98 Fountain. Twin 502 mpi. Both temp gauges go up to 210 and the red light comes on. Exhaust manifolds do not feel super hot, I can put my hand on them ... maybe 95 degrees. Impellors are new. New temp senders. New 160 thermostats. Water hose at thermostat housing ... small hose cool, large one warm as expected. Temp readings the same with gauge connected to sending unit, even with a new gauge. Any ideas? Would a 140 thermostat help? I have the ohm readings: 488 ohms at sender with engine cold, about 88 ohms with engines warmed up and reading 210 on gauges.

My 92 502 Mag runs a consistant 150. What does yours normally run at?

could be debris in the lines if the impeller came apart before

This is what the Mercruiser service manual says the sending unit resistance should be. Maybe the proper gaskets (with rivets) were not used on the thermostat housing?

Dave

Thanks for the input, guys. I put a 140 degree thermostat in this am. Now the temp reads and seems to hold at 179 ... much better. I think temp senders may not be the right resistance for the Gaffrig gauges, so the gauges read high. I'm going to test it with an infared on Monday just to be sure. Thanks, Dave.

Those MPI motors are supposed to have a 170* theristat. If the computer doesn't see 170* temp. it will run it full rich. Do you have a scanner you could hook up? It will tell you what the computer is seeing for temps.

I have same boat and motars, also had same problem when purchased boat, reverse flushing wasn't good enough, had to take hoses off all coolers reverse flush and move my finger around other end to get peices of old impellor out.Put hoses back on and temp., read about 170 in beginning of season and 180 in the middle of season as water warms up.With injection I am told there suppose to run that hot for the sencors to function properly.

JMPH

Thanks guys. There was not debris in the cooler lines or anywhere. Infared showed engines are running proper temp, so its all ok now.

I have a new 496 HO and had to switch water temp. sender to get the right readings on my gauge. I wonder if this will affect the information to the computer, or does it get the temp. info from another sender. I´m a little bit worried that the motor won´t get the right fuel mixture all the time.

The engine does not get its info from the water temp sender (assuming they are separate on your engine). If it runs a little cool, it will just run a little rich. That's not going to hurt. Plus, a little cooler is probably better for the life of the engine. You could always check the actual engine temp with an infa-red temp gun.