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  1. #1

    Default Omc cobra, mercruiser 5.7 problems

    I have been chasing a running issue with this 5.7 "vortec" since I bought the boat. It has turned into more issues. I will try to provide everything done since day one. Looking for others thoughts, opinions, insight.
    Bought boat and "gone through" engine, out of the boat, last year. Set valves. Checked compression.
    Installed mercruiser 350 vortec. No spark.
    Installed new Pertronix ignitor and flamethrower coil. Added electric fuel pump.
    Hard to start( fires for a few seconds and dies), but once warm will idle and start easily. Has quadrajet carb.
    Took to lake couples times. Engine would reach 4000+ rpm, then back itself down to around 2500-3000.
    Checked valves while engine running.Looked at head castings and found, they are not vortec heads but rather a 64cc(if I remember right) chamber, older heads with center bolt valve covers. Used boat in this condition for a few more outings until stored for winter.
    This spring, installed higher GPM carter fuel pump. Took to lake and reached 4000+ rpm without ever backing itself down.

    Found only 4-9 volts at positive terminal on coil with ignition on. Was able to pull 1-3 plugs wires off cap without a difference to engine rpm.(specific 3, not any 3). Also found adding jumper wire from Batt pos. Terminal to pos. Terminal on coil smoothed out rough idle somewhat.
    Replaced spark plugs with autolites for vehicle(they didn't have the marine ones). Took to water and with 4 hours of run time, engine lost power at 3500rpm. Not quite a backfire, but was making noise through carb and possibly exhaust. Found one spark plug had no center electrode left. Did not look like electrode burned off.
    Put old plug in when returned to home. Tested with water muffs and engine gave over 4000 rpm and sounded healthy.(no load) Still rough idle and starting.
    Added relay so coil could receive full battery voltage. Took to water. Boat reached wot a couple times. Later in the day, would not go over 2600rpm without engine sounding bad. Backfiring, noise in carb. Brought home.
    Yesterday checked compression. 5 cylinders were 165. Other 3 were 130, 115, and 0.
    So after reading my novel, thoughts? Valves? Gaskets? Timing? Issues with spark? Maybe more than one problem? Thanks

  2. #2
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
    Posts
    5,147

    Default Re: Omc cobra, mercruiser 5.7 problems

    Found one spark plug had no center electrode left.
    Yesterday checked compression. 5 cylinders were 165. Other 3 were 130, 115, and 0.
    Ayuh,.... My guess is, it's been runnin' lean the whole time, 'n you've detonated it to dead,....
    Any Grease is Better,... Than No grease at All....

  3. #3

    Default Re: Omc cobra, mercruiser 5.7 problems

    Thanks for the response. I have brought engine to higher rpm and checked plugs that appeared to be black/brown, but still very possible. I do not like quadrajet carbs. The engine does seem to run rich at idle. when its died, you can smell the gas fumes. Either way, 0 psi is not a good sign, checked valves by sight to see if there was something obvious. The electrode of spark plug that disappeared happened to of been in cylinder with 0 psi. Engines ready to pull and will do in the near future, expecting the worst. Does anyone think i have more than one problem occurring? i should say, the boat got on the trailer under its own power this last time. so it does run, just continued to idle rough and couldnt give it any high rpm. maybe theres been a hole in a piston for some time now

  4. #4
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
    Posts
    5,147

    Default Re: Omc cobra, mercruiser 5.7 problems

    maybe theres been a hole in a piston for some time now
    Ayuh,.... Ya won't know til ya pull the head,...

    Were ya gettin' Massive crank case pressure, lotsa fumes outa the vent tubes,..??

    If not, the zero compression is probably a badly burnt valve,....

    Carbs, all of 'em have several circuits or actions,...
    idle, mid-range, wot, as well as choke, 'n accelerator pump,...

    The color of the spark plugs only reflects the last few minutes of operation,....
    Wot spark plug checks, ya run Wot for a few minutes, 'n chop the throttle as quickly as feasible, 'n shut it off,....
    Any Grease is Better,... Than No grease at All....

  5. #5

    Default Re: Omc cobra, mercruiser 5.7 problems

    No excessive pressure from case. Yes ive checked plugs that way. It does make sense pistons could be heating up at high throttle. expanding and increasing friction, causing a rpm reduction. I dont know if that idea could last very long but engine has plenty of power, and always starts and runs the same since purchased, after everything thats happened. Other than loss of high rpm. Black plugs could have been from trailing fuel entering chamber after spark was shut off, i dont know. last year i believed fuel pump was starving engine at high rpm which most likely aided in the issues occurring currently. This is a 28' cruiser boat set up for salmon fishing. Only reason to go wot has been for test purposes, 10 seconds at a time. appreciate your help bondo, anxious to tear into it. What causes an electrode for disappear from a plug? have never seen that before. it looks almost if it broke out, but may have melted. I ruled on a faulty plug since it only took 4 hours and the old plug has lasted after incident in same chamber. But i may be wrong

  6. #6
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
    Posts
    5,147

    Default Re: Omc cobra, mercruiser 5.7 problems

    What causes an electrode for disappear from a plug? have never seen that before.
    Ayuh,.... A Bad case of Detonation,....
    Any Grease is Better,... Than No grease at All....

  7. #7

    Default Re: Omc cobra, mercruiser 5.7 problems

    Well so far the side with 0psi, the head has two similar intake valves, and two different. 3 similar exhaust and one different. The one with 0 psi, is cupped in, to where it's not seating correctly. Giving 0 psi. Another cylinder is leaking minimally into the other at gasket explaining 115psi. Cylinder walls look perfect and pistons good. Can see ring gap and looks within spec. How the intake valve cupped inwards? No idea

  8. #8

    Default Re: Omc cobra, mercruiser 5.7 problems

    And I found flat tappet lifters...so there's that. Unless they make single roller lifters without needing the cross brace to the other.

  9. #9
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
    Posts
    5,147

    Default Re: Omc cobra, mercruiser 5.7 problems

    this 5.7 "vortec"
    Ayuh,.... So are the heads Vortec,..?? are they salvageable,..??

    Sounds like the rest of the motor is earlier,...

    Still sounds like massive detonation from a lean condition,...

    Ya might consider a long block crate motor replacement,....

    'n, ya, they make retro-fit roller lifters,... check the better hot rod parts distributors,...
    Any Grease is Better,... Than No grease at All....

  10. #10

    Default Re: Omc cobra, mercruiser 5.7 problems

    Based on head #s- 14096217. 87+ 65cc 1.94 & 1.5 valves, centerbolt. the engine block number does show it to be vortec.

    I think im understanding what the guy did. this 88-89 boat, which i dont think vortecs were around then, had a frozen block. He said he bought this Vortec mercruiser from a guy and went through it. I think he just bought the block or block and lower assembly, and put the original heads on it. this might explain the one intake valve being dished in so bad. Water may have been in the cylinder during freeze?? And either stopped seating when ive had it or i never did compression test that cylinder when purchased. Im wondering if he put the original cam/ lifters in this vortec block. im not convinced it has a hydraulic roller cam. I didnt pull a lifter out, but they sure look like flat tappets ive installed on older engines.
    Its hard to explain to someone without them seeing for themselves.

    Basically i have a red headed stepchild of an engine. Vortec block, older but fairly decent heads, and possibly older cam and crank assembly.

    As far as are the heads salvagable, not without some time. ive found a couple cracks around the spark plug hole and another on the other head. all on the outside. They may never spread, or they may one day, one month, one year from now. Assuming they have been like this since I got it. And engine hasn't seen over 180 185 deg.

    Looking at getting other heads, or buying valve and trying to see what can be done with cracks. And also have looked into 350 long blocks and 350 vortec short blocks.

    After all of this, i agree something else is going on. could be lean on top end, could be an erratic pertronix igntor. What has never changed is a rough idle, hard cold starting, smooth running above idle, and plenty of power.

  11. #11

    Default Re: Omc cobra, mercruiser 5.7 problems

    'n, ya, they make retro-fit roller lifters,... check the better hot rod parts distributors,...
    Typing in retro fit roller lifters, these are not. they dont have that brace to keep the pair from moving

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