Merc 74 Inboard Overheating

I have the port engine showing

I have the port engine showing 190 at idle and 180 at 1500RPM's. This is the port engine the starboard engine runs around 165. The system is closed coolingThe following has been done:
- Replaced thermosat with 143
- Rebuilt raw water pump.
- Raw water exhaust elbows are new.
- Have changed gauges and same happens.
- Water temp. sending unit has been replaced
- It seems to be pumping the same amount of water as other motor. The motors have 400 hrs on them.
Once in awhile the starter will not engage and I get a clicking noise from the engine circuit breaker reset. Any thoughts?

First of all (here I go ag

First of all (here I go again!) buy a laser-pointed temp gage (under 50 bucks) and see what's really happening down there. Sweep the entire engine and exhaust system to see if/ where/ what is getting hot. You might be surprised by the results.

Jeff

"Jeff,

Thanks. i just did t

"Jeff,

Thanks. i just did that today and they both are within 1-2 degrees of each other. I did all the gauage checks including swiching them around. Still the same diffence. Ground on the battery to engine looks good as well as battery Any other thoughts.

Thanks,

Mike"

"Mike: use a 16 ga. jumper wi

"Mike: use a 16 ga. jumper wire from the sender to the gauge. First remove the wire connector at the sender and at the gauge. If other wires are attached to the gauge, leave them on during the swap. If the gauge reading is good with the jumper, you have found a wiring problem. If there is no change, then use a jumper on the ground side. With multiple wires connected to the + post on the gauge, you can remove them one at a time to see if that corrects the reading.

Could also be a corroded connector at the sender. You have 2 engines to compare the wiring continuity with a digital ohmeter - get a cheap $8 one from Radio Shack. Disconnect all batteries first. Check the resistance of the wire on each engine from the sender wire (disconnected) to the disconnected engine barrel plug pin. If they are the same check it from the male barrel plug to the disconnected gauge wire. Locating where the bad section of wire on the hotter engine will be simple. If the engine wire is the problem, first try cleaning the engine temp sender with a 3M scuff pad and replacing the connector. Look for dark copper wire when you strip it to make the connection. If it is very dark copper and not like an average penney, that is most likely your culprit. Polish the stripped wire with the scuff pad before adding the connector. Also look at both ends of the barrel plug for dark or bluegreen corrosion on the pins and between two or more of them. That condition can provide the short that is putting more restance to the gauge causing it to read incorrectly. I saw this occur in the military with SAM missle radars in high moisture environments--some are hard to find. The very 1st rule when troubleshooting is LOOK, LOOK & LOOK for the obvious. Look at each wire and not the harness--See each tree not just the forest. Pull on the wires and feel them with your fingers while tracing along it. You may find a scuffed peice of wire insulation that allowed corrosion to develop or a pinch point. If the problem is between the engine and gauge, it is easier to just replace the one wire--solder all the splices and seal with liquid electric tape and shrink tube over it.

I have replaced three harness assemblies on my 87 Baretta due to old corroded wiring. Made a difference. Boats built with tinned wiring usually don't have these problems but that raises the cost.

Good luck, Guy"