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496HO sensor issue - overheat alarm

Old 07-14-2014, 09:37 AM
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Default 496HO sensor issue - overheat alarm

I have an issue on my port engine - alarm will sound in constant tone, ECM cuts back power and only lets the engine idle. Shop took a look at it, had an overheat code and it looked like low water pressure before the sea pump was causing it. They changed:
-Sea Pump
-Impeller Housing (it was scored, had the aftermarket ebay kit in it)
-Impeller (didn't look terrible but it was due for a change)
-Temp sensor

This still didn't solve the problem...I'd get the alarm/guardian mode within 5 minutes of startup...brought it back in where they checked pressure, it was fine at idle but would drop right away when under load..they suspected the hose that goes from drive up through the transom was the issue. It was replaced..this required the engine to be pulled so obviously a gigantic labor bill to replace a cheap part. After replacing this pressure was good at idle and under load.

Took it out this weekend where it ran perfect for an hour, then - constant alarm and ECM shutting down the party, Same as before. Pressure is fine after the water pump, so closed loop system is working as it should (I suspect), raw water side is for the most part brand new. I haven't gotten the code on what its about this time but shop should call me to let me know.

Engine is a 2005 496HO with CMI headers. Any ideas out there? Getting frustrated with this so any help appreciated.
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Old 07-14-2014, 10:01 AM
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Do you have a fresh water flush connection in the engine compartment for flushing with a garden hose?
If so, the Tee that enters into the system between the transom and the SWP is known to gum up with sand, weeds, and other crap. This could cause the issue you are having.

At idle, do you have same water flow out of both sides of the exhaust?
You may need to run the boat with the scanner hooked up to see if one side of the motor is heating up more than the other.

You can buy the scanner for $500 and do a lot of this yourself. www.rinda.com There are lots of posts on this.

How old is your antifreeze? Is it still bright pink, or is it starting to turn metallic/gray? The antifreeze is only good for 5-7 seasons, TOPS. Once it starts to turn color, it's silting up and does not flow/cool as well. You could also have an issue with the circulation pump (the typical water pump that a car has, which actually flows the antifreeze though the closed part of the system).

Are you getting any water in the bilge? You may also have one of the two water actuators (that open to drain the water out when you apply air pressure to the schraeder valve) going bad. This is rather common as well. They have springs in them that get tired over time.

It would help to know exactly what codes the engine is throwing.
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Old 07-14-2014, 10:07 AM
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I do have the garden hose connections for flushing. Shop said they did flush out the whole system but I'll give this a go just in case..called the shop to get them to check out the tee. I asked if maybe the flush was causing issues but they dismissed it right away. I'll ask the to take a better look.

I do get water out both sides of the exhaust.

I just called up Rinda to order up the scan tool, Its a bit frustrating to wait on the shop to hook it up all the time. Are there separate temps sensors on each side?

Antifreeze is fresh this year, bright pink in color

No water in the bilge.

Haven't gotten to the circulation pump yet, all the focus has been on the raw water side and not the closed loop side.

Appreciate the ideas, as soon as I get the codes I'll post them.

Last edited by chiefy; 07-14-2014 at 10:20 AM.
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Old 07-14-2014, 10:19 AM
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Originally Posted by chiefy
I do have the garden hose connections for flushing. Shop said they did flush out the whole system but I'll give this a go just in case.

I do get water out both sides of the exhaust.

I just called up Rinda to order up the scan tool, Its a bit frustrating to wait on the shop to hook it up all the time. Are there separate temps sensors on each side?

Antifreeze is fresh this year, bright pink in color

No water in the bilge.

Haven't gotten to the circulation pump yet, all the focus has been on the raw water side and not the closed loop side.

Appreciate the ideas, as soon as I get the codes I'll post them.
There are several temperature sensors, port and starboard.

Those Tee fittings that Baja installed from the factory have a spring loaded check ball fitting in them. Just remove all of that crap inside the Tee. It's not needed. As long as you have the cap on snug with a garden hose washer so it can't suck air, you don't need all that check ball crap. THAT'S what gums up with debris and restricts water flow. We found this out with a buddy's 496 Mag 30' Baja which would overheat only at 45mph or above.
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Old 07-14-2014, 10:24 AM
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When were the headers installed? Did all of this start right after they were installed? Any other significant event that this 'started right after'? (change spark plugs, IAC replacement, etc. etc) There are so many sensors on these motors, that it's kind of easy to disturb one. Breaking a wire, or damaging a harness happens, which is why Mercury sells replacement splice kits for everything.
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Old 07-14-2014, 10:36 AM
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Dude they need to unhook each hose and clean/flush it. I get money you have debris in front of the fuel cooler. Took three shops and me telling them where the issue was to find it. Back flushing alone may not supply enough pressure.

I too just went through this.
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Old 07-14-2014, 11:04 AM
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Originally Posted by CrownLPX
Dude they need to unhook each hose and clean/flush it. I get money you have debris in front of the fuel cooler. Took three shops and me telling them where the issue was to find it. Back flushing alone may not supply enough pressure.

I too just went through this.
Me three!


I had debris in the cool fuel tee. Back flushing didn't have enough pressure. Took a month to find issue.
Kept changes sensors, water pump, etc.
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Old 07-14-2014, 11:32 AM
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The best way to see if you have cooling problems and or restrictions and or air leaks in your system is, 2 tests one is a Mercruiser spec on water output volume test motor side and the next test is take our certain sections of your cooling rubber hoses and install clear ones in there place for testing purposes only. You keep changing out certain sections until you find your problem area.

Restrictions and or air leaks and or a bad water water circ pump will show a lot of air bubbles in the cooling water stream. You are always looking for nice moving solid water flow in the clear hoses. Again testing only with clear hoses.
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Old 07-14-2014, 01:05 PM
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Thanks guys...yeah I'm a month into this and losing patience. Also paid ridiculous amount of $$ to change out everything. Here's what I'll try:

-Get the check value stuff out of the tee in the flush system.
-Will get them to pop open the tee in front of fuel cooler and clear it all out.

They did backflush but thanks for the feedback that you guys are giving saying that backflush probably won't be enough.

Sydwayz, the CMIs have been on there for a couple hundred hours so no changes there. But now that you mention it I did change the batteries out the weekend before it started acting up. I did make sure power was off etc. before the battery swap. also My oil pressure guages were not lighting up (ie: the actual light was not lighting up, they were functioning) so had someone check and wiring was loose on those.

-
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Old 07-14-2014, 10:43 PM
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Sydwayz, were are thise actuators, I want to check mine. Thanks
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