Merc 350 mag mpi HELP! 200°

[Timbolex]

I'm running out of options, I have a 2003 merc 350 mag with bravo3, runs incredible, water temp gauge reading 180 while at low rpms and as soon as i get on lake jumps to 200 plus, changes impeller, tstat (160) temp sender, did some IR readings and the area where the tstat goes and the ecm sender/temp sender reads a perfect 160s, but the sides of motor reading 200plus?! Is this accurate? Wouldn't it go into limp or alarms with it close to 210 on sides?

 

[Timbolex]

Is this normal

 

[dubs283]

did some IR readings and the area where the tstat goes and the ecm sender/temp sender reads a perfect 160s, but the sides of motor reading 200plus?! Is this accurate? Wouldn't it go into limp or alarms with it close to 210 on sides?

Looks like based on pictures you're taking temp readings at the exhaust manifolds. Yeah, they get hot. If you are getting 160 degree readings at all rpm/load ranges at the senders you are good.

The outsides of the exhaust manifolds get extremely hot during normal operation

 

[Bondo]

I'm running out of options, I have a 2003 merc 350 mag with bravo3, runs incredible, water temp gauge reading 180 while at low rpms and as soon as i get on lake jumps to 200 plus, changes impeller, tstat (160) temp sender, did some IR readings and the area where the tstat goes and the ecm sender/temp sender reads a perfect 160s, but the sides of motor reading 200plus?! Is this accurate? Wouldn't it go into limp or alarms with it close to 210 on sides?

Ayuh,..... Did you back-flush the in-comin' line from the drive, when ya changed the impeller,..??
Is the impeller housin' still perfectly smooth,..??

 

[Lou C]

In those pix you appear to reading the temps of the exhaust manifolds not the engine. If that engine uses the warm manifold system they will be somewhat warmer than the engine because the manifolds don’t get cooling water till the engine thermostat opens. I’ve found that the best place to take readings is on the intake manifold right under the thermostat housing. That is usually 10-15 degrees lower than my dash gauge. That said yes your temps are too high. Perhaps check for Bravoitis? I would also test the thermostat make sure it opens at the right temp & opens all the way. Might want to test the raw water output flow from the impeller. Last thing would be clogging in the exhaust elbows but if you’re in fresh water it’s not that likely.
Temps I get on the rest of the engine are 145-155 or so. Manifolds (cold manifold style system) are 100* at idle and the highest they get is 135* after coming off plane.
These open cooling systems are low pressure but high volume. Anything that restricts water flow in or out will cause overheating.
A less common cause of overheating can be leaky head gaskets where combustion gas gets into the cooling water. This can be checked by replacing the hoses feeding the manifolds with clear hose. With the boat in the water & fully warmed up look for bubbles in those hoses…

 

[Timbolex]

Looks like based on pictures you're taking temp readings at the exhaust manifolds. Yeah, they get hot. If you are getting 160 degree readings at all rpm/load ranges at the senders you are good.

The outsides of the exhaust manifolds get extremely hot during normal operation

 

[Timbolex]

I was hoping that was the case

 

[Lou C]

However your dash gauge temp is still too high…
Here’s mine

 

[Timbolex]

I agree, I believe the dash guage is whacky, it's inconsistent

 

[Timbolex]

and wouldn't you think if temp was actually what it says on guage, i would have alarms sounding or gone to limp mode?

 

[Timbolex]

In those pix you appear to reading the temps of the exhaust manifolds not the engine. If that engine uses the warm manifold system they will be somewhat warmer than the engine because the manifolds don’t get cooling water till the engine thermostat opens. I’ve found that the best place to take readings is on the intake manifold right under the thermostat housing. That is usually 10-15 degrees lower than my dash gauge. That said yes your temps are too high. Perhaps check for Bravoitis? I would also test the thermostat make sure it opens at the right temp & opens all the way. Might want to test the raw water output flow from the impeller. Last thing would be clogging in the exhaust elbows but if you’re in fresh water it’s not that likely.
Temps I get on the rest of the engine are 145-155 or so. Manifolds (cold manifold style system) are 100* at idle and the highest they get is 135* after coming off plane.
These open cooling systems are low pressure but high volume. Anything that restricts water flow in or out will cause overheating.
A less common cause of overheating can be leaky head gaskets where combustion gas gets into the cooling water. This can be checked by replacing the hoses feeding the manifolds with clear hose. With the boat in the water & fully warmed up look for bubbles in those hoses…

The head gasket issue worries me bad, the good signs is my oil looks clean, no white smoke and runs really well, I'm going to check my hoses today, when checking with IR the entire engine block seems right temp, but that 200 plus on exhaust manifold freaked me out, newbie!

 

[Timbolex]

Ayuh,..... Did you back-flush the in-comin' line from the drive, when ya changed the impeller,..??
Is the impeller housin' still perfectly smooth,..??

I paid someone to do impeller, so can't really answer those questions

 

[Timbolex]

Apparently it's not so much the exhaust manifolds getting hot that's an issue, just want to make sure risers are not showing super hot, I'm guessing these are risers?

 

[Lou C]

Technically speaking they are exhaust elbows. A riser is a spacer that goes between the manifold and elbow if you need more height on the exhaust to prevent the exhaust backing up into the cylinders. That depends on the static water line of the boat (how high the water is in the exhaust Y pipe behind the engine).
Normally on a cast iron exhaust system the elbows will show corrosion inside faster because they are exposed to hot exhaust gas and water. When the water passages start to clog up you can have very hot exhaust even if the engine doesn't overheat (yet). That is dangerous because the rubber exhaust hoses can start to melt, fail and let water in the boat. The temps of the elbows must be kept below the melting point of the rubber. Ideally less than approx 160 or so. Mine never got that hot, unless there was a problem.

 

[Timbolex]

Technically speaking they are exhaust elbows. A riser is a spacer that goes between the manifold and elbow if you need more height on the exhaust to prevent the exhaust backing up into the cylinders. That depends on the static water line of the boat (how high the water is in the exhaust Y pipe behind the engine).
Normally on a cast iron exhaust system the elbows will show corrosion inside faster because they are exposed to hot exhaust gas and water. When the water passages start to clog up you can have very hot exhaust even if the engine doesn't overheat (yet). That is dangerous because the rubber exhaust hoses can start to melt, fail and let water in the boat. The temps of the elbows must be kept below the melting point of the rubber. Ideally less than approx 160 or so. Mine never got that hot, unless there was a problem.

So if the part that I circled is not super hot, but just the sides of sides that I was taking temp of is only high, and thermostat housing is also around 160, will that be ok?

 

[Scott Danforth]

go back to the person you paid and ask if they found all the impeller bits

if not, start pulling things apart yourself

overheat is a direct result of lack of cooling water flow.

 

[Lou C]

Super hot meaning how hot? How hot is it where the rubber hose fits on? And you should take readings right after the boat comes off plane that will be the hottest it gets (usually). Idling is not a test esp if on the water hose/muffs not in the water. Remember it is the rubber exhaust hoses you are concerned with.

 

[Timbolex]

go back to the person you paid and ask if they found all the impeller bits

if not, start pulling things apart yourself

overheat is a direct result of lack of cooling water flow.

It isn't really overheating, just hot at exhaust manifolds, block cool and thermostat housing also 160°

 

[tpenfield]

Keep in mind that the exhaust manifolds are going to be getting the 160˚F water from the engine as their cooling water. So, probably 185-200˚F is not bad. The riser/elbow gets cool water directly (which then mixes with the water from the manifolds.

The top of the riser/elbows should be in the 100˚F range at idle, because of the cool water it is getting.

 

[Timbolex]

Keep in mind that the exhaust manifolds are going to be getting the 160˚F water from the engine as their cooling water. So, probably 185-200˚F is not bad. The riser/elbow gets cool water directly (which then mixes with the water from the manifolds.

The top of the riser/elbows should be in the 100˚F range at idle, because of the cool water it is getting.

That makes perfect sense, heading out in a couple hours, I can't remember if I checked the temp of risers, but I'm really hoping they are gonna be around 100 when I check, fingers crossed! I'll keep ya posted