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Gil wet exhaust tubes getting hot

Old 08-02-2013, 09:54 PM
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Default Gil wet exhaust tubes getting hot

I have a 1999 Mercury HP 500 Carbureted engine with the factory Gil Exhaust manifolds and polished stainless 4" tubes. recently while running on a poker run, I smelled burnt rubber and immediately thought mine or someone running with our group, was loosing their coupler. It was not that however when I noticed after slowing down and running the hatch up, that we had blown a hole in the side of one of the short exhaust hoses that connects the end of the pipe to the thru hull tips. Anyhow, we ended up patching up and trying to run back to the hotel and pull the boat out and popped the other side in this process! We idled back and cleaned everything up, I noticed they were burnt inside, I knew something was wrong. I just went through this engine last year, it is all back like it was originally, no modifications were done, and it has been running perfect.
I replaced the tubes and got everything back together, we took it out and when you run up on plane, the engine temp goes up to just shy of 200, then when you slow down it immediately drops right back down to 120 or so. I have checked every hose for restrictions, none were found, pulled the oil cooler, cleaned it even though I only found minimal particles, replaced the impeller, other one was still perfect, I use a cross over rather than a circulating pump and it has always worked perfectly, no problems. Now the engine at idle runs about 125 or so degrees but the exhaust pipes are getting so hot, you cannot touch them. I can touch the engine block and it isn't like that at all.
Could there be a problem with the internal water jackets inside the pipes? I pulled them and ran water through the front where the hose attaches and water easily ran through, but didn't seem to come out of all of the water jackets at the end, mainly one.
This is blowing my mind and I have been a Marine Mechanic for years and work on these type boats an many others as well. Plenty of water flow, yet I have overly hot exhaust? Suggestions please?
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Old 08-02-2013, 10:06 PM
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Do you run a water pressure gauge? You've definitely have a flow or pressure issue.I would have said you've got a bad manifold that has either a restriction or or burnt through, but that wouldn't explain both sides getting hot.
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Old 08-03-2013, 08:37 AM
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Originally Posted by neversatisfied23
Do you run a water pressure gauge? You've definitely have a flow or pressure issue.I would have said you've got a bad manifold that has either a restriction or or burnt through, but that wouldn't explain both sides getting hot.
No water pressure gauge, and the manifolds are perfect. I checked the water jackets this morning by running a hacksaw blade from the tip up into the water jackets of the pipes and found nothing like a restriction or clog. The flow seems to be strong, I even pulled the hoses while the engine was running and it comes out strong also. This one has really got me stumped!
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Old 08-03-2013, 08:42 PM
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Any chance you got a bad fuel pump, filter or something leaning out the motor?
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Old 08-06-2013, 05:39 PM
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No bad fuel pump, pump works just fine and the engine has never been modified or the carburetor messed with, still set at factory settings. Motor runs perfectly, and after checking every single hose, replacing impeller and even using a tool I made to run strait to the intake side of the belt driven water pump itself from the hose, pipes are still getting hotter than I think they should be. Engine runs right at 150 degrees idling and there seems to be good flow. Water is getting pretty hot after a short time on hose I am still baffled.
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Old 08-06-2013, 05:49 PM
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Your engine water temp should never get that high. It is borderline close to setting off the alarm.
It should have a 140* Tstat and should never run higher than about 150* on the guage.
I'm betting you have a restriction/clog somewhere.
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Old 08-06-2013, 07:26 PM
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No restriction anywhere, checked everything there is to check. I am running a crossover set up rather than a circulating pump, and before yesterday did not have any thermostat installed at all. I installed the one that came with the engine back in it with 4 holes drilled around the outer edge like I have done before. It is hotter than hell down here in Gulf Shores Alabama right now, water temps are in the 80's in the midday hours. Hose water however, is not! I am almost beginning to believe that I am going to have to re-install the circulating pump and thermostat housing and use top feed hoses as well as bottom hoses on the manifolds to correct this issue. Boat ran fine before and quite cool with the crossover set up. Water flows through the pipes, they just seem to me like they are hotter than they should be even with the thermostat out and the engine running barely over 120 degree's.
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Old 08-06-2013, 07:55 PM
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When you say they seem to hot,,,how hot are they ?
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Old 08-10-2013, 10:02 AM
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[B]I reinstalled the orginal circulating pump and thermostat housing using a 140 degree thermostat, everything seems fine as far as engine temp goes. Engine temp is at 120 degree's idling on hose, you can actually touch the block and cyl heads and they aren't even hot to the touch. The only thing I see that's getting hotter than I believe they should be, is the pipes themselves. The manifolds are warm but not hot, the stainless pipes are hot to the touch. This is the factory Mercury HP 500 Gil exhaust system with no modifications done to anything. I have been told that it could actually be the water passages inside the pipes themselves that have either moved, or shifted in some odd way not allowing enough flow of cooling water, I have never heard of such a thing happening to these, this engine was designed as one of the orginal offshore vee bottom race class engines and were built tough. I have never over run my engine since we went through it last year and it was fine before, it's really testing my experience and I have been a Merc Tech for nearly 30 plus years. The pipes are right around 110 degrees after running 30 minutes or so. The exiting cooling water is about the same temp as well.

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Old 08-10-2013, 12:02 PM
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Yes it is possible that something moved inside the risers. The inner pipe needs to be centered inside the outer pipe otherwise water won't reach all the way around and will burn holes through the rubber. This can happen even with the correct temps. Where the holes are will tell you what's not getting water.
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