[Elbonk]

OK, so I saw 350#'s as the spec compression numbers on this forum and after talking to a diesel tech he did the math and said 318 would be fine. Other people at the injection shop were saying 400 to 450 would be ideal.
Post up your mileage and compression and oil pressure numbers, condition- rebuilt/reman/original and age of your motor. Thanks!

 

[Paul(in NY)]

From Cummins Shop Manual

This is a Quote from the Cummins 3.9/5.9 shop manual on compression testing;

"Due to variables such as; starter and battery conditions that effect 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 @ 250 RPM) 2413 kPa ( 350 psi)
* Used Engine ( cranking speed @ 250 RPM) 2068 kPa ( 300 psi)

It is recommended that the compression pressure be checked and compared on all cylinders. All Cylinders should be within 690 kPA (100 psi) of each other "

Paul

 

[Elbonk]

This is a Quote from the Cummins 3.9/5.9 shop manual on compression testing;

"Due to variables such as; starter and battery conditions that effect 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 @ 250 RPM) 2413 kPa ( 350 psi)
* Used Engine ( cranking speed @ 250 RPM) 2068 kPa ( 300 psi)

It is recommended that the compression pressure be checked and compared on all cylinders. All Cylinders should be within 690 kPA (100 psi) of each other "

Paul

I suppose using a $25 import compression tester is another variable too!

 

[dieselcruiserhead]

Cool.. I got anywhere from 320 to about 350-360ish when I did mine. the engine was cold, the more I cranked it the warmer it seemed to get and the compression went up slightly.. It went up incrementally as I moved from front to back... Engine has 186K miles on it, Frito Lay used Chevy model...

 

[Elbonk]

Oh yeah, mine.

I ran 310 to 330 on the first three jugs, then by #4 I had 360 if I remember right. It had been sitting for a week. I ran through again and had 340 to 350 +. I have 30 psi at idle and 55 at rpm warmed up. Mileage is unknown. Chevy/Penn 20'er.

 

[jahmes143]

Reviving an old thread. Paul, what's the bulletin # of the manual you're quoting from? Neither of my manuals list any compression numbers. I've got the Shop Manual 3666017-01 and the Troubleshooting and Repair Manual 3666087. Thanks.

 

[AcIdBuRn02ZTS]

^Paul hasnt been logged on since 2012... he prolly wont answer.

Not sure what manual he got the info from.

 

[Biggsyj]

I also read somewhere that 300 was minimum and 350 was good but I don't remember where. It may have been a past thread from here.

 

[Ppump01]

Depending on temp, elevation, and engine speed the pressures will vary.

 

[jahmes143]

Yes, there will be some variance. Let's assume 70 degrees, sea level, and 200rpm.

 

[Ppump01]

It's hard to put a correct number as it varies depending on your overall CR. Anything about 300 is said to be good. Most I came across are seeing anywhere from 325-500 psi.

 

[scout4bta]

It's hard to put a correct number as it varies depending on your overall CR. Anything about 300 is said to be good. Most I came across are seeing anywhere from 325-500 psi.

How many revolutions of the crank or pulses should it take to get to these numbers?

 

[Ppump01]

Pull all injectors and disconnect FSS. Crank over till the pressure stops rising. Depending on how well your adapter seals it should only take a few seconds per cylinder

 

[Biggsyj]

When I checked mine I got 375 from all four. The motor had been rebuilt three years prior from reviva but it had also been sitting for a year. So it was not warm and the oil had a long time to drain away from the Rings.

 

[AcIdBuRn02ZTS]

And to get accurate reading, be sure to crank the engine the same number of rotations for each cylinder.

 

[Ppump01]

And to get accurate reading, be sure to crank the engine the same number of rotations for each cylinder.

At 200-250 rpm it'll be hard to count lol. Maybe the same time frame? Say you crank the first one for 5 seconds, then do each one the same. Also try not to take to long doing it to keep temp the same across the board. There's a lot of variables to this and I believe there is up to a 100 psi differential between the lowest number and highest one. The closer the better. The blue by test is the method that Cummins suggests but this will tell you which cylinder it's the culprit if your having issues