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Pulling her out of hibernation and everything going very well. After running in driveway for an hour one riser seems normal (115-120 on IR gauge) other is signifigantly cooler (90-95) with gauge cluster temp around 160-170.

I did have an impellor let go late last season, removed all the parts and pieces from the thermostat housing and didn't notice any problems last few times out.

My plan is to keep an eye on it for a few hours on the water then maybe pull the suspect riser. They haven't been done since I have owned the boat. Any opinions on how much difference there can be between risers and what other problems might it be causing or anything else I am not thinking about? Not particularly interested in pulling the riser but not afraid of it either....just seems like an opportunity to break something while pulling it off.

_________________
Jon Miller
1998 four Winns Horizon H200
Volvo Penta 4.3 GL
Newtown, CT

untitled-545 by millerjont, on Flickr

My 5.7 volvo and now my 8.1 mercs all had different temps and both a surveyor and my mechanic stated this is not out of the norm especially this time of year with the cold water. I actually went through my engines with the mechanic over the weekend and we had this happen. He stated the obvious, water will follow the easiest path. As long as the hotter side is within spec you are okay. Can you put your hands on the risers? It should be warm but not uncomfortable to touch. If its too hot to touch then you need to pull the risers.Check the thermostat housing with the IR gauge to confirm your dash reading.

If you haven't pulled the risers it may be time. Multiple mechanics recommended pulling the risers(not too difficult and only a gasket on each) every few years. The gaskets between the risers and manifolds can go bad over time and leak down as well as inspecting the manifolds for cracks and rust if you don't have closed cooled.

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--2002 3880 Regal Flybridge | Twin Merc 8.1HO **Seven**
--2003 AB 13 VST Console Inflatable | Tohatsu 40hp TLDI **Phish'n ski**
--310 Mercury Inflatable | 1982 9.9 Johnson

When I rebuilt the top end of my old 4.3 after the head gaskets blew back in '16 I knew I was also due for new exhaust, and had been changing them about every 5-7 years. The old OMC one piece manifolds are no longer available (and have not been for years) so I had been researching converting it to the Volvo center riser style which is the same as what you have. I wound up using Barr Marine aftermarket manifolds and elbows, for the hoses I used Volvo OE since they are made to fit the 90* adaptor piece that adapts from the 4" elbow hose to the 3.5" Y pipe hose. It all fit perfect. When running it on the hose, with good water pressure I don't get more than 100 anywhere, except the rear part of the starboard side adaptor piece between the elbow 4" hose and the 3.5" hose, the rear of this part might hit 110 or so. More typically the tops of the elbows and manifolds are 75-85* on the water hose. I'm not sure if there is a difference in the water flow from my Cobra system to the Volvo but as it stands the only difference is the drive mounted impeller vs the engine mounted one on the Volvo. The exhaust and thermostat housing are the same. I think you may still have pieces of impeller somewhere, or the hotter riser is starting to clog. Yeah it can be intimidating to take exhaust apart but us salt guys do it all the time. Your cooler one is right in spec from what I've seen with both the OMC batwings and the Volvo style center riser exhaust. The other one seems a bit hot to me. However, see what you get on the water because not all muffs give good water volume. When running my boat with the old exhaust, at idle they were never over 100*, often as cool as 85*. After coming off plane, the hottest they would get was about 135 and then they'd cool down to 100 pretty fast.

After the rebuild...a pair of reman heads, all new gaskets, and center riser exhaust.

https://www.dropbox.com/s/55j6h4dd1u8fv ... 3.MOV?dl=0

Here's a pic of it running on the water hose. My next project is converting from the old Quadrajet to a Holley 4160 as used on the later OMC's from 1990 on and the carbed Volvo versions of the same engine. Don't really want to fiddle with a 30 year old carb anymore.

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

Thanks for the info......warm riser is about as warm as it always is.....just the cold one threw me a bit. (Ir readings confirmed everything) will get out on the lake today and see what it does....was just more curious about uneven warming cooling of the motor leading to other problems down the road.

reluctant to tackle the risers and such as it seems like once I start unbolting/breaking things it will lead down a rabbit hole ... ..

thanks again for the replies.

_________________
Jon Miller
1998 four Winns Horizon H200
Volvo Penta 4.3 GL
Newtown, CT

untitled-545 by millerjont, on Flickr

The real rabbit hole is if the riser gasket leaks and lets water into a cyl via an open exhaust valve. That will cause rough running missing and low compression due to rusted exhaust valved not sealing.
Do you have rust stains on the outside of the riser/manifold joint? If not then good if so then it's time for at least an inspection. Otherwise you can wind up having to do a valve job.

_________________
88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

hmm...interesting....I have very slight rust stains next to the bolts but nothing too drastic....same on both sides for what its worth..will look closer next time I am out.

I know the correct answer...which I have pretty much ignored for 5 years owning the boat....I suspect I will dig into it when I finish up the rest of spring startup (lawn, mulch, pool, tractor,)

All that said took it out on plane for an hour + today and its running as good/better than it ever has.(no doubt helped by the free advice here)....temp on the gauge was perfect whole trip though still had the disparity on riser temps.

Do the riser bolts generally get seized up and break when trying to remove, or will 24 hours of penetrating oil help things out....also is the warm riser the blocked one or other way around? (I think water only gets into the risers when the thermostat opens up?)

either way its tied back up to its summer home waiting for braver water skiers.....

_________________
Jon Miller
1998 four Winns Horizon H200
Volvo Penta 4.3 GL
Newtown, CT

untitled-545 by millerjont, on Flickr

I never had trouble removing my old OMC manifolds but they used zinc plated studs and nuts so no corrosion. I used gasket sealer on the threads when I installed the new system last summer since I've been doing this with the thermostat bolts and they never seize up (16 years in salt water). Impossible to say if you'd have an issue but try warming up the engine first then drain the manifolds. With the Volvo system the water goes right out the manifolds with a cold engine and closed thermostat. When the engine warms to about 160 the stat starts to open and allows hot water to leave the engine via the manifolds. Cold water from the impeller then gets sucked into the engine. Try it in the water first and take temp readings at idle and after coming off plane.

_________________
88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II