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Mercruiser 496 HO Low pressure, high temperature problems

Mercruiser 496 HO Low pressure, high temperature problems

Old 06-15-2018, 02:06 PM
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Default Mercruiser 496 HO Low pressure, high temperature problems

The boat: 2008 Formula 350 SS (dual engine)
Engines: Mercruiser 496 HO Seacore (w/ Axius)
Drives: Bravo III
Exhaust is captains call (not sure if that could matter)

I bought the boat last September and quickly realized this was a problem. I had a 1999 330 SS (w/ fresh water cooled 454s) before this and I figured it would be a simple fix like a bad impeller. After the battle of getting my pump out (those green and white flush hoses are too short!) I found the impeller was in great shape but there was some serious "swirl damage" to the housing which I learned is from sand/rocks getting stuck behind the impeller. To remedy this I bought a rebuild kit from http://www.marine-496pump.com. Unfortunately this still didn't solve the problem. Here's the symptoms I'm seeing:

Port water pressure: 4-5psi
Port temperature while cruising: starts around 162 and then slowly climbs to 167-175F
Starboard water pressure: 17-19psi
Starboard temperature while cruising: 162F
Edit: One more symptom I see occasionally - After cruising for a bit and then going at slow speeds (docking or anchoring) I will see a little bit of steam come out of the rear port side of the boat.

With the temperature difference I'm pretty sure this isn't just a bad sensor, so what else could be the problem?

Ideas of cause:
  • Those green and white tubes don't have a good seal, the tube has come out of the metal end a few times while trying to install/remove pump, could this be allowing air in?
  • Thermostat?
  • Pump failing for other reasons?
  • Something wrong with the Seacore heat exchanger?
Ideas ruled out (IMO):
  • Intake clogged - the engines have both drive and hull intakes connected via a T connector just before the pump, even if one part was clogged the other should supply enough water (it doesn't seem to be clogged, water gushed in when I disconnected the pump!)
  • Blockage in the cooling path after the pump - I may be wrong here but I would expect this to cause a higher than normal pressure, not lower, right?
  • Water pump failure - This just pumps the coolant through the engine, not the fresh water, right?
What other reasons could cause this?Is the seawater pump under the engine the only impeller in the system? I was talking to my neighbor and I learned that his boats Alpha One Gen 2 has an impeller in the lower unit, but looking at the diagram for the Bravo 3 It doesn't look like that's the case.

Last edited by DracoDan; 06-15-2018 at 03:08 PM.
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Old 06-15-2018, 04:20 PM
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Pull the Seapump out and check the impeller. if it came apart it could be blocking before the pressure sender.
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Old 07-10-2018, 12:46 PM
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the 496 had the early version location for the water pressure sensor impeller housing area and then the later 496 had the water pressure sensor moved to the power steering cooler. Alot of times the pressure sensor can be bad or contamination filled the hole of the sensor as well

Last edited by BUP; 07-10-2018 at 01:04 PM.
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Old 07-10-2018, 09:18 PM
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Take off a hose from thermo hsng to manifold. Same hose both engines.run engines and compare the water streams from each hose. If same size stream of water squirts just as far before it starts to curve down, the pressures are very close. Your reeadings of 5 and 20 should be a stream and a dribble.
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Old 06-02-2019, 11:40 PM
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My 496 is doin the same thing. How did u fix it.
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Old 06-03-2019, 08:55 AM
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I am dealing with the SAME issue! Here is what I have found so far. I am using the same rebuild kit as you. I installed 2 kits and now my temperatures are running at 200 degrees. Those particular rebuild kits SUCK! The impeller is not stock and it is a lower durometer it is a lot softer. There is another rebuild kit that offers a plate for the bottom and a shim for the exit/flat plate part of the housing that allows you to use the stock impeller. I just ordered another brand and will update this as soon as I install.
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Old 06-03-2019, 02:02 PM
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have not read the new comments / nor whats asking here but I do know if you have flucuating coolant temp swings - 9 out of 10 times its the T stat and or the water circ pump. And yes they go bad in marine closed systems.
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Old 06-03-2019, 02:21 PM
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I use the kit that is a OEM correct matched sized impeller and the correct sized SS plate that you drop in. The impeller is the correct sized because of the space the drop SS plate takes up, Impellers and housing are all about the correct tolerances for proper flow and cooling. If you need to know where to get them let me know.
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Old 06-04-2019, 12:58 AM
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Originally Posted by Keith Atlanta View Post
I am dealing with the SAME issue! Here is what I have found so far. I am using the same rebuild kit as you. I installed 2 kits and now my temperatures are running at 200 degrees. Those particular rebuild kits SUCK! The impeller is not stock and it is a lower durometer it is a lot softer. There is another rebuild kit that offers a plate for the bottom and a shim for the exit/flat plate part of the housing that allows you to use the stock impeller. I just ordered another brand and will update this as soon as I install.
I also m dealing with this. I replaced both impellers with Merc quicksilver. I noticed when @ West m they had an OEM & a ???. I could easily notice the West marine after market brand was a much softer durameter. So I installed quick silvers, but I now have up to 180 temps in 50 degree water when on plane when last year they were 170 max. I did notice the temps dropped from 190 to 180 max after the impellers we're broken in after 30 minutes. I have the brass pumps with stainless plates. Monitoring to see if it improves.

Last edited by drumerjb; 06-04-2019 at 09:42 AM.
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Old 06-04-2019, 01:02 AM
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Originally Posted by BUP View Post
I use the kit that is a OEM correct matched sized impeller and the correct sized SS plate that you drop in. The impeller is the correct sized because of the space the drop SS plate takes up, Impellers and housing are all about the correct tolerances for proper flow and cooling. If you need to know where to get them let me know.
So a stock quicksilver impeller won't work in my brass stainless drop plate application?
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