Mercruiser 5.0 MPI Overheating

[MattKarins]

I own a 2013 Tahoe Q7I SF and it has a Mercruiser 5.0 MPI in it and late last season it started running hot out of no where. By hot I mean about 200 or around there, 180 is center on the temp gauge. I think it was the second to last time we had it on the lake that it heated up and I really didnt pay all that much attention to it because it would end up cooling itself down. I winterized it and put it in storage, then this summer we got it out dewinterized it and took it for the first time a couple weeks ago and right off the bat it started heating up but this time it got pretty warm around 210-220. So trailered it and called a local boat maintenance shop. They suggested the impeller might need to be changed it never has been and might have gotten brittle. So I buy the impeller and change it myself (also impeller was actually in great condition) and put the lower unit back together and take it back out on the lake for a test run. This time within about 10 minutes the temp gauge is at 220 and not stopping, this is at a basic idle to get out of the no wake zone. As I am turning the boat around the temp gauge hits around 230-235 and kind of holds there and bobbles up and down as I am going back to the dock to trailer it. I decide to open the two drain vents it has installed to winterize and see what I get as water flow, it has great water flow and the water to me seems warm but not 240 degrees hot. So after trailering it and getting it back home I decide it might be the thermostat so I pull it and it looks horrible and wont move. I put it in a pot and heat it up with a thermometer and its almost 195 before it even budges and it never really fully opens. So go down to the parts store and buy a replacement thermostat and put it in and its pretty windy so instead of taking to the lake I decide to put the muffs on and hook it up to the hose and at idle it still heats up to around 220-230. So shut it off and now am at a loss. This is where I need you guys help please.

 

[alldodge]

Howdy

Lets find out if its the gage or the motor. Pick up an IR temp gun and check temps at the thermostat and other locations. Could be your gauge or sender is going bad

 

[Bt Doctur]

If you can keep your hand on the manifold ,therm housing and riser without melting your skin your not overheating.

 

[MattKarins]

I really appreciate the replies ALLDodge and Bt Doctur, the thought of an IR gun had crossed my mind the very last time I ran it and it overheated but I am very new to boating and I really didnt want to restart it and do damage if possible. Also yes I can put my hand directly on the thermostat housing and hoses all day long just about when the gauge is reading pretty high.

 

[Bt Doctur]

LIke I said ,if you can hold your hand on it your not running hot. suspect a temp sender or wiring issue. Check the ground connections at the gauges. A missing /loose/corroded ground can affect the temp gauge that way. Can test by running a temp wire from the neg battery post to the black common gauge wire

 

[MattKarins]

Would it still react like its picking up a temp, because its still a semi slow gradual climb? Even takes about 30 seconds to a minute to show any temp at all. It just never really stops when it starts and thats what worried me is that it was dang near to top of scale at one point.

 

[QBhoy]

Dodgy gauge aside....
Pull the exhaust rubbers off each side of the engine and look down the cow horns to check the exhaust flappers haven't broken up and restricting flow. Easy check for one of the possible causes.

 

[alldodge]

Would it still react like its picking up a temp, because its still a semi slow gradual climb? Even takes about 30 seconds to a minute to show any temp at all. It just never really stops when it starts and thats what worried me is that it was dang near to top of scale at one point.

Yes, motor has to have time to get up, and is looking like the sender

 

[Silverbullet555]

Dodgy gauge aside....
Pull the exhaust rubbers off each side of the engine and look down the cow horns to check the exhaust flappers haven't broken up and restricting flow. Easy check for one of the possible causes.

Is Merc still putting flappers in?

Did you get any temp alarms north of 200?

 

[QBhoy]

Think so...pretty sure mine has them in my 5.0 mpi. Might stand corrected.
I have a corsa captains call fitted, but when in silent mode you can hear them chatter.
Perhaps the op doesn't have flappers.

 

[Silverbullet555]

Think so...pretty sure mine has them in my 5.0 mpi. Might stand corrected.
I have a corsa captains call fitted, but when in silent mode you can hear them chatter.
Perhaps the op doesn't have flappers.

I'm not sure either. Volvo Penta stopped using them a while back. At least in the engine in my boat. That's why I asked.

 

[MattKarins]

Is Merc still putting flappers in?

Did you get any temp alarms north of 200?

See thats the thing, I have never gotten any audible alarms or any lights on the dash ever until the very last time I tried it after replacing the thermostat. And that was an audible alarm directly after start up, it stayed on for about 10 seconds and went off, no other signs at all on any gauges or anything. Then I revved it up just a hair to about 1500 rpms just to see if it would build heat. And it literally just beeped at me and went off. Then it started heating up and never stopped.

 

[MattKarins]

I cant thank you guys enough for the info, again I am definetly new to all of this so I greatly appreciate it. I have been on this forum a million times just never joined until the other day.

 

[alldodge]

Merc still uses flappers

 

[QBhoy]

Check the flappers then.

 

[RichRab]

I think if the flappers were bad you would have seen them when you did water pump and had drive apart. When I had same problem I took hose off power steering cooler and a tub under drive and stuck a garden hose in cooling line you just removed to flush backwards. I got mud out it solved problem. If it is overheating and alarm is working it would be a solid alarm. Been all through it.

 

[Nutcase76]

This is totally automotive related so take it with a grain of salt. A few months ago my son said his ford focus was overheating at highway speeds only. I took it for a drive and everything I got up around 70 mph, the gauge would steadily climb into the red. Slow down, and it would go back down. Pulled over to check and nothing seemed extremely hot. No boiling, etc. It made no sense at all to me. If there were a blockage or issue, it should manifest itself more at lower speeds or stopped as there isn't any airflow through the radiator. Several forums suggested the temp sensor and even though it didn't make sense in my head, I replaced it and have no issues sense. I still don't understand what would cause the sensor to keep rising with speed but it solved the issue so it might be worth a shot for you. I'm just thinking that if the manifold doesn't burn your hand that it's not really above 200 degrees

 

[Lou C]

Might want to pick up an IR temp gun, its a great tool for troubleshooting overheating issues. When taking readings, I've found that on the GM V6 or 8 when you take a reading right under the t stat housing on the intake manifold, your reading is the closest of anywhere on the engine to the temp gauge on the dash. When my gauge said 160, I would get 150-155 in that spot on the intake. At idle the manifolds were usually at 85-90 and the hottest they would get after coming off plane was about 135. I'd say if you can keep your hand on them, its gotta be below 140-145. Above that it feels HOT.
​When you replaced the stat, was it for sure a marine stat? They are supposed open much lower than an auto stat, marine starts to open about 160, auto not till 195. Did you test the new one first? Might want to try that. Also if the old stat was crudded up, did you check the outlet ports that feed the risers, if they are clogged it can cause the engine to start to overheat and the risers to get hot. These open systems depend on a lot of water flow in and out. There is very little pressure in them so the water can boil easily if it gets to 200 degrees, which is why alarms are set to go off then. Get the IR temp gun and take readings first, verify that the dash gauge is OK. Then check the stat and housing again. Next I'd do the raw water volume test to make sure the impeller is pumping enough water. Each manufacturer has their own spec for this.

 

[tpenfield]

replace the gauge sender ( not the ECT sender) and see if that makes a difference. The sender ( or gauge) may be playing tricks on you since the engine seems to be cool to the touch.

 

[MattKarins]

Thank you guys for all your help, I just wanted to provide an update. I had one day off today so to be honest I was to scared to run it if the gauge was true to even trouble shoot anything else, (being the rookie I am.) So I called the local marine mechanic shop a little over an hour away and told them what was going on and they said to bring it over but they were 5 weeks out on things since they were basically the only decent ones around. So I pull up and give the guy the whole spiel of what I had done and what the boat is doing and he looked a couple things and asked my wife and I if we were going to be in town very long. I replied sure we were going to go grab lunch. He said "heck if its something simple I can fix Ill stick it right in our tank and run it a bit and see what I think and let you know." So we went and ate lunch and piddled for a bit and he called me about 45 minutes later and he said he had hooked up to it with his computer and ran it for a while in their tank and shot it with their IR gun and checked several things and everything checks out great except the gauge. So we went to go pick it up and when I get there it was still in the tank running and he said come check this out. As I round the corner he taps on the gauge thats pegged out and drops completely off and then jumps all over the place. So he ordered me a gauge that will be here tomorrow and thats it. Again I really appreciate all your help I am certian I will be here again.