Overheating at idle
 

My sbc with Alpha outdrive overheats at idle.
I replaced the impeller and housing in the outdrive, and it still overheats, I then did a flow test per Merc manual and it flowed over 4 qt's at 1000 rpm-- ok per manual.
I tried 3 different thermostats and then checked them in 140 degree water to make sure they opened ok, I put the thermostat in that opened the most at 150. Still overheats, I took the waterpump off and checked the impeller it's ok.
If I run at 1500 plus rpm it does not over heat, If im on plane it reads 150 on guage but when I idle down the temp rises.
I'm thinking about getting a high volume water pump will it give me other problems if I use one?

Thank You Scott

Scott
I had the same problem with the same drive a few years ago."mechanic" had serviced the unit including a new water pump he refused to believe there was a problem.When I suggested that he might have forgotten to key the new impeller he looked at me as if I had been smoking the same stuff he was known to partake in regularly :D
Anyways he snuck up to the boat one day ,split the unit and took it away.Later after re-installation the problem was gone and he never said what it was.Needless to say I have'nt used him for a long time :crazy:

Sometimes the rubber holding the tube from the pump/impeller gets pnched when the drive lower is put back on. Only way to know is to re do it. I don't know if you replaced the whole pump or just the impeller. If just the impeller was replaced the problem could be excessive wear on the housing or plate.

check the inlet side of your powersteering cooler and see if its full of old impeller parts and probally some trash!and did you replace the housing with the impeller?

Yes I replaced the entire pump in the outdrive and I put the key in for the impeller. I've got plenty of flow from the outdrive. The sending unit checks out ok.
I pulled the exhaust manifolds off and have muratic acid in them now. I got some big chunks of scale out of them by flushing. The power steering cooler is clear.
It really acts like a stuck thermostat but after using 3 different ones it still overheats at idle.
OK when on plane and wot

try drilling a small 1/8 or 3/16" hole in the base of your thermostat, this lets some water flow through before the thermostat opens, it worked for me

I'm not sure how the alpha drives are but my last boat had a OMC Cobra drive and coming out of the impeller housing was a weep tube. I had a tiny piece of seaweed about the diameter of a pencil lead and 1/8" long get caught in the weep tube which caused the engine to overheat. Something to check if their set up the same.

In the early thermostat housings there were ball shaped pressure valves that caused some backpressure, forcing water through the engine (thermostat open). When the thermostat is closed, water would bypass the engine and be routed by the valves to the risers. If the shaft or springs on these fail, most water would bypass the block. Just pull off the front hoses from the thermostat housing to the risers. See if they're functional.
Also check and see if both exhausts manifolds are the same temp.
Gary

Thrillseeker I tried that also I have 3 holes 3/16 in one Tstat And 6 holes in another, Tried both still overheats
Maybe cleaning the exhaust manifolds will help I will put them on this PM and try.
Thanks

Gary
I have those ball things, they seem to be ok.
One side seems to be hotter than the other but not alot
I wonder if I would tighten the nut on the end of the shaft if it would force the water through the block.
I did see that the thermostat housing has been changed on newer models. Mine is an 1986 they changed in 88 I think. Wonder why?
Thanks
Scott