One manifold hot (5.0)

[Lou C]

Well it could have been in the salt but I wouldn’t worry about it too much. I have a lot of experience with salt water use & what I’ve found is that you can get a good 15-20 years of use from the engine before troubles start but the exhaust system needs replacing (at least the elbows) every 5-7 seasons. What eventually happens is that the cooling passages in the cyl heads start to get thin & either don’t seal well against the head gasket or the part of the head behind the valve seats can rust through & hydrolock the engine. I found this out from doing an oil analysis & found sodium in the oil from salt water (2011). A few years later (2013) had an overheat but it ran fine for 2 more seasons & then the head gaskets starting leaking at the end of the third season after the overheat (2016). So I took it apart had the heads checked & the machine shop found that there were cracks in the exhaust valve seat area (overheat) and the cooling passages were getting eroded (salt water use but by then it was in the salt at least16 years). So I installed a set of reman cyl heads with Fel Pro gaskets & ARP head bolts. Also installed new center riser style exhaust. Came out fine still running well 5 years later.
However when I repower this boat FOR SURE the new engine will have a closed cooling system on it! There is no way that'd I'd put a new or reman engine in and leave it raw water cooled here in the Long Island Sound region...many benefits, the only drawback is cost and complexity.
So don’t worry replace the exhaust & go boating! Here's a couple of pix of that job. Not difficult actually just have to keep all the parts organized and CLEAN! I used a thread chaser to clean out the cyl head bolt threads in the block, and new ARP cyl head bolts, the old ones were quite rusty due to the years of raw water cooling. Speaking to a couple of marine mechanics, they said that it is not uncommon for the cyl heads to rust through after that many years of use but the blocks rarely if ever do. So I'll run this one as long as I can but when the time comes I'll repower it. I priced the same boat (Four Winns H1 is the new version, my old one is a F/W H-200) brand new and it's like $67,000. Crazy.

 

[ryno1234]

Thank you so much for your help @Lou C. I just went out to the garage to inspect the other riser and it is flaking so bad there is a good 2x2in flake of rust separated. Reminds of phyllo dough, lol.

 

[Lou C]

If you want decent quality made in USA aftermarket exhaust use Barr Marine. I used them on mine and they fit perfect with no leaks. The paint doesn't hold up great on my salt water mooring so I painted them with Rustoleum.
I orginally had the OMC one piece V6 exhaust manifolds like Merc used for a few years and when those became NLA I converted it over to the center riser style that OMC and Volvo later used. Easy with V/P parts and the Barr manifolds and elbows. Fits like factory.

 

[Searay205]

Being an older engine I bet your thermostat is 140F. 140F is key to preventing crystallization of the salt on engine internals. New fuel injected engines like to run 160F. I had a 1988 Mercruiser 3.0 liter in salt water its entire life. Saw it last week out in Galveston bay. Original engine and spark plug wires!! and alternator. 34 years old. I found the Mercruiser OEM manifolds would last 10 years the aftermarket anywhere from 7 to ? I always replaced at 10 years. Funny sometimes after 10 years they still looked great when I broke them apart with a sledgehammer. The manifolds corrode faster because of the hot exhaust gases cooking the salt on the manifolds, engine blocks don't see that temperature so corrosion is much slower.

 

[Lou C]

I actually tried going from a 160 to a 140. Found condensation in the manifolds so went back to the 160. When I had the heads off 5 years ago found no evidence of salt build up in cooling passages

 

[ryno1234]

Ok - Update: I bought new OEM manifolds, risers, exhaust shutters for both sides and a new thermostat.

Installed everything and the boat *runs* good, but... the SAME manifold is still running hot.

I held my hand on the port riser and could hold it there indefinitely, I tried to do the same on the starboard riser and I could only hold it there for about 2 seconds.

Very frustrated at the present moment over this.

The only things I could think that could cause this at this point are:

• Blockage in the thermostat housing (I didn't notice anything, but I also didn't flow test it)
• Blocked in the hose which goes from the thermostat housing to the manifold (again, didn't flow test it but I bent it, twisted it, etc...)
• Something deep inside the Y-pipe causing a backup of exhaust keeping it very hot (the previous exhaust shutter was totally toast and I did find one half of it when I removed the outdrive a month ago.. .the other half could still be in there or the previous owner could have removed it)
• That side of the engine running extra hot for some reason.
• Something else I can't think of....

Any ideas? I'm obviously itching to take my boat out, but also don't want to do damage to it by running a hot manifold.

I do know water is flowing through it because I did have a water leak on the Y-Pipe on that side, but perhaps not much?

 

[ryno1234]

Update to above:

• I tested the thermostat housing, there is no blockage
• I pulled both hoses from the thermostat that go to the manifolds and hooked a garden hose up to them. Both hoses flow water through the entire system out the outdrive.... so I don't have any water flow issues from the thermostat all the way out the back.
• I put a borescope down the Y-pipe. I found (on the hot side) 2 halves of an exhaust flapper. I fished them out. This is in addition to the 1/2 an exhaust flapper I found in the outdrive when I pulled it out 1 month ago. That means there is another 1/2 a flapper that likely was removed previously from a previous owner when the outdrive was pulled off at some point I'd guess or it's still stuck somewhere in the Y-Pipe. The borescope didn't show anything else though.

I still have 1 hot manifold and I've tested pretty much everything I can think of regarding the water flow and making sure I have clear passages for exhaust exiting.

The only thing different about the starboard and port manifolds is I do see that the hot one (starboard) has a flexible metal pipe attached to it that goes to the intake manifold. I imagine this is an EGR type pipe. It's HOT, but I would guess it's supposed to be. I've attached a photo - you can see the pipe going to the manifold on the left side of the photo, wrapped in a black sleeve.

So, at this point, is it possible that the exhaust is simply hotter on that side for some reason? Running lean? (although engine seems to run fine).

I'm beating my head against the wall on this one.

 

[ryno1234]

I have a separate post but it's not getting much love likely because it's long and drawn out, so I wanted to do a new post summarized.

I have a 1998 5.0 Mercury with a raw water system.

I have one manifold / riser running 30+ degrees hotter than the other (~110 degrees at 1,500 RPM vs ~145 degrees). I took temp readings with an IR gun off the riser.

The hot manifold side has visible exhaust from its port on the outdrive while the other port does not have hot exhaust visible. The water coming out the port is also hotter (to be expected)

Some details:

• Brand new OEM manifolds and risers
• Tested water flow by removing the hoses that come off the thermostat and attaching a garden hose to them. Both sides flow evenly.
• I used a borescope down both sides of the Y-Pipe and removed old exhaust flappers that fell in there. Both sides are now clear (as far as I can tell).
• My thermostat "T" housing is clear. The flow to both manifolds comes from a shared area in the thermostat housing so there is no favoring of one manifold vs. the other.
• The hoses coming off the "T" housing have different temperature (again, despite having a shared area where the water flows through). This is a real head scratcher...
• Brand new thermostat.
• Brand new exhaust flappers

I'm at a loss as to what could cause such a temp difference when the flow to the manifolds should be the same.

Maybe this isn't a problem? I was about to ignore it, but then when I saw the exhaust visible from just one of the 2 ports on the outdrive, I felt this might end up melting my flapper again.

 

[Lou C]

That, is very curious, you have a Vortec engine and there isn't supposed be an exhaust crossover in the intake like on the Pre-Vortec engines. I'm wondering just what is the purpose of that?!

 

[Lou C]

from the end of your last thread, that tube that connects the intake and exhaust manifold got me very curious so I did some snooping around....

Intake Manifold

Intake Manifold

www.mercruiserparts.com

Exhaust Manifold and Elbow

Exhaust Manifold and Elbow

www.mercruiserparts.com

OK you got me curious, looks like some of them have this tube that goes from the exhaust manfold to the intake manifold. Since the Vortecs had no exhaust cross over to warm the area under the carb, I'm wondering if this intake used a hot water passage there, that exits via the starboard side exhaust manifold.
Is it then possible that, the manifold on that side will run hotter because of this and it may be normal?
achris?

 

[ryno1234]

from the end of your last thread, that tube that connects the intake and exhaust manifold got me very curious so I did some snooping around....

Intake Manifold

Intake Manifold

www.mercruiserparts.com

Exhaust Manifold and Elbow

Exhaust Manifold and Elbow

www.mercruiserparts.com

OK you got me curious, looks like some of them have this tube that goes from the exhaust manfold to the intake manifold. Since the Vortecs had no exhaust cross over to warm the area under the carb, I'm wondering if this intake used a hot water passage there, that exits via the starboard side exhaust manifold.
Is it then possible that, the manifold on that side will run hotter because of this and it may be normal?
achris?

Hrm, interesting! That would definitely answer why it is hotter on that side....

I thought it was an EGR tube and just recirculating exhaust gases back into the intake manifold to be reburned. *shrug*.

I like your answer better because it would at least explain what's going on... but that wouldn't explain one of the hoses going to the manifold being hotter than the other I don't think.

 

[Lou C]

no EGR valve on marine engines, that I have ever seen.
If your thermostat has those poppet valves, I'm wondering about them...
an obvious explanation would be if those springs don't have equal tension you won't have equal flow....

see....springs, poppet valves etc....

Thermostat and Housing

Thermostat and Housing

www.mercruiserparts.com

I'm not sure why Mercruiser has this fascination with making a raw water cooling system so complex. You should see the raw water cooling system on the new 4.5. My brother has one. Runs great but a bunch of hoses, plastic fittings, etc. Meanwhile my old OMC and similar Volvos do just fine with 5 or 6 hoses in total, no poppets, springs, cross over pipes, plastic fittings to make it easier to drain that break etc.
Here's a news flash boat companies, your designs are horrible for mechanics, make the damn engine easy to get at, so Merc can trash all the new plastic junk hung on there engines because the boat companies insist on making the engine in-accessible.

 

[ryno1234]

see....springs, poppet valves etc....

Thermostat and Housing

Thermostat and Housing

www.mercruiserparts.com

You must be talking about #13 - #15 in that link... I don't have any nylon balls, etc. I can stick a screwdriver straight through the "T" which goes off to either manifold. It is just freely open.

By the way, I've really appreciated your help. Thank you.

 

[ryno1234]

@Lou C, I found a separate thread which says that tube that goes from the manifold to the intake IS an exhaust gas line: https://forums.iboats.com/threads/mercruiser-5-7-exhaust-tube.675274/

 

[Lou C]

You must be talking about #13 - #15 in that link... I don't have any nylon balls, etc. I can stick a screwdriver straight through the "T" which goes off to either manifold. It is just freely open.

By the way, I've really appreciated your help. Thank you.

So your thermostat housing is a different style then...

 

[Lou C]

The idea then is to heat the area under the carb, to better atomize fuel.
But, doesn't explain why the starboard side runs hotter still!
I know for sure that the standard GM cast iron intakes for the Vortec engines do not have an exhaust cross over like my old Pre-Vortec does. This must be a special intake made up to add back the exhaust crossover. Since the Vortec cyl heads have no provision for an exhaust crossover, they had to take the exhaust gas from the exhaust manifold then.
Are you measuring the temp on a trailer? Does it sit higher on the hotter side?

 

[Lou C]

Here's a pair of pre-vortec intakes, these are for the 4.3 but the 5.0 is similar. You can see the cross over port between the 3 intake ports. There is a matching port in the cyl head to feed in the exhaust gas. The tin cover is to prevent oil from burning off the hot cross over
passage.
This was eliminated in the Vortecs but I guess Merc decided to add it back, till they went EFI.

 

[Bondo]

I'm at a loss as to what could cause such a temp difference when the flow to the manifolds should be the same.

Maybe this isn't a problem? I was about to ignore it, but then when I saw the exhaust visible from just one of the 2 ports on the outdrive, I felt this might end up melting my flapper again.

Ayuh,..... Is the motor/ boat Dead Level,..?? If not, I'm guessin' yer seein' water comin' out the lower side, 'n less water comin' out the higher side,....

Yer chasin' a ghost, that isn't there,.....
Yer stated temps are well within what would be expected,.....

"n yes, I merged yer threads,..... Please read our postin' rules,....
One thread per topic,.....

 

[ryno1234]

@Bondo, the boat isn't level, however it is close to level and the low side is actually the hot side, so I don't think that plays into it here.

I have a new theory however - my thermostat housing takes water from the port side and flows it to the starboard side where it is being sucked in by the engine driven water pump.

I've found that all of the ports on the thermostat housing connect. There is absolutely no hard walls inside the housing that would prevent water from traveling from any one port to another. That said, there is a natural flow due to the pushing and sucking of water and the thermostat itself opening making certain flows more likely to be used (I don't fully understand exactly how this works).

I'm wondering if that Port to Starboard flow of water is what is creating a bias for the hot water to flow out the Port side.

I'm graspin' at straws, but, maybe I should just go boating and call it good.

 

[Bondo]

Ayuh,.... It's not a balanced, equal flow system,....

Go enjoy the boat,....