Spring Engine Maintenance

Information on this page primarily refers to 4 cycle gasoline fueled inboard and I/O marine engines

This is the exciting time of year boat owners start thinking about getting their boats shipshape for the new season.

It is a good time to give the engine and it's systems a thorough safety check. Always make sure all repairs you make or permit others to make are done properly. Using a qualified marine repair facility is your best insurance that quality work will be done. Qualified marine dealers are aware of the requirements that must be followed when performing repairs and are kept abreast of repair issues by service publications issued by the manufacturers. Most dangerous conditions we have seen are the result of owner's tinkering, fly-by-night service operations that work out of the trunk of a car, or unqualified "friends" unknowingly or carelessly making modifications or repairs to boats that are not up to Coast Guard standards.

However, if you have good mechanical knowledge, know when it's time to take the work to experts, and are aware of the safety issues we have mentioned, you certainly can safely do much routine maintenance yourself.

Your boat and engine manufacturer spent a lot of money to make sure that your vessel is safe and dependable for your pleasure. With proper maintenance your engine will continue to serve you safely and reliably for many seasons to come. Good maintenance doesn't cost, it pays, in the safety and reliability of your boat. Most people have limited time to use their boats what with all of the other demands of today's lifestyles. A well taken care of boat will reward you by being ready to go when you are. Also, many expensive major failures can be prevented by proper care.


Basic Engine Maintenance:

Make sure your engine is clean. A clean engine is safer to operate and easier to work on. Paint any bare areas and examine the entire engine for any excessive corrosion. Also clean the bilge. It is illegal to pump oil into the water so make sure that your bilge is clean before launching so that when your pump comes on you will not produce an oil slick. Make sure your engine has no serious oil leaks. Some slight seepage is normal, but large amounts of oil in the bilge indicate a problem that should be looked into by a professional.

Check wiring for corrosion and chafing. Make sure any accessories that have been installed have proper fusing. All wires powering accessories if connected directly to battery must be fused at the battery. Accessories must be either supplied from a fused power strip or in-line fuses installed at the source of power. We have seen many owner-installed accessory wires short and burn because there was no proper fusing. We even had a boat burn to the water line last summer because of an owner-installed depth sounder power lead that shorted out. Remember, the fuse at the unit only protects the unit, not the wires supplying the unit. If your boat contains any such wiring, contact a professional to get this fixed now.

Check batteries for proper charge and if they are more than three of years old, replace them with marine rated units. Make sure battery terminals are clean, tight, and coated with an anti-corrosive spray.

Don't run engine without water. Most water pump impellers will be destroyed in less than a minute's running without water. Outdrive mounted pumps are particularly susceptible to this problem. Always use a flushing connection or a muff when running a boat on land. Never race the engine, because the hose will not be able to supply the amount of water required to keep the engine cool.

Check all water hoses for deterioration and clamps for tightness. Replace any suspicious hoses and clamps now. Do the same for exhaust hoses and/or bellows. Make sure all water drain plugs, hoses, and fittings loosened for winterizing such as water pump covers have been replaced and are tight. If it has been more than about three years, replace the water pump impellers.

Check belts and replace them if there is any sign of deterioration. Make sure they are tensioned properly.

Make sure your engine has the correct thermostat in place. Do not remove the thermostat or substitute an automotive unit of a higher temperature. Salt precipitation and corrosion accelerates at high temperatures. If your engine has a closed cooling system, make sure it has at least a 50/50 mixture of permanent antifreeze to retard corrosion. If it has not been changed in a more than a couple of years, change it now. It is best to keep antifreeze in these systems year round. Catch old antifreeze in a bucket and take it to a recycling facility. Don't dump it in the bilge as it is illegal to pump it overboard.

Change the oil and filter using a premium SAE 30 or SAE 40 oil depending on your area's temperature range. Most engine manuals recommend SAE 40 for marine applications on most inboard and I/O engines. Check your manual. Remember used oil must be disposed of properly at a recycling center.

Check your transmission, power steering, power trim, and trim tab fluids, using the procedures recommended in their manuals, using the recommended fluids


Fuel System:

Always try to buy gasoline without any alcohol. Alcohol promotes separation of water and gasoline in the tank, and accelerates corrosion and deterioration of most fuel system components. Of the two types used, Methanol is the most damaging. Although the new fuel hoses are rated alcohol resistant, fuels with alcohol tend to reduce lifespan of fuel hoses. If you can find gas without it, buy it. Also avoid "reformulated" or "low emissions" fuel if possible. These fuels are usually oxygenated and can also speed deterioration of fuel system components and hoses. Oxygenated fuel can make the engine run leaner than normal and this could be a problem in some cases.

Never use "dry gas" potions sold in auto part stores. The are simply methanol, used as antifreeze to prevent slugs of water in automotive fuel lines from freezing and blocking them in winter. This serves absolutely no purpose in a boat. As mentioned above, methanol is not something you want to add more of to your fuel system. If you have a large amount of water in your tank, it must be pumped out. There are marine additives that do not contain alcohol that will cause small amounts of water to combine with the fuel and be burned in the engine.

We also suggest buying at least 89 octane fuel for a typical marine engine. A lot of the fuel sold today is of marginal quality at best, and looses even more octane just sitting around in your tank. One grade higher than the manufacturer recommends is good insurance. We have seen more otherwise good engines ruined by detonation than any other cause. On the other hand, unleaded fuels have not been the problem many people expected. Most any marine engine built since the mid-seventies has hardened valves and induction hardened, or hardened insert valve seats. Unless you run your engine at 80% or more power for long periods of time, unleaded fuels should give you no excessive valve seat wear. As long as the octane level is sufficient to prevent detonation and the fuel contains as few unwanted additives as possible, your engine will be happy.

Check all fuel lines and fittings for leaks. Check rubber hoses for brittleness and cracks. Make sure ty-rap or other fasteners are in place and there is no abrasion or chafing on any hose. Make absolutely sure that all hoses used in your fuel system are marked with USCG approval for the purpose used. There is a Coast Guard Approved hose for fuel fill, vent, and fuel feed use.

Make sure that nobody has substituted automotive type hose for any marine rated hose. If you find any substandard hose, better get it replaced before placing the boat in service this spring. Also check all hose clamps and fittings for proper tightness. Make sure all fuel filters are the proper marine rated unites for your engine. Now is the time to replace the elements and clean out sediment. Replace any filter cans showing corrosion with the correct stainless marine unit. Spin on filters must be replaced at least annually with the exact replacement because they can rust out.

We have seen many dangerous, improperly installed automotive fuel filters that have been used by people who don't know the difference. We have even seen in-line automotive plastic fuel filters spliced into the pressure line near the carburetor with cheap spring type hose clamps and rubber hose! In general, unless they are specially rated by the manufacturer for the purpose, hoses of are not permitted on the pressure side of the fuel pump of an inboard gasoline engine and any fuel filters placed there must be marine grade.

Also check fuel tanks, filter bowls, fuel pumps, steel fuel lines, and carburetors for corrosion. This is a particular concern in salt water. These items are typically made of steel or aluminum and both of these metals can be eventually corroded through by saltwater. All metallic fuel system components should be checked at least annually, especially if the boat is older. Replace any of these items showing excessive corrosion with marine grade replacements before placing the boat in service for the year.

Make sure that the carburetor is clean and free of corrosion and leaks. Spray all linkage with a protective lubricant such as WD-40. Make sure linkage works freely. A dirty flame arrestor can greatly reduce performance and increase fuel consumption. Take the flame arrestor off and clean it thoroughly with carburetor cleaner spray. Note: this spray is very flammable. Only use it outdoors, away from the boat, and observe all safety precautions. Allow flame arrestor to dry and then re-install it on engine. The flame arrestor is a safety component in that it protects your boat from fire/and/or explosion in case of an engine backfire. Never operate your boat unless your flame arrestor is present and in good condition. Never substitute a hot rod or automotive type air cleaner for the flame arrestor

Check vents on the outside of hull to make sure they are not obstructed. Spiders love to nest in these things and a plugged vent will make for aggravation at the gas dock when the tank will only take gas at a snail's pace. Another item to check is make sure your vent line does not contain sags that cause fuel to puddle, interfering with the venting. Use ty-raps to support hose properly so tank will fill quickly. Also make sure that the vents are angled down and back at approximately a 45 degree angle to reduce the possibility of water entering the vent. Also check the condition of the O-ring on the fuel filler cap. Most modern caps depend on this O-ring to keep water out of the tank. Did you know that most of the water we have found in fuel tanks can be traced back to leaky filler caps or vents that are turned forward and literally pump water into your tanks when in rough water? Most complaints about water in fuel supposedly from gas docks we have instead found to be due to these two items.

Tip: Put some grease on the threads of your filler cap after each filling. This will make it easier to remove next time and will help keep water out of the tank also.


Ignition System:

The ignition system is probably the least understood and most problematic system on the engine. This, like the fuel system, is a system that requires special marine rated parts for most all components. Also be advised that some marine engines rotate opposite to automotive and have nonstandard firing orders. Since most caps are not labeled, remove spark plug wires one a time to transfer them to a new cap or when replacing wires. Remove wires one at a time when changing spark plugs. We get calls often from hapless owners who yanked all the wires off of their distributor cap and then had no idea how to figure out the firing order. If you feel confident to tune your own engine please pay close attention to your owner's manual. As well as bad fuel, overheating, or overloading, detonation can be caused or contributed to by improper timing, crossed wires, or distributor advance problems. It is ABSOLUTELY IMPERATIVE that manufacturer's tune-up specifications be followed to the letter. If your marine engine does not seem to be performing like it used to and you do not know why, see a professional because continued running of an engine that is out of tune can cause big problems. You usually cannot hear detonation in a marine engine because of the good insulation typically used on marine engine compartments. On a flying bridge, you usually cannot hear the engine at all. Detonation can destroy an engine in a few minutes. NEVER "POWER TIME", OR LISTEN TO PEOPLE UNFAMILIAR WITH MARINE ENGINES WHO SUGGEST YOU CAN "GET MORE POWER OUT OF HER" BY CHANGING THE TIMING FROM SPEC'S. More marine engines are probably ruined by this than almost any other cause.

The following items should be replaced at least every spring, more often with heavy use :

Spark plug wires should be replaced at least every two to three years, or even sooner if they are showing any signs of deterioration.

Distributors should be checked carefully to make sure the advance mechanism is working properly. Lubricate the advance mechanism with distributor lube as well as the cam. Put a couple of drops of motor oil on the felt under the rotor. Be careful not to over lubricate, as the grease will get onto the points and cause them to burn out prematurely. Due to the damp conditions that most marine distributors experience, they are much more prone to have stuck advance weighs, springs that have rusted off, and cams that are so rusty that they eat the new points up in just a few hours running. If your distributor has any of these symptoms, it must be professionally rebuilt or replaced. It is impossible to properly time an engine if the advance in the distributor is not functioning to specifications. Remember to always set dwell first, then timing. For most 8 cal. Engines, set point gap to about .017 then recheck with dwell meter while running. Remember: less gap will give you a higher dwell meter reading and more gap will give you a lower dwell meter reading. Tweak the points a thousandth or so, retighten, then restart the engine to recheck the dwell. The only reliable dwell reading should be taken with the engine running at idle speed. Set dwell to engine manufacturer's specs. Cranking dwell is only good for an approximate setting. If you use this method, always recheck with the engine running. Set timing to manufacturer's specs at an idle speed of 500 rpm unless otherwise instructed by your service manual, or your distributor's advance coming in will make your timing inaccurate.

The Mercruiser electronic ignition distributor has electronic advance, so there are no weighs to deal with, but if the pickup unit in the distributor shows a lot of rust and corrosion, it should be replaced at tune up time. This is an extremely reliable ignition system, but it depends on special components. Use only factory replacement coils, caps, rotors, pickups, and modules for continued proper performance. We have seen several cases of customers pulling their hair out because they used a cheap automotive replacement coil on these systems and then started having all sorts of performance problems. Use only factory replacement parts and you'll avoid these troubles.

Marine spark plugs are also of a special heavy duty design of a different heat range than used for automotive applications. You may be able to find them in an auto store, but make sure they are the exact part number your marine manual recommends, not what the same engine in some automotive application might take. Tighten plugs to as instructed on box. Never clean spark plugs, always replace them. Sandblasting roughens the insulator on a plug, making it much more likely to foul again. Gap the plugs carefully with a gapping tool, not a screwdriver so as not to chip the insulator. Try not to drop plugs as this may crack the insulator or close the gap and cause a misfire. If you do drop a plug, always recheck the gap and carefully inspect the insulator for cracks before installing it. Also make sure you are using a special spark plug socket with a rubber insert to protect the insulator. You can crack a plug on installation and never know it until later the engine just doesn't run right and you just cannot figure out why.

Make sure spark plug wires are fully pushed on plugs and fit properly. If not, replace them with exact replacement original equipment marine wires. We see many instances of people running their boats with plug wires off and dangling in the bilge. This not only is very bad for the engine, it presents a source for the ignition of any gas fumes that may accumulate in your bilge.



Make sure that steering, throttle, and shift controls operate smoothly and without excessive or unusual free play. Some steering systems have grease fittings on the end of the cable and on the power steering unit. Grease these fittings before using your boat. Some outdrives cannot be shifted when they are not running. Do not force the shifter. MerCruiser Bravo drives are one example of a drive that should not be shifted when the engine is not running. Stiffness or slop may mean cables or other parts need replacement. See a professional before operating boat if you notice any of these conditions.

Make sure all gauges read properly. They are there for your safety and convenience as well as to protect the engine. Water temperature, volts, and oil pressure must be all at normal levels. If not, find out why. It might NOT be just the gauge!

By making you more aware of the necessity of proper maintenance, and some of the safety concerns we have noticed over the years servicing boats, we hope we have helped you have a better, safer boating season.