Hot vs. Cold Compression Testing

[JHZR2]

Hi,

Curious of the pros/cons of hot versus cold compression tests. For example, on the MB TM, the compression for diesel engines should be done with engine temperature no less than 70C.

However, Ive heard that a cold compression test can be more telling in some ways than hot. Also, for example, that a hot test tells you some things, but a cold test tells you everything.

FWIW, I was changing glow plugs on my 82 and got the following cold:

Cyl 1 380
Cyl 2 385
Cyl 3 375
Cyl 4 355
Cyl 5 375

Not sure if I had a slightly poor seal on #4, there is an issue, or it is just a valve adjustment issue...

The 617.95 service manual lists normal compression as 24-30 bar. Minimum is 15 bar. Maximum difference between individual cylinders is 3 bar. (These figures are with the engine at normal operating temperature.)

Your thoughts?

Thanks!

 

[river_rat]

Originally Posted By: JHZR2
...but a cold test tells you everything.

Interesting. What more is it supposed to tell?

 

[JHZR2]

part of the reason why Im asking...

 

[SAATR]

Well, at temperature you should get your absolute best ring seal as the pistons have expanded to their working diameter and closed the piston to wall clearance, as well as the rings having the smallest end gap. Compression tests should therefore show what the engine is actually working with rather that a condition (cold) that is only found at startup.

The cold test will give you the best idea of how well the rings are sealing by themselves, in other words how well their doing at their worst (dead cold). This could reveal a stuck or broken ring that tightens up with temperature, or simply a cylinder with a poor seal that improves marginally with the thermal expansion of the piston.

 

[Shannow]

I used to use cold, as there was less variability in conditions than warm/hot/used to be hot/cooling down.

Although it's off topic, I've seen Wankels that needed to be tow started, that once warmed once in the day would run fine. Seen Valve Seat Recession on a propane fueled V-8 that wouldn't suck hard enough to get the gas carby producing, but worked fine warm.

 

[berniedd]

If your engine is aluminum, it is a good idea not to remove the spark plugs until the engine has cooled down somewhat. Otherwise, you might strip the threads on the head. Ergo, you'd test compression at lower than operating temps.

 

[JHZR2]

Iron, old MB diesel. Did cold because of convenience, but also chasing an intermittent cold start miss that happened only for the first 15 seconds or so of operation. new GPs and secondary FF made first cylinder smooth start happen this AM, so time will tell if it was just a weak GP that a resistance test couldnt show...

 

[Scott_Tucker]

I have found that a cold compression test can exaggerate compression problems and therefore can be of benefit. When I do a compression test I don't care how 'good' a cylinder is, I only care about what is bad.

 

[JHZR2]

thanks! would you say my #4 cylinder is "bad"?

 

[Torino]

Originally Posted By: Shannow
I used to use cold, as there was less variability in conditions than warm/hot/used to be hot/cooling down.

Although it's off topic, I've seen Wankels that needed to be tow started, that once warmed once in the day would run fine. Seen Valve Seat Recession on a propane fueled V-8 that wouldn't suck hard enough to get the gas carby producing, but worked fine warm.
Yes, cast metal (iron or alum.) seems to move around a bit erratically when hot. John--Las Vegas.

 

[Scott_Tucker]

Originally Posted By: JHZR2
thanks! would you say my #4 cylinder is "bad"?


No, is there a problem with the way the engine runs?

 

[Steve S]

Check the valve lash. How does the engine run? For 35 psi difference between highest cyl and the lowest are you going to tear the engine down? See what happens by not running Amsoil and doing an oil change every 3,000 miles.