Low compression in 1 cyl.

Discussion in 'Small Block Tech' started by partsrparts, Jan 15, 2020.

  1. partsrparts

    partsrparts Silver Level contributor

    So I decided to do a comp check on my engine just because I was curios.
    170-180 all cyl. except #2
    #2, 130 dry, 135 wet, 40% leak down, can really hear air escaping into crank case, ( with one valve cover off, removed oil cap off other and you can not only hear it but actually feel the air escaping), and you can hear it slightly in the exhaust pipe.
    The engine is a 1968 350 4bl out of a Le Sabre, It sat for probably 15yrs before I got it.
    I had a stock rebuild with low comp v6 pistons put in it and was not happy with the outcome so I took it to a performance machine shop and had it done again with 10-1 comp at zero deck, custom ground cam from TSP.
    The engine has always had a low, slightly shaky vacuum at idle with (10") and a slightly rough idle. ( it has had this with the low comp stock rebuild and the latest one. (less than 10,000mi).
    The engine runs like it always has with nothing seeming to be wrong, I wouldn't have known there was a problem if I hadn't of done the comp test.
    Could there be a crack somewhere in the head causing this? I ask this because it has always had the slightly shaky somewhat rough idle after both rebuilds.
    No water in the oil or vise versa, no bubbling in the radiator, nothing seems amiss. spark plug readings have always been inconsistant with this engine.
    Thoughts?
    Keith
     
  2. 1972Mach1

    1972Mach1 Just some M.M.O.G. guy.....

    During a leak down test, if there is air leaking into the crankcase/block, you've got a piston/ring sealing issue. Coming out the exhaust is an exhaust valve issue. See if you can get a bore scope and look down that cylinder.

    EDIT: is that 10,000 miles on the current build with the newest pistons?
     
  3. partsrparts

    partsrparts Silver Level contributor

    I was under the belief that if the wet test was only slighter better than the dry it might not be a piston/ring sealing issue, Dry 130, wet 135. I also did a wet test on the cyl next to it, #4 and got 170 dry, 175 wet.

    The current build has 8,400mi on new JE pistons.
    The Buick chassis manual states not to disassemble engine for oil consumption because of rings not sealing until at least 8,000mi. It has always used a little oil.
    The manual also states that with 180 being the highest that the lowest of 130 is acceptable, but I know that there is a problem somewhere. I have a hard time believing that the rings haven't seated yet.
    Stacked ring gaps?

    Does anyone know of a good affordable bore scope out there? there's alot to chose from but reviews are all over the place on which ones are good.
    Thanks, Keith
     
  4. 1972Mach1

    1972Mach1 Just some M.M.O.G. guy.....

    Chrome moly rings can take a while to seat, but your rings should definitely be seated by then, and it'd be very odd to have one cylinder that hadn't seated yet and 7 others had. Possibly have a compression ring installed upside down, too, but I'd be getting a look down that cylinder before I went tearing into it.
     
  5. alec296

    alec296 i need another buick

    It may be head gasket. You could pull intake and air up cylinder. Try to find air movement
     
  6. 1972Mach1

    1972Mach1 Just some M.M.O.G. guy.....

    Head gasket should have low compression/bad leakdown into the two cylinders and not just one, and air should most likely be coming out the intake or exhaust of whatever the adjacent cylinder's intake or exhaust valve is open (depending on what stroke the cylinder next to it is on). Also he said he's getting a lot of air out into the crankcase which means rings are leaking or the piston is damaged in some way. Even if a head gasket is leaking between two cylinders, the piston rings on those two cylinders should still be sealing and preventing air from entering the crankcase/block.
     
    Last edited: Jan 15, 2020
    GranSportSedan likes this.
  7. alec296

    alec296 i need another buick

    Should but not always between 2 cylinders. Especially with the only 4 bolts per cylinder, clamping force may not be as even. Although this is more common on boosted engines, what to say it didn’t leak into valley.
     

    Attached Files:

    1972Mach1 likes this.
  8. philbquick

    philbquick Founders Club Member

    I worked on a 64 Chevy 327 that had bad compression in several cylinders. Pulled the heads, valves looked great, all cylinders held gasoline. Put it back together, still ran like crap. Took it to the other shop I worked at an put it on the Sun machine and it turned out to be a cracked distributor cap. Replaced the cap, drove it about 10 miles and redid the compression check and it was good in all cylinders. An ignition problem can crud-up the valves and affect compression. Check the wire going to that cylinder, it should read less then 10K ohms per foot and replace that plug. I would also re-torque the head hot. Go from there.
     
  9. partsrparts

    partsrparts Silver Level contributor

    Does anyone know of a good bore scope for a good price? I do want to get a good look in there before I go any farther. Moving farther south next month so I don't want to tear into it yet because I don't want to move it in pieces.
    Thanks, Keith
     
  10. partsrparts

    partsrparts Silver Level contributor

    Looked in with a borescope today, no cracks, holes or chunks missing from top of piston, no scratches or gouges on cyl. walls. Took a look in a couple other cyls. and they all had a very light carbon build up except the one with the low compression, it has more of a wet carbon build up on top. All head bolts have proper torque.

    You have a good point there Andy, that could be my loss of compression, air into crankcase, oil consumption problem it's had, and the wet piston.
    It's just going to be awhile before I can tear it apart.
    Thans, Keith
     
  11. sean Buick 76

    sean Buick 76 Buick Nut

    I would re-do the leak down test on that low cylinder, you should be able to determine where you are leaking.

    1. Piston ring?
    2. Intake valve ?
    3. Exhaust valve?
     
    Mark Demko and Freakazoid like this.
  12. UNDERDOG350

    UNDERDOG350 350 Buick purestock racer

    Sounds like what I had when 1 piston broke in the middle.
     
  13. partsrparts

    partsrparts Silver Level contributor

    I want to re-do the comp and leak down tests when the engine is warm. Now that I think about it the first leak down I did where I could hear the air in the exhaust the air had pushed the piston down and had a 70% loss so I'm thinking the exhaust valve had started to open. The second one I did I put a breaker bar on the crank bolt and tied it to the frame so it wouldn't move, and ended up with 30% leak down. Didn't check the exhaust that time.
    Keith
     
  14. philbquick

    philbquick Founders Club Member

    They recommend doing a leak down test with the piston up. It's kinda tricky holding it there on an automatic.
     
  15. Mark Demko

    Mark Demko Well-Known Member

    When doing a leakdown test, remove the rocker shaft to assure none of the valves open.
     
    1973gs likes this.
  16. philbquick

    philbquick Founders Club Member

    If the piston is at TDC on compression stroke the valves will be closed.
     
  17. partsrparts

    partsrparts Silver Level contributor

    So I got a chance today to warm the engine up a bit, not a run down the highway for 20mi. type warm up but fast idle to 180* run it around the block warm. Pulled the #2 plug, (the one with 130 comp) and did another comp check, 170lb. This time all I did was pull the 1 plug, stick a screw driver down the carb to hold butterflies open, disconnect power to dist. and crank it till it stopped rising. The first time when I got 130 out of the #2 cyl. I had pulled all the plugs, removed the carb and the engine was stone cold, hadn't been started in a few weeks.
    So is a crack closing up with metal expansion when warm, or something to that effect?
     
  18. Mark Demko

    Mark Demko Well-Known Member

    Yes parts do expand a bit when warm/hot.
    I would take the "hot" reading as fact, as that's where the engine operates.
     
    MrSony and 1973gs like this.

Share This Page