TimG'
I had a problem like yours and it turned out to be a piece of wood lodged in the rubber hose going thru the transom assy. It got jambed thru the LWP on the front of thr drive. When back flushing with the drive removed , full hose pressure had the water hitting ground about a foot behind the boat. I added 150 psi air thru a T connection and tried again , now the water was hitting ground about 3 free out and my temp problem was gone

scarab22 , do you have silent choice?
could there be a problem with IT maybe?
reason im following this thread is i have the same warm on one side , hot on the other with both my 502 efi's. pulled one impeller last night and it looked fine. i also have very little water coming out of my exhaust on one side.
same situation on the other motor but very little water out the opposite side.
motors never overheat but i get steam like you.
im going to try Tim G's garden hose trick also and see what i get.

Yes, I do have silent choice and its works perfectly so I don't think thats a problem. I'm also going to try and do some back flushing , and tighten up hose clamps in case I'm sucking air.
J-Bonz, the manifolds and engine have very low hours and are run in fresh water.When I took off the exhaust manifold elbow, I checked inside, looked good.

Well, now that we're in the off season I decided to do some checking. Followed the water flow and found that my impeller was bad, even though I had lots of water out the pipes and running 180 on the temp guage.( ran one time in some shallow water, sand/gravel bottom) Also found some impeller chunks, seaweed, sticks,and a small rock all stuck at the entrance to the oil cooler!.This should cure my problem. I still think it will steam more out the starboard side though. Merc says it normal.