Engine Overheat

[Themanofsteel]

I have a 1982 Blackmax Mercury 150hp. V6.

I’ve replaced both Head Gaskets, and surfaced them, and Exhaust Plate
I’ve replaced the Poppet
Ive completely redid the impeller and housing.
I replaced the Thermostats.
I’ve flushed the engine with Vinegar, Slat-away and Muriatic Acid (No lectures on the acid please.)

When I have the thermostats in, the engine slowly rises to just about 175 degrees. At throttle it reaches almost 200 degrees. Water feels very hot.

When I take the stats out it stays cool at idle but still overheat at throttle.

 

[Blb2003]

Could be running lean. That causes a lot of heat. Do the carbs have a fuel mixture screw?

 

[Themanofsteel]

Hmm Ok. Didn’t think about that. I’ll check them out. Thx.

 

[Faztbullet]

Does the alarm go off or you using a gauge?

 

[Themanofsteel]

Alarm, gauge and hand. It’s gets very hot to the touch. Like I can barely touch the water.

 

[jimmbo]

Is there any blockage between the water inlets and the water pump? Is the tube/pipe that goes from the water pump to the engine clear? Grommets for the water tube good?

 

[Faztbullet]

Install a water pressure gauge in top of block and see if pressure low.

 

[Themanofsteel]

Ok. I recently ran the engine and separated the water out put to see what the flow is like coming from the left and right cylinders. The left has a very very weak flow but the right seems good. See pic.

 

[Themanofsteel]

Also, where can I find the right pressure gauge and sensor for my engine?

 

[Themanofsteel]

Im waiting for my pressure gauge to arrive. In the mean time I removed the lower unit and attached a hose to push water up the water shaft into the engine. Water seemed to poor out from the exhaust shaft. See pics. Is this normal?

 

[havoc_squad]

Unless your exhaust cover is leaking water in a cylinder via exhaust port or leaking on the outer cover part, that should be normal.

If my memory is correct for most outboards, except for some of the water cooling the exhaust cover/plate being sent out the midsection, all other water flow is governed by the thermostats and poppet valve (not including bypassing holes/ports at the thermostats)

In short, all that water should only be from exhaust cooling if the engine is not up to operating temp.

If water pressure is good on left and right and not running lean, I'd be concerned about exhast gasess from the cylinder being expelled into the cooling by a crack if it is like most Merc 2 strokes that have no cylinder heads.

However, since you said low flow on one side, probably want to focus on that first with a water pressure gauge to diagnose.

 

[Themanofsteel]

If water pressure is good on left and right and not running lean, I'd be concerned about exhast gasess from the cylinder being expelled into the cooling by a crack if it is like most Merc 2 strokes that have no cylinder heads.

I unscrewed and removed the plug where the pressure sensor goes at the top of the engine. I started the engine to inspect the water flow. I noticed some white smoke blowing out before the water filled to the top. Can I expect some smoke in there or none at all?

 

[Themanofsteel]

So I removed the exhaust plate manifold and removed the lower unit. I used a water hose to push water up the water pump shaft. In the pics you can clearly see water escaping into the exhaust probably from a blown gasket judging from the picture of the gasket.

Can I conclude that this is probably the reason for my overheating issue.

I'm curious, is age the most likely reason the gasket blew? I don’t want this to happen again. I bought the engine a year ago for $100 and it always had a overheat issue. It seemed to get worse with time since I use my boat a lot.

also is there a high performance gasket I can buy?

 

[jimmbo]

Remove the block and inspect the gasket, I think you will find it intact, but with an arch where it is leaking. Also check that the gasket surfaces are flat. That Gasket is coated with a sealer, or it used to be, any additional sealer will often act as a Lubricant and the exhaust gas pressure will tend to push the thin strip forward, resulting in that arch. Improper torque and torquing sequence will also affect the gasket's seal.

People often sell engines that have problems, without disclosing the info to the potential buyer. Does it look like the engine was apart to replace this gasket before(paint chipped off corners and/or washers of Bolts) ?

 

[Themanofsteel]

That I can tell, the paint surrounding the area of the gasket is in tack perfectly. I don’t think the gasket has ever been replaced before.

The previous owner knew nothing about the engine because he picked it up, along with a boat, from someone else but never fixed it up and used it. When I picked it up and inspected the engine it looked like it was in perfect shape. With the exception of wires and hoses becoming brittle, it almost looked brand new.

Anyways. I ordered a new gasket. What kind of sealer do I use for the gasket?

 

[jimmbo]

That I can tell, the paint surrounding the area of the gasket is in tack perfectly. I don’t think the gasket has ever been replaced before.

The previous owner knew nothing about the engine because he picked it up, along with a boat, from someone else but never fixed it up and used it. When I picked it up and inspected the engine it looked like it was in perfect shape. With the exception of wires and hoses becoming brittle, it almost looked brand new.

Anyways. I ordered a new gasket. What kind of sealer do I use for the gasket?

Myself, I would install it dry. Others may have a different opinion.

 

[Themanofsteel]

So I separated the block from the driveshaft housing and removed the exhaust plate assembly and old gaskets. The gaskets was blown on the exhaust plate assembly, where my finger is, circled in red, and I was loosing water and pressure. I put new gaskets on all sections, total of 3 gaskets. I thoroughly cleaned and inspected everything before reassembly.

It idles cool now, about 140 or less, where as before it got as hot as 175 degrees at idle. It still gets hotter that expected at full throttle but doesn't trip the alarm. I believe I need to replace the impeller. Otherwise it runs good now.

 

[Themanofsteel]

After much toil, it turns out my overheat was due to bad cylinder heads. I replaced both and the engine never ran cooler.

The cylinder heads looked fine but for some reason they caused my overheat issue. I even had a machine shop surface them but no luck. I finally got 2 heads from another engine, surfaced them myself and put on. Engine doesn’t overheat anymore.

My original cylinder heads, I took to a machine shop to surface. Still overheated after installing. After watching a video of how to surface yourself, with a large, perfectly flat block, and spray gluing 80 grit sand paper on to it, I was able to surface the heads, and the engine block as well.

I can’t recommend surfacing yourself because you could damage your engine block or heads but I took the chance, with a lot of caution, since I was at my wits end. All in all Worked out.