Cooling System - What else could it be?

I've have a lingering cooling issue I thought was a simple thermostat issue, but... The symptom is the water temp gauge bounces from 170-100 after the thermostat opens. Some times it drops at 130F, and some times it sticks at ~110F. What I've done:

- Replaced the thermostat w/ a genuine Merc part, including all gaskets; twice in case the first one was bad out of the box.
- Replaced both port & starboard temp sending units on the thermostat housing.
- Replaced the impeller in the sterndrive.

Current Status:
- Today I broke down and purchased a new Water Temp gauge. The old gauge appears to be bad. At the same time, with the new gauge, the engine reaches 160F, then drops to 120F and stays there.

Starting cold, the engine comes up to temp (160F), thermostat opens, temp drops to ~120F. This is in the driveway, connected to ears & garden hose. Using a laser heat gun, the top of the intake manifold (roughly under the carb) is reading ~175-180 while running at ~1,500rpm. The exhaust manifolds are reading 120-140F (starboard side always runs hotter). I have good water flow out of the sterndrive, and I've even open the drain plugs from the block and manifolds (carefully) when it is idling to ensure I have good flow.

I would swear in the 23 years I've owned this boat (original owner, '97) it would warm up to 170F and stay there during normal operating conditions. The only thing I haven't done is replace the actual water pump on the engine.

What am I missing? (4,3L V6, Alpha Gen+)

Andy T.
Tacoma WA

More info...
I pulled off the thermostat housing and plugged the outlet ports (with wine bottle corks) that feed cold water to the manifolds. If I run water into the inlet (from the kitchen faucet), I would expect it to block the outlet to the engine water outlet (largest hose outlet) by the thermostat. In other words, if thermostat is closed, no water should get to the engine cooling outlet port. However, the water is (somehow) bypassing the thermostat. That helps explain why the engine isn't maintaining temp; if cold water is bypassing the thermostat.

I make sure, validate, I remoted the thermostat, and ran the faucet at just past a steady drip to fill up the inlet side. Water was existing the engine outlet without it filling up the thermostat housing. I THINK... perhaps the housing has a crack/hole that is allowing water to pass between the water inlet and the engine outlet, without being regulated by the thermostat.

Any thoughts?

Sounds like the water dam and flller block are missing or damaged in the outdrive.

Docksidemarineservices said:

Sounds like the water dam and flller block are missing or damaged in the outdrive.

Click to expand...

Thanks Chris. I know the filler block is in place, however I did not replace it when I installed the new impeller. How does the filler block effect the thermostat with respect to regulating the engine temp?

And can you confirm the thermostat housing should NOT allow cold water to pass between the fresh water inlet and the engine side outlet without passing through the thermostat?

First, you need bypass water to cool the exhaust from the first seconds of start up. Is your system 2 or 4 hoses? Do you have the t-stat installed correctly?


If the filler block and water dam are missing or damaged they will let exhaust gases into the cooling stream and will cause large fluctuations in temp. The Aluminum water dam that is part of the lower unit can rot away and cause the same issue.

Docksidemarineservices said:

First, you need bypass water to cool the exhaust from the first seconds of start up. Is your system 2 or 4 hoses? Do you have the t-stat installed correctly?


If the filler block and water dam are missing or damaged they will let exhaust gases into the cooling stream and will cause large fluctuations in temp. The Aluminum water dam that is part of the lower unit can rot away and cause the same issue.

Click to expand...

Yes, The t-stat is installed correctly. It is 4 hoses (fresh water, port/stbd to manifolds, outlet to engine). My recollection from when I replaced the impeller over the winter is the filler block appeared to be in good condition, and the lower unit is clean. Mostly the boat has only been in fresh water.

PROBLEM FOUND: Water Circulating Pump Failure:
I pulled off the water circulating pump and the pully is not rotating the impeller, thus no proper water flow. I found a similar YouTube video with a bouncing water temp issue. Although the issue was not exactly the same, it was a similar failure of the impeller.

Good find.

FYI, a 2 hose system has just two hoses to the exhaust, to the bottom of the manifold. A 4 hose system has 2 hoses to the bottom of the manifold ans 2 hosed to the exhaust elbow or riser block. We don't count the water pump hose or the water supply hose.