AQ170 overheating fix

"I fixed my run hot problem by

"I fixed my run hot problem by drilling two or 3 (can't recall how many) holes in the flat plate part of the thermostat. I did this after checking and or repairing ALL of the usual suspects for causes of overheating in this motor as discussed on this forum. Seawater pickup neck on up through both pumps. New head and head gasket etc. . ALL is as it should be and the thing still ran a little hot according to the gauge in the dash. So, I got out the drill. These holes allow a little water to circulate into the head even when the heat activated valve in the 'stat is fully closed. My theory as to why these "iron pigs" run hot is that it is a long way from the incoming cool water at the front of the head to the temp. sender at the back of the head. By the time the 'stat at the front of the head realizes that the head is hot and opens up, the back of the head is REALLY HOT! and the sender is letting us know it through the ga. in the dash. With my modification, the needle on the ga. stays in the green ---always--- and if I am really on the throttle sometimes the needle on the ga. will just barely touch the pointy tip of the red wedge. My theory is just my theory, but the fix I devised based on the theory works for me. Very satisfying."

Many marine T'stats have s

Many marine T'stats have such holes.

"Got a 1994 Four Winns with a

"Got a 1994 Four Winns with a 5.0 Volvo engine, serial # T01273537, Model # 502ACPMDA. The boat operates in fresh water and at the last outing it immediately heated up to over 240. The raw water impellor was slipping on the shaft, replaced it and the thermostat. I lubed the new impellor good with glycerin to aid in dry startup. With the boat sitting in the water, it immediately heated up to 240 again, and I couldn't detect water in the exhaust at the prop. (Probably melted the new impellor!)Where did I miss it? Is there a way to prime the pump before starting the engine? The inlet ports on the lower foot look perfectly clean, however this boat by necessity runs in some rather shallow water to get to the boat lift where it stays. How do I check water flow to the pump, and where can I down-load a diagram of the cooling system for this engine? Thanks in advance."

I would like to see a picture

I would like to see a picture or diagram of where you drilled the holes. Also what size holes did you drill?

"Casey P, I don't have any

"Casey P, I don't have any photos to put up, but I can describe what I did for you . The thermostat is basically a generally flat disc of metal about 2 inches in diam. There is typically a rubber ring type gasket that slips over the outer edge of the disc so there is rubber material around the outer edge of the disc on both the "heads" and "tails" side of the disc. The center of the disc is taken up by the heat activated valve. There is an obvious coil spring there that holds the valve closed until there is enough heat present to cause the valve actuator to expand and work against the spring to open the valve. If you are holding the thermostat in your hand it will be in the closed position...unless your hand is VERY hot, like 160 deg. F. .. With the therostat closed and installed (assuming a good gasket seal) no water can get past the thermostat until heat causes the valve in the disc to open. I simply drilled 2 or 3 approx. 1/8 inch holes (might have been 7/64 but close enough ) through the disk of the thermostat in places where they would not interfere with the rubber gasket around the outer edge, nor interfere with the functioning (open/close) of the heat actuated valve. If you pick your spots to drill so that you don't mess up the gasket's ability to seal at the edge and don't mess up the opening/closing of the valve in the center I don't think you can go too far wrong..... it ain't rocket science. You are just trying to let a little water pass through the disc when the big valve in the disc is still closed. I hope that helps, and hope you weren't offended by my description of stuff you already may know. If you have more questions about this just post up here."

"Got it! My only concern would

"Got it! My only concern would be a slow warm up. However, if that's not a problem and it works that's great. It's certinly something to keep in mind for future reference.
Thanks for the enginuity and explanation."

Matt Broyles- That is a good e

Matt Broyles- That is a good explanation and good idea also.

anything to prevent the water

anything to prevent the water from flowing to the heads is detrimental.relief holes .excellent idea.

Glad to help out guys. As for

Glad to help out guys. As for warm up problems--I have had none. I generally don't run this boat when water is real cold though. Here in South Central Oregon I run it from mid April to Oct. Never in the ice. I have a duck boat w/ outboard for that but that's another story!