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VG30E Compression Test Results


Albeitt
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I finally got around to pulling my plugs and performing a compression test. 

 

Here they are as seen from the front of the vehicle:

 

              BACK

5: 158 PSI  6: 165 PSI

3: 115 PSI  4: 110 PSI

1: 152 PSI  2: 149 PSI

             FRONT

 

The FSM states 168 PSI as the standard, 121 PSI for minimum, and a 15 PSI maximum differential between cylinders. 

 

1, 2, 5, and 6 are acceptable enough, but that doesn't mean anything (as far as I know) since 3, and 4 both have such poor compression and a high differential between their adjacent cylinders, as well as being on opposite sides of the engine.

 

My next step is to sequentially replace the spark plugs and crank the engine to see if I get bubbles in the coolant. 

 

Any suggestions or pointers as to what the issue may be? I know it's a coolant leak somewhere, but I'm working on diagnosing what the exact cause is. 

 

EDIT: When testing for bubbles in the coolant, I only found bubbles when cranking cylinder 6. I'm not sure what this means, as this was the cylinder with the highest tested compression. 

Edited by Albeitt
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12 hours ago, Albeitt said:

I finally got around to pulling my plugs and performing a compression test. 

 

Here they are as seen from the front of the vehicle:

 

              BACK

5: 158 PSI  6: 165 PSI

3: 115 PSI  4: 110 PSI

1: 152 PSI  2: 149 PSI

             FRONT

 

The FSM states 168 PSI as the standard, 121 PSI for minimum, and a 15 PSI maximum differential between cylinders. 

 

1, 2, 5, and 6 are acceptable enough, but that doesn't mean anything (as far as I know) since 3, and 4 both have such poor compression and a high differential between their adjacent cylinders, as well as being on opposite sides of the engine.

 

My next step is to sequentially replace the spark plugs and crank the engine to see if I get bubbles in the coolant. 

 

Any suggestions or pointers as to what the issue may be? I know it's a coolant leak somewhere, but I'm working on diagnosing what the exact cause is. 

 

EDIT: When testing for bubbles in the coolant, I only found bubbles when cranking cylinder 6. I'm not sure what this means, as this was the cylinder with the highest tested compression. 

I had this issue with a Mazda engine, BP01, it's only a 4 banger but the fastest and easiest way to determine if you have bad rings or a bad head gasket is to run the compression test and record result, then add a few milliliters of fresh clean engine oil (displacement will determine how much you need, keep in mind cylinder bore is more important than stroke here). If your compression improves significantly after adding a small amount of oil to the cylinder, the piston rings are shot. My little BP01 is supposed to have an 8.9:1 compression ratio, atmospheric pressure @ sea level is 14.7psi, 14.7x8.9=130.83 so a perfect cylinder should show 130psi I had >123psi in cyls 1&2, 97psi in cyl 3, and 63psi in cyl 4. 2.5ml of oil in cyl 4 brought compression up from 63psi to 108psi (head gasket was also bleeding pressure out the end of the block, however I did not have exhaust gasses in my coolant or in my oil but you could see where it was blowing thru the side) cyl 3 I added 2.5ml of oil, compression came up from 97psi to 121psi. While doing the head gasket I popped the pistons out, had them cleaned. Had the head pressure tested, surfaced and cleaned, block was surfaced, cleaned and honed, and of course new rod end bearings and rolled in new mains. I'm bumpin out 130 psi in all 4 cyls now, perfect!

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