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  1. #1
    Join Date
    Mar 2013
    Location
    Lake Murray, SC
    Posts
    65

    Default Overheating Issue

    Boat: 1992 Supra Comp w/ PCM 351 Pro Boss converted to electronic distributor ignition (530 hrs)

    My boat is having an overheating issue. When I first take it out, it runs absolutely fine for a couple hours. After a while of driving around, it will start to stutter and hesitate while in forward and then soon after it will just die altogether. When this happened, I lifted up the doghouse engine box and found that the engine as a whole was putting out some serious heat. I touched the part of the engine where the fresh water is pumped into (hose from top of water pump into engine) and it was hot. I can start the boat and run it in neutral just fine and I can actually feel that part of the engine slowly cooling down. After sitting in neutral for a while I can then reengage it into forward and it will work for a while, and then the cycle repeats.

    This happened to me last weekend, and I needed to replace the impeller anyways for the season so i went ahead and did that. Yes, I made sure to reinstall the water pump correctly and not upside down. With the new impeller, same problems.

    Could this be a faulty thermostat? http://www.skidim.com/prodinfo.asp?number=R026002

    At the beginning of the day my temp gauge will sit at the bottom of the gauge (120) and then will sit around 150-180 after the boat is warm and I am driving around. The gauge will stay in this range even when I have this problem and i'm sure it much warmer than that. I'm guessing that this means that either my sending unit or gauge is bad.

    Has anybody had issues like this or suggestions? Any input will be greatly appreciated.

  2. #2
    Join Date
    Apr 2013
    Location
    KC, MO
    Posts
    685

    Default

    When you replaced the impeller, were any of the vains missing? Have you ever had missing vains from an impeller? If so, check around the thermostat and make sure there's nothing blocking the flow of the water, that's where they'll usually get stuck. Also check to make sure there's not any trash or weeds, or other blockage from your water pickup all the way to the impeller housing (and anything else between there). I'm not familiar enough with the older supras to know all the components, but there might be a transmission cooler along the way. Pull all the tubes out and make sure there isn't any blockage.

    I wouldn't just assume the guages are bad. 150-180 is pretty normal operating range for the 351 (I think they have a 160* t-stat in them). These motors will always feel hot. They're sitting in a tiny little doghouse with very little airflow.
    -Mike
    2006 Supra Sunsport 20V

  3. #3
    Join Date
    Mar 2013
    Location
    Lake Murray, SC
    Posts
    65

    Default

    None of the vains were missing from the old impeller. Thanks, I'll pull all the hoses and check for blockage.

  4. #4
    Join Date
    Sep 2004
    Location
    Hudson, WI
    Posts
    2,651

    Default

    First things first -- we need to find out if you're actually over heating. The gauges rarely go bad and the fact that you're this close to "normal" operation on it makes me think that it's probably OK. A sender is a cheap way to double-check or you could get one of those infrared/laser thermometers and double-check.

    I suspect that you are not overheating at all and that your hesitation/hard starting issue is something else.

    Are you using last seasons gas? Have you checked/changed your fuel filter?
    Former owner of a 1987 Supra Saltare. Current owner of a Malibu 23LSV.

  5. #5
    Join Date
    Mar 2013
    Location
    Lake Murray, SC
    Posts
    65

    Default

    I plan on picking up an infared thermometer from harbor freight sometime this week.

    Just to clarify, I'm not having hard-starting issues. I can start the boat fine and run it without engaging the transmission all day long. It's just after continuous use in forward that this issue turns up.

    I'm not using last seasons gas and I have checked the fuel filter. There was no debris or junk in the filter and the boat runs fine in neutral so I didn't think the fuel filter could be the culprit.

  6. #6
    Join Date
    Apr 2013
    Location
    KC, MO
    Posts
    685

    Default

    Does it die as soon as you put it in gear, or will it idle in gear as well and then die as you give it more throttle? Can you rev the motor while it's in neutral without it dying or will it sputter off?
    -Mike
    2006 Supra Sunsport 20V

  7. #7
    Join Date
    Apr 2013
    Location
    KC, MO
    Posts
    685

    Default

    Here's some more things to check from another forum I'm on. This is related to an MC with the pcm 351.

    1. Vacuum leaks around carb base. If air is leaking underneath the carb, you are going to be lean no matter what you do. Test this by spraying some carb cleaner around the vaccuum lines.
    2. Point gap check minimum (prefer to go with dwell as it is substantially more accurate) Replace if necessary....bad points surface will look pitted.
    3. Condition of distributor cap & rotor. If any crud on contacts...scrape it off. Replace if necessary.
    4. Verify timing.
    5. Verify correct spark plugs, condition and correct gap. Replace if necessary.
    6. The balance between mixture screws and idle speed screw is easily thrown out of whack. If the idle mixture screws are turned in too far, not enough fuel can be provided for adequate idle speed and the idle speed screw needs to be turned out far enough to open the throttle blades to the point where fuel is provided by the main operating circuit. If the idle screws are out too far, it is hard to get decent idle quality as it will be rich no mater what you do. Screwing them in all the way, then backing out 1 1/2 turns is where to start. Small adjustments from this point.
    Last edited by Zim; 05-13-2013 at 12:11 PM.
    -Mike
    2006 Supra Sunsport 20V

  8. #8
    Join Date
    Apr 2013
    Location
    KC, MO
    Posts
    685

    Default

    Some other things to verify as well...

    Pull the flame arrestor off and make sure the carb is dumping fuel when you put it into gear.
    Inspect your fuel line - Ethanol eats away at these and they will eventually collapse and cause blockage. Has your fuel line ever been replaced?
    Is your ignition system in verified good working condition? How old is the coil? Plugs? Wires? You said you have electric ignition, which setup do you have?
    -Mike
    2006 Supra Sunsport 20V

  9. #9
    Join Date
    Mar 2013
    Location
    Lake Murray, SC
    Posts
    65

    Default

    It will die as long as its in gear, regardless of what level of throttle i'm at. Yes, I can rev it up when I lock out the transmission.

    The electronic ignition/distributor is brand new. I bought the kit from skidim (http://www.skidim.com/prodinfo.asp?number=RK107025A) and put it on last month. I had a marine mechanic install it and verify the timing.

    I thought about the spark plugs, so I replaced those this weekend as well (OEM from skidim), still having the same problem but they did need to be replaced.

    The fuel line could need replacing, I could only inspect the outside of the fuel line and there was no debris from degrading fuel lines inside the fuel filter (tank to filter line). Is there any good particular way to inspect it?

    I'll adjust the idle and mixture screws as they definitely have the potetial to be out of whack and see if that helps. I know the carb is dumping fuel when in neutral, however I haven't checked after I put it in gear.

    I just assumed something was overheating since the boat will work fine from cold to 2-3 hours and then it'll act up, then i'll just let it sit for a while and it will start working again and then a cycle begins.

  10. #10
    Join Date
    Apr 2013
    Location
    KC, MO
    Posts
    685

    Default

    It doesn't sound like overheating just because you're in the normal operating range of engine temps, and you can run it so long before the problem persists. To me that sounds like fuel delivery issues since eit runs so well at all times besides under any sort of load. It makes sense that it will run for a while since any debris/buildup in the fuel line would settle, then get jarred around and shaken loose when in use, finally creating enough blockage to restrict fuel flow enough to stall your boat. If you don't know when the fuel line was replaced, you might as well just do that anyway. A 20+ year old boat needs it anyway. Try that and see where it gets you.
    -Mike
    2006 Supra Sunsport 20V

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