1998 DT-100 overheat ??

[corvette]

Bought a 1998 pontoon with 98 DT-100 on it. The engine started and idled fine for a few minutes then the tell tale would stop and turn to steam then overheat light and buzzer would go off. I changed water pump and stats and it still would overheat. After reading post on this subject I corrected the overheating by changing the lower unit exhaust gasket or grommet. This curred the weak tell tale and overheat condition while running in a large tub. The temp would not exceed 117 degrees around cylinders and water jackets. I was eager to get pontoon in water. Yesterday I run it about 5 minutes to 10 minutes Was running at 4000rpm then I run it up to 5300 and back down to 4000. About 4500rpm I thought I would try to trim it up a little. This raised the RPM and slowed it down then the overtempt light cam on and the engine lost RPM. Not sure why. After I got back to dock which was near. I took cover off and felt the block. It did not seem hot at all to me . Yes it was very warm but not hot. I decided to leave well enough alone for now. I went back to lake today with thermometer so I could check the temp of block for sure. I also decided not to trim the motor at all and run it full down. Motor started soon as turned key. Smoked like a two stroke should at idle. I brought rpm up to 3600 rpms and ran motor steady for 20-25 minutes before the horn sounded one time ( no lights came on) however engine begin to lose rpm. I shut it down immediately and checked the temp all over the motor. could not find a temp over 120. Without a doubt it was NOT overheated. after about 5 minutes I restated it and drove it back to dock about 3000rpm a very short distance. OK, My next concern is oil. I filled the oil tank up but it does not seem like it has gone down much. Im not sure how much oil the engine uses. I ran the motor 20-25 minutes. If was low on oil would it not overheat as well? Can the oil system work at idle but not at a higher rpm? Im thinking of adding oil to the gas to see if it does any better. If the motor was starving for oil would it not burn it up after 20-25 minutes? Would adding 50-1 oil to gas be ok? I just hope its not too late after running motor today. Any advice is very welcome!!!

[Solarman]

If not sure add oil to gas. If oiling system is dirty (old thick oil in lines, and flow switch sensor) flow of oil could be affecting flow sensor causing the problem? Better safe than sorry, it may smoke more than normal but extra oil in motor won't hurt. Plus you can test oil flow sensor. Yes, oil flow sensor will throw motor into safe mode, but should send a light code per manual.

You can clean/flush oil system, and that may make system work again, but if oil switch sensor doesn't read properly (per manual) then should change it. Make sure you bleed oil system completely and know oil is being used out of tank (mark tank oil level), before going back to just injection system. Motor will smoke more, and you might need to clean plugs frequently, but won't hurt motor. Also, extra oil could affect rpms when running.

Yes, the exhaust system can interfere with water pump flow if exhaust pressures blow into water passage (good call on finding that).

Good luck, post back when able to let us know what you find out.

[corvette]

OK, today I did add oil to the gas. Also checked compression. It was same as before running motor so it appears no damage. I ran the motor at 3800RPM trim down for 30 solid minutes today. It ran fine no issues or shut downs. Motor does run rough in gear at low throttle just above idle but does smooth out when RPM increase above 2000-2500. One thing I notice is that the throttle was that if I move it just a little it seems to want to go higher its REALLY sensitive. I am waiting for the oil tank to show its working. Any idea how long motor needs to run to show a decrease in oil in the tank? Even with the oil in gas I really cant tell much difference in the smoking. Seems a little heavier at idle but everywhere else seems normal. does the sensor actually affect the oil flow to the motor or just relay info to lights. I have not oil light when key is turned on. I assume it should come on like the temp light does at key on. Would a bad sensor affect the oil light? Going out again today to change plugs in motor and run it some more. Thanks for the reply.