How to Inspect Exhaust Manifolds and Risers on a Sterndrive or Inboard

Check your records or ask the previous owner when the manifolds and risers were last replaced. No records on an older boat means you should assume they are due for inspection now. Note whether the boat lives in salt, brackish or fresh water and whether it is kept in the water or on a trailer. Put the date and the next inspection in your BoatMend maintenance plan.

With a flashlight and mirror, look at each manifold, riser and the joint between them. Warning signs:

• Rust streaks, bubbling paint or flaking metal, especially at the top and aft end of the risers.
• Weeping, rust or white salt crust at the manifold-to-riser joint. That means the gasket is leaking to the outside, and a leak to the inside often follows. Do not wait on this one.
• Cracked, soft or scorched exhaust hoses, and rusty or loose clamps.

Rust on the outside is a strong hint that there is more rust on the inside.

Pull the dipstick: milky or gray oil means water has reached the oil (it can come from other leaks too, but it always needs a pro). Hard starting, a rough first few seconds, white smoke at start-up, or a knock or clunk when cranking after the boat has been sitting are classic signs of water in a cylinder.

If you pull the spark plugs, compare them. A plug that looks steam-cleaned, wet or rusty while the others are normal points to water getting into that cylinder.

Do not keep trying to start an engine that knocks, locks up or only turns part of a revolution. Each attempt can bend a connecting rod. Remove the key, disconnect the battery and call a marine mechanic. A pro can remove the plugs, clear the water and test the engine before more damage is done.

With the engine running on the water (or on a flushing device approved for your drive), let it reach normal temperature. Flushing setups differ: Mercury notes that its MerCruiser flush kit for Alpha drives can only be used with the boat out of the water, Bravo and inboard kits work in or out of the water, and the engine must be running whenever a flush kit is used. Follow the instructions for your exact kit and keep clear of the propeller. Use an IR thermometer to compare the same spots on the port and starboard manifolds and risers. A side that reads much hotter than the other usually has clogged water passages. On engines with a separate exhaust outlet per side, less water coming out of one side is another clue.

Also watch for overheating at higher speed or under load. Clogged exhaust water jackets are one cause, though the impeller, a clogged sea strainer or intake and the thermostat are more common. Our engine overheating guide covers those.

Removing a riser for inspection is a reasonable DIY job on many engines if you are comfortable with tools: a handful of bolts and two hose clamps. It is not a beginner job on boats with tight engine bays, V-drives, closed cooling or rusted fasteners. If a bolt snaps in the manifold, the repair gets expensive. If in doubt, have a mechanic do the inspection and replacement together.

The boat must be out of the water. Disconnect the battery. Drain the water from the block and manifolds as described in your winterizing procedure (or your manual) so it does not pour into the exhaust when you lift the riser. Take photos from every angle and label the hoses, sensors and any brackets you remove.

Loosen the exhaust hose clamps and work the hose off the riser. Plug the open exhaust hose and tie it up above the waterline so you cannot forget it. BoatUS reports that owners and even mechanics sink boats every year by leaving exhaust openings unplugged, sometimes overnight. Remove the riser bolts in a criss-cross order and lift the riser off. Keep the old gaskets to compare with the new ones.

• Water passages: they should be open. Heavy scale or rust that blocks them means the casting is near the end of its life.
• Probe the metal with a large screwdriver. If you hit solid metal right away, that is a good sign. If chunks of rust flake off easily, replace it.
• Gas side (the center passage): expect black carbon. Any rust in the gas passage means water has been getting in. Replace the parts and have the engine checked.
• Mating faces: look for pitting or eroded spots, especially at the thin areas between water holes. A pitted face will leak again even with a new gasket. Replace the part or have the face machined by a shop. Owners who find a little water sitting in the manifolds after a hose flush, with gaskets that "looked fine," usually trace it to corroded sealing faces like these.

If one riser is bad, the other is usually close behind: replace risers in pairs. Manifolds cost more and are harder to remove, and their insides are hard to judge even when they are off. Many owners replace manifolds and risers together when the risers are due, especially in salt water, because a manifold failure can cost an engine. Use only parts listed for your engine's model and serial number, and always new gaskets.

Clean the mating faces carefully with a gasket scraper (not a grinder, which removes metal). Fit new gaskets in the exact orientation shown in your service manual; some are directional. Tighten bolts in the sequence and to the torque in your manual, not "as tight as possible." Reattach the exhaust hoses with new clamps. Remove the plug from the exhaust hose before you forget, and refill or reconnect the cooling system as your manual describes.

Run the engine on the water (or on approved flushing) with the engine hatch open and nobody in enclosed spaces. Check every joint for water drips, steam or exhaust leaks, and confirm normal temperature and water flow. Recheck the joints and clamps after the first couple of outings. Test your CO alarms and replace any that are out of date.