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Old 01-16-2011, 03:53 AM   #1 (permalink)
maverick_driver
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Compression test on 6.5

Hi,
I searched for a detailed write-up for a compression test procedure on a 6.5 and I found only incomplete informations. Can somebody add all the steps preferably with pictures and also the desired compression test results?
Thank you

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Old 01-16-2011, 09:59 PM   #2 (permalink)
orionthade
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A stock 6.5l in our application will have a compression ratio of 21.5:1 - 22.5:1. If you do the math, this means that in a perfect world, each cylinder will read between 301 and 315 PSI on the compression stroke.

To do a compression test, you'll need first need a diesel application compression testing gauge. They will typically read between 350 and 400 PSI.

Next, you'll want to take the GP out of the cylinder you are currently testing and thread the nipple into it's port. Connect the gauge to the nipple.

I'd imagine, like on a gasser, you'll want to disable the fuel delivery system.

Crank the engine for 6 or 7 revolutions. The gauge will read the compression. I would do this a couple of times, clearing the gauge after each test. Write down the cylinder number and the reading. Rinse and repeat for all 8 cylinders.

Unless your motor is brand new/rebuilt with freshly rebuilt heads and new rings, it's doubtful that your cylinders are actually going to read a true 21.5:1 to 22.5:1. They will likely read less.

The key is that you want all the cylinders to to be fairly close to each other in their reading. If you have 7 cylinders that read between 285 and 300 PSI and 1 that reads 200, then that cylinder is leaking. Either you have extremely worn compression rings, a blown head gasket, burnt valves, damaged valve seats, a cracked head or a cracked bock. It's also possible that air is leaking out from around the injector or the glow plugs.

To determine which of these issues it is, you can do a leak down test.

To perform this, you'll need an air compressor and a fitting that will thread into the glow plug port. Then, you'll want bring that cylinder to top dead center, then release compressed air into it.

If you hear bubbling it the cooling system, then you likely have either a blown head gasket or a cracked head that is into the water jacket.

If you hear air coming out of the intake, you have a damaged intake valve/seat.

If you hear air coming out of the exhaust, you have a damaged exhaust valve/seat.

If you hear air in the oil pan, then you have worn rings or a hole in your piston.

Clear as mud?

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Old 01-16-2011, 10:41 PM   #3 (permalink)
NVW
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The compression on a healthy engine will be more like 400 psi. They should all be within 10% of each other.

It's also recommended that all the glows are removed. I don't do it myself however.
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Old 01-16-2011, 10:56 PM   #4 (permalink)
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Quote:
Originally Posted by NVW View Post
The compression on a healthy engine will be more like 400 psi. They should all be within 10% of each other.

It's also recommended that all the glows are removed. I don't do it myself however.


I've pulled the glows when doing a comp test. Less strain on the starter/batteries with the glows removed.

My pickup had 400 psi in 5 out of the 6 cyl i tested(couldn't get to the ones with the GP heat shields). the 6th cyl had 390.

My blazer was all over the board from 360 to 400, but that engine sat a long long time unused.
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Old 01-16-2011, 10:58 PM   #5 (permalink)
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Quote:
Originally Posted by NVW View Post
The compression on a healthy engine will be more like 400 psi. They should all be within 10% of each other.

It's also recommended that all the glows are removed. I don't do it myself however.

400 psi would be 28.5:1 compression at 14 PSIA ... Of course what I didn't take into account was elevation. In the math, I assumed sea level barometric pressure of 14 PSIA. 14 x 21.5 = 301. 14 x 22.5 = 315.

I'm in Salt Lake, so my barometric is actually closer to 12.5 PSIA. This means that in my case, 12.5 x 21.5 = 268.75 and 12.5 x 22.5 = 281.25.


As for the glow plugs, I didn't know this. As I said, I've never run a compression test on this motor before.
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Old 01-16-2011, 11:02 PM   #6 (permalink)
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How can I make sure injection pump will not add fuel in the cylinders?
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Old 01-16-2011, 11:06 PM   #7 (permalink)
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Quote:
Originally Posted by orionthade View Post
400 psi would be 28.5:1 compression at 14 PSIA ... Of course what I didn't take into account was elevation. In the math, I assumed sea level barometric pressure of 14 PSIA. 14 x 21.5 = 301. 14 x 22.5 = 315.

I'm in Salt Lake, so my barometric is actually closer to 12.5 PSIA. This means that in my case, 12.5 x 21.5 = 268.75 and 12.5 x 22.5 = 281.25.


As for the glow plugs, I didn't know this. As I said, I've never run a compression test on this motor before.
Remember that's 14.7 psia, or lbs per square inch absolute. The cylinders are larger than 1 square inch, which is where your numbers and math don't account for.
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Last edited by IamDave0887; 01-16-2011 at 11:06 PM.
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Old 01-16-2011, 11:06 PM   #8 (permalink)
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Quote:
Originally Posted by maverick_driver View Post
How can I make sure injection pump will not add fuel in the cylinders?
If you are 94 and newer then you can either unplug the pmd/fsd or pull the ecm fuse or the fsos fuse
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Old 01-16-2011, 11:08 PM   #9 (permalink)
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Quote:
Originally Posted by maverick_driver View Post
How can I make sure injection pump will not add fuel in the cylinders?
What year is the truck? 92-93 is mech injection and removing one wire will prevent fuel from going into the cylinders. 94+ is electronic injection and you'd have to remove a fuse(i pull the fuse for the fuel solenoid). 94+ you can also unplug the PMD and it won't add fuel.

Quote:
Originally Posted by NVW View Post
If you are 94 and newer then you can either unplug the pmd/fsd or pull the ecm fuse or the fsos fuse
Guess i type too slow.
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Last edited by IamDave0887; 01-16-2011 at 11:09 PM.
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Old 01-17-2011, 12:53 AM   #10 (permalink)
orionthade
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Quote:
Originally Posted by IamDave0887 View Post
Remember that's 14.7 psia, or lbs per square inch absolute. The cylinders are larger than 1 square inch, which is where your numbers and math don't account for.

D'Oh! You're right.

Disregard my math.



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