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Compression & Leak Down Results On Rb26

Don Dada

By Don Dada,
in Engines & Forced Induction

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Don Dada Mentor

Don Dada

Posted
Posted

Hi guys i've done a compression and leak down test on my rb26 to see if i can determine why it burns about a qt of oil between changes (5000Kms)

Ive somewhat ruled out valve seals because it doesnt smoke on start up even after sitting for some days, neither on deceleration, neither on idle.

There is slight white/blue on hard accel but i cannot see it in from inside the car....i was told this by a friend driving behind.

It fills the catch can with less than a table spoon of oil during the period also.

Anyway here are the results...

...Compression________________Leak down

....DRY........WET____________DRY...........WET

1) 155.........200 __________1) 12%..........10%

2) 151.........200___________2) 8%...........7%

3) 160.........205___________3) 10%.........10%

4) 157.........200___________4) 11%.........11%

5) 145.........190___________5) 15%.........14%

6) 152.........198___________6) 20%.........14%

Did not hear any air escaping through the intake,exhaust,through the coolant etc, only through the dip stick hole and the oil fill cover...about the same amount for all cylinders

So what do u guys think Rings or turbos or....???

James_03 Mentor

James_03

Posted
Posted

Yeah I thought that might have been the case.

It would show different symptoms usually for turbo, as you have mentioned. It would more than likely get smoke on startup after leaving it overnight. Which would lead you to either stem seals or turbo, but the test there pretty much points to rings for me anyway, that will be the smoke under acceleration too.

Did you perform your own comp test prior to purchasing? How long have you had it for?

Don Dada Mentor

Don Dada

Posted
  • Author
Posted

I started the engine when it was on the ground and did a comp test, no. 5 was a little lower that the rest...same as it is now, and then did a next test after about 5-6mths of running in the car and it was more or less the same....just 1-2psi difference

I used a different gauge this time tho so i cant compare it to the last results, and it has been about another 6mths since the last test.

Going from the difference between the wet and dry test on no. 6 it points to rings, but is that difference so bad??? ie. from 20% to 14%.....

I would of thought there might have been a larger difference in leak down between wet and dry on no. 5 seeing that it had the lowest comp.

The car still makes good power tho.

James_03 Mentor

James_03

Posted
Posted

Yeah I know what your saying re no 5, I would have expected a different result there.

Maybe take it to a reputable garage and have another test done, can't hurt having another set of results to compare with. Should cost sub $100, and you can then get another opinion as well. Was the test performed at WOT?

Personally I think your looking at combined blow by across all cylinders. Not anything major, but enough to cause that amount of oil loss over x period of time. In any event, whether it's one cylinder, 2, or all of them, the end result is the same, it will need a rebuild in order to resolve it.

If it's making good power and still pulls strong, then leave it for a bit longer and just watch your oil.

You could even investigate running a different grade oil in order to minimize it.

Don Dada Mentor

Don Dada

Posted
  • Author
Posted

Don't mind getting a second opinion at all but there is no one locally to do it that's why i had to buy my own equipment to do it!

Its a cheap gauge from harbor freight in the states though, so i dont know how accurate it is......did each test about four times though to ensure a consistent result!

The comp test seems more or less ok for a rb with some ks on it....just no. 5 is a little lower but yet doesn't show any improvement in leak down when it was done wet.

Oil control rings was suggested as a probable cause but neither a comp test nor leak down would determine this right???

Test was done wot on a warm motor using redline 20w50

Don Dada Mentor

Don Dada

Posted
  • Author
Posted

Yeh....

I got one of those small greddy copy catch can filled with copper scuff pad thingy with one 1/2" hose from the cam covers running to it then back to original spot on the rear turbo

Is it possible that the 1/2" hose is too small??

The rest of the pcv system is stock.

I dont think there excessive pressure in the sump tho....It does not blow out the dipstick and it isnt on that tight

  • 4 weeks later...
GTScotT Grand Master

GTScotT

Posted
Posted

2 things to try:

Cold comp test (not on a warmed up motor)

Comp test with a cap full of oil down the cylinder (also cold)

Doing it cold may show you less impressive results, and major variances in the results from a cap full of oil in the bore shows worn rings.

Perhaps remove your front pipe and look for evidence of a greasy burn, all turbos with worn seals have been a little greasy in the exhaust housing that I have worked on.

Realistically you are going to have to make your own decision here.. Another thing to note, is your breather setup stock (I noted you said you had a catch can) and how is it setup.

Unfortunately these things are not as simply diagnosed as they are said, GL.

Don Dada Mentor

Don Dada

Posted
  • Author
Posted
Do the plugs show any signs that it's burning oil?

Just to make sure....If its burning oil what color will the plugs be??

Also going by the comp and leak down results is ok for me to rule out the bottom end as the problem or is it still possible for it to have good comp and oil getting past bad oil control rings and being burnt???

tranman Enthusiast

tranman

Posted
Posted
2 things to try:

Cold comp test (not on a warmed up motor)

Comp test with a cap full of oil down the cylinder (also cold)

Doing it cold may show you less impressive results, and major variances in the results from a cap full of oil in the bore shows worn rings.

With regards to the cap of oil, isn't that what the "wet" and "dry" readings referred to?

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