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Engine Exhaust Maintenance (MerCruiser)


Bad exhaust components will ruin your engine. Most stock exhaust components are made of iron and rusts. If the water passages rust through to the dry exhaust passages then water will enter the engine and do serious damage.

Water intrusion damage from bad, leaking exhaust components is the number one reason for marine engine failure – according to many mechanics.

Exhaust elbow gaskets tend to start leaking after several months and if they aren't tended to right away, will cause rust to the valves and cylinder walls.

Problem Symptoms

If you notice an excess amount of white bellowing smoke (steam) coming from the exhaust then there is a good chance that someplace you have a leak.

Sometimes, customers will complain of hard starting or a "hydro-lock" situation. Water in the cylinders won't compress and this could cause serious damage.

Overheating at high rpm could be an indication that the exhaust water passages are getting clogged with rust. If the exhaust is rusted that bad then it's getting ready to leak if it isn't already!

Any visible external leakage at the riser gasket area is a serious indication that damage is about to occur.

If you remove the spark plugs and find rust or water then you may have an exhaust water leak.

If you are getting an arcing (snapping noise) from the ignition coil, then you might be getting water in a cylinder.

Smart Decisions

According to experienced mechanics, exhaust elbows last approximately 2 years. Exhaust manifolds tend to fail at about 4 years. These periods will vary depending on the environmental factors where the engine is run.

If you suspect an exhaust problem, don't ignore it! You may only need some new riser gaskets installed.

It's very important to prevent further engine damage. Fogging the engine works well to keep the cylinders and valves from rusting further.

You may want to pull the plugs and fill each cylinder with Marvel Mystery Oil which works great.

Don't run the boat with bad exhaust! It will cost you $$$! Get the good stuff. Generally, in the marine industry, you get what you pay for. Don't get cheap when it comes to exhaust. NEVER buy or install used parts.

The experienced techs in the industry always get a good laugh when customers ask about buying used exhaust parts. The aftermarket junk has improved somewhat but still, NOTHING beats quality factory replacements.

If one side is leaking... then the other side should be replaced too. If you remove the elbows and the metal surfaces are rusted bad, thin walled or brown in color then the casting is junk. Throw it out and get new.

Acid boiling of castings should ONLY be to clean off surface rust. Boiling the castings in acid will only thin the walls more, thus adding to the leaks!

Disassembly, Inspection and Replacement

The manifolds HAVE to be drained before disassembly. There is usually a drain cock or brass plug(s) in the bottom of the manifolds and elbows. Some manifolds have a hose attached at the bottom. Removing the hose will also drain the manifold. Use a screw driver or 5/16" socket and loosen all the hose and bellows clamps. Remove the shift bracket from the elbow and any modules or mounting brackets. Remove any hoses which are attached to the manifolds and/or risers. Unbolt the riser from the manifold. You may have to tap it with a soft board to knock it loose.

Inspect the gasket surface area after scraping the gasket material off. If you see rust in the central exhaust passage area then a problem exists. If you think the gasket may have been leaking, then you need to really really clean the mating surfaces. If the metal has a mushy brown look after cleaning, then throw the castings out. The gasket surface areas should have a shiny metal appearance to be usable.

Removing the exhaust manifolds is pretty straight forward stuff. Just unbolt them. Sometimes the manifold mounting bolts are rusted away. This makes it tough and you may find that it helps to hammer a smaller size metric socket on the heads to get them out. Really bad bolts may have to cut off with a really good carbide cutter.

Look into the heads exhaust ports and inspect the exhaust valve stems. There should be no rust on the valve stems! If you see rust, then spray some Liquid Wrench or PB Blaster on the stems. Before reassembling the engine, check the compression!

Clean all the gasket mounting surfaces before installing the new exhaust. Make sure all the fittings and pipe plugs are installed with Permatex 2B or better sealant. The MerCruiser manifold and riser gaskets need NO SEALANTS! Reinstall all the accessories and make sure you mount the ignition module ground wires!

Recheck the bolt tightness and the hose clamps after the engine has been warmed up once or twice. You might smell some weird smells while the new coatings are burning-in. Keep these new castings clean and inspect for leaks quite often. To help them last, flush the engines with fresh water after every single use.

A MerCruiser manifold will typically cost about $400.00 each A MerCruiser elbow will average around $200 each.

Excerpted from www.sterndrives.com

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