overheating

Well you were right,

old muffs busted so water flow was not getting through properly. My new/old impeller did burn up.
I am replacing impeller now and going through the hoses to find all debris. is there a certain amount of time /rpm that I should have the muffs on for?
I’d like to find out if it is overheating but is 10minutes to long to run it only on the muffs?

I imagine the pully seized up briefly when the impeller shredded .

but also how would I check the exhaust to see if the old shutter peices are still in it? Would that cause overheating?

Btw is anyone experiencing boat ramp shut downs due to the virus? I am in Jax Fl

Not familiar with the 5.7, I have only worked on 7.4 and 496 (8.1), but I think your exit hose coming off your raw water pumps next stop is the heat exchanger. Should be able to pull the end cap off the exchanger (no need to drain coolant/antifreeze in block) and there you would find a bunch of the impeller pieces. Save them so you can puzzle them back together to see if you can account for entire impeller. You can also back flush your various cooling units to make sure everything is out. Since you are on the hard, make sure your transom plug is out and remove hoses off units and rig up connections to back flush oil, fuel, steering and heat exchanger units. Do them individually to be sure you don't get water in places it does not belong. This will also let you know for sure you have an unobstructed path thru each of these units. If you do all this, (which sounds like a lot, but isn't that bad, I did both my motors in a day), you will know your entire raw water intake system is open and unobstructed as long as you also did what Epos4 stated earlier. That is remove the water intake hose from inside the transom to check if it is open or closed down due to Bravoitis. I have attached a pic of what Bravoitis looks like. If there is restriction there - that would be the root of all of your problems. Be sure to do this before restarting your engine. I had this happen on one of my 496mags and it struck hard and fast. There is an updated kit from Merc that replaces that hose that goes thru the drive with a hard tube to prevent this from re occurring. After some investigating, I elected to install thru hulls and strainers on both motors and T'd them into the outdrive line. I now strain the water coming in via the thru hulls - very little comes thru the drives now - and can see the water moving thru the strainer for a visual verification. I also installed drive showers on my out drives to assist with cooling them, although the water that goes through the drive on its way to the engine has very little cooling effect on the drive.
So far as how long you can run on muffs. I have ran mine all the way to operating temp at just a little faster than idle for as long as 20-30 minutes. Just need to keep an eye on water flow out of the drive (no water flow out, shut down immediately). If you have a raw water pressure gauge, make sure it is reading some pressure. Doesn't need to be a lot, even 1.7psi is plenty. Also, check your water as it is coming out to see if it is warming up as well as your exhaust elbows. Should be able to lay your hands on them while motor is running. After shutdown, they will get hotter - this is normal. Sounds like a ton of work, but you will come away with piece of mind knowing that your raw water system is unobstructed and flowing properly.

Thanks, going to check through the list and get back with you about my findings

After checking and cleaning the cooling system and checking for bravoities I took the boat out on the water today.

off the ramp I overheated about idle speed . Shut off engine and cooled it down. started back up and ran it above idle speed then stayed around 1400rpm temp went to 165
I brought it up to 2100rpm and temp climbed to 188 then I opened up to 3000rpm to get in an area where I could shut off the engine. To my surprise the temp cooled back down to 163 after raising the Rpm’s. I continued to cruise around then raised up to 4000 rpms and ran for 10minutes or so. I slowed down to 1500rpms and immediately the temp started to climb.

having trouble with this overheating issue.

Km renovations said:

After checking and cleaning the cooling system and checking for bravoities I took the boat out on the water today.

off the ramp I overheated about idle speed . Shut off engine and cooled it down. started back up and ran it above idle speed then stayed around 1400rpm temp went to 165
I brought it up to 2100rpm and temp climbed to 188 then I opened up to 3000rpm to get in an area where I could shut off the engine. To my surprise the temp cooled back down to 163 after raising the Rpm’s. I continued to cruise around then raised up to 4000 rpms and ran for 10minutes or so. I slowed down to 1500rpms and immediately the temp started to climb.

having trouble with this overheating issue.

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You may have already addressed these two, but are you running without a thermostat to eliminate any issues with that?
Also, have you validated your temperature gauge?

I do have a thermostat in (167degree)
I did replace the temp sensor on the tstat housing

I saw a post that says air leak can be a cause most likely from the water pump gasket.
Going to attach a clear hose tomorrow and run water to see if I get any air bubbles in the lines.
Any tips or suggestions are greatly appreciated!

Sounds like you have pretty much ruled out blockages and such on the raw water side except the actual gauge itself which measures engine temp, not raw water. What are your raw water PSI readings at the different rpms you mention. (Assuming you have that gauge on your system) They should be 2-3psi at idle, and then gradually rise with rpms. WOT should be at least in the high teens to mid 20s, maybe a bit higher. This will tell you how efficiently water is moving through your system. If your psi is say 2 at idle, then 7 at 1400, then 12 or so at 2100, then 18 at 3500, this is normal escalation. If the numbers are staying low or there is variation such as going up to 12 and then not increasing much as you get to the 2100-2300 rpms, that indicates starvation for water. On the other hand, if the psi goes up to the higher numbers at the slower rpms, this indicates restriction. These numbers are very useful in diagnosing if you have a water starvation or water flow restriction issue. Also check - How do the elbows feel when running? Do they feel hotter when the gauge says they should be? You can get an IR temp gun to measure actual temps on various parts of the motor as well. Relatively inexpensive and nice tool to have. As mentioned earlier, you should be able to put your hands on them while running. It is normal for them to be hotter when you come down off plain, so don't let that concern you. (Side Note: Get in the habit of letting your motor run at idle or just above after coming off plain. Don't shut it down right away, give it some time to pump some water through so it will cool down. The cast manifolds hold and absorb a ton of heat.)
Just as a thought, you have been looking hard at the raw water side and you had mentioned the center water pump (I am assuming you mean the coolant pump) had seized up in an earlier post. I am not one to throw $$$ at a problem with out justification, but to rule out the coolant side and after having that pump seize up, maybe consider changing that pump out would be warranted.
There is probably something really stupid we are overlooking or not checking into deep enough. This is usually not this difficult to rectify.

Check water flow from the drives to the sea pump.

Check the flow from the sea water pump as per the Mercruiser service manual. You basically need to connect a long hose to the outlet of the pump put it into a 5 gallon bucket, bring the engine to 1000 rpm and measure the volume - it should be minimum 7.5 qts per 15 seconds.
This will tell you if you have enough water coming from the drive to the sea pump.

On my 496 the water goes from the pump to the power steering cooler first ( Big hole in cooler ) then to the oil cooler with tiny tubes. I had a problem when I first boat the boat with overheating I found sea weed in the oil cooler, After removing it I was good.

Since the problem appears to intermittent, is a varying obstruction possible? Can a hose breakdown on the inside and separate internally? If so, the main hose coming in from the drive could be variable in the obstruction. As more water is pushed through, the internals cause more blockage but not always the same amount. Pulling and inspecting that hose would probably be a PIA, but it would be cheap/no cost. Unless you already did this.

Oh...and another cheap diagnostic is to pull the thermostat. Run without one. Will take only a new gasket and twenty minutes. Rarely does a tstat go bad, but you would know for sure if this still happened without one.

DWABoat said:

I think you should pull those manifolds immediately, whether that is the cause of the overheating or not.
Since you just got the boat, it has lived in salt water, and you don’t know how old the manifolds are, you need to evaluate.
If the manifolds are severely rusted/corroded inside, they can still look clean on the outside. At some point, the insides can corrode and cause water leakage into your engine cylinders and blow your engine by hydro lock. I don’t know the year of your boat. You could have dry exhaust or wet. Either one can have corrosion on the mating surfaces.
Since pulling them is simple and only costs the price of gaskets, it is a logical step in your evaluation and just good business.
Also, pulling the manifolds gives easier access to the backside of the motor for the other work.
Pull the manifolds/risers. Take lots of pictures from all angles. Post them here for these guys to take a look.

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Good point the manifolds on our boat plug up slowly in salt water and after 3 years they get full pf rust/salt/grunge. Also the thermostat gasket has a by pass cut out on one side. The last one I got did not have the cut out. I cut one in. It worked fine. Make sure you put in the correct thermostat ours is 142F a mechanic put in a 160F and engines got hot above 1200 RPM. Your heat exchangers could be pugged up a rad shop can refurbish it for about 200. We have small heat exchangers on the oil and transmission . You probably have one on the oil only. New one from mr cool is about 100.

You can do.the heat exchangers while on the boat. This allows you to do it without draining antifreeze. Just pull the raw water hoses off and create a loop using barnicle buster. I have done this and worked great. Used a drill pump to circulate, and a 5 gallon bucket as a troft. Let it circulate for an hour. Used wire tie to keep grill at proper speed.

Something I thought I read. You said you changed oil and not filter but added 5 quarts of oil. I believe that engine takes 5 w filter change. Probably nothing but check level.
do you know when manifolds and risers were last changed.

DWABoat said:

Oh...and another cheap diagnostic is to pull the thermostat. Run without one. Will take only a new gasket and twenty minutes. Rarely does a tstat go bad, but you would know for sure if this still happened without one.

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A thermostat also acts as a "restrictor" in a motor so that the coolant takes some time to sit in the intercooler/radiator to actual cool down. Most thermostats fail in the open position. Of course I would love to be proven wrong on this to get some resolution. But, just wanted to throw this information out there.