Steam usually an indication of reduced raw water flow. Bad impeller, clogged U tube, or delam hose are also possible.
As for changing risers:
The first and biggest challenge is removing the old units. I would strongly recommend first cutting off the exhaust tubes attached to the outer elbows (assuming the outer tubes terminate to a 45 degree fiberglass fitting). Wireless tubes can simply be cut off with a utility knife. Wire tubes will require a Dremel tool to cut the coils to simplify removal. If the boat is in the water the exhaust outlets at the transom must be plugged or expect to be terrified when a deluge flows in as soon as the outer exhaust tube is removed. I found a rubber cap used for PVC tubing that had an OD exactly correct to plug the exhaust at the transom. To minimize water/coolant intrusion into the exhaust manifolds during removal, attempt to drain any residual raw water from the elbows by removing the brass plugs from the base of the elbows, and disconnect the coolant hose from the manifold and drain as much coolant as possible to get the level below the elbow mating surface.
Unbolting the elbows would seem to be straight forward – eight long bolts and eight nuts on studs. My problem was one bolt that was seriously stalled due to corrosion between the bolt and the mounting bore. A sheared bolt would have been a major setback. A few minutes with the blue tip wrench on the lower boss and the bolt backed out fine. One nut was heavily rusted to the stud and sheared at the bearing surface – no big deal since I was replacing all the hardware. After removing the bolts and nuts is when it gets a little crude. It is very common for rust to form between the stud shanks and the associated mounting bores of the elbows. You cannot merely lift the elbows off the studs ! Women and kids should not be present for the next phase of the operation. The school solution is to employ a four-pound plus sledge and whack the elbows from side to side to rock the units up the studs. A heavy sledge is essential since low velocity impacts still carry much energy (KE=1/2MV**2 for the Dilbert types). The studs were heavily corroded in my case, however, numerous whacks and much Deep Creep later the old Blackfin finally yielded all four elbows.
The studs were easily unscrewed from the manifold using a stud puller. The old gaskets were scrapped off and the surfaces on the manifold and block-off plate were cleaned with acetone. All the threads were chased and new studs were installed dry and tightened in place with the stud puller. Marine grease was smeared around the shanks of the studs in hope of avoiding the Thor routine next time. I researched the issue of sealer on the gaskets on the internet and did not find any reliable direction. Many recommended installing gaskets dry while others recommended Permatex. One troop went dry but later regretted it when leaks developed. I elected to use a light film of Permatex around all openings.
Low profile OEM elbows are no longer available so I researched aftermarket options. The wrench-benders at my local marina were down on the elbows made offshore. Their biggest dissatisfaction was a high degree of porosity which limited longevity. I went with Barr units (CR20-98126) ordered from Boaters Plus. They appear to be perfect replacements – already painted Crusader blue, great fit, include new gaskets and a complete hardware kit, and made in the USA. I also ordered four additional gaskets since two are required per elbow (FWC manifolds). Service from Boaters Plus was excellent – ordered Sunday night and had them in hand to do the job the following Saturday.
With the gaskets gooped and in position, I maneuvered the elbows into the exhaust tubes while slipping the elbows on to the studs. I elected to put anti-seize on the bolt threads which some do not recommend. I also greased the bolt shanks. After tightening the hardware in sequence, the job was pretty much over. The final issue was the 1.0 inch water hose fittings on the elbows (CRU97911). The only way to remove the fittings is with a pipe wrench. I managed to remove all four but cracked three of them when they collapsed slightly under the load of the wrench. OEM units are available but cost $25 plus. I went with Mercury Part Number: 22-807155 which is compatible and only cost $7. It also has a hex fitting which simplifies removal/installation.
Hope anyone taking on this challenge finds this helpful. Every three years or so the elbows need to be changed and patience and planning will make the job just a little easier and predictable. Everything considered, it was not that bad. Not looking forward to doing it again any time soon !!