crank each tested cylinder 4 revolutions
• record each of the 4 pulse readings of each of the 4 revolutions
• the 1st pulse of the needle should be more than 1/2 of the 4th pulse
• for example, if 185 is the target: 30-60-90-120= there's a leak all the time and uniform in size like a burnt valve, or severely burned out head gasket
• 90-100-110-120 = that cylinder is sealing up to a point but begins to leak - rings, leaking head gasket....
• 120-145-165-185 = is normal
• adding a little oil to a weak cylinder and re testing will only show some sealing by the rings - but indicate a direction to look
• ...next proceed to the Cylinder Leak Test