GM305 1978 temperature problem

[sarantis]

I installed a new thermostat housing together with new thermostat and gaskets.I started the engine and there was water flow at the two holes of the bottom of the transom.Is this normal?
When the temp.needle past the center I show it started going back ,then again ahead,again back,the same flow of water from the transom holes BUT after one minute the needle was ready to enter the red region .What do you suggest ?The thermostat housing is the one with the 8 hoses.The hoses right installed,the risers old but checked and cleaned ,the manifolds too,the thermostat with the nose up.I had problems in the past with the tamperature but i had bad flow of water.Previously i worked it without thermostat and at least in low turns the temperature was low.What hapened now?

 

[sarantis]

Re: GM305 1978 temperature problem

I noticed that the top cover of the thermostat housing stays cold.What does this mean?

 

[sarantis]

Re: GM305 1978 temperature problem

In addition the prop in neutral with the earmuffs on doesn't turn and i think the water coming out of the prop is little.Where from should I have more water coming out? The two holes or the prop?

 

[POINTER94]

Re: GM305 1978 temperature problem

Well, it is normal for water to be coming out of the prop and the top of the lower unit if it is a merc. just a little comes out of the prop on mine. The thermostat housing should heat up. Have you changed the thermostat and installed it in the right direction? (Start with the obvious, sometimes) Also, I encountered a similar situation on a 305 and we changed the temperature sensor to clean up the problem. If you have a temperature gun you can check the engine temp to verify.

When was the last time you replaced the impeller?

 

[sarantis]

Re: GM305 1978 temperature problem

Thanks for replying
new impeller with its housing,water pocket(above the impeller),new hose .
The thermostat housing is hot but not the top cover of it.The thermostat is new and checked

 

[POINTER94]

Re: GM305 1978 temperature problem

We chased this issue for weeks, and came up with the senor sending erroneous info to the gauge making it appear to be running hot when it wasn't. You can buy a cheap lazer temp gauge at a hobbie store for 50-75 bucks american. You point the lazer at the block and hold for a few seconds. It gives you a digital readout of the temp. I found mine at a place that sells those remote control cars.

Normally I would expect a majority of the water to be coming out of the top of the outdrive. Do you have a weep hole on the starboard side of the outdrive? I would also assume you have check the hoses. The warmer they get the more flexible they feel. Are they all recieving warm water? Or just some?

Next step check the exhaust elbow water passage. Use air or water to make sure that it is not blocked. Check the exhaust hose to make sure it isn't being overheated. If it is, your manifolds may need to be removed, cleaned (with acid) or replaced.

 

[sarantis]

Re: GM305 1978 temperature problem

I don't believe it's the temperature sender because last time at sea it worked fine.I have only the plug starboard.Not the bolt behind it.But I've cleaned all this area.New gaskets,seals,etc.

 

[sarantis]

Re: GM305 1978 temperature problem

But i think that in neutral the prop should turn slowly.Shouldn't it?

 

[POINTER94]

Re: GM305 1978 temperature problem

I have seen happen, but not always. If you rev the engine on the muffs the prop typically turns very slowly. It's rotation is caused by, I believe, the water passing threw. The old omc units almost always did. But they exhausted more of the water out the prop.

At idle it wouldn't surprise me in the least if the prop didn't turn.

Did you pull off the brass plugs on the side of the exhaust manifolds and check for sludge?

The rest of my knowledge would be to look at the water pump and make sure belt is tight and working. Otherwise is the water pump working at all? Sounds like it is. Loose connections between the outdrive to the pump introducing air into the system and not allowing the pump to work efficiently if at all? Did you do a tune up or change the wires? Could the timing be off? Is she running lean?

The rest of the stuff can be downright pricy. Water in the cylinders? Warped head?

 

[sarantis]

Re: GM305 1978 temperature problem

I did again a test without the thermostat.It gets hot again but after some time.Is it normal for the engine to get hot when running it at 1300 -1500 rpm
with the earmuffs?Anyway i found the port riser to get more hot than the other.I 'm going to pull it out and check it again.Also i have to check again the impeller because in a try i forgot the garden hose closed but only for some seconds.As about the engine it runs good.No water in oil.By the way what should be the oil pressure? The needle is a little more than the center.

 

[Haut Medoc]

Re: GM305 1978 temperature problem

Is thias a raw water of closed system?.....
By all means, you will have to recheck the impeller in the drive, if the no water condition was more than 30 seconds......
Not knowing how old this motor is & if it was used in salt water, the condition of the engine circulating pump could possibly be an issue....
Sometimes the vanes rot off in salt water use.....
Just a thought......
PS, it is normal for one side to be slightly warmer than the other.....
The engine, however should never get hot regardless whether if on the muffs or in the water at any RPM's ......
All overheating problems are caused lack of flow into the engine, poor circulation, or exhaust restriction......
A mechanical overheat from lack of oil pressure would do it as well....
If you have oil pressure, I would rule that out.......​

 

[sarantis]

Re: GM305 1978 temperature problem

Thanks for appearing Haut,
I took out the circulating pump and it was in a very good condition.I also took out the two hoses of the top cover of thermostat,(they lead to the risers)and i think the water presure wasn't so good.If i take out the inlet hose that goes to the thermostat housing will i understand anything or the circulating pump makes the inlet flow strong?Can you explain me the route of water?I have at each side a hose from the riser to th.housingandtwo hoses from the manifold to the th.housing.You can see my th.housing at the following adress : http://www.marinepowerservice.com/B...*********************************************
What happens when the thermostat opens?Where does the water goes?

 

[Haut Medoc]

Re: GM305 1978 temperature problem

The link does not go to your specific engine.....
However, with the tstat open, the water is sent to the exhaust elbows.....​

 

[bruceb58]

Re: GM305 1978 temperature problem

However, with the tstat open, the water is sent to the exhaust elbows

Water is being sent to the exhaust manifolds/elbows at all times no matter what the thermostat is doing.

When the thermostat opens, water within the block is sent to the thermostat housing where it mixes with the incoming cool water. The circulating pump sucks water out of the thermostat housing back into the block. At the same time, water is leaving the thermostat housing out to hoses leading to the exhaust.

You need to check the condition of the risers and manifolds to determine if they are causing a water flow problem.

 

[Haut Medoc]

Re: GM305 1978 temperature problem

Water is being sent to the exhaust manifolds/elbows at all times no matter what the thermostat is doing.
True......

When the thermostat opens, water within the block is sent to the thermostat housing where it mixes with the incoming cool water. The circulating pump sucks water out of the thermostat housing back into the block. At the same time, water is leaving the thermostat housing out to hoses leading to the exhaust.
Not true..
The thermostat just allows more coolant to exit the system when it reaches temp......

You need to check the condition of the risers and manifolds to determine if they are causing a water flow problem.

Agree......with that.....
Is BIG OIL robbing your your eyeballs out in Greece?....
Sarantis, you are free to PM me ......

 

[bruceb58]

Re: GM305 1978 temperature problem

The amount of water leaving the thermostat housing to the exhaust is always the same...thermostat open or close. The thermostat is only allowing the water to leave the block...has nothing to do with water going to the exhaust system.

If you look at the thermostat housing you will see the incoming raw water inlet goes into the same chamber that the exits to the exhaust come from. The thermostat does not control that flow of water.

If the thermostat was controlling that amount of water that got sent to the exhaust, all the rubber parts of the exhaust system would be burnt up during warm up.

 

[Haut Medoc]

Re: GM305 1978 temperature problem

One more time....
In English......
There is a certain amount of water that is sent to the risers at all times to prevent the overheating of rubber exhaust parts.....
Once the engine & tstat has warmed to the proper temp, the excess water is sent out to the exhaust elbows thru the tstat.....
It only recirculates when the cooling water is sufficient to close the tstat to a smaller opening....
Thus restricting flow, & recirculating it back through the engine....​

 

[bruceb58]

Re: GM305 1978 temperature problem

Ok...I stand corrected.

Looked at a coolant diagram in my Volvo manual and you are correct.

 

[sarantis]

Re: GM305 1978 temperature problem

****************PROBLEM SOLVED!!!!!**************************

Indeed!When i pulled out the lower unit to check the impeller i realised that I had made a mistake when installing it. Its housing was provided together with a rubber seal
that should be put in a gap between the stainless cap and the top of the housing and i had put it over the top of the housing. Now i have more water from all of the three outlets.The temp.needle doesn't pass the center by any means.
THANKS to all that helped and to the inspirator of this site!
By the way, Haut Medoc ,what does this mean?"Is BIG OIL robbing your your eyeballs out in Greece?...."

 

[Coors]

Re: GM305 1978 temperature problem

Lol; Big Oil Haut.