Overheating Mercury 200

[BlueHeron]

Two years ago, I purchased a totally rebuilt late 90's Mercury 200 outboard (with fuel injection), in Ontario, but then moved to Nova Scotia before having time to test it out. After 2 years of frustration I am still trying to get it working properly. The problem seems to be about overheating. The motor works great in the yard, but when I put the boat in the water and start to run it with any resistance at any speed other than a bare crawl, the alarm goes off. I have had it to a marine mechanic several times with no luck. He has replaced the impeller, the thermostats, and the sensors and yet the motor still overheats. Water is coming out of the engine as it runs, but apparently not enough. I am tired of wasting my money on repairs that don't work and am hoping someone will have a suggestion. Thanks.

 

[andy6374]

Re: Overheating Mercury 200

Do you have a water pressure gauge? If so what are your water pressures at various RPMs? If you don't have one, get one and it will help make diagnosing this problem easier.

Did the mech replace all the components of the pressure relief system? (poppet, diaphragm, grommet, ect....).

Are the cylinder heads warm to the touch? Or is the alarm just going off?

Other things that can cause the engine to overheat when you are sure that it is not the waterpump, tstats, and pressure relief system are:

-Clogged water passages. This could be around the cylinders and you'll have to pull the cylinder heads to take look. Or you could have a piece of old impeller stuck in the water pickup tube that runs from the base of the powerhead down the midsection. This is the pipe that is inserted into the waterpump housing. Try dropping the lower unit and stuffing a hose down the tstat holes and see if any debris falls out of the water tube.

- The grommet that connects that same water pickup tube to the powerhead can get swollen or turned over and result in inadequate water flow or blowed water flow.

- A blown head gasket can allow some exhaust gases exscape into the water jacket and cause the cooling water to boil and then you'll overheat. You'd probably see water on your spark plugs in this case.l

-Running lean will cause you to overheat.

Gave you all the info I could think of, now it's time for you to try some new things and report back.

 

[BlueHeron]

Re: Overheating Mercury 200

Do you have a water pressure gauge? If so what are your water pressures at various RPMs? If you don't have one, get one and it will help make diagnosing this problem easier.

Did the mech replace all the components of the pressure relief system? (poppet, diaphragm, grommet, ect....).

Are the cylinder heads warm to the touch? Or is the alarm just going off?

Other things that can cause the engine to overheat when you are sure that it is not the waterpump, tstats, and pressure relief system are:

-Clogged water passages. This could be around the cylinders and you'll have to pull the cylinder heads to take look. Or you could have a piece of old impeller stuck in the water pickup tube that runs from the base of the powerhead down the midsection. This is the pipe that is inserted into the waterpump housing. Try dropping the lower unit and stuffing a hose down the tstat holes and see if any debris falls out of the water tube.

- The grommet that connects that same water pickup tube to the powerhead can get swollen or turned over and result in inadequate water flow or blowed water flow.

- A blown head gasket can allow some exhaust gases exscape into the water jacket and cause the cooling water to boil and then you'll overheat. You'd probably see water on your spark plugs in this case.l

-Running lean will cause you to overheat.

Gave you all the info I could think of, now it's time for you to try some new things and report back.

Thanks for the advice. I will print it out and take it back to my mechanic to see if it helps him sort out this mess. I will let you know if I am successful in a week or so hopefully.

 

[andy6374]

Re: Overheating Mercury 200

If you have to print this out so that your mechanic has a clue.......... FIND A NEW WRENCH!

I am sure you can do it yourself too with the help of everyone here at iboats.

 

[xtraham]

Re: Overheating Mercury 200

make sure your engine is mounted properly to the boat ie: the water pickup is fully submerged in the water while on plane, A/V plate should be even with or just above the lowest part of the haul