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Cummins Blowby Test

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I had heard from several sources that the way to determine the condition of a Cummins engine is with a blow by test. I'm not sure if it's done this way because it's easier or more accurate.

I do know compression tests are a PITA to do on the 24v trucks because you have to pull out the injectors and use a special adapter.

I did some searching and found information on Cummins Forum about how to do this, so I built a setup with that information.

Here is the website: http://www.cumminsforum.com/forum/94-98-powertrain/8684-how-much-blowby.html

The setup is very simple; you don't need to get fancy with it. You just make a U with tubing and put water in it. This is a simple manometer and that will measure pressure in inches of water column.

You attach the manometer tubing to your vent tube. On my truck this was easy since my tube is relocated down by the fuel tank.

Somewhere in the line between the vent tube and the manometer you need to have a fitting with a 15/64" hole. This is a controlled leak. If you didn't have that, the blowby pressure would just push the water right out of the manometer.

You just put a mark on the tubing where the water is without the truck running and measure from there.

Here are the #s I got:

Idle - 1" rise

2200rpm - 2" rise

2500 rpm - 2 5/8" rise

2800 rpm - 3 7/8" rise

The specs that are posted are:

New Engine

63 L/Min (2.5" rise) @ 2200rpm

76 L/Min (3.5" rise) @ 2500rpm

85 L/Min (4.5" rise) @ 2800rpm

Worn Engine

126 L/Min (10.5" rise) @ 2200rpm

152 L/Min (14.5" rise) @ 2500rpm

170 L/Min (17" rise) @ 2800 rpm

Now I'm not sure if by new engine they mean brand new without the rings seated? I'm asking a few questions on Cummins Forum to clarify because the numbers don't seem correct.

Also I'm not sure how they are converting inches of water column to liters per min. The 1" = 27L/Min and then add 3 for every 1" doesn't work out to the specs posted.

Inches of W.C. is a measurement of pressure and L/Min is a measure of flow. As far as I know, this can't be easily converted.

I would be interested in hooking it up to other Cummins trucks to compare the numbers. I can bring it to the dyno event if any one wanted to volunteer their truck.




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:think: :think: It says BANNED...

Good post Nate.

:lol: yeah,,,,,,,,,,, dont post dtr or cf links cuz at least half of us cant get in to read those forums :td::lol:

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Didn't know you all were banned from Cummins Forum too.

Here is the info copied from that thread from illflem:

The Cummins blowby orifice tool is simply a tee with one .221" (15/64-in) outlet. Connect one end of the tee to the end of the blowby tube. Put a manometer on the last tee outlet. That is your blowby tool. They sell them at Cummins, but I have made my own plenty of times, less than $10. A simple manometer can be made by looping into a 'U' 6 feet of clear tubing, fill it with water in it half way. Measure how high the water level rises with a tape measure, multiply it by 2, convert it to LPMs.

Rough conversion is 1"= 27 lpm, add 3 lpm for each one inch (1/2'' of rise in the tube) of water

The reason for multiplying by 2 is that inches of water equals the water rise in the open end of the tube plus the inches the water is pushed down on the engine side of the water tube. For simplicity my numbers below are the measurement of rise only.

Cummins new 5.9 engine numbers are:

63 liters per minute(2.5" water rise) @ 2200rpm,

76 L/Min (3.5" rise) @ 2500rpm

85 L/Min (4.5" rise) @ 2800rpm.

Worn engine that needs rebuilding are roughly double i.e.

126 L/Min(10.5"rise) @ 2200rpm

152 L/Min(14.5"rise) @ 2500rpm

170 L/Min(17"rise) @ 2800 rpm

Beside indicating a compression problem the valves could also be out of adjustment.

Another way (mine), same idea, is to block the blowby tube with a 1/2'' pipe nipple with a cap that has a 15/64 hole drilled in it. Use 3/8'' id looped clear tubing with water in it slipped over the oil dipstick tube. Other tubing end remains open. Use a sharp tipped felt marker to mark the water level with the engine off, have a helper start an already warmed up engine and run the rpms up to 2.2, 2.5 & 2.8k rpms. Mark each water level with the pen, measure the distance from engine off mark then multiply each by 2.

This is all very simple to do, just hard to explain with words.

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I was reading this Nate and wanted to point out that to correctly measure a manometer, you measure the separation between the low and high water levels, not just one side from your initial mark. If you only measured one side, then you need to double your numbers.

This is explained in the copy you posted.

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