Mercruiser 305 V8 running hot at low RPM fine at Speed

[thomascrown]

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.

I used the Cork Gasket on top of the thermostat with the copper piece on top of that. I followed the diagram on the parts diagram.

 

[thomascrown]

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.

This leads me to: hose from the bellhousing to the gimbal housing.

How do I check this for obstructions and or kinks??

 

[nola mike]

This leads me to: hose from the bellhousing to the gimbal housing.

How do I check this for obstructions and or kinks??

First quantify your output. We're looking at a crappy Internet video. Eyeball the hose for starters. Assuming your old pump was in one piece, low flow issue most likely coming from your out drive. If it was running ok prior to you messing with it, it's probably an issue with the impeller assembly or tube to water pocket.

 

[Fun Times]

I used the Cork Gasket on top of the thermostat with the copper piece on top of that. I followed the diagram on the parts diagram.

No warning horn sounding when hot?

Engine serial number handy?
Yours design has the 2 check balls in place? If yes, Yours requires an O-ring vs the cork gasket and using the wrong one will cause an overheating condition at idle RPM’s.

STANDARD COOLING SYSTEM(DESIGN I)

STANDARD COOLING SYSTEM(DESIGN I)

www.mercruiserparts.com

STANDARD COOLING SYSTEM(DESIGN II)

STANDARD COOLING SYSTEM(DESIGN II)

www.mercruiserparts.com

STANDARD COOLING SYSTEM(DESIGN III - ALPHA)

STANDARD COOLING SYSTEM(DESIGN III - ALPHA)

www.mercruiserparts.com

 

[thomascrown]

No warning horn sounding when hot?

Engine serial number handy?
Yours design has the 2 check balls in place? If yes, Yours requires an O-ring vs the cork gasket and using the wrong one will cause an overheating condition at idle RPM’s.

STANDARD COOLING SYSTEM(DESIGN I)

STANDARD COOLING SYSTEM(DESIGN I)

www.mercruiserparts.com

STANDARD COOLING SYSTEM(DESIGN II)

STANDARD COOLING SYSTEM(DESIGN II)

www.mercruiserparts.com

STANDARD COOLING SYSTEM(DESIGN III - ALPHA)

STANDARD COOLING SYSTEM(DESIGN III - ALPHA)

www.mercruiserparts.com

No Alarm, not even sure where I would hear it from, or where it would be wired.
Engine SN# 0A688183
My Thermostat Looks like Design 1, so I didn't use the rubber 0 ring. Design 1 shows gasket on top of Thermostat, is that where the black 0 ring should go? between top of thermostat and copper holder?

When I took the old thermostat out, I didn't find a gasket or 0 ring, so I followed the diagram.

 

[thomascrown]

No warning horn sounding when hot?

Engine serial number handy?
Yours design has the 2 check balls in place? If yes, Yours requires an O-ring vs the cork gasket and using the wrong one will cause an overheating condition at idle RPM’s.

STANDARD COOLING SYSTEM(DESIGN I)

STANDARD COOLING SYSTEM(DESIGN I)

www.mercruiserparts.com

STANDARD COOLING SYSTEM(DESIGN II)

STANDARD COOLING SYSTEM(DESIGN II)

www.mercruiserparts.com

STANDARD COOLING SYSTEM(DESIGN III - ALPHA)

STANDARD COOLING SYSTEM(DESIGN III - ALPHA)

www.mercruiserparts.com

I just noticed that Diagram one does not show the T valve balls, which mine has. Top of my thermostat is marked 47440

 

[Fun Times]

No Alarm, not even sure where I would hear it from, or where it would be wired.
Engine SN# 0A688183
My Thermostat Looks like Design 1, so I didn't use the rubber 0 ring. Design 1 shows gasket on top of Thermostat, is that where the black 0 ring should go? between top of thermostat and copper holder?

When I took the old thermostat out, I didn't find a gasket or 0 ring, so I followed the diagram.

If you had the alarm and working correctly, it should sound from behind the dash area normally.

Yes looks like design 1 so the cork gasket is to be used here.... Just thrown off because check balls was mentioned being there and not seen in design 1.

Your serial number is for the sten drive...MERCRUISER MC1 1.65:1

@Bt Doctur , could you look in the manual and double check the gasket orientation please.

 

[thomascrown]

If you had the alarm and working correctly, it should sound from behind the dash area normally.

Yes looks like design 1 so the cork gasket is to be used here.... Just thrown off because check balls was mentioned being there and not seen in design 1.

Your serial number is for the sten drive...MERCRUISER MC1 1.65:1

@Bt Doctur , could you look in the manual and double check the gasket orientation please.

Found this serial number A612838

 

[thomascrown]

Thanks for everyones input.
Went and fired it up let it warm up at dockside for 20 minutes, temp never went past 170. Took it out for another hour. Temp hit 180 once, then came back to 170. To it back to the dock and let it idle another 15 minutes, stayed at 170. I went all speeds, but since it was mostly a low speed overheat, I kept it at trolling speeds, idle, with a couple full throttle runs.

No Idea what changed. T stat balls were tightened. I flushed some intake lines and cooler, and bam running cool. Unless I flushed some mud or silt from the cooler, I never saw anything come out.

I will take it as a win.

 

[Scott06]

Thanks for everyones input.
Went and fired it up let it warm up at dockside for 20 minutes, temp never went past 170. Took it out for another hour. Temp hit 180 once, then came back to 170. To it back to the dock and let it idle another 15 minutes, stayed at 170. I went all speeds, but since it was mostly a low speed overheat, I kept it at trolling speeds, idle, with a couple full throttle runs.

No Idea what changed. T stat balls were tightened. I flushed some intake lines and cooler, and bam running cool. Unless I flushed some mud or silt from the cooler, I never saw anything come out.

I will take it as a win.

Not trying to overcomplicate things but If you are running a 140 thermostat and the gauge is accurate there is still something going on here.

I run a 160 thermostat and it stays at 170-175. When I used to run a 140 thermo in my old boat was sitting at 155 or so. I think the alarm went off at 170

Probably the ball tightening removed some bypass of water to the exhaust which is ok unless the exhaust gets hot before thermostat opens as it can melt things. Its definitely better but would really look at why there isn.t that much water flow to thermostat housing as this may get worse in the summer with warmer water temp

 

[QBhoy]

Surely that’s the wrong t stat ? Not likely your issue, but almost sure that a raw cooled like that, should likely be a 160 t stat. Unless I’m getting mixed up.

 

[thomascrown]

Surely that’s the wrong t stat ? Not likely your issue, but almost sure that a raw cooled like that, should likely be a 160 t stat. Unless I’m getting mixed up.

Honestly when I bought the boat, it had a new T stat setup in it. Based on the Serial number of the engine, I can never find anything about the engine on any parts sites. The boat is a 1987 so I buy parts for it based on that year range. The serial sticker says: A612838

 

[nola mike]

Surely that’s the wrong t stat ? Not likely your issue, but almost sure that a raw cooled like that, should likely be a 160 t stat. Unless I’m getting mixed up.

Typically the raw waters get 140's, I think some of the efis have 160.

 

[QBhoy]

Typically the raw waters get 140's, I think some of the efis have 160.

I won’t doubt a man like yourself, but I was almost certain that a raw water was a 160 for some reason. It’s a 160 in mine for sure. But she’s an mpi, similar to what you have mentioned

 

[Scott06]

I won’t doubt a man like yourself, but I was almost certain that a raw water was a 160 for some reason. It’s a 160 in mine for sure. But she’s an mpi, similar to what you have mentioned

My2004 carbed raw water came factory with a 160

 

[QBhoy]

My2004 carbed raw water came factory with a 160

Love a school day on here. Fair play.

 

[nola mike]

Love a school day on here. Fair play.

Ok, well... Looks like it's pretty random what got what. Newer v6/V8 trends towards 160, but many engines come with either. Older were mostly/all 140 that I see. No clear cut crossover if vortec or 4bbl. The fi models I looked at were all 160. So I stand corrected and idk if it matters.

 

[Fun Times]

Found this serial number A612838

0A612838 suggest the engine to be an 1986 Stern Drive (200) Model number 02002346... Your engine should be running the 140 degree, https://www.crowleymarine.com/mercr...-drive/1986/-02002346/standard-cooling-system

1986 Stern Drive 200 [02002346] - Parts Lookup | Crowley Marine

View parts diagrams and shop online for 02002346 : 1986 Stern Drive 200. Offering discount prices on OEM parts for over 50 years. FAST $4.95 shipping available.

www.crowleymarine.com

Mercruiser Service Manual Number 9 for sale | eBay

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[dingbat]

Didn’t read through the whole thread but I have to ask, how cold is the water?

Used to drag my old boat (I/O) up to Lake Ontario to troll for Steelhead for a week in in early April.

Same exact overheat scenario every year. Bring the home and drop it in the Bay and had no problems at all

Locals (NY) said to get a lower temp thermostat or just remove it altogether. Just removed my every year for the trip and never had an issue

 

[thomascrown]

Didn’t read through the whole thread but I have to ask, how cold is the water?

Used to drag my old boat (I/O) up to Lake Ontario to troll for Steelhead for a week in in early April.

Same exact overheat scenario every year. Bring the home and drop it in the Bay and had no problems at all

Locals (NY) said to get a lower temp thermostat or just remove it altogether. Just removed my every year for the trip and never had an issue

River is 48, lake is 40.