ive got twin 557s w/innercollers, 871s w/twin dominators when i run it one motor goes to 170-180 on the gauge the other shows about 140- flows plenty of water out the exhaust, run crossovers, last year it was the same side heating up maybe the gauge is bad?vacuum leak, timing?

running thermostats? maybe one is bad.

no thermostats

swap the senders...?

How are your impellers??

all of the above ,also if you have bravos, check for bravo fever, the inlet hose that runs from the gimbal helmut to the transom assy has that little nylon insert. sometimes the corrosion builds between it and the housing itself closing the inlet hose and she may run hot. also check for debri in the oil cooler on the inlet side. and chech all connections , especially on the suction side of the water system
.

good luck

fixed it, the lower unit had a nose cone welded on that was restricting flow