Suggestions or tricks to reduce blow-by?

[Diesel JD]

MNR, at 500K miles, one would expect a lot of blowby. The Cummins might last a bit longer than that though. Acceptable compression on a 6.9/7.3 is anything in the 300-500 range as long as all cylinders are within 25%. As Mel and others stated, the important reading is the difference. You really are supposed to test for compression hot, but that involves burning your fingers quite a bit. It can be done but I'm not sure the difference justifies it. The readings probably would be closer if you drove it then let it cool down to at least a comfortable working temp <100 block and coolant. Still better than having sat a few days!

 

[Cheaper Jeeper]

MNR, at 500K miles, one would expect a lot of blowby. The Cummins might last a bit longer than that though. Acceptable compression on a 6.9/7.3 is anything in the 300-500 range as long as all cylinders are within 25%. As Mel and others stated, the important reading is the difference. You really are supposed to test for compression hot, but that involves burning your fingers quite a bit. It can be done but I'm not sure the difference justifies it. The readings probably would be closer if you drove it then let it cool down to at least a comfortable working temp <100 block and coolant. Still better than having sat a few days!

Well, you guys are making me feel at least little better about it!

How's this for a plan....

Take the readings for the other bank (driver's side 5-8), then put it all back together, start it up and get it up to temp, shut it down and take another reading off a couple of the cylinders that are easiest to get to and don't require tearing everything apart (like maybe the front one on each side).

I'm thinking that if the 5-8 bank are all with 10% of each other (just like the readings I got for the 1-4 bank) and they are within 10% of the 1-4 bank readings, then the same should still be true when its warmed up, right?

So if the #1 and #8 measure out in the 400 range with the engine warmed up, then I think I can be reasonably confident that I have a failrly accurate picture of what kind of compression they all have, and I should be able to do this without frying my fingers....

 

[h2odrx]

i thought odds where on one side and evens on the other?
2-4-6-8 1-3-5-7

 

[Cheaper Jeeper]

i thought odds where on one side and evens on the other?
2-4-6-8 1-3-5-7

I'm not sure and don't care enough to look it up. For the purposes of this discussion it doesn't matter - I'm referring to them as passenger side being 1-2-3-4 with #1 in the front and driver's side being 8-7-6-5 with #8 in front

I just got done checking 5-8 and they weren't significantly different (or better) than 1-4

5 - 390
6 - 360
7 - 350
8 - 360

So it looks like my overall spread is 70 psi - from 320-390, or about 18%, unless the guage is wrong - which it may be since it is a Harbor Freight cheapo.

I'll pick up another one from HF on the way home tomorrow and test #1 & # 8 (two front ones) to see if the readings are any different. Then I'll warm it up and test 'em again to see if the readings are any different.

Kind of interesting that the two rear cylinders (what I'm calling 4 and 5) have the highest readings in their respective banks (#4 - 360 and #5 - 390)

 

[Diesel JD]

Very interesting, since they see the most blowby of any cylinders thanks to the CDR design. Honestly, I think those readings sound good, maybe not quite what you hoped for but certainly not enough to be a problem. I really think most inaccurate compression tests come from a poor seal at the glow plug adapter but since you got all in the neighborhood, I'd not worry about it.

 

[Cheaper Jeeper]

Very interesting, since they see the most blowby of any cylinders thanks to the CDR design.

Yeah, I kinda expected the two rear ones to be lower having read about how most oil that gets by the CDR ends up being burned in those two cylinders. On the other hand, burning oil is only supposed to be a problem because the higher BTUs cause excessive heat and lead to head gasket failures on those two cylinders. I'm thinking that maybe because they get more oil, those two were less "dry" than the other 6 (remember it hasn't been started in nearly a week) so the numbers were a little better.

Honestly, I think those readings sound good, maybe not quite what you hoped for but certainly not enough to be a problem. I really think most inaccurate compression tests come from a poor seal at the glow plug adapter but since you got all in the neighborhood, I'd not worry about it.

Yeah, I was hoping for numbers in the 400 + range - like I got on my other truck. I'm less pleased with these numbers, but I'm getting less and less "concerned" about them. Especially since it starts and runs so well. I'll try reading a couple of them after I warm it up tonight and see what I get then. I'm also going to do an oil change with some good quality oil - hopefully that will reduce the blow-by significantly...