Leak down test results
07-28-2014, 07:02 PM
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Leak down test results
I believe I have a bad ring, so I did a leak down test. I started on cyl 8 because that's where the engine was sitting. I tested at 90PSI and all cylinders held 86 to 89 PSI on a cold engine. But I had strange results on cyl 1.
After testing cyl 2 (at the 90 timing mark on my streetdampr), I lowered the pressure to only 20psi and rotated the engine to see when it would quit holding pressure. I got past the 270 timing mark and then rotated the engine backward to get back to TDC to test cyl 1 (which I was testing last). I set up to cyl 1 and opened the tester's air valve. There was a lot of hissing and it wasn't holding pressure. I found moving the engine slightly in the forward direction sealed the rings.
I did some testing rotating the engine forward from 90 mark to TDC and it would hold pressure. Rotating backward from 270 to TDC and it wouldn't hold pressure until I rotated the engine slightly forward. The problem is very repeatable. I tested cyl 2 similarly and it held pressure in both directions. I haven't retested any others yet.
Do I actually have a problem with the rings on cyl 1? Why would it hold pressure one way and not the other? The piston is moving up and down the cylinder either way. The rockers remain loose and there's no valve movement when I get to 270 degrees, so its not valve related.
This is a '92 350 TPI engine with less than 50k miles, with heads, cam and a recent procharger. I haven't driven it much with the blower yet but my WB says the mix is good enough (AFR around 12 under boost).
After testing cyl 2 (at the 90 timing mark on my streetdampr), I lowered the pressure to only 20psi and rotated the engine to see when it would quit holding pressure. I got past the 270 timing mark and then rotated the engine backward to get back to TDC to test cyl 1 (which I was testing last). I set up to cyl 1 and opened the tester's air valve. There was a lot of hissing and it wasn't holding pressure. I found moving the engine slightly in the forward direction sealed the rings.
I did some testing rotating the engine forward from 90 mark to TDC and it would hold pressure. Rotating backward from 270 to TDC and it wouldn't hold pressure until I rotated the engine slightly forward. The problem is very repeatable. I tested cyl 2 similarly and it held pressure in both directions. I haven't retested any others yet.
Do I actually have a problem with the rings on cyl 1? Why would it hold pressure one way and not the other? The piston is moving up and down the cylinder either way. The rockers remain loose and there's no valve movement when I get to 270 degrees, so its not valve related.
This is a '92 350 TPI engine with less than 50k miles, with heads, cam and a recent procharger. I haven't driven it much with the blower yet but my WB says the mix is good enough (AFR around 12 under boost).
07-31-2014, 03:16 PM
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Re: Leak down test results
No one familiar with the behavior of bad rings that can confirm that cly 1 really has a problem?
No theories on why it would seal when the engine is rotated one direction but not seal when rotated the other?
No theories on why it would seal when the engine is rotated one direction but not seal when rotated the other?
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