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SubTopic :: riser and exaust manifold overheat
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dan de francesco
posted 07-22-2009 2:53 PM

I have a 5.7 v8 omc being replaced on 2007, when I launch this summer I noticed that with the engine idle running fine, but at 2500rpm or more the exaust manifold and riser became very hot. Eventough the temperature gauge is normal 160 so seems that the block engine is fine, by the way I have already replaced the thermostat. If anybody has any idea what could be wrong please let me know. Thanks

Thanks Dan. You think could be the impeller on the drive? What I am surprised is why the block engine is not overheating,but only the risers are very hot. By the way it starts fine. This coming weekend I will go to the marina and I will check for bubble on the inlet hose. Thank you Dan for the input ,I will keep in touch.

dan long
posted 07-23-2009 6:38 PM

at 2,000 rpms your impeller could be slipping on the shaft and thus not sending water to the top you need to disconnect hose comming to inlet side of t stat that comes from the back and see if water is gulping out my elbows were bad on my 5.7 and it overheated right away you also need to eliminate t stat but if its at 160 or higher it is opening does it start hard usually when manifold or risers go you get water in cylinders cause water gets where gases should be and goes by head into valve area and then wont start?

dan de francesco
posted 07-27-2009 4:22 PM

Thank you Wayne I will go this weekend to the marina and I am going to check the exaust manifolds and risers and clean with muriatic acid, they are only 3 years old but i keep my boat in the water the all summer. I think there should be the problem. I will let you know how is going

wayne elsworth
posted 08-19-2009 4:42 AM

sounds like the same as mine.

just been down the shop and they have informed me that i should start with the inpelller,

I also took out the thermo boiled it in a sauce pan of water and that works fine,

also ran the motor without the thermo and still got hot,

i will lett you know after i have made the change at the weekend ,



dan de francesco
posted 08-19-2009 3:25 PM

Thank you Wayne I will go this weekend to the marina and I am going to check the exaust manifolds and risers and clean with muriatic acid, they are only 3 years old but i keep my boat in the water the all summer. I think there should be the problem. I will let you know how is going

Bill McKay
posted 08-20-2009 8:12 AM

doubt the risers are plugged - that would overheat the engine. Most likely the impeller. In fresh water manifolds and risers can go 30 years without scaling up. If you are in salt, probably not wasted to be be boiling them.

dan de francesco
posted 08-20-2009 2:34 PM

Hi Bill so you think is the impeller? NOT the risers exaust manifold. Would it overheat the all block engine with a bad impeller for lack of sufficient water? I am in salt water. Please let me know. Thank

Bill McKay
posted 08-20-2009 4:00 PM

Generally speaking...The exhaust manifolds get cooled by the water that is leaving the engine and has already picked up engine heat. When the impeller is new, there is excess flow of cold lake water. Therefore, the thermostat spends a lot of time almost closed, and all of that excess cold water goes to the manifolds. Now, if the impeller is weak, there is little excess flow. The low flow of new cold water results in the t-stat staying nearly wide open. The manifolds only get the hot water from the engine as there is no excess cold water. All kinda moot now though, knowing you are in salt...depends on flushing practices how 3 year old manifolds will fare. They could be clogged.

dan de francesco
posted 08-23-2009 1:00 PM

Hi Bill Thanks for your clear explanation I will check the impeller first. I will let you know.

dan de francesco
posted 09-15-2009 3:04 PM

Well Sunday I was checking the temperature with my laser therm. and I pin point the all engine temp. The only high temperature (above 250 degree)was on the center connection of the exaust manifold and the head starboard side. Could be just the gasket the went bad or some serious damage to the internal parts of the block like valve, piston, cam, ecc.?

Bill McKay
posted 09-15-2009 3:15 PM

did you change the impeller?

Under what conditions were you checking the temps with the IR therm.?

dan de francesco
posted 09-15-2009 4:01 PM

I did not change the impeller because I have to haul the boat out of the water and I was waiting the end of the season, but I disconnect the hose fron the thermostat and I have I good pressure of water

dan de francesco
posted 09-15-2009 4:03 PM

the high temperature was checked at 2000rpm in the slip

Bill McKay
posted 09-16-2009 8:49 PM

Cobra drive, or stringer drive?

If cobra, you can change w/p impeller in the water.... not ideal, but possible.

If stringer, grease the swivel bearing - quite possible the w/p is sucking exhaust gases due to dry swivel bearing.

dan de francesco
posted 10-12-2009 3:20 PM

Well today finally seems that my overheat problem is solved. I pulled the boat out of the water and I replaced the impeller and I cleaned the exaust manifold with muriadic acid. The intake screen on the outdrive was covered with algees and dirt,so I cleaned that too.I put the boat back on the water seem that everything is fine. I checked with my IR therm. and on all the parts of cooling system the temperature stay below 160 F. By the way the connection where the exaust manifold and the block are connected the temperature on the starboard is around 250 F, on the port side is around 250F to 350F. I talked with the mech on the marina He said that is normal because of the exaust gases coming out from the pistons. I would like to give thanks to all the fellows who replayed this forum especially to Bill. Thank you

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