Diagnosing/Fixing Cooling Problem Help

[minuteman62-64]

1982 Mariner, 30 HP. First run after bringing back to life (after 20 year sleep) - ran good, but overheated when running at high speed. Now in "fixit" mode.

Pulled lower unit. Everything looks OK. Impeller that was replaced in April, w/only about 2 hrs. running time, in good shape, flexible, no sign of wear/deterioriation. Ran water through thermostat housing. Good flow through hole where thermostat sits, exits through lower unit, nothing blown out that would indicate an obstruction. Ran water through other hole in thermostat housing (appears to go down cylinder head and into block). Water came out tell-tale and lower unit exhaust port, but not real good flow.

Would the next step be to pull the powerhead and look for an obstruction where the powerhead bolts to the driveshaft housing?

If I then pull the cylinder head will that give me a clue if there is an obstruction in the block?

Any advice/insight appreciated.

 

[Sunfish12]

Re: Diagnosing/Fixing Cooling Problem Help

It sounds like you have decent flow, run it without the thermostat in, cover on, thermostat could be bad.

 

[minuteman62-64]

Re: Diagnosing/Fixing Cooling Problem Help

Checked the thermostat today - in a pot of water on the stove, with a candy thermometer. It began to open at 130 degrees and was fully open at 150 degrees. I don't know how accurate the thermometer is, but I have to assume the thermostat is operating as it should (or pretty close to as it should).

 

[Texasmark]

Re: Diagnosing/Fixing Cooling Problem Help

Checked the thermostat today - in a pot of water on the stove, with a candy thermometer. It began to open at 130 degrees and was fully open at 150 degrees. I don't know how accurate the thermometer is, but I have to assume the thermostat is operating as it should (or pretty close to as it should).

I like that way to check a stat rather than running without it. On some engines you can't fully fill the block with water without the stat and the top cylinder can overheat.

Numbers are close enough. Most Merc pellets I have seen in mid and upper hp engines has 143F stamped on them and that is the opening temp. Your 130 is close enough for me.

Do you have any dirt-dobbers around there? What was the condition of the impeller when removed? Were all the blades intact?

With the stat out, peering down in the hole what did the water jacket look like? Any sign of foreign objects, corrosion, dirt dopper dirt????? Looking back at your post, looks like you did that.

Is your lower unit such that it gets adequate water flow into the water inlets when running at WOT?

I'd pull the water jacket cover off the heads and pull the water jacket cover off the exhaust and check them out with your eyes and external water hose before I'd pull the whole power head.

HTH,

Mark

 

[minuteman62-64]

Re: Diagnosing/Fixing Cooling Problem Help

Do you have any dirt-dobbers around there? What was the condition of the impeller when removed? Were all the blades intact?

With the stat out, peering down in the hole what did the water jacket look like? Any sign of foreign objects, corrosion, dirt dopper dirt????? Looking back at your post, looks like you did that.

Is your lower unit such that it gets adequate water flow into the water inlets when running at WOT?

I'd pull the water jacket cover off the heads and pull the water jacket cover off the exhaust and check them out with your eyes and external water hose before I'd pull the whole power head.

HTH,

Mark

I don't know if we have dirt-dobbers here (in San Diego). The motor sat unused from 1992 to 2011, so it is possible something took residence somewhere in the cooling passages. Nothing blew out when I injected water in every apparent orifice. Right now it is in a barrel while I'm doing some testing at idle and the fastest idle speed I can get with the throttle over-ride (going to Harbor Freight today to pick up the on-sale laser thermometer). When I remove the barrel I am going to remove the screens over the water intake and see if there is any problem in there.

When I checked the water pump the impeller was intact and very pliable - no sign of wear or deterioriation. I did notice that the mechanic who had brought the motor back to life had replaced the impeller with the water pump gaskets dry. When I re-installed it I used gasket sealant, as prescribed by the Service Manual. Don't know yet if that will make a difference.

I was thinking about the water intake issue at WOT. In the barrel, the pump would not prime until the water level was up to the connection where the lower unit drops (just about the level of the water pump). At WOT, on a plane, only the intake is in the water. Before I take the motor out of the barrel I'm going to run the motor and drop the water level to where it would be if on a plane, with the motor still running at fast idle, and see what happens.

 

[minuteman62-64]

Re: Diagnosing/Fixing Cooling Problem Help

Ran the motor today with my newly bought Harbor Freight non-contact temperature gauge in hand. Ran it at idle for 15 minutes or so and after about 5 minutes the temperature at the top of the cylinder head stabalized between 150 - 153 degrees F. Thought I'd up the ante, so I used the throttle over-ride to increase RPM's to 1500 - what the hell! The flow from the tell-tale began to dribble and then stopped. I measured the temperature at the top of the cylinder head at 200 degrees F. and rising.

After I cut the engine off I noticed that the loss of water from the tell-tale flow had drawn the water level in the barrel down to the top of the cavitation plate. So, it pumps when the water level is up around where the lower unit separates from the drive shaft housing (which is right about the level of the water pump housing) and won't pump when the water level is at the cavitation plate, but still above the top of the water intake (which is where it is when at a plane at WOT).

I think I have a water pump problem.

 

[Texasmark]

Re: Diagnosing/Fixing Cooling Problem Help

Several things that you need to understand:

When running in a barrel the water can get very hot. Keep this in mind in your determinations. BTDT

When the engine is at rest, the level of the outside water needs to be over the height of the water pump normally. Think about it. When your boat is in the water, your 20" transom is half way submerged....so is your water pump (submerged). It will pump submerged but it is a centrifugal pump and not necessarily sealed so it essentially has no "lift" capacity like a "jet" water pump for supplying your home with water from a shallow water well in a rural area.

Next, when up and running, the water intakes are positioned such that ram water forces it's way into the water pump and actually at high speeds, the soft rubber blades of the impeller fold back and the pump essentially "free wheels" and water flow is based upon ram water pressure. You won't get this in your barrel.

And last, if you put her in gear in the barrel you will run the water out of the barrel.....good for hot water (to be replaced with cool water) but bad for the pump trying to find water to suck up if it goes out faster than your garden hose can replace it which is usually the case. BTDT too.

So sir, you may be generating a lot of potential problems that aren't problems at all.

Get it out and run it and take your laser thermometer with you and leave the cowl off. And, when on plane up to WOT, look at the water inlet holes in the lower unit and ensure that none are visible. Anti-vent plate should be near +/- (half - one inch roughly) depending upon a lot of things, right at the top of the water that is running out from under your transom. If it is, you won't be "showing" any water pick up holes and won't be loosing valuable dynamic pressure from the force of the engine going forward in the water which you are relying on to cool your engine at the higher speeds.

Then come back and tell us how it went.

HTH,

Mark

 

[minuteman62-64]

Re: Diagnosing/Fixing Cooling Problem Help

Damn, just when I thought I was understanding some of this stuff. I had no idea the cooling water flow was entirely dependent on the "ram-jet" effect at high speeds.

This opens up a whole new area to look at. When I re-installed the motor I did not remember the tilt angle setting - so I set it in the middle. I also changed the trim of the boat by moving the fuel tank forward, into the bow area. One of the tasks on the shake-down cruise was going to be to adjust the motor tilt angle. However, with the euphoria of getting the boat underway, after almost a 20 year sleep, followed by the concern with the overheating, we never got to that step. I'm wondering if I had the wrong tilt angle (like front end "plow"), and it resulted in the transom being further out of the water than required for full immersion of the water intake?

 

[minuteman62-64]

Re: Diagnosing/Fixing Cooling Problem Help

I've fiddled around with the boat and trailer for the last couple of months, addressing things that I noticed during the initial shake-down cruise. I've shot water all through the OB's cooling system and run it in a barrel at about 2200 rpm for about 20 minutes with no over-heating (checked using the HF remote sensing thermometer). I've also installed a cylinder head temperature gauge, with the indicator at the pilot station in a easy sight line, and run the engine to verify the gauge reading. I also checked the tilt angle and it looked to be off by one adjustment hole (towards the "plow" position). So, also adjusted that.

It's in the mid-70's here and will be for the next couple of days. Second shake-down cruise tomorrow. I'll see how it goes.

 

[phebus1]

Re: Diagnosing/Fixing Cooling Problem Help

In the process of waking your motor up, have you cleaned/rebuilt the carb? Overheating issue may not be cooling water related, but may be caused by leaning out on the top end.

Just fuel for thought.

 

[minuteman62-64]

Re: Diagnosing/Fixing Cooling Problem Help

Braved the daunting weather conditions on Mission Bay today. Temperature, at 11:00 a.m. in the mid-70's, 3 inch chop, 1-2 knot winds (just threw this in for you guys in the mid-West). Ran perfectly all the way up to 5000 rpm, which is as fast as I'd run it anyways. Only significant difference was the change in motor tilt angle. Just to check, I put the tilt back where it was on the first test run and trimmed the boat like it was at that time. At about 4000 rpm the cooling water intake lifted half out of the water and the flow from the tell-tale stopped. I didn't push it, but I'm guessing that if I ran the throttle up further it would continue to rise up. So, I'm thinking that the problem on the first run was just a dumb mistake on my part.

Overall the motor ran smoothly. No lingering effects from the previous overheating were apparent.

On the carb: Yes, the guy who brought the motor back to life cleaned it out. However, he didn't rebuild it. I'll probably go ahead and do that sometime this winter just to make sure I have ethanol-safe components.

 

[fisherguy123]

Re: Diagnosing/Fixing Cooling Problem Help

Braved the daunting weather conditions on Mission Bay today. Temperature, at 11:00 a.m. in the mid-70's, 3 inch chop, 1-2 knot winds (just threw this in for you guys in the mid-West)................... I'll probably go ahead and do that sometime this winter just to make sure I have ethanol-safe components.

what winter? I`m jealous

 

[Texasmark]

Re: Diagnosing/Fixing Cooling Problem Help

Braved the daunting weather conditions on Mission Bay today. Temperature, at 11:00 a.m. in the mid-70's, 3 inch chop, 1-2 knot winds (just threw this in for you guys in the mid-West). Ran perfectly all the way up to 5000 rpm, which is as fast as I'd run it anyways. Only significant difference was the change in motor tilt angle. Just to check, I put the tilt back where it was on the first test run and trimmed the boat like it was at that time. At about 4000 rpm the cooling water intake lifted half out of the water and the flow from the tell-tale stopped. I didn't push it, but I'm guessing that if I ran the throttle up further it would continue to rise up. So, I'm thinking that the problem on the first run was just a dumb mistake on my part.

Overall the motor ran smoothly. No lingering effects from the previous overheating were apparent.

On the carb: Yes, the guy who brought the motor back to life cleaned it out. However, he didn't rebuild it. I'll probably go ahead and do that sometime this winter just to make sure I have ethanol-safe components.

Great! Glad it was something simple.