1996 Volvo Penta Cobra SX outdrive plugged with flapper valve

I bought a boat in Dec 2022 from my neighbour. Apparently, he never had the outdrive off boat for servicing. I was told he only ran the boat around 3000 rpm, so I figured he babied the boat. Well he did baby it, but clearly the boat overheated above 3000 rpm. When using the boat for the first time this summer, I opened it up to around 4500 rpm and it immediately overheated the 3.0 l engine. The temp gauge jumped past 200 F. All hoses were pressurized and too hot to touch including the raw water line connecting the lake to the pump.



I originally feared a plugged block/manifold, manifold leak or blown head gasket. I tried cheap and easy repairs - replacing impeller and thermostat. No difference at all - although I'm sure the impeller was a little worse for wear from the overheat and probably pumping nothing but hot exhaust. I suspected a stuck flapper valve and removed the exhaust riser to inspect. Only the pivot shaft was there. The flapper was missing. Since the riser had been replaced to try and solve an overheating issue (was an invoice for it), I had no way of knowing whether the flapper was deleted during that servicing (volvo penta had apparently put out a service bulletin about deleting these years ago and no longer sells replacements).



I wanted to replace the bellows, lube the u-joints and inspect the gimbal bearing. I pulled the outdrive and was disappointed to see no remnant of the flapper in the exhaust bellows. Next day, I saw the pieces lodged in the exhaust passages of the outdrive. Mystery solved. Both main exhaust passages into the leg were almost completely blocked by the rubber and fiberglass components of the original flapper. They've been there for years. At low rpm, the exhaust could leave and the engine ran cool. The previous owner has driven the boat only to 3000 rpm for all these years to prevent overheating and I can see from the invoices this has always been an ongoing issue. If he had taken the drive off, it would have been an easy fix and he would have had full enjoyment of the boat's capabilities! Crazy that the technicians working on the boat never wrote that suggestion on the invoices.



Fortunately, he only used the boat 2 or three times a summer for the last 20 years (l live next to him). I can see from the invoices that his impeller shredded at one point. My guess is that this overheat may have melted the flapper, which then fell down the exhaust pipe. As time went on it probably lodged itself into the exhaust passages so that 3000 rpm was the max before overheating. Then I took the boots to it (WOT) and wedged them in so tight only 1600 rpm could push enough exhaust through to keep it cool enough. Fortunately the engine is all iron. I don't see any evidence that anything was warped. Compression is great, oil is clean, and he really maintained the boat apart from outdrive removal. I ended up only using 1/2 tank of gas all summer as I could only do 1600 rpm without risk of over heating. So it was more of a beer drinking barge with a nice stereo this summer! Hope this helps someone with the same overheating issue. I still may replace the manifold because it is 27 years old!