Hello,

Due to some pretty bad blowby, I did a compression check on the 96.
Numbers:

1- 285
2-260
3 290
4- 290
5- 285
6- 300

This engine has 300k on it. Do these numbers seem low to anyone else. Checked for vac leaks etc..

thanks for your help

cole

 

Yes they are low should be 350 Min.

 

Your numbers are within spec.
What's most important is the cylinders are within 10% of each other.

 

Are those numbers taken on a hot engine, or a cold engine?

 

Cold. Heck by the time you get all of the injectors out it would be cold anyway I think. Thanks for the help, I think I'll run a blowby test next.

Cole

 

All you really need to worry about is oil consumption.
If you don't have leaks and start using more than 2 quarts every 5000 miles then you can worry.

Many people who aren't experienced with crankcase ventilation directly to the atmosphere think what is normal looks like excess.
After all gasser engines haven't vented to the atmosphere since the '60s.

 

Hi, Number 2 is a little low, compared to number 6, what happens to the compression when you add a table spoon of oil to each cylinder,then retest, is there is a huge increase in compression , but to be honest, I would not worry about it. how bad is your blowby, all Diesel have a little, and where are you seeing it from? can you see blueish smoke from the filler cap when running ?

 

Thanks, but I've heard the wet compression testing a diesel is not such a good idea. I am going to do a blowby test tomorrow to verify, but it could be as simple as a vac leak or valves. This is a fleet truck so has seen it's share of work. I pulled it out of the fleet to restore it for myself. Thanks, for your help.

 

Dieseldemon is right. According to my Diesel Engines I instructor you want at least 350 psi of commpession although 400 is better when cranking at 175 rpm.

 

Yep, putting flammable substances in a compression ignition engine leads to exciting moments when you roll her over.....

I have not found hard and fast Compression PSI numbers in the Cummins service manual, just blow bye measurements, for our 5.9's it is 63L/Min for new at 2200 RPMs and 126 L/Min at 2200 RPMs for a worn engine.

 

Mine was 360psi on all cylinders last time I tested. This is with a cam and oversize head gasket though. a little less than normal.
Motor has 220k on it now.