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| Low Compression, But Good Leak Down....WTH? https://slantsix.org/forum/viewtopic.php?t=62804 |
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| Author: | Poboyross [ Wed Sep 05, 2018 11:59 am ] |
| Post subject: | Low Compression, But Good Leak Down....WTH? |
I've been futzing with this engine for over a year now, two steps forward....one (sometimes 3) steps back. The latest is having a good leak down test on a known low compression cylinder. At first I thought it was a bad head gasket, which I replaced. The compression has gone up since then, when it was a paltry 35, now it's about 60-70....so now instead of ALL dead, it's just MOSTLY dead. At some point in the engine's past, it did overheat. There are scuff marks in cylinder 2, but not gouges. When I replaced the head gasket, I looked at the valves and none looked burnt. I redid the lash per instructions. While I haven't measured rocker travel, they all *appear* to be the same....same goes for lifters (albeit still visual). I figured it a cam lobe were gone, I'd be able to possibly see some difference in travel....maybe not. The only thing that really leaves is a bent connecting rod, correct? I went back and looked at photos of when I had the head off, and it appears to top out at the top of the cylinder in the same place as do all the others. I *do* realize I'd have to get some definitive measurements, but with compression that low, I figured something should throw up a red flag visually. I also don't know how bent of a rod it would take to knock down compression to less than half of what it's supposed to be. Any ideas of where to look next? I'm not planning on rebuilding it any time soon, but replacing one piston with it still in the car is a possibility. |
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| Author: | Rick Covalt [ Wed Sep 05, 2018 4:03 pm ] |
| Post subject: | Re: Low Compression, But Good Leak Down....WTH? |
Pretty sure it is not a connecting rod issue. How would one even get bent? Unless there was some kind of a catastrophe? |
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| Author: | Poboyross [ Thu Sep 06, 2018 2:10 pm ] |
| Post subject: | Re: Low Compression, But Good Leak Down....WTH? |
Yeah, that's what I was thinking. I checked the push rods, lifters, and cam lobes today on the offending cylinder...no banana...they all look good with no wear at all and push rods are straight. The cylinder does have some scuffing on the walls, saw it when I had it cracked open last year, but the scuffs weren't deep enough to even catch my fingernail. I'm at a loss! |
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| Author: | SpaceFrank [ Fri Sep 07, 2018 3:33 pm ] |
| Post subject: | Re: Low Compression, But Good Leak Down....WTH? |
Compression tests can be tricky, and leakdown tests can be even trickier. Can you describe exactly how you did these tests and what numbers you got? For the compression test: was the throttle held open, was it wet or dry, how many revolutions of the engine before you recorded the pressure or how long before it topped out. For the leakdown: what was your source pressure, what position were the pistons in, how did you ensure both valves were closed? Also, if the engine is running, what are your symptoms? |
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| Author: | MrFusion [ Fri Sep 07, 2018 8:33 pm ] |
| Post subject: | Re: Low Compression, But Good Leak Down....WTH? |
I had a similar problem with my truck. It was a farm truck or sat for the last 10 years or so. When I acquired it the carb had been taken off and it sat that way for at least a year. It started and ran but barely. Upon further investigation there were some sticky valves and the intake valve on #1 cylinder was all but frozen. I got that fixed by filling the intake runners with ATF and letting it sit for a week. After that I still and poor compression (less than 60 psi) on #1 cylinder although the valves were working properly. A friend opined that the rings were stuck so I bottomed the piston in the cylinder and filled it with a mix of ATF, kerosene, MMO, and a splash of acetone. I let that sit for a week plus. I drained it from the cylinder put the plug back in and ran it for a hundred or so miles (short trips around town and some highway). Eventually the miss cleared up and the plug read like the other 5. The next warm compression test it pumped up to 105 PSI. Which leak down tester did you use? If it only uses 15psi (Harbor Freight) its pretty much useless. Get one that uses 100psi. That will more accurately identify problems. Bad rings or a poor ring seal can hold 15psi all day. Not the case with 100psi. You'll have air whistling out of the dipstick tube and oil fill. |
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| Author: | MitchB [ Wed Sep 19, 2018 11:50 am ] |
| Post subject: | Re: Low Compression, But Good Leak Down....WTH? |
You don't have to do a leak down for your diagnostics. Pressurize the cylinder in question with shop air - greater than 100 psi with the piston right around TDC with the valves closed and see where you hear air leaking: out of the oil fill cap on the valve cover, the tailpipe, the carb/intake or is the cooling system building pressure? Mitch |
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