Over heating issue

[jtim108]

We have been fighting an over heating issue for a bit now first the impeller was bad and not pumping water second the thermostat was stuck closed third the passages on the exhaust side of things were gunked and we weren’t getting good flow fixed all of that and went out on the water and were good all day long till the end of the day it started getting hot again around 185.

we used some rydlyme to try and clear out the rest of the calcium build up and such and are getting lots of exhaust flow now, however we went out yesterday and the water pump hose that’s about 2.5in blew a hole in it.
Found a new one threw it on and hoped it went from being old and not in the best shape, but now the boat is over heating again and that hose has a fair amount of pressure in it however the hose going to the exhaust manifold does not so it seems to me like we have an issue on the engine side where water isn’t flowing? Any ideas on common problem areas or ways to try and clear out that side of things?

mercruiser 140 1984
serial number 6805225
alpha 1 gen 1 stern drive

 

[Rick Stephens]

You have a blockage. The cooling system is open, if it isn't blocked there isn't anything that will allow pressure build up. Time to back flush or clean things out more. A 36 year old motor could have enough scale built up inside to plug up anywhere.

 

[jtim108]

You have a blockage. The cooling system is open, if it isn't blocked there isn't anything that will allow pressure build up. Time to back flush or clean things out more. A 36 year old motor could have enough scale built up inside to plug up anywhere.

So we just blew through the water pump line and everything came right out of the thermostat housing with very little effort think it’s possible the thermostat is bad and not opening causing the over heat and pressure

even though it’s new we did overheat once with it in because of the exhaust blockage?

but from what we gathered we have no blockage on the engine or exhaust side

 

[jtim108]

I’m gonna try to break this down better cause there’s a lot going on

first we have water to the thermostat housing from the impeller

second since the boat is in the lake we removed the boot off of the riser and started it and have really good flow coming out of the riser so no worry there for blockage

third we removed the water pump hose and blew through it with our mouth and water comes through the head and back into the thermostat housing no problem

fourth we removed the thermostat and ran the boat and the boat still creeped up and started getting close to 185 so we killed it

we inspected oil and found clean oil with no water in it, the spark plugs are dry and burning good, rechecked the timing and are at 6 btdc, we haven’t ran the compression test yet because the boat fires right up idles perfect at 750 rpm, put it in gear and it has no issues and doesn’t have any misfire or signs of lack of power at wot

the only thing that has changed from this issue to the last time we ran the boat with no issue is we flushed the exhaust side of the cooling system with rydlyme and fixed the timing / did an ignition tune up

i feel like I’m running in circles and that there isn’t anything there to be causing this over heat but I can’t figure this one out

 

[Rick Stephens]

The scale build up would as likely be in the block as anywhere. The flow of water comes into the thermostat housing and then directly out the exhaust except what is allowed into the engine by the thermostat. So the exhaust can be cool but the engine hot. The thermostat has a hole in it to allow a small amount of water to flow into the engine even with the thermostat closed. Some thermostat housings have check balls that keep more water flowing in the motor. alldodge has a diagram of flow for most older motors. It gets a lot more complicated on the newer cooling systems. Not always for the better. Your '84 3.0L is pretty simple system.

 

[Scott Danforth]

how much flow do you have from your raw water pump?

with the boat in the lake, and the motor running just above idle (about 1000 RPM), you should be able to fill a 5 gallon bucket up in 20 seconds.

 

[jtim108]

how much flow do you have from your raw water pump?

with the boat in the lake, and the motor running just above idle (about 1000 RPM), you should be able to fill a 5 gallon bucket up in 20 seconds.

The flow is pretty good we just hooked a clear hose from the thermostat housing to the exhaust manifold to watch water flow and it filled with some good pressure and has one big air bubble in it?

 

[Rick Stephens]

Pretty good is different from detaching the in line from the thermostat housing and running that into a bucket and knowing it is to spec.

 

[Scott Danforth]

if you dont want to put it into a bucket, get a flow meter. you should be flowing about 16gpm at 1000 RPM

 

[Rick Stephens]

Here's the cooling system diagram for that motor.

 

[jtim108]

if you dont want to put it into a bucket, get a flow meter. you should be flowing about 16gpm at 1000 RPM

Pulled the hose off of the thermostat and it shot water to about the driver seat and stayed constant we don’t have a bucket or flow meter at the moment we also noticed we are getting a random miss every once and a while then it goes away I think we might have a head gasket issue causing air pockets/ fowling a cylinder from any water that might be getting in the cylinder

 

[jtim108]

Here's the cooling system diagram for that motor.

Also that diagram is a bit different then what we have we don’t have that cooler #5 and we have a dif style thermostat

 

[Rick Stephens]

Pull plugs and look at them.