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?

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!

Any chance you got a bad fuel pump, filter or something leaning out the motor?

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.

When you say they seem to hot,,,how hot are they ?

[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.

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.

Had the exact same problem running Gill or Gill/Mercury manifolds and Gill tail pipes. Our problem ending up not being in the exhaust. The issue was a factory installed plastic thread protector plug that had been pushed in and was bounceing around inside the stock Mercury thermostat housing and finaly got wedged in a internal water passage. We had some water flow in the exhaust but not enough to cool the engines at high RPM. It only took us an entire boating season to find the problem. Good hunting...

Oh, how did we find it? We finally Cut the Merc housing in half with a band saw.

We were also not running a thermostat. The issue was a blocked passage in the Mercury thermostat housing that was restricting water flow to the exhaust. This only showed up as an issue at high RPM. To test, check your water pressure at the hose connection at the tail pipe and see if it is less than at the block.

Forgot about of an issue we had with the Gil manifolds. Some of the powder coating inside the manifold broke loose and was restricted water flow so the tailpipes ran hot. This is much more likely than the the thermostat housing, especially in salt water.

Another thought....before you remove the Gil manifolds, remove the jump hose fitting on the manifold and see if you see an obstruction. A small mirror might help.

I found a couple of small pieces of what appeared to be plastic like material which I removed out of one ( starbd. ) side. The other ( port ) was clear. The manifolds themselves seem warm to the touch and not hot which they should be, but the pipes seem to hot. I am thinking maybe this could all end up being a timing issue, do you happen to have the initial timing specs for idle and advance?

Thanks

Too much timing will definitely cause high exhaust gas temps and the condition which you describe . Take some timing out of it and I bet your problem goes away . My HP500 timing is set at 34* total @ 3000 rpm .

Yeah I know this is true, do you happen to know what the exact idle timing should be? it starts fine but has a slight hesitation when I initially turn it over. Someone told me 8 degree's at idle and I have also heard 10.

Thanks

The initial timing is not generally important, especially when you are concerned about excessive heat. Connect a water hose while on the trailer, remove the prop and run up to 3000-3500 RPM while holding the timing light. As others have said, 34 degrees total is a good safe # on pump gas.

Your engine was designed to run on 87 octane . You will gain nothing but a lighter wallet by running a higher octane . Set your total timing as has been described and you will be a happy boater .

have you pulled the hose off the oil/p/s cooler(s) ? make sure there is not a bunch of old impellor pieces blocking the flow there...seen that many times...

Yes is was clear. I ordered a brand new Impeller kit complete with housing, wearplate, everything. I also have a brand new module from Mercury Racing and I plan to get this all installed in the next day or so. I will post how it turns out.

So what happen good bad what, we were flowing this and then nothing, is it fixed what did you find? thanks.

+++

Follow up?

Head gasket?

I had to read through the entire post to make sure it wasn't mentioned but What about transom assembly water hose corroded? It seems like the only thing not mentioned and his symptoms are almost identical for that problem. Good enough flow at low rpm and at high rpm the exhaust gets hot. Be worth looking into?

^^^^^^^^^yep Bravo fever!!!!