3.0 Mercruiser Still Overheating

[JimS123]

Worked fine on the muffs and most of the first day's outing. Then came in a no-wake zone and docked at a restaurant when it started to overheat. Got up to 180 or so on my gauge.

Replaced the entire water pump and the thermostat. Next time out the gauge showed about 150-155, vs the 140 it had always run at. I spent an hour at various speeds and it never wavered, so the T-stat surely is holding it at a constant temp. Then, went in a no-wake zone for another trial and after about 5 minutes at dead idle the temp started to climb again. Bumped it up to 1300 rpm or so and the temp came right back down.

Could the original overheat have damaged the temp sensor that it is not showing 155 instead of 140?
Obviously, at 750 rpm its not providing enough cooling water. Is that normal?

This is a fresh water motor and I see no signs of internal corrosion.

 

[Silverbullet555]

When is the last time you changed the impeller?

 

[alldodge]

Could the original overheat have damaged the temp sensor that it is not showing 155 instead of 140?
Obviously, at 750 rpm its not providing enough cooling water. Is that normal?

Only way to know that its not the gauge or sender is to measure the temp at the thermostat housing.

There should be no issue with the alpha drive keeping the motor at temp.

 

[JimS123]

When is the last time you changed the impeller?

Two years ago before the problem. Then again yesterday, after the problem reappeared.

 

[harringtondav]

My temp gage climbs to 175 degrees w/a 160 degree thermostat each time I start it cold w/muffs at idle. Once I increase throttle to around 1500, it drops like a rock, then then hunts up and down until it stabilizes at 160. Same when I come off a 3000+ rpm cruise to a no wake.

According to my Merc. 4.3 manual, a 1.98:1 drive (likely your 3.0L) supplies 3.0 gal/min @ 1000 rpm. I suspect close to 3 more gpm for each add'l 1000 rpm. Most of this cools the exhaust. So when you drop from a high rpm to a low rpm, it may take a while for the engine, and hot heads to to stabilize with 70-80(?) degree seawater. No doubt you know this, but just asking if you wait long enough for it to stabilize.

Also, are you sure you were sold a 140 degree stat?

 

[harringtondav]

...ps, I can't imagine the engine circulating pump loosing efficiency, but a maybe....

 

[JimS123]

The T-stat was marked 140 on the package, as well as engraved on the unit.

I'm probably being paranoid here, watching the gauge constantly, after having an over heating condition.

Truthfully, I don't often run at idle and never look at the gauge when docking, so it may have done this all along.

 

[harringtondav]

JimS123 This may be a stretch, but these same engines operate at 190 degrees normal in autos, and I think an overheat warning occurs around 220. I'm guessing our boat engines have 16 psi thermos, which will prevent boiling far over this. I upped my 4.3 from 140 to 160 to gain better fuel efficiency that the autos get @ 190 degrees. I think the 140 is a NMMA and SAE std to keep the engine compartment cooler for safety and to avoid carb vapor lock. So I think 150-155 operating is safe, and spikes at throttle off are OK, as long as it stabilizes w/in a min. or so. But knocking, off engine run-on, or hot fuel stumbling would say not OK.

Also, I've noticed all my gages change when I turn my running lights on. Temp gets higher. I noticed my alt. belt was loose and tightened it. It was slipping, leaving dust in the engine compartment carpet. Haven't checked it yet w/the extra load, but I hope to see a fix.

 

[Rick Stephens]

Open cooled engines will start to form bubbles at 160,and those also can form crystals. Hence the goal to keep under 160. If closed system, then 15 PSI can be maintained, 200 or 210ƒ is more efficient.

Jim, I wonder if you have a blockage somewhere in your line coming in from the impeller to the T-stat? I Know AllDodge talks now and then about putting a short piece of clear to check water flow.

 

[Weirdnerd]

I would check the hose from the lower unit ( impeller water) to the engine, if it is kinked, it can restrict the water flow.

 

[tpenfield]

To me . . . the symptoms sound like there is a slight blockage somewhere in the cooling system flow, and at idle speeds there is not quite enough pressure to bring enough raw water into the engine. Once the RPM goes up a bit there is enough pressure.

cooling system problems are difficult to chase around, but you could start with backflushing and looking for possible flow restrictions. Exhaust manifolds and risers can also restrict flow as they become corroded.

Running on the muffs can be deceiving, as the garden hose puts its own pressure into the raw water intake

 

[Scott Danforth]

pull the line from the outdrive. should be able to fill a 5 gallon bucket in about 18 seconds with motor at 1200 rpm.

 

[alldodge]

pull the line from the outdrive. should be able to fill a 5 gallon bucket in about 18 seconds with motor at 1200 rpm.

Only if sitting in the water right?

 

[Scott Danforth]

Only if sitting in the water right?

correct....LOL

 

[harringtondav]

Rick Stephens is correct. Rick Stephens mentioned open cooling systems. I thought of my reply today and realized I was wack. There is no pressure cap on a full sea water Stern Drive. So I regret the gaffe. At sea level water boils at 212 degrees, less as you elevate. I wait forever to get my pasta water to boil, and it seems to happen at the last minute. Anyway, my 160 thermo hasn't caused a problem in 20 yrs., so I'll put changing back to a 140 on the 'B' list and work on the annual, eventual boat issues that come up.

Oh yea, I just remembered why I upped mine to 160. My '96 Merc #18 4.3 engine manual showed the 160 degree stat in the EFI 4.3s. I knew higher temps improve fuel economy so I went for it. This wasn't restricted to heat exchanger systems.

 

[AlabamaNewbie]

I have a hijack question. In cars, you dont run straight water for a couple of reasons (mainly freezing), but also antifreeze/coolant makes the cooling fluid not so abrassive to the fins in the water pump. Having ran vehicle with straight water and seeing how it wore away the fins in the pump, I get why you don’t want to do that.

From what I can tell and gather, the water circulator is basically the pump that forces the water through the engine that it gets from the impeller, right? Obviusly a car going 15,000 miles a year would move a lot more water around, but a boat that is 30 years old moving fresh water around all the time — whats to keep the water from abrasing down the internals on the water circulator over time? Is that something I need to think about during my restoration?

 

[Silverbullet555]

I know VP switched to 160 degree thermostats in the mid 2000s OJ some engines did merc make the same change?

I spent way too much time chasing a cooling issue. Some things I did might benefit you.

Backflush everything.

Put clear hose into lines to see water flow. I used clear hose from lowes and a combination of PVC pipe and copper fittings to create the necessary connection points. You can see air bubbles in the lines which can help determine where to look.