1978 5.7l pre-alpha with alpha gen 1 drive over heating

[matt0107]

Got the boat with a broken water tube (brass tube through gimbal housing) from my brother in law. So i replaced it, along with gimbal, bellows etc. Water tested the boat. It kept cool at lower rpms (>2500). But when tried to get it up on plane it started climbing. No sure the temp but was getting near the red. So I limped it home. So then I replaced the thermostat. Guess she didnt have one. So put in a 160 degree. Then replaced the impeller, The old one still looked good. Also i pulled the exhaust elbows and cleaned them out. Had little blockage, but not bad at all really. Then I put it on the muffs and started getting warmer at idle. Still in the green but closer to red. So then I pulled the water pump. Flushed the block manifolds everything had good flow. Still same temps. So then swapped temp sensor. Same result. Was getting about 400 ohms cold, then 40 ohms warm. But when i was shooting the motor with the temp gun the readings around the thermostat was 160. Around the block was under 140. Exhaust manifolds were between 180 and 200. I pulled the raw water hose of before the power steering pump cooler while running and water was coming up. No crazy pressure but like a garden hose at Idle. Im newer to boats but pretty much worked on everything in-between. Kinda stumped. No water mixing in oil. So think head gaskets are ok. Also haven't done a compression test though. Any one have any ideas?

 

[itsathepete]

Gauge is not accurate or was originally specified to have a 140 thermostat

 

[Rick Stephens]

Drop to a 140ƒ thermostat.

Check in the bell housing that the water inlet hose isn't kinked.

 

[Bondo]

Ayuh,....... Welcome Aboard,....... Were there any wear marks inside the impeller housing,..?? at All,..??

 

[matt0107]

Thanks for the replies. The water hose isn’t kinked. I double checked it. I will toss an 140 in there and see if it helps. I know mecruisers usually run 140. But figured a 160 would be ok. Feel like SBCs like some more heat to them but never know. I replaced the whole housing when I did the impeller. Does any one have the resistance charts for a mercruiser temp sensor? So I can rule out the gauge.

 

[Rick Stephens]

Gas engines are most efficient at 200-210ƒ. Unfortunately, that is only possible with closed cooling where the coolant is pressurized to 14 pounds or so. An open cooled Mercruiser motor at over 160ƒ will start building up steam bubbles on the walls of the cylinders and heads around the exhaust valves. Those bubbles prevent heat transfer from the cast iron to the water. So the motor gets hot spots and runs even hotter. Hence 140º thermostats to keep under those mid 160's.

In your case you also have an issue with not enough flow. There is no doubt of that since your exhaust manifolds are getting hot.

 

[matt0107]

Gas engines are most efficient at 200-210ƒ. Unfortunately, that is only possible with closed cooling where the coolant is pressurized to 14 pounds or so. An open cooled Mercruiser motor at over 160ƒ will start building up steam bubbles on the walls of the cylinders and heads around the exhaust valves. Those bubbles prevent heat transfer from the cast iron to the water. So the motor gets hot spots and runs even hotter. Hence 140º thermostats to keep under those mid 160's.

In your case you also have an issue with not enough flow. There is no doubt of that since your exhaust manifolds are getting hot.

Ok thanks. what temps should the exhaust manifolds be around?

 

[Rick Stephens]

Ok thanks. what temps should the exhaust manifolds be around?

Cool enough you can rest your hand on them, no problem.

 

[dubs283]

Pretty important to replace the base assy and impeller housing on early style alpha/mc 1 drives and pressure test as well. Base gasket failure is quite common on those style drives. As mentioned a 143 t stat is recommended

 

[kenny nunez]

Did you inspect the plastic cover that the copper pipe is in which connects to the drive pump? If it ever melted from not enough water exiting the exhaust it will melt and it causes the water out leak out from under it. Since you stated that the through transom water tube was broken when you got the boat this could be your problem. With the boat in the water disconnect the incoming water hose at the thermostat housing and try to stop the flow at idle with your hand. You should not be able to hold it back, if you can then the leak is probably the plastic cover.

 

[matt0107]

Pretty important to replace the base assy and impeller housing on early style alpha/mc 1 drives and pressure test as well. Base gasket failure is quite common on those style drives. As mentioned a 143 t stat is recommended

ok thanks the drive is a alpha gen 1. Came off my other boat but did the whole kit base, impeller, cap and all. The gimbal housing is MC 1 though. Will drop it to 140. See how it goes.

 

[nola mike]

Also make sure you check the water pocket in the upper, if melted it lets exhaust gas into your cooling stream

 

[matt0107]

Did you inspect the plastic cover that the copper pipe is in which connects to the drive pump? If it ever melted from not enough water exiting the exhaust it will melt and it causes the water out leak out from under it. Since you stated that the through transom water tube was broken when you got the boat this could be your problem. With the boat in the water disconnect the incoming water hose at the thermostat housing and try to stop the flow at idle with your hand. You should not be able to hold it back, if you can then the leak is probably the plastic cover.

The tube that broke was up in the gimbal housing. So the impeller was always pushing water up to the bell housing. So should have stayed cool. But I did the whole housing kit base cap gaskets and all with impeller. But I will try holding the water back to see if it’s loading pressure along the way. Thanks for the information.

 

[matt0107]

Also make sure you check the water pocket in the upper, if melted it lets exhaust gas into your cooling stream

What do you mean by water pocket?

 

[kenny nunez]

That is what I was referring to in my post. It is mostly known as the water pocket cover.

 

[Rick Stephens]

What do you mean by water pocket?

The water pocket is the plastic housing in the upper gear case that the copper water pipe inserts into. It is held in with 4 1/4 - 20 bolts. It is also in the direct path of the exhaust gases and if any loss of cooling water will melt, right there.

 

[matt0107]

After new 140 thermostat and checking for water pressure on the water intake hose. Nothing seemed to work. So did a compression check. So it’s blown head gasket or cracked head. Yay.

 

[Lou C]

You might want to investigate further there are many causes of low compression. Could be valves not seating due to leaks from the manifold to elbow gasket, or worn rings. To further investigate leaky head gaskets replace the stat housing to ex manifolds hose with clear hose. Run the boat in the water and warm it up. Look for air bubbles suggesting exhaust gas in the cooling water. That, and water in cyls & oil suggest leaky HGs.
Was this a hot compression test with the throttle valve wide open? If not may not be accurate.

 

[Rick Stephens]

Need to pressure test the cooling system.

Should rent/buy a leak down tester and track down your low compression.

May very well be two different problems.

 

[matt0107]

The motor was cold and WOT with all plugs out when I tested for compression. The reason I am thinking this is the issue because I know the motor was over heated from the broken water tube when I first bought it from my brother in law. Once it gets warm it steams pretty good in the drive way out the exhaust. Also had washed plugs in the low compression plugs. The odd part is the motor still runs great and no water in the oil either. It also gurgles after I shut it off after it starts to get warm. On the side with the suspect gasket. I did order new gaskets and have a buddy who will pressure test the heads and check for warp. Hoping this takes care of it.