Mercruiser 305 V8 running hot at low RPM fine at Speed

[thomascrown]

New Impeller, water pocket is good. New Thermostat 140 degree, installed correctly. New temp gauge, did not install the new temp switch.

Hit the fresh water lake while making my way out to the lake the boat temp was about 180, over about 30 minutes it started to creep up to 230. Give it some RPM, it quickly drops back to 180. Took it for a fast cruise, it dropped back to 160 and ran fine. Back to the port, at no wake speeds temp stays around 180-200, idle or trolling speed 220-230.

So my plan is to:
Check water flow by disconnecting raw water line to thermostat and check water flow. If not good, plan to check oil cooler for blockage. If no blockage, I plan to pull the hose at the transom and look for blockage. If I am good there. I am not sure where to go. Check my balls to make sure they aren't loose.

I bought a new engine water Circulation pump as a last ditch effort. It seems like these don't fail, and I don't know the symptoms they show when the fail. I would guess overheat at high speed, hate to mess with it if I don't need to.

Can I blow out the oil cooler with Air? Or just back flush it? It doesn't look like it will be easy to get to, to completely remove it.
Can I flush the engine/ run the engine with a hose thru the thermostat raw water intake? What would this prove, if anything.
Where else would I want to look for blockages, as I imagine the oil cooler is going to catch anything of size that would cause problems.
This doesn't seem like riser/ exhaust issue, but water issue. So I haven't pulled them, they look good, no rust ect.
I did not replace the water pump base when I did the impeller, not sure if that would cause the issue I am having.

Not sure what else, or where else to look.

 

[nola mike]

Check my balls to make sure they aren't loose.

I...I just can't. But yes, loose balls are ready to check.

Can I blow out the oil cooler with Air? Or just back flush it? It doesn't look like it will be easy to get to, to completely remove it.
Can I flush the engine/ run the engine with a hose thru the thermostat raw water intake? What would this prove, if anything.

How do you drain the cooler to winterize? Assuming it overheats on muffs? If so putting a hose into the t stat intake will isolate the problem to one side of the hose.
If it's on the supply side of the drive, check for both flow and with a section of clear hose between the supply and stat looking for bubbles. Also have you used an IR temp gun at various points? Can compare both sides of the engine/manifolds.

 

[thomascrown]

I...I just can't. But yes, loose balls are ready to check.

How do you drain the cooler to winterize? Assuming it overheats on muffs? If so putting a hose into the t stat intake will isolate the problem to one side of the hose.
If it's on the supply side of the drive, check for both flow and with a section of clear hose between the supply and stat looking for bubbles. Also have you used an IR temp gun at various points? Can compare both sides of the engine/manifolds.

I have it winterized by a marina. So I don't know how they do the cooler.
I will try the clear hose.
I Have an IR Temp gun, but what am I looking for exactly, hot spots?
I will have more info after I do some testing tonight.

Thanks

 

[Bondo]

Ayuh,..... Welcome Aboard,....... I had that same issue years ago with a 4.3l that was some motors built into one,....
It was the check-balls in the t-stat housing,.... or in my case, a lack of them,....

 

[thomascrown]

Ayuh,..... Welcome Aboard,....... I had that same issue years ago with a 4.3l that was some motors built into one,....
It was the check-balls in the t-stat housing,.... or in my case, a lack of them,....

Thanks for the welcome.
I know my springs are rusted, How tight are they supposed to be, can you over tighten them, or under tighten them, my manual doesn't really say anything about it

 

[Bondo]

Thanks for the welcome.
I know my springs are rusted, How tight are they supposed to be, can you over tighten them, or under tighten them, my manual doesn't really say anything about it

Ayuh,..... Just snug enough to seal,.....

The idea is to hold water in the block,.....
'n I truly doubt it's the circulating pump,....

 

[nola mike]

I meant to say check balls *easy* to check. I think it would be difficult to overtighten, you want them closed at idle, they prevent the cooling water from going straight out the elbows.

 

[thomascrown]

Ayuh,..... Just snug enough to seal,.....

The idea is to hold water in the block,.....
'n I truly doubt it's the circulating pump,....

The only reason I keep thinking circulating pump is how quickly the temp drops at high RPM, plus the boat is 1987, I imagine it's the original, and I am hoping for something easy. What are the symptoms of a faulty circ pump?

I am heading over after work to try to troubleshoot some of these things. Any ideas are greatly appreciated. Boat is sitting in the water at the slip.

How would I check the transom side / by bellows water line for a kink without removing the drive? I had the bellows all redone one season ago, can't imagine it's kinked. I even tried raising and lowering the trim to see if it would cool the engine faster, which it did not.

 

[Bt Doctur]

When was the impeller replaced?

 

[thomascrown]

When was the impeller replaced?

1 week ago, along with thermostat.

I did not replace the water pump base, just impeller plate and gaskets

 

[Scott06]

I have it winterized by a marina. So I don't know how they do the cooler.
I will try the clear hose.
I Have an IR Temp gun, but what am I looking for exactly, hot spots?
I will have more info after I do some testing tonight.

Thanks

If you have the check balls on the thermostat housing they can pass raw water from the impeller right out the exhaust starving the engine.

Somewhere in the merc manuals there is a flow test for quantity of water coming up to the thermostat housing. Easy test to do to verify water is getting there.

What impeller did you use ? some of the aftermarket ones have had a high failure rate or slip on the shaft collar that is part of them.

is the water tube up from pup into upper in good shape - not leaking water out.

Is it sucking air around the base due to corrosion

What drive year is this gen one or two?

 

[thomascrown]

If you have the check balls on the thermostat housing they can pass raw water from the impeller right out the exhaust starving the engine.

Somewhere in the merc manuals there is a flow test for quantity of water coming up to the thermostat housing. Easy test to do to verify water is getting there.

What impeller did you use ? some of the aftermarket ones have had a high failure rate or slip on the shaft collar that is part of them.

is the water tube up from pup into upper in good shape - not leaking water out.

Is it sucking air around the base due to corrosion

What drive year is this gen one or two?

I used Mercury parts from my marina.
Water pickup tube looked good.
Not sure how I would check if it is sucking air around the water pocket, so I am unsure.
Gen One

 

[thomascrown]

Did some tests. Here is what I found.
First I ran the engine with the raw water line off the thermostat. Water was flowing at idle Lots of water with throttle. I pulled the line off the PS oil cooler, checked flow same results flow at idle and lots at throttle. I checked the cooler for obstructions, stuck my finger in and is clear through. I disconnected the water line from the hull and oil cooler, started and was gushing like an upside down water hose at idle probably a 1-1 1/2" arch. Put everything back on.

Second I pulled the hoses off the t Valve, and started the engine, water was splashing around alot. Ball time. balls were loose, like really loose. SO I cranked em down almost all the way. Started engine and had some water not much. Put hoses on and ran engine.
Result: was running 230. After adjusting my balls, ran no hotter than 200 at idle. With a little more RPM drops to 180. With lots of throttle drops to 160.

Couple notes: the hoses that run to the risers are cold. Hoses from thermostat to exhaust are hot. Thermostat is hot. Raw water in to thermostat is cold. Hose from circ pump to thermostat is not hot or cold.

Throttled up with higher RPM checked hose pressure on the hose from circ pump to thermostat and was squishy like at idle. Both risers are hot. I forgot my temp gun, so will check tomorrow. Hot enough I couldn't keep my hands on them long. Side note, I never drilled the holes in my thermostat, but it did have some sort of relief thing.

I forgot to bring a bucket to count flow, and of course now my bilge pump isn't working, so I didn't flood the boat to much as it is in the slip right now. Will fix sump tomorrow.

Lots of info, not sure where to go from here. I have a new Circ pump, but I am not sure if I should put it on, and if it would really make a difference.

 

[thomascrown]

When was the impeller replaced?

The alpha pump will push just under 20 gpm. The pump deadhead pressure is about 20psi However as Bt doctur stated,

At what RPM is it pushing this flow?

 

[nola mike]

Not sure how I would check if it is sucking air around the water pocket, so I am unsure.
Gen One

That's what the clear hose I mentioned is for. Bubbles if you're sucking air from anywhere.

 

[thomascrown]

View attachment 339461313_1388885535209421_3906650783579554776_n.mp4

 

[Fun Times]

Does your engine have the audio warning horn system option installed for overheat? If yes, did the warning horn sound once over 196 degrees?

AUDIO WARNING SYSTEM

AUDIO WARNING SYSTEM

www.mercruiserparts.com

Did you use a cork gasket or O-ring when installing the Thermostat? The correct position of the O-ring mattress in these cases.

You mention a new gauge and old sensor... "Possibly" the Ohms resistance might not be a correct match and giving/receiving false readings going on there if no horn sounded and the warning system is installed.

Did this issue seem to start before or after all the service work was completed?
Need your infrared temperature gun to confirm it's truly overheating.

 

[Scott06]

View attachment 377189

that doesnt look like a lot of water at idle. I'll see if I can find the water flow test from merc manual

If balls were loose (try to set NOLA up good here...) they would be passing too much water at idle directly to exhaust then overboard. Thats why the lines off the T are cold - they are always passing water to cool exhaust regardless of what thermostat is doing.

 

[Scott06]

This thread from 2012 has the flow test numbers in post #4

water flow/volume test?

im having overheating problem with my 73 gm 250 in line 6 165hp, with alpha 1 drive unit. ive read some threads about checking how much water you get from stern drive. i get 3 quarts in 15 secs at 800 rpm. thats from the hose disconnected from thermostat housing. is this good? should i have more...

forums.iboats.com

 

[nola mike]

View attachment 377189

That looks like low flow to me at idle, and just a mess above idle. You want something like 20 gpm, probably a bit less at idle and on muffs.

Edit: Scott beat me to it, what he said.