Mercruiser 305 V-8 Temp issues

[AnOldMedic]

Want to see what everyones thoughts are.

My boat runs 180 to 200, up and down. I typically troll and it remains at 180, sometimes it creeps to 200, but goes back down. If it doesn't I give it a little throttle, and it drops down. Cools rapidly to 160 when wide open for a cruise, but then comes back up to 180.

Every year I have replaced the impeller. My water pocket looks good. I have good water flow coming from the impeller. I have lake tested it, and it pumps water out like crazy.

I have replaced the thermostat 140 degree. And this made no difference. I have adjusted the T valve balls on my thermostat so they are tighter, and this keeps temps closer to the 180.

I have cleaned/ checked the power steering cooler for any blockage. I don't know where else to check for blockages. I never pulled the thermostat completely off, just took the old one out and put a new one in.

Not sure if I didn't tighten the hose that runs from the steering cooler well enough when I checked it for blockages, but two times while running wide open the hose popped off. So I started to pay attention to hoses a lot closer, and started to squeeze them. Pressure is building up, but I don't know why or where. I have noticed the hose that runs from the circulating pump to the thermostat housing gets extremely hard at times, and others feels normal. This is where I think my problem lies. Just don't know what it means.

I am worried about this pressure, and My risers get hot, sometimes to hot to hold your hand on them for any amount of time. Which I think is increasing my engine temp also.

I had the boat winterized, and asked them to check my hose that runs thru the hull for any kinks, they say everything looks good. Had the bellows done 3 seasons ago.

So heres where I am at.
I pulled the risers and the exhaust manifolds. I didn't think they looked that bad, they have been on the boat since I owned it for 8 years now. I have attached pictures. The PORT side exhaust manifold is extremely rusted on the bottom, not sure why. starboard looks fine. No water intrusion in the exhaust parts of either manifolds or risers.
I never noticed any leaks, my bilge usually has some water, so I don't know if I would notice a leak or not. Directly underneath that exhaust manifold is my power steering cooler.
I pulled the water circulation pump, because it sounded like the bearing was a little sketchy.

I have ordered all new exhaust manifolds, risers, and water circulation pump and a new thermostat. I would like to send some of this stuff back if I can immediately rule it out.

Please look at my pictures, and let me know what your thoughts are.
I appreciate it.
Let me know if I need to clarify anything.

 

[Chris1956]

What do the top of the risers, under the rubber sleeve look like? That is usually an indicator of how much the manifolds and risers are corroded.

Picture 2 shows a lot more rust that I thought I would see, especially on a freshwater boat. Was she used in the salt, before you bought it? She clearly had a leak there at some point, and it looks like saltwater damage, to me.

Unfortunately, cast iron is porous and once exposed to the saltwater, running it in freshwater slows down the saltwater corrosion, but does not stop it, as the salt in in the cast iron itself.

 

[AnOldMedic]

What do the top of the risers, under the rubber sleeve look like? That is usually an indicator of how much the manifolds and risers are corroded.

Picture 2 shows a lot more rust that I thought I would see, especially on a freshwater boat. Was she used in the salt, before you bought it? She clearly had a leak there at some point, and it looks like saltwater damage, to me.

Unfortunately, cast iron is porous and once exposed to the saltwater, running it in freshwater slows down the saltwater corrosion, but does not stop it, as the salt in in the cast iron itself.

Top of risers under rubber looked good, not nearly as rusty as I thought they would. Just rust stained, no major flaky rust. Checked the flappers and they were all good.
This boat has always been in Freshwater.

 

[Lou C]

I agree with the statement about that manifold's corrosion, even my 100% salt water used manifolds don't look like that!
You say pressure is building up between your circ pump hose and the thermostat housing, are the passages in that housing that feeds the elbows open?
Are the thermostat housing's outlets all open and not necked down by corrosion? Does your thermostat open at the right temp and as far as it's supposed to? Looks like you have the warm manifold system so your manifolds won't get water flow till the stat opens so they can run at about engine temp. But your elbows should be getting cool water all the time.

 

[Chris1956]

MerCruiser impellers will cause overheat at idle, if the bottom of them is damaged. You state that you replace it every year, so I will assume that is not the issue. Do you replace the wear plate, and the two gaskets as well? If the wearplate has scratches or the gaskets leak, that can cause overheat. Also check the waterpump base for melting and the waterpump cover for scratches in the SS cup. Replace all defective items you can find.

Did you check the rest of the manifold water supply hoses? They can get rust in them, especially with the heavy rust spot, and partially clog.

 

[AnOldMedic]

MerCruiser impellers will cause overheat at idle, if the bottom of them is damaged. You state that you replace it every year, so I will assume that is not the issue. Do you replace the wear plate, and the two gaskets as well? If the wearplate has scratches or the gaskets leak, that can cause overheat. Also check the waterpump base for melting and the waterpump cover for scratches in the SS cup. Replace all defective items you can find.

Did you check the rest of the manifold water supply hoses? They can get rust in them, especially with the heavy rust spot, and partially clog.

I replaced the whole water pump and seals, last year.

I will take all the hoses off and check those for blockages

 

[AnOldMedic]

I agree with the statement about that manifold's corrosion, even my 100% salt water used manifolds don't look like that!
You say pressure is building up between your circ pump hose and the thermostat housing, are the passages in that housing that feeds the elbows open?
Are the thermostat housing's outlets all open and not necked down by corrosion? Does your thermostat open at the right temp and as far as it's supposed to? Looks like you have the warm manifold system so your manifolds won't get water flow till the stat opens so they can run at about engine temp. But your elbows should be getting cool water all the time.

Not sure why I would get so much corrosion in that spot, so badly like that.

I was looking for a diagram of my thermostat, as I never took it off to inspect it for any blockages, just took out old thermostat and put new one in. I attached the diagram of the thermostat for reference.

I also attached a picture of the brass sleeve that I thought I may have put in wrong. I put the cork seal then the thermostat then the brass sleave as pictured in. I never really thought the thermostat was the primary issue, but now I am thinking there is a restriction in it..

 

[Scott06]

If the hose is getting stiff and the elbows and risers are hot then I would think water flow out through the exhaust is impeded and manifolds an risers will help.

Did you verify that the ps cooler is not blocked?

Some where there is a flow test for alpha one drives. I think like 3 -qts in 15 sec. ? Of course if you lacked water flow the hose wouldn't be pressurized...

 

[QBhoy]

Did you check that your exhaust flappers were still in place and in one piece ? If part or all of one has seen a “thermal event” in the past history, you can often find it further down the exhaust horn and restricting water flow/exit.
But given what you’ve said. We all also know that gauges can be fairly wild in reflecting a true reading. Have you an alarm sounding ? If it’s of an age to have such a thing…it should have definitely sounded, should the temps reach the figures the gauge is suggesting

 

[Lou C]

With an open (raw water cooled) system anything that inhibits the flow of water in or out will cause elevated temperatures.
Another often missed cause of overheating is leaky head gaskets, they can allow exhaust gas to get into the cooling water and even if nothing else is wrong, the engine will run hot.

 

[AnOldMedic]

With an open (raw water cooled) system anything that inhibits the flow of water in or out will cause elevated temperatures.
Another often missed cause of overheating is leaky head gaskets, they can allow exhaust gas to get into the cooling water and even if nothing else is wrong, the engine will run hot.

This crossed my mind, how would I go about looking at this, without pulling the heads.

 

[Scott06]

This crossed my mind, how would I go about looking at this, without pulling the heads.

Pressure test the cooling system- block of inlet and outlet hoses with pipe fittings, T in a gauge, rig up a way to inject compressed air. Should hold 15 psi relatively indefinitely. If head gasket is blown would expect to see air coming out a cylinder via sparkplug hole and or into crankcase (you would see milk in oil via crankcase)

 

[dubs283]

Agree with most inspections recommended here. Would look at the t-balls again.

IME the balls often get worn/tapered at the housing side and turning them around along with retensioning the springs helps for overheat at lower rpm.

The problem is too much water passes through the t valve and out the exhaust not allowing for enough cold/sea water to stay in the system at lower rpm. Think low volume supply at lower rpm

With the sea water pump it's important to replace the base/seal assembly and gasket with appropriate sealant next to the exhaust passage along with the entire pump housing/assy. Normally a base gasket failure causes overheat at higher rpm but can create your issue as well. Would recommend only using oem parts for sea water pump service on older alpha/mc1 style drives

Not sure your situation is caused by a cracked head but it could very well be. Normally a running issue and overheat at all rpm would be symptoms of this failure

 

[AnOldMedic]

Agree with most inspections recommended here. Would look at the t-balls again.

IME the balls often get worn/tapered at the housing side and turning them around along with retensioning the springs helps for overheat at lower rpm.

The problem is too much water passes through the t valve and out the exhaust not allowing for enough cold/sea water to stay in the system at lower rpm. Think low volume supply at lower rpm

With the sea water pump it's important to replace the base/seal assembly and gasket with appropriate sealant next to the exhaust passage along with the entire pump housing/assy. Normally a base gasket failure causes overheat at higher rpm but can create your issue as well. Would recommend only using oem parts for sea water pump service on older alpha/mc1 style drives

Not sure your situation is caused by a cracked head but it could very well be. Normally a running issue and overheat at all rpm would be symptoms of this failure

This begs the question. What is the proper running and operating temp of this Mercruiser engine. I don't see one in the book.
I didn't think 180 was an overheat, just running hot.

 

[Lou C]

One thing I haven't figured out is why Merc used the check balls but OMC & Volvo never used this system, everything is free flowing and I've really not had overheating problems with this boat even in salt water. The only time was from marine growth in the water intake holes and to solve that, I wound up splitting the drive and removing that water intake screen because stuff always grew on it no matter what I did.

 

[Scott06]

This begs the question. What is the proper running and operating temp of this Mercruiser engine. I don't see one in the book.
I didn't think 180 was an overheat, just running hot.

depends on what thermostat 140 or 160

140 thermostat should be 150 or so

My 160 thermostat runs about 165-170

not being under pressure like in a automotive radiator they run cooler temps so it doesnt boil and lessens mineral scale buildup

 

[Lou C]

This begs the question. What is the proper running and operating temp of this Mercruiser engine. I don't see one in the book.
I didn't think 180 was an overheat, just running hot.

Can't say for what Merc would say but I can tell you 20 years worth (all salt water) on my OMC, with the 160 stat, it runs mostly at 155-160. It might rise up to 175 but only after coming off plane when the engine has heat built up in it but the raw water flow slows down. But it will cool back down to 160 after a few min of idling.

 

[Scott06]

One thing I haven't figured out is why Merc used the check balls but OMC & Volvo never used this system, everything is free flowing and I've really not had overheating problems with this boat even in salt water. The only time was from marine growth in the water intake holes and to solve that, I wound up splitting the drive and removing that water intake screen because stuff always grew on it no matter what I did.

the balls are part of the warm manifold system, allows some cold water to flow to elbows when thermostat is closed (cools exhaust) then should shut when warm water off the thermostat comes up through manifolds.

Theoretically cuts down on condensation in cold manifolds by just supplying cold water to elbows.

 

[Lou C]

This crossed my mind, how would I go about looking at this, without pulling the heads.

Another way to do it is to find some clear reinforced hose the same size as your t stat housing to manifold hose. Temporarily replace the regular hoses with these. Then run it on the water till the stat opens and bring it up on plane. When you bring it back down to idle, if you see bubbles in the water flow that's a good sign of exhaust gas getting in the cooling water flow via a blown HG. I had this on mine about 10 years ago, a few years after the overheat, the HGs really started leaking and put water in a cyl, which led to a top end overhaul.
Also look for signs of water in cyls (might not see it in the early stages)
water in the oil (same thing)
and do a compression test (believe it or not blown HGs don't always show in a comp test, mine didn't, the results were normal)
the signs were the bubbles in the cooling water
some water in a cyl, that wouldn't show up right away, but after the engine sat a few hrs after blowing out the cyls sure enough as it cooled off water would seep back into #2 cyl.

 

[Lou C]

the balls are part of the warm manifold system, allows some cold water to flow to elbows when thermostat is closed (cools exhaust) then should shut when warm water off the thermostat comes up through manifolds.

Theoretically cuts down on condensation in cold manifolds by just supplying cold water to elbows.

That's kind of what I thought, make sense because with the totally open OMC/Volvo set up the manifolds and elbows run really cool at idle, like 95-110* and a max of 135-140 only after coming off plane. Yet, I haven't seen signs of rusty exhaust valve stems when I pull the exhaust off for the normal checks we have to do in salt water (5-7 years).