Mercruiser 496 (8.1L) overheating

cbruck

Member
Jul 5, 2016
40
Ocean Springs MS
Boat Info
2010 SR Sundancer 390 I/O's with 496 MAGs and Axius Drive
Engines
496 MAG's
I'm having a problem with overheating and the engines going into "safe mode". It only happens over time when running pretty well full rpms. The temp gauges read 175 when it drops to guardian mode. I live 100 miles from any mercruiser dealer that can work on my boat. I replaced the water pumps just last year. I just had the boat out and the raw water seems to be fully flushing. I am going to replace the thermostats and try to drain the old (don't know how old) coolant and replace. There seem to be multiple temp sensors. Does anybody know how many sensors and at what temps they are set for? I assume that the sensors in the exhaust manifolds are set much higher than the coolant sensors. Also, does anyone know the "shop" manual for merc 496's (2010 model) that will show how to replace the thermostat and how to drain the coolant. I have the service manual #30.
 
It only happens over time when running pretty well full rpms.

What do you mean by "full rpms"?
Are you sure that cooling is the problem?
There are a lot of conditions which will trip the engine into Guardian mode. Did you pull the codes?
 
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What do you mean by "full rpms"?
Are you sure that cooling is the problem?
There are a lot of conditions which will trip the engine into Guardian mode. Did you pull the codes?
This.

If it went into guardian, there is a reason. While the codes won't necessarily tell you what, specifically, is wrong, it will get you into the right system to start diagnosing. It could just be a wonky sensor... could be something else. Right now, it's just a guessing game.

How hot were the top of elbows - you should be able to keep your hands on them, at least for 3-5 seconds depending on how hard you were just running the engine and how quickly you check.

"Hopefully", boating in Missouri, he shouldn't be seeing too much salt water and the manifolds should be OK. :)
 
Outdrives or v drive? if outdrive, could be bravoitis - if v drive check to be sure the scoop/grate is clear - if close cooled, are old impeller pieces or some other restriction blocking the exchanger - there are two sensors in the water neck IIRC the one with two wires is the smartcraft one that could put it into guardian mode....good luck and please report back when the problem is solved
 
tiara in the snow 01.JPG
A little off topic, but when you say you are running at full RPM, that sounds like a pretty extreme way to run 8.1s. Do you do this routinely? If so, that is going to reduce engine life and create issues before they fail completely. Impellers would be high on the list of suspects. Assuming the boat has not been run in salt water for extended periods, manifolds and related plumbing should last the life of the motors. I would also look at hose failures where a critical component is soft and collapsing under load or where a piece of impeller has been sucked into a spot and is impeding circulation.
 
The 8.1 can have several cooling issues. If your gauge showed 175 degrees and you went into guardian, then it is not the antifreeze side of the system as 175 is fine.
I had this issue on my 8.1 and got the exhaust overheat alarm each time. Turns out the plastic collar in the outdrive that connects the water suction from the raw water pump to the outdrive was partially melted and restricting flow. So similar to Bravoitis in results, but it was not from saltwater. Best anyone could tell it was from running on muffs and overheating.
 
Replacing coolant if you choose to (but it is not going to solve your issue) is simple. There is a drain plug in the block on each side of the engine just above the oil pan level half way back. Depending on model year it is likely a hex plug. Pull those and drain. Port side is easy, starboard side is tough as there is other plumbing in the way. Refill into the reservoir a 50/50 mix of Dexcool and distilled water while keeping the front of the motor higher to burp it. Takes about 18 quarts total.
 
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Thanks for your input everyone. I'll try to go in order on my responses.

1) The boat is a SR 390 DA and 10 years old. It was always in salt water but winterized up north until the last 3 years where it has been in brackish water here in Mississippi continuously. The risers have not been replaced, could be the issue.

2) I'm running the boat around 4000 rpm's and after about a half hour the analog gauges read 175 and maybe rise to 177 when one engine drops into Guardian mode. Boat has vesselview and I just went to look at the alarm history but I just replaced the batteries so have wiped them out. I plan to run the boat soon and capture the alarm history.

3) The boat is on the Gulf coast. I plan to take the boat out and use an optical thermometer to test the various areas to see if I can determine the temp at the thermostat and at the manifolds.

4) They are sterndrives and so far it looks like (as you said) there is one sensor next to the thermostat but the sensor on the far side goes to the emc and vesselview. And then there is a sensor on the manifold. Also there is a water pressure sensor in the power steering and when I take it out again, I need to verify the PSI readings on vesselview. From what I can determine, they should be around 3-5 at low rpms and 15 or so at higher rpms. When I had the boat out we have an internal flushing system and running the boat out of the water the raw water system seemed to be pushing plenty of water. I realize there is the water pump in the sterndrives but the drives are 1 year old.

5) I do run the boat around 3800-4000 rpms as if I go much lower this boat tends to settle into the water and work the engines that much harder. The boat has run fine until about 6 months ago with this started to happen. I initially thought it was just barnacles over the water inlets on the sterndrive. We just had the bottom done and cleaned and checked the sterndrives. It's a 20000 lb boat. The sterndrives are new and as well as the raw water pumps (1 yr old). I certainly understand the impeller issue and depending on what we find on temps, water pressure and fault codes we plan to drain the closed coolant, replace the thermostat and remove and clean the heat exchanger. Might find something in the exchanger.

6) I'll replace the coolant as the maintenance indicates it should be done every 5-7 years. I agree that will probably not be the fix but I was curious if anyone changed their coolant and fixed an overheating issue? Is it possible to pull just one plug and flush the system with water at the thermostat? Getting to both plugs is a real tight fit in my boat.

Once I take it out and run it again, I'll report back as to what we find.
Thanks for all your input.

If anyone has any specific temps, such as at the thermostat (should be 160-170 if the stat is working)? The manifolds should run much hotter, how hot? And is the water pressure at around 5 at low rpms and 15 at high rpms correct?
 
Coolant temps should be 165 to 170. Manifold temps should be 200 but not much higher or you will go into limp mode.
You can pull one drain but you will leave a few quarts in the block- probably ok but not ideal.

Running the engines on the flush ports is what I was told melted my plastic sleeve and caused the exact issue you are reporting here. Theory is without the cooling water coming through the exhaust temps get too high in the leg and melt the sleeve. Water pressure should be 3 at idle and over 20 at WOT, with cruise around 12-15 psi. Check those closely as they will be indicative of the issue.
 
(I realizethere is the water pump in thesterndrives but the drives are 1 year old)

Bravo III’s don’t have a water pump in the outdrive.

Only a passageway for raw water to be drawn up by the engine mounted raw water pump.

I find a few barnacles in mine every year. My Winter service includes splitting the drive and cleaning out the barnacles.

Water pressure readings are a good place to start. Lots of good people here that will point you in the right direction if you provide some data.
 
You have Bravo 3 drives - ALL Bravo series drives DO NOT have an impeller/pump in the drive. That is only in the Alpha series of drives.

Getting the codes will help tremendously and save us from guessing and possibly putting you down the wrong track.

Actual temperature numbers are good, to a point. But don't get overly concerned with the actual number - especially don't try and compare "exact" numbers from your engines/sensors to another. It's not an exact science. Plus, the number can sometimes change (when using an IR gun) simply by moving the gun 1/4" one way or the other. However, as I mentioned above, using your hand is a great/quick tool to see if the engine is "actually" overheating, or if a sensor is just going wonky.

If I'm understanding what you wrote correctly... for the first 6 or 7 years of it's life it was in salt water? So that definitely brings the intake hose, where it comes through the transom, into question... "Bravoitis" as mentioned above, amongst other things. Google that a bit.

Here's a good test that can be done to check the amount of water coming to the engine:
-- This must be done in the water at exactly 1,000RPM's for 15 seconds. Repeat it 4 times.
-- Take the output hose off the raw water pump and use another hose and plumb it to a bucket. You could also plumb off the power steering cooler (a little easier to get to).
-- A good pump will flow 7.5qts in those 15 seconds.
 
Hopefully the codes will help narrow down where to look.

4,000 rpm for sustained periods is also interesting. I believe with matched props your boat/engine combination tops out at a rated 4500 rpm. Are the throttles full forward to get to 4,000 rpm?
 
Same motors and drives. 2006 340. Same problems arose in our 2nd season, primarily on Port motor, which also had high H2O PSI numbers Did all of this to both motors, although only one was having the issue. Started with impellers, no change, the old ones appeared fine and in tact. Then pulled heat ex changers, OMG were they all plugged up. There is no strainer on the intake from the drives, so whatever has been sucked up over the years goes straight thru the raw water pump and into the ex changers. Cleaned with muratic acid, the crap that came out was amazing. Put everything back together, saved and reused existing coolant. Temps and pressures back to normal, for me 170 at what ever RPM. 3rd season, last year - started showing low raw water PSI on Stbd motor. Removed hose at transom, Diagnoses Bravoitis. (See Pic) Elected to change over to thru hulls and T the outdrives into the thru hulls. Also added drive showers. Now have strainers that catch the crap before it gets into the system and both motors run @ 170 degrees with PSIs @ 3-5 @ idle and 15 @ 3250 (my cruising RPM) and low 20s @ WOT. My first fault on the SmartCraft was the exhaust manifold temp sensor, as this is the last to get raw water, which before doing all of the above this water was already hot and had little cooling effect.
This was my experience, hope it helps you with your investigating. Also, for future reference, if you do get some wonky faults that do not make any sense - check your batteries condition. The computers on these boats are extremely sensitive to voltage changes. I chased a ghost for 2 seasons, kept getting a Fault 40 MAT on the SmartCraft. First I swapped sensors between motors, no change. Then changed it out - still no change. Finally gave in and had it put on diagnostics, showed voltage changes but not enough that the tech thought that was the problem. Changed out batteries on that motor, problem solved.
18318.jpeg
 
Again, thanks everyone for the help.

I didn't realize that there was not a water pump in the Bravo 3 drives. I'm used to my old Alpha's.

1) Thanks for the temps, it's what I expected. I was curious if anyone knew at what temps the sensors indicate the overheat condition. I don't believe I overheated anything as when I ran it out of the water, with the internal flush going, I only had the boat running 1-2 min to align the drives for removal.

2) I will get the water pressure numbers and alarm codes

3) I understand taking temps. Just an interesting note that when I was warming up the engines the port side of the engines (mqnifolds) warm up considerably faster that the stb. I first that that was a problem but both engines do the same thing.

The boat was in NY for it's first 7 years where it lived in the water for six months a year and pulled and stored for the other 6 months. Then I brought it to Miss where it has lived in the water for the last 3 ish years. It was out for nearly 9 months of that time getting new drives put in.

My understanding on Bravoitis is blockage where the raw water passes through the transom. About 1.5 years ago, we hit something like a rope and it took out both gimble bearings which ended up damaging the transom assembly etc. From the engines out, everything (transom assembly and drives) are new.

Your waterflow test is good information, thanks.

4) If and when I drain the coolant that I will pull the heat exchanger apart and clean since I'm right there replacing the thermostat.
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I do have a problem with wonky codes as I get a "non critical-thrust vector module" error randomly with just the key turned on without the engines running. They say the lower CCA on the batteries are the problem and the solution is $1600 worth of odyssey batteries. I do have a question, if I replace the batteries do I then have to have the software reloaded or does simply installing the high CCA batteries solve all my problems? If anyone has comments in this regard, I'd appreciate hearing them.

Again, thanks everyone for your comments and when the weather improves and I can get the boat out for a run, I will get back here with more data.
 
Yes, it's absolutely normal for one manifold to warm up quicker than the other. It's not specific to your engines, by the way, it happens to all of them.

TVM fault... Yes, if memory serves, that is one of the faults that you get when the battery voltage is lower than what the computer likes to see. DTS/Axius... all of those kinds of things are very voltage sensitive - a lot like higher end cars and even some newer "normal" cars. I believe the voltage needs to read at least 12.0V for normal function. What does it read before you turn anything else on? Does it drop siginificantly?

Try starting the generator, first. Do you still get the TVM fault?

As with all, newer, voltage sensitive things, correct voltage is key. Without that, all sorts of weird and sometimes unexplainable things can happen. You mentioned you just replaced the batteries... is that roughly when the issue started? That very well could be the issue if the batteries you put in there aren't of the proper rating. Double check me on this but I think, if memory serves, that the batteries need to be rated at a minimum of 1000MCA and 180 reserve minutes. Check out the Deka Intimidator series.

If the batteries aren't up to snuff, I would take care of that first. It might fix the guardian issue.

Edit: Oh, one more thing... double check that your alternators are putting out correctly - especially the engine that's going into safe mode. One of the reasons for guardian mode is to try and preserve enough juice for shifting and steering, since those are not analog.
 
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cbruck. Sounds like you can rule out the bravoitis. I would start at the heat ex changers. Just pull the end caps off n look at what's behind them. If they are plugged up, they will need cleaning. Never done it this way, but you may be able to do them on the boat by removing the hoses and circulating Barnacle Buster thru them using a small bilge pump and 5 gallon bucket, just letting it recirculate. Will take a little engineering to adapt the hoses to fit etc, but certainly would beat taking everything apart. Why I would start here is due to your comment that your port side of your engines get hotter faster than your strbd side. This is due to when the sea water comes in and goes thru the ex changers, if they are plugged up, not enough water will flow thru to effectively cool the ex changers and that decreased flow of water will also heat up hotter and faster. The Port side of your motors is the final place the raw water has the job of cooling. If it is already at 160 - 170 degrees, it will have little cooling effect. I would do this to both motors before anything else. Another indication of this is high raw water PSI on your gauge. Before I did mine my pressures were in the high 20s - 30s at relatively low rpms. After, they dropped to what they should be as already mentioned. High numbers means there is a blockage somewhere, low numbers mean engine is starving for water, i.e. bad impeller / blocked sea water intake.
When I was trouble shooting mine, in the spring when the water temps were in the 60s and low 70s, I would run warmer than usual, but no alarms. It was August, when the sea water temps were 84-87 degrees, I started running hotter as warmer August water didn't cool as effectively. Be careful not to damage the rubber seals that are on each end. I made new ones from rubber gasket material from AutoZone. But you may not need to replace yours. I just did mine because I had it apart.

Now $1600 for batteries. OMG you are getting raped. Most of us locally with 320 - 380s, and I have read it in these and other forums as well, run Group 31 AGM Marine Batteries. They are available at SAMs Club for $179.00 each with a core turn in. They are also available at Batteries and Bulbs online at $219.00 each and have a 30 month replacement warrantee. Order on line and pick up at store and you get 10% off. If your batteries are more than 2-3 seasons old, you should consider replacing them. Batteries and Bulbs will also test your old ones so you can see what there condition is if you want to go that route first. Not sure of your charger set up, you will need to make sure it is set properly for the AGM batteries. The difference in CCA should NOT make a difference. It should already be set properly, but worth checking.

One more thing - your motors will run/test cooler when on muffs and hose water. Most hose water is way cooler than sea water, therefore has better cooling effect.
 
I didn't reply to the question about rpm's. You are right that the top end of my boat is 4500 rpm. When I purchased the boat on out sea trial, the marinemax broker took it up to 4500 for a short period per my request. It probably hasn't been above 4200 since and I generally run it around 3800.

1) Yes the TVM fault has been there since I installed new batteries a 1.5 years ago. The batteries are consistently around 13.5 volts but I did put a oscilloscope on them and cranked the engines and it dropped down to 11.5 volts for a short (100 ms) time. My MCA is listed at 800. They are not beefy batteries. I was told the TVM module in the Gen 1 Axius has a voltage problem. Good idea about starting the generator first and see if I get the fault. The thing is, I can just turn the keys on (without starting the engines) with batteries at 13.5 and I will get the fault randomly. That was my question about the software. Does the software get permanently damaged with low voltage conditions and has to be reloaded with the G3 diagnostic software? If I replace the batteries with it fix the problem or will I have to have the software reloaded? The alternators have both been replaced and are putting out 14+ volts.

2) I was told by a friend/mechanic that we have to remove the heat exchanger if we want to replace the thermostat and we will pull the end caps off and clean them out. He said he has used a gun cleaning rod that works really well to clean out each of the internal tubes.

And on the batteries, I mentioned the $1600 and I have 4 batteries at $400 apiece for the Odyssey's which are supposed to be the highest MCA that you can buy. If I change them out, I think my charger has a AGM setting that I'll need to set.

Again, thanks for the comments.
 
Sounds like you have several options to start with / check out. Best of luck and let us know how things are going. This way we all get the education. We can only give you what our experiences are, yours are going to be unique to you and your boat. Thanks for sharing and I am sure those that have already shown interest will follow your progress.
 
You shouldn't be getting 13.5V without the engines running, though. A brand new battery is 12.8V. 13.5 to 14-ish is with an engine (alternator) running. Looks like your alt's are good, though.

FYI, you likely have the normal 496's (not the HO's). Their WOT range is 4400 - 4800. I prefer to see engines being able to run closer to the max for best efficiency, but 4500 is OK. Likely that's just the best you could get with the load you have in the boat and the conditions.
 

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