93 200hp mercs over heating & over revving

[hps]

recently purchased 27' cape horn with 1993 twin 200hp mercury(s),they have new water pumps and ran great on route to local island.temp stayed @140-150 on both. on return trip boat difficult to plane and starboard motor was revving up to 5,000+ rpm and sounded/felt like it was slipping. the port motor was fine staying @4000rpm +/-. i backed off the rpms and tried to plane off again,successful this time. in a matter of minutes the starboard motor overheated and i backed off again.made it home running both motors at low rpms and temps on starboard motor 170-180. at idle motors return to 140+/- degrees. can you give me an idea of what happened?

 

[jim dozier]

Re: 93 200hp mercs over heating & over revving

Did the overheating/slipping motor put out a good pee stream while it was acting up. The causes of overheating might be<br />1. the water pump, which you said was new. Verify that its still working. This wouldn't make the engine slip.<br />2. Sucking some crud causing a blockage, like mud or sand when you run in shallow water.<br />3. Could you have run into some seaweed or a plastic bag that would have caused cavitation and slippage and might have sucked air into the cooling system.<br />4. Was the motor trimmed up too much? Causing ventilation and sucking up air.<br /><br />There are a lot of reasons why and engine would overheat, impeller, thermostat, fuel mixture, blockages,engine damage, etc. but they aren't usually associated with the engine slipping.

 

[grandx]

Re: 93 200hp mercs over heating & over revving

I've nearly the same engines, 96 model 150's and they consistantly run 170 - 180 F at higher throttle maybe even up to 190 at times, and generally around 140-150 at idle. I've beat my brains out trying to get them to run cooler. Replaced entire pumps, impellers numerous times (only to find they were in perfect condition), thermostats (same thing, they tested good). Only thing I haven't messed with is the poppit valves.. My Clymer manual states something like "should never run hotter than 20 degrees more than the thermostat rating", well my new Merc service manual does not say this, it says the audible alarm should sound at 240 F and turn off at 210 F. If the engine should not exceed 165, why in the world would the alarm not sound till 240 F. I've been told to run em and quit looking at the temp gages (which I've checked and are accurate) and I'm beginning to lean this way myself. Seems odd that both engines do the same thing if something is broken. Please tell us more JIM...

 

[jim dozier]

Re: 93 200hp mercs over heating & over revving

It gonna take a bigger brain than me to get to the bottom of this. Overheating is a common issue on this forum. The puzzler here is the "slipping" issue and whether it is related to the overheating or just coincidental. Slipping is usually due to a spun prop hub or otherwise damaged prop or ventilation from something in the trim or hull bottom or trash interfering with flow and prop bite. Trying to connect the two is well, what I said before.<br />Incidentally, my Suzuki DT85 runs at 190-200 F. at WOT and about 140-170 at lower speeds.

 

[hps]

Re: 93 200hp mercs over heating & over revving

sorry for not relaying water was coming out the pee hole with force,also we had just had to rock it off a sand bar prior to leaving the island so that may explain the slippage. appreciate all the info and am relieved to hear that temps over 170 are something the motor can live with. i plan to take it out this weekend and try it again without the sand bar,if y'all can think of anything else it may be or something i can check please let me know.