LNF bad piston/ring. What should I do next? - Cobalt SS Network



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Old 01-03-2018, 05:06 PM   #1
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LNF bad piston/ring. What should I do next?

I think I have a bad piston ringland or ring in my LNF. I want to fix the engine but i'm not sure what I need.

This last weak I've observed more and more misfires either upon deceleration or cruise, but none under acceleration. The car felt OK but my scan gauge said otherwise. I did a dry compression test with the engine cold, I got ~162psi on all cylinders except 83psi on #2. Then i did a wet compression test on #2 and got 158psi, then repeated the test an hour later and got 131psi. I did a compression test about 3,000 miles ago - all cylinders were within 4psi. I looked at the #2 piston with my $15 smartphone borescope camera, didn't see any obvious problems.

My cobalt has 100k miles on her, I've owned it since 65k, and I've had it bolted and tuned since 75k. My goal is to keep it running reliably as is with the stock turbo and custom tune. Should I just order some new stock pistons and rings and slap them up into the bottom of the motor? Do I need to spend a bunch on money on Wiseco pistons? I'll check but I'm hoping my cylinder walls are not scored. Do I need to start thinking about replacing my valve train or timing chain or anything else? As far as I know the engine is all original.
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Old 01-03-2018, 05:22 PM   #2
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LDK

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Old 01-03-2018, 05:37 PM   #3
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yupp LDK, contact Jeff Reiland. He offers a brand new complete LDK for $2,700 shipped.
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Old 01-03-2018, 05:42 PM   #4
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LDK

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Whoa whoa whoa lets talk about this haha. The LDK sounds like a nice motor and all but I'm trying not to spend thousand of dollars and rebuild my whole motor. I live in Alaska now, the closest used LDK is probably a few thousand miles away.
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Old 01-03-2018, 05:44 PM   #5
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this wont be cheap. you cant just slap forged pistons in an engine without doing work to it. and forged pistons will not last as long as OEM ones.
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Old 01-03-2018, 05:45 PM   #6
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You could source a single good LNF piston, pull the rod out and hone it with a drill hone then get some new rings. gap them and see what happens.

I wouldn't do that.

but you could
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Old 01-03-2018, 05:50 PM   #7
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It kinda depends on how bad the damage is. You could pull the piston out, then inspect; but I really doubt you wouldn't need at least a hone. It also depends on if you're able to do the work yourself. It sounds like you're comfortable doing it yourself so that is helpful.
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Old 01-03-2018, 05:54 PM   #8
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if the ringland cracked and broke, and you cant find the pieces. I'd be worried. I'd pull my oil pan off and check to see if anything ended up in there.
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Old 01-03-2018, 07:17 PM   #9
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..

Also there is no way in hell a piston is going up thru the bottom. Aint happening. Head has to come off and down thru the top. They make 86MM ring compressor rings that work perfect for install. If you do go that route, and I see a few have, I would wad up a bunch of clean rags and shove them down the bore to minimize debris circulating thru the engine. Also mask of all the top of engine except the bore like a surgeon. Once done, wipe the bore a zillion times with automatic transmission fluid or a clean towel. Keep repeating till its clean. The ATF will grab and hold the dirt from hone job. Lots of labor, but it could be done for $500 or so and a good bit of time. Change the timing chain while your in there too
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Old 01-03-2018, 08:26 PM   #10
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So i could start by removing the oil pan and looking for metal, but it sounds like i'll end up pulling the head and pistons soon anyways. It's temping to spend $2,700 on a new LDK but freight shipping to Alaska will probably take three times the cost and time. So assuming the cylinders are OK with just honing, I could nearly have a new LNF for under $1,000. ZZPerformance sells a set of 4 "take out" LNF pistons and rods for only $300 and new head with valves/springs for only $300. Timing chain is only $100 plus gaskets and fluids etc...
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Old 01-03-2018, 08:55 PM   #11
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At least you have some options. If you are keeping the car till it rusts to the ground I would go with the ldk setup. If your head is ok (you could go with a takeoff new head), you could always just throw in a ldk shortblock. Block is a lot better and you wouldn't have to worry about pourus block later.
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Old 01-04-2018, 06:51 PM   #12
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So i could start by removing the oil pan and looking for metal, but it sounds like i'll end up pulling the head and pistons soon anyways. It's temping to spend $2,700 on a new LDK but freight shipping to Alaska will probably take three times the cost and time. So assuming the cylinders are OK with just honing, I could nearly have a new LNF for under $1,000. ZZPerformance sells a set of 4 "take out" LNF pistons and rods for only $300 and new head with valves/springs for only $300. Timing chain is only $100 plus gaskets and fluids etc...
Top ring land bits can't get past the piston. There in you cat now.
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Old 01-05-2018, 03:34 PM   #13
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yupp LDK, contact Jeff Reiland. He offers a brand new complete LDK for $2,700 shipped.
Tell more about this guy please. Where is he located? ZZP sells a complete LDK Long Block with stock turbo for $2,600 and quoted me $960 for freight shipping to Alaska from Michigan. Eeeek!

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Top ring land bits can't get past the piston. There in you cat now.
What cat? haha no state inspections where i'm at. But really I wouldn't think piston pieces broke off because last time I ran the car it was ok, just sounded a little off at idle and showed lots of misfires but none under acceleration. But who knows without taking it apart. I'll check for pieces in my muffler haha

Last edited by Natomic; 01-05-2018 at 03:36 PM. Reason: fergot sumthin'
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Old 01-05-2018, 06:54 PM   #14
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He lives outside Pittsburg Pa
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Old 01-05-2018, 08:37 PM   #15
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What cat? haha no state inspections where i'm at. But really I wouldn't think piston pieces broke off because last time I ran the car it was ok, just sounded a little off at idle and showed lots of misfires but none under acceleration. But who knows without taking it apart. I'll check for pieces in my muffler haha
Boroscope it and see if there are any bright reflections from the top ring showing.
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Old 01-05-2018, 08:47 PM   #16
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I was in your shoes about a month ago. Looked into rebuilding but decided it wasn't worth it. I am in the midst of swapping in an LHU from a rearended Buick Regal.
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Old 01-11-2018, 10:28 AM   #17
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Nobody here has mentioned a leakdown test.... yet. Everybody wants to put all kinds of work in w/o proper diagnosis. May be simple as a bad valve or head gasket... We will never know without a leakdown test...C'mon. Baby steps here...
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Old 01-11-2018, 11:03 AM   #18
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He did a pretty definitive wet compression test, so...
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Old 01-11-2018, 11:16 AM   #19
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So..... what? Where is the PSO (pressure squirting out)? Through the cyl. bore? Through the head gasket? Through a valve (can you hear air coming from the intake or exhaust)? Where? THAT tells you what to fix. Not a "wet test".
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Old 01-11-2018, 11:19 AM   #20
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And btw... chances are if he detonated it via tune, bad gas etc... he knows this but is not sharing that info--- just straight to a new piston.
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Old 01-11-2018, 11:22 AM   #21
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you do a wet test to see if the issue is ring seal. if it's valves or head gasket, the number wouldn't change. if i'm wrong, please explain how it's possible that a wet compression test would change if the valve seals are faulty.
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Old 01-11-2018, 11:30 AM   #22
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you do a wet test to see if the issue is ring seal. if it's valves or head gasket, the number wouldn't change. if i'm wrong, please explain how it's possible that a wet compression test would change if the valve seals are faulty.
I don't give a [email protected]#$ about the "valve seals". And I am helping the op PROPERLY diagnose his issue. Step by step. Not saying you are wrong... or disagreeing with you. Pull the motor down. I'm outta here..
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Old 01-11-2018, 11:45 AM   #23
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Holy butthurt batman
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Old 01-11-2018, 11:49 AM   #24
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Holy butthurt batman
This. Seriously.

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I don't give a [email protected]#$ about the "valve seals". And I am helping the op PROPERLY diagnose his issue. Step by step. Not saying you are wrong... or disagreeing with you. Pull the motor down. I'm outta here..
You said a leakdown test is more proper than a wet compression test. My only point is that the wet compression test already showed the piston rings or cylinder bore to be the issue. What would he gain by doing the leakdown? There's not really a need to further diagnose.
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Old 01-11-2018, 11:52 AM   #25
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Guess that was uncalled for (it was meant to be sarcasm)....as it was probably a typo.... sorry. And exninja you are almost surely correct but this IS the innernet and proper diagnosis is always a good thing.... (maybe the valves could use a freshen as long as the head's off, ect...) --- unless he's going through the bottom.... (more sarcasm)....
[email protected]%$

Trying to help... not offend.
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