1G Leak down test results clarifications

[greddy_1700]

Hey everyone, just wanted a bit of a sanity check on the results of my boost leak test, compression test, and leak down test. This is on my built 2.3 engine, but not a new rebuilt, I have had this engine for about 10 years, but have probably only put 20,000 miles on it since the rebuild. I updated the engine a bit over this last winter, changing the turbo, intake manifold, cams, springs, new valve seals, gaskets, changing to speed density, etc., but I didnt touch the bottom end since it was always a healthy engine.

Since putting the car back together I have had some issues with the engine. I recently tried to do a boost leak test, but was having significant blow by to the point where I was almost popping the dipstick tube at only around 5psi of system pressure. I verified that the PCV is working correctly, plus I have a check valve in the line as well between my manifold and catch can. This made me think that I either had issues with the valve seals, or worst case the rings.

I did a compression test last night (on a cold engine, throttle closed), and the results were decent considering the engine was cold. Biggest takeaway for me was that the engine seems to have good compression, and even across all cylinders.
Cyl 1: 155 psi
Cyl 2: 160 psi
Cyl 3: 150 psi
Cyl 4: 157 psi

I then did a leak down test, which was more concerning.
Cyl 1: 15% leak to crank case
Cyl 2: 5-7% leak, nothing noticeable
Cyl 3: about 23-25% leak to crank case
Cyl 4: about 7-10% leak, nothing noticeable

I am confident in my leak down procedures, ensuring I was TDC on each compression stroke with the valves closed, and made sure that my tester was calibrated correctly to inlet air pressure. There was no leaking into the coolant jacket or other cylinders, but there was a strong steady airflow blowing out my PCV valve during the test on cylinders 1 and 3.

My question is, do these results indicate that my rings are leaking significantly causing the blow by? If so, how do I have decent compression / even compression considering these leak down numbers? I dont want to tear this engine down again if I can avoid it, but just looking for any sort of additional sanity check that isnt pull the head. Would repeating the leak down test with the cams out tell me anything different? Or would it be possible for valve seals to leak into the crank case causing this issue?

Just looking for any advice while I continue investigating.

 

[99dsmer4g63]

The air shouldn’t be escaping far enough to get to the valve stem seals. If it is you either have valve issues or the valve sealing to the seat on the head issue. You would notice it coming out the exhaust or intake manifold too.

When I leak down test usually repeat it about 3-4 times per cylinder just because of the variables in setting the tester and if the air compressor turns on or anything.

Pretty sure you already know what you need to do unfortunately. You could dump a small amount of oil into that cylinder and re test. If it passes well that’s not good. Make sure to remove the oil as it could hydrolock and bend a rod if too much is left in.

I’m sure someone will chime in and tell me that’s incorrect and I spelled something wrong but anyways that’s my opinion.

Good luck

 

[97egl]

The only thing that taking the cams out will do is verify that you don't have a lifter hanging a valve open.

The valve seals can't leak to the crank case on a leakdown test. But on a leakdown test the definitely can.

If you are popping the dipstick out at 5psi on a boost leak test I would suspect something other than the rings is causing that. Did the dipstick pop out when you did your leak down test or when you are driving the car and under boost?

I wouldn't worry too much about the leakdown test myself. Normally they are not used to determine the health of an engine but used to pinpoint a problem after finding bad compression. Piston rings rotate in cylinders as the engine is running so the #1 and #2 rings could be closer to being lined up on 1 & 3. You also might have rolled the engine a hair past TDC and that would cause the rings not to be seated on the pistons when testing. (I would think that the air pressure would have seated them when applied but still a possibility)

 

[greddy_1700]

Thanks for the feedback and sanity check. The fact that I am getting crank case pressure on both a boost leak test and a leak down test are what is troublesome.

I should note I didnt actually pop the dipstick out, more so felt like it was about to pop. When I had 5psi of pressure built up on my BLT gauge, i pulled the dip stick tube and there was a massive rush of air, doubt the seal on the dip stick would have held much longer at that rate. This happens on the leak down test too if I block off the PCV valve and valve cover breather, making me think it has to be in the combustion chamber and cant be caused by anything else like PCV, turbo seals, etc.

I will probably repeat the tests on a warm engine and see if there is any change, double check everything, and try to get a bore scope down each cylinder before committing to tear down the engine.

Regarding doing the tests with cams out, I know its a long shot, but knowing I changed the cams, lifters, and valve seals in the winter makes me think maybe it could be something there? Wishful thinking I am sure...

 

[DSMPT]

do these results indicate that my rings are leaking significantly causing the blow by?

Could be yes, could be no. The rings are not the only thing that affect amount of blow by.
As you mentioned that you ran the test cold and I assume you have forged pistons. The ring lands condition, amount of oil on rings and cylinder wall, ring gap, PTW clearance and cylinder wall wear would affect amount of blow by.

how do I have decent compression / even compression considering these leak down numbers?

It is possible and it would be like that. It's because those compression test numbers are just the recent numbers with those leak down test result. You have to compare with the compression test numbers from before like when you finished breaking in the engine, when the engine was healthy. For instance, before when you finished the break-in period, the compression test numbers could have been above165 psi and just a few % leak down result for all cylinders when the engine was healthy. If that's the case, then those compression test numbers would make sense.
And also the compression test is dynamic and the leak down test is static. That would make difference, too. Technically those are two different tests.
This is the reason why I recommend to note the compression test number, leak down test number and intake vacuum when the engine is healthy/new or when you change cams with different profile or when you buy the car. So it can be used as a base line to compare in the future.

Would repeating the leak down test with the cams out tell me anything different?

You can know the valve seating/sealing condition without being affected by lifters (or wrong valve timing). And you can run leak down test with different piston position. (But if in the middle of stroke, you have to hold or lock the crank while pressurizing the cylinder).

The fact that I am getting crank case pressure on both a boost leak test and a leak down test are what is troublesome.

That's kind of normal that the crank case gets some air while running a boost leak and leak down test, especially if you run the test cold. And if the oil filler cap, PCV valve and breather ports are blocked while running the test, you would see more air come out from the dip stick tube.

I don't know how much blow by you have usually had, but if it's significantly more than before or suddenly started blowing more, then maybe it's time to rebuild the bottom end. But you know that maybe it's just time to upgrade the crank ventilation, too.

 

[greddy_1700]

So, small update, but just completed a warm compression test, and generally speaking the results are the same as cold, or slightly worse... That's really making me suspect the rings are failing if there is no warm improvement.

My compression tester broke on the last cylinder of course, so couldn't do a test with a bit of oil in each cylinder. I think the next step is likely borrow a decent bore scope and confirm if there are any obvious issues with the block.

 

[greddy_1700]

Well, got my borescope this weekend, and didn't see much. Some scratching for sure, but doesn't look that bad to have such terrible leak down results. Starting to lean towards stuck piston rings, but open to other thoughts or opinions.

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[danl]

I know borescopes tend to magnify things but that looks pretty bad. I don’t have images from my 4g63 but it’s much worse than my s2000 which are the only images I have saved.

 

[greddy_1700]

So, late follow up. I disassembled the engine, took a look and the cylinder walls actually look really good. The scratching that came up on the bore scope was defintely magnified, and I cant feel any of those scratches, none catch a finger nail which is a good sign.

What I did notice was there was a bit of play in the pistons front and back. I could rock them in the cylinders back and forth a bit. Now I know that there obviously needs to be some clearance, but seems like a tad much to me? I would assume it should be pretty snug, and not move within the cylinder wall so easily. Would that potentially confirm my suspicions of stuck rings causing my blow by?

Regardless, engine is tore down (aside of the rotating assembly), and going to my machine shop to get inspected, cleaned up, measured, and reassembled.

 

[chrysler kid]

I use compression test to verify engine health, only use leak down test on motors that are unable to be compression tested

I had a 7 bolt that was down to 120 psi in cylinder 2 and it would pop the dipstick out under boost, but no other ill side effects or lack of power

There shouldn’t be any slop. The shop could have either over bored the block or you needed them to spec you oversized pistons for the larger hone. Or maybe has the wrong sized rings for the pistons that are gapped correctly. Hard to say I’m not super familiar with the 4g63 this is just my backyard knowledge of 350 Chevy rebuilds

 

[greddy_1700]

I use compression test to verify engine health, only use leak down test on motors that are unable to be compression tested

I had a 7 bolt that was down to 120 psi in cylinder 2 and it would pop the dipstick out under boost, but no other ill side effects or lack of power

There shouldn’t be any slop. The shop could have either over bored the block or you needed them to spec you oversized pistons for the larger hone. Or maybe has the wrong sized rings for the pistons that are gapped correctly. Hard to say I’m not super familiar with the 4g63 this is just my backyard knowledge of 350 Chevy rebuilds

Yeah thats fair, I would anticipate some movement, but I thought the spring force behind the rings would make it more difficult to move by hand, which is leading me to thinking it is the issue of stuck rings as suspected. Either way, hoping to reuse pistons and get away with just a hone, but time will tell.