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Thread: Pistons in otherwise stock lower end

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    Pistons in otherwise stock lower end

    Hi,

    My 91 VR4 has been smoking on deceleration and after eliminating pretty much everything else I'm 99% sure that it's got a broken oil ring. I've been trying to convince myself otherwise (replaced the heads and inspected the cylinder walls at the time which look absolutely perfect) as well as replacing the turbo's (they had 115k miles on them and a fair amount of shaft play so they needed replaced anyway), but it's still smoking on decel and a little at idle.

    I'm looking at hopefully just replacing the piston's and rings and honing the block, but not actually boring it since everything else looks great (the motor still as 165 psi compression (+/- 5 psi depending on cylinder).

    I'm wondering if you guys think I'll be ok just honing it and replacing the piston's and rings and if so what you would consider to be the best piston for an otherwise stock lower end.

    I was looking at the CP piston's on ips's website and they look nice do you know if they produce a stock size that will fit the stock bore?

    Thanks,
    Andy
    Mods ------------------------- 1991 Mitsubishi 3000gt VR4---------------My website
    If you live in the Delaware, Maryland, or Virginia area check out our 3si chapter forum

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    1st ever COTM and COTY verified Feedback Score 9 (100%) green-lantern's Avatar
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    Sounds like a bad PCV valve or you have the rear vent plugged off or something.

    Quote Originally Posted by HilbillyHomeboy View Post
    I bet she smells of old mustard and sawdust.
    Jeremy

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    Quote Originally Posted by green-lantern View Post
    Sounds like a bad PCV valve or you have the rear vent plugged off or something.
    I don't think it's the PCV valves as I've had 3 different ones on over the last couple of months thinking that there was a very good chance that might be the case!

    The rear vent might very well be a problem though because I'm not exactly sure what you're talking about, haha. Do you have any pictures?

    Also one other thing to add to the equation all of the smoke (at least at idle) is coming from the rear bank. I've ran the car with the downpipe off the engine and all of the front bank is completely clean, all of the smoke is coming out of the rear!
    Last edited by andyvr4; 07-24-2011 at 01:11 PM.

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    1st ever COTM and COTY verified Feedback Score 9 (100%) green-lantern's Avatar
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    Quote Originally Posted by andyvr4 View Post
    I don't think it's the PCV valves as I've had 3 different ones on over the last couple of months thinking that there was a very good chance that might be the case!

    The rear vent might very well be a problem though because I'm not exactly sure what you're talking about, haha. Do you have any pictures?

    Also one other thing to add to the equation all of the smoke (at least at idle) is coming from the rear bank. I've ran the car with the downpipe off the engine and all of the front bank is completely clean, all of the smoke is coming out of the rear!
    The rear vent is the hose that goes from the rear passenger side head to the bubble. I've seen people plug that off before or put some sort of obstruction in there (a small Krank Vent can do it). If you want to eliminate the possibility of gasses building up in the crankcase then pull the hose off on the drivers side that connects the heads together and take it for a run. If the smoke goes away you have some kind of blow by issue.

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    Quote Originally Posted by green-lantern View Post
    The rear vent is the hose that goes from the rear passenger side head to the bubble. I've seen people plug that off before or put some sort of obstruction in there (a small Krank Vent can do it). If you want to eliminate the possibility of gasses building up in the crankcase then pull the hose off on the drivers side that connects the heads together and take it for a run. If the smoke goes away you have some kind of blow by issue.
    I guess I didn't know what it was called, haha. I actually haven't even had that hose or the one between the heads connected through out the testing to prevent crankcase pressure buildup.

    I also went ahead and removed the pcv valve completely just to eliminate it out of the equation. The car seems to be smoking just the same with these removed as before.

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    1st ever COTM and COTY verified Feedback Score 9 (100%) green-lantern's Avatar
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    Quote Originally Posted by andyvr4 View Post
    I also went ahead and removed the pcv valve completely just to eliminate it out of the equation. The car seems to be smoking just the same with these removed as before.
    When you removed the PCV valve what did you do with the hose that goes to the lower intake?

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    Forum User Feedback Score 0 JasonY's Avatar
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    You would be fine, stock rods are good to 600tq reliably.

    Probably would want to get a forged crank in there too if you have it apart.

    Jason

    Silverstone Metallic 94 R2 FD and a Silver 2015 F250 Lariat Ultimate CCSB 6.7PSD
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    1st ever COTM and COTY verified Feedback Score 9 (100%) green-lantern's Avatar
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    When you pulled the heads did you check the combustion chambers to see if they looked darker or had oil on them? What about the exhaust manifold? You should have seen oil or dark soot if one cylinder was burning oil.

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    Quote Originally Posted by green-lantern View Post
    When you removed the PCV valve what did you do with the hose that goes to the lower intake?
    I put a cap on it to block it off so that no air could get pulled through.

    Quote Originally Posted by JasonY View Post
    You would be fine, stock rods are good to 600tq reliably.

    Probably would want to get a forged crank in there too if you have it apart.

    Jason
    I'll definitely consider this, but I'm not sure how much I gain. I'm never really planning on more than around 15 psi of boost on the 16t turbo's, do you think that I gain enough to replace it?

    Quote Originally Posted by green-lantern View Post
    When you pulled the heads did you check the combustion chambers to see if they looked darker or had oil on them? What about the exhaust manifold? You should have seen oil or dark soot if one cylinder was burning oil.
    I did look at them and all 6 looked fairly normal (no discoloration), there were slight vertical scratch marks in the walls, but they where small enough that you couldn't feel them with your finger nails. Nothing that I would consider abnormal for a 115k miles motor.


    I also just ran another compression test to see if anything's changed since I hadn't ran one in around 25 miles and the results came back like below.

    1=162
    2=175
    3=165
    4=165
    5=167
    6=163

    The 175 one is the only one that stands out, but only because it's compression actually seems high. These numbers were all taken on a completely cold engine with nothing added to the cylinders (I.E. oil through the spark plug hole).

    I'll be glad for any other information that you guys could give me!

    Thanks for all the help

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    Forum User Feedback Score 0 JasonY's Avatar
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    Cast cranks are a funny thing, some break in half at stock power, some people run the shit out of their cars that dont even know they have one, without failure.

    Its like a piece of mind thing and if you can get a good used one, not that hard of a hit in the wallet.

    Jason

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