Typical compression test results.

lmorris · April 20, 2015

Has anyone done enough compression tests to get some idea as to what is normal? WSM states minimum of 300 PSI. I am getting 300-310 on one that has ingested dirt up to the CAC.

Steve Mutter · April 21, 2015

I had a 2014 6.7 getting around 360 psi last time I did one.

mchan68 · April 21, 2015

The compression ratio on these engines is only 16:1 if I'm not mistaken. That said, I would think compression pressure readings on the lower end of the spectrum to be the norm.

lmorris · April 21, 2015

Hotline has tech reporting 360-380 as normal. This one is definitely dusted. It left with an oil change and new air filter. Most likely going to auction. Hate to be the guy that buys that one.

Bruce Amacker · April 23, 2015

What altitude are you at? It has a large effect on comp readings.

:grin:

lmorris · April 23, 2015

What altitude are you at? It has a large effect on comp readings.

2116 ft. I'm thinking 350 is the bench mark here. Typical 6L and 6.4L come in just under 400.

cbriggs · April 24, 2015

That sounds right. I'm at 2600 ft and usually see 370 to 390 on 6l and 6.4l on good holes. 300 sounds pretty low for any diesel....

lmorris · April 24, 2015

So I contacted hotline about crankcase pressure testing and they advised me to put in a "Report a problem" case in the PCED. Here is the reply I received back:

Your concern was closed for the following reason:

Leon, way back in 2009 during development, we tested engines that had cylinder wall/ring damage using crankcase pressure similar to how it was done on the Navistar engines. The damaged engines had one or two cylinders that were messed up. On these engines the compression test reviled the bad cylinders had as much as 50% more leak down than the good ones. Comparing good engines to ones with damage, we found the crankcase pressure test inconclusive. We found compression and cylinder leak down to be a superior way to identify the condition of the cylinder seal. I was present daring the above tests and still have the prototype tool we attached to the oil fill cap.

Brad Clayton · April 24, 2015

That's complete crap. A dusted engine will pass a compression test all day long but blow 50 inches of water on a magnehelic gage. Each test has it's own purpose and should be done according to what your looking for. One test should never replace the other test as far as a superiority issue.

You could have a brand new engine with one cylinder wall gouged out, it will pass the crankcase pressure test with no problems and probably blow a 2 in" H2O. However the affected cylinder will flag an issue on a compression test and definitely show the problem on the subsequent leak down test. This truck would run rough and smoke for sure and the compression/leak down test would be the valid way to test for base engine issues.

A truck that runs perfectly normal but exhibits an overloaded crankcase ventilation system with heavy oil would be a situation where a crankcase pressure test would be in order to help pinpoint or rule out base engine problems.

I would dare so from the way that quote is worded and the spelling, that the actual hotline engineer that was at his desk took a break and left his computer logged on and a hammer head walked by and cobbed up that reply and fired it off before getting caught.

Brad Clayton · April 24, 2015

Of course I'm speaking about the Navistar engines, but I find it hard to believe that the 6.7 is so different that it can't be applied. I know they have a much different crankcase depression regulator, but how much pressure can they hold/mask?