OMC 30 overheating

"I recently purchased a boat w

"I recently purchased a boat with an 1988 4 cyl OMC 3.0 litre GM engine. The engine overheats when allowed to idle. If you rev the engine up to above mid throttle the temp drops to where it should be quite quickly and stays there until revs are dropped back down. I replaced the Thermostat and the problem remains although it seems to take a bit longer to overheat. When idling a good deal of water is coming out of the exhaust output area and the exhaust manifolds dont seem to be overheating(touch method). I am trying to decide where to go next, water pump or curculation. Looking for advice...."

I had a v8 with sim prob. I f

I had a v8 with sim prob. I found that the idle mixture was to lean. It was .5 turn and needed to be 1.5 to 2 turns. Cheap to check.

Does the circ hose from the pump to the block (thermo housing) get hot?

"It is winter up here, another

"It is winter up here, another 2 months before the ice goes away. I will check the idle mix screw while in storage but cant confirm if the circ hose is getting hot until early May. What is the thought, if it's not hot the circulation pump is toast? Would that mean circualtion is being driven by the raw water pump when it gets to a higher pressure due to speed increase?"

"Be sure to check with the eng

"Be sure to check with the engine/carb manufacture about setup settings before making adjustments. I have a Holey and if I call Holley they will give me everything that I want to know about that specific carb.

I would think that if the hose is getting hot with the engine that the circ pump is doing its job and that the plumbing is not blocked. Do you see any signs of a failing circ pump? Loose shaft, noisy, belt noise, or leaking water?"

"The carb is the stock Rochest

"The carb is the stock Rochester, will check the service manual re setting. I did have the plugs out and they looked like the engine was on the rich side, not lean.

No visible signs of circ failure..nothing out of the weeping hole. The belt was very loose but when tightened it made no change other than to make me feel better!! Thanks for the info re the circ pump hose, will check when I can start it again.

I think I will go after the raw water pump..seems like they have a petty short life and I am pretty sure the seller started it once or twice out of the water without a garden hose adapter and as per other postings I read on the site that would be the quickest way to kill the impeller....maybe the volume or pressure is low.."

"I had a similar problem with

"I had a similar problem with my 88, I removed the housing it had several small cracks around the three boltholes, they were letting air into system. When you test run on the muffs, you might take back cover off and keep an eye out for this, good luck."

"Thanks Don, I am more than an

"Thanks Don, I am more than anxious to tackle this problem now!! I will focus on the pump, wish I didnt have to wait for the snow to go."

"If Your manifold stays 'c

"If Your manifold stays 'cold' even at idle, I would have taken down the circulation pump and checked the impeller for wear. Does not take much of corrosion and cavitation dammage to reduce the circulation. The original ones I think had steel/casted impellers, Sierra has brass impellers. If You have the boat on land, take out the rear drain cock, attach a garden hose and backflush the engine. (take out the thermostat before doing this!)"

"When the engine is up to oper

"When the engine is up to operating temp would the manifold be the same temp as the block or slightly cooler? Will have to check this out also as soon as the ice clears, doesnt look like a big effort to pull the circ pump. Is there a fitting you can buy to attach a garden hose to the rear drain cock? When you backflush do you disconnect the raw water connection where it connects at the thermostat housing?
Appreciate all your help!!"

"The manifold should stay quit

"The manifold should stay quite a bit cooler than the engine itself. If not You may have a capacity problem with the impeller pump. Check that impeller and that the housing has not partly meltet. If so, change it! Take out the drain cock, go to a hardware store and by a hoose fitting, there is no special fitting for backflushing."