Thread: blow by

  1. #1

    Cool blow by

    Hello all,

    I have a 90 wrangle with a 2.5 . I have been seeing some oil on top of my engine as of late. I replaced the cpv valve and the grummit. But i still have oil comming out . Can some one tell me what i should do next.

  2. #2

    Re: blow by

    Clean the engine with some degreaser to start with.Then start it up and look for the leak.Could be you valve cover casket or you oil cap itself.

  3. #3
    rubijeepchick04's Avatar
    rubijeepchick04 Guest

    Re: blow by

    I have a 4cyl. Iron Duke motor in my 80 CJ. It has bad blow by. Although it can come out a number of places, I found that mine was coming from where the dipstick goes into the block. I used some of that high heat silicone and gouped it up around that area. Seems to have stopped most of it but I still get a little every now and then. I have also noticed that if I let the engine warm up before I start driving it I get a lot less. This engine has been rebuilt but the cylinder walls were scratched when it was torn down. Took it to a machine shop and they couldn't do anything with it cause its been bored out to the max so its basically a throw away engine now. I'm just getting by with it until I can do an engine swap; hopefully a Chevy 350 is in my CJ's future.

  4. #4

    Re: blow by


  5. Re: blow by

    Poor crankcase vacuum can also aid in leaks. I would also check and make sure all hoses are tight, not dry rotted and such. It's more than likely isn't the problem but good to make sure.
    supercharged 4.7HO. Cams, bigger injectors, Meth injections, F&B throttle body, SCT custom tune, 180* T stat, AUX electric fan, True dual exhaust, shift kit, stall convertor, 275 60 R17 rubber.

    New motor: Ross Racing ceramic coated pistons, Billet H beam con rods, forged crank, larger supercharger, One off sheet metal intake, Ported big valve race prepped heads, cams....

  6. #6

    Re: blow by

    term clarification.

    "blow by" is when the rings are worn to excessive gap or stuck in the ring grooves not allowing them to expand against the cylinder walls, or cylinder walls are scored/gouged allowing compression pressures to "blow by the rings" into the crankcase...oil becomes prematurely black with carbon abrasive and piston skirts show signs of carbon build-up.

    oil on the external engine is a leak...not "blow by"... per se.
    it can come from anything from a clogged PCV system to a faulty gasket, to a cracked block or head.

    in the first case, it's compression pressures, in the 2nd, it's downward piston pressure. a compression test can show low compression indicating "blow by", if the numbers are good it's a breather problem... if the internal engine shows carbon it can be a blocked passage. since the EPA got involved, they generally use the intake vacuum to draw a slight vacuum on the crankcase.

    you can squirt a teaspoon of oil on the pistons and if the compression numbers improve, it suggests rings not sealing

    "thing-a-ma-jig" and "doo-hicky" are not exceptable in any case to mechanics...there's a universal extra charge for mis-use of terms by the illiterate... and we have a zero tolerance policy.

    if you have compression pressures in the crankcase and actual "blow-by, then you need to address the rings and mic the piston skirts/cylinder bores...there's no other fix.

    mystic marvelous clearance clogger additives are not acceptable unless your intent is to pass the problem off on a sucker... in which case you should have a conscience problem
    Last edited by yo momma yj; 08-17-2009 at 09:24 AM.

  7. #7

    Re: blow by

    Clogged breather tube will mess it up too!

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