LNF bad piston/ring. What should I do next?
#1
New Member
Thread Starter
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.
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.
#4
New Member
Thread Starter
#6
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
I wouldn't do that.
but you could
#7
Senior Member
iTrader: (1)
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.
#9
Senior Member
iTrader: (6)
..
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
#10
New Member
Thread Starter
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...
#11
Senior Member
iTrader: (3)
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.
#12
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 $100 plus gaskets and fluids etc...
#13
New Member
Thread Starter
Originally Posted by HGT
Top ring land bits can't get past the piston. There in you cat now.
Last edited by Natomic; 01-05-2018 at 02:36 PM. Reason: fergot sumthin'
#15
What cat? haha no state inspections where i'm at. But really I wouldn't think piston pieces broke off because last time I ran the car it was ok, just sounded a little off at idle and showed lots of misfires but none under acceleration. But who knows without taking it apart. I'll check for pieces in my muffler haha
#16
Senior Member
I was in your shoes about a month ago. Looked into rebuilding but decided it wasn't worth it. I am in the midst of swapping in an LHU from a rearended Buick Regal.
#17
Nobody here has mentioned a leakdown test.... yet. Everybody wants to put all kinds of work in w/o proper diagnosis. May be simple as a bad valve or head gasket... We will never know without a leakdown test...C'mon. Baby steps here...
#19
So..... what? Where is the PSO (pressure squirting out)? Through the cyl. bore? Through the head gasket? Through a valve (can you hear air coming from the intake or exhaust)? Where? THAT tells you what to fix. Not a "wet test".
#22
I don't give a s@#$ 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..
#24
Senior Member
iTrader: (1)
This. Seriously.
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.
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.
#25
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)....
POW!?!@%$
Trying to help... not offend.
POW!?!@%$
Trying to help... not offend.