8.3 compression test

[MAdams]

I searched and didn't see a lot of info about this so I will share what I know. While I had my injectors out I adjusted the valves and checked compression. I was going to use a old hollowed out injector but I bought the proper gauge set.

Cold engine, batteries fully charged, all the injectors removed I was getting between 385-395 psi

Anyhow, leaving this here for someone to find in their future search.

 

[WillWagner]

There is a procedure for that but if there were a loss of compression issue in a compression fired engine, you would have other visible signs of an in cylinder failure, like, lots of smoke out the bottom/blow by tube, popping in the intake, chuffing in the exhaust. In 30 years, never a need to take a comp test, but for midrange engines, there is a spec and procedure to do it.

In short, it is kinda a waste of time to do.

 

[MAdams]

Well that explains why I couldn't find any data lol. I bought the compression test kit because I have a Deutz engine in my bobcat that's smoking and I need to diagnose it. So far I have ruled out any faulty spark plugs

 

[74M35A2]

Cummins list advisable compression figures for some engines but not all. They prefer to do it by blow-by volume measurement. But, fortunately, for our 8.3's, they do give some #'s as guidance. Looks like you have some carbon build up working in your favor. Here is what the free online Quickserve program lists for the serial number of my M925a2 8.3L engine:

Note : Due to variables such as starter and battery conditions that affect engine cranking speed, it is difficult to establish an absolute value for compression pressure; however, the following values can be used as guidelines:

• New engine (cranking speed at 250 rpm) 2413 kPa [350 psi]
• Used engine (cranking speed at 250 rpm) 2068 kPa [300 psi]

It is recommended that the compression pressure be checked on all cylinders and then compared. All cylinders should be within 690 kPa [100 psi] of each other.

​

 

[Jbulach]

Interesting, I just checked and my 8.3 with 7 year old batteries cranks exactly 250rpm according to the tach.

 

[74M35A2]

You’re amazing.

 

[Mos68x]

Good to know, hopefully I won’t have to use this info anytime soon