Welcome to our first issue of THE EXPRESS, the Marine Parts Express newsletter.

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This offer includes all workshop manuals, parts books,
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OVERHEATING by Scott Huber

What do I do if my engine is overheating?

There are three common reasons why marine engines overheat. 1) Low flow or no flow of raw water (either lake water or sea water); 2) Lack of circulation of engine coolant or anti-freeze; and 3) Compression gases or air in the cooling system.

1. The symptoms of low flow on your raw water cooling circuit are generally characterized by a slow overheat. The engine will usually maintain temperature at low RPMs, but will begin to slowly overheat at higher RPMs. A good place to start is with your raw water or impeller pump. Check for broken or missing fins on the rubber impeller. The second place to look for restrictions is on the intake side of the pump. Any coolers (such as power steering coolers, transmission coolers) are suspect. These coolers are a good place for debris, grasses, etc., to restrict water flow into the engine. If the intake side is found free and clear, make sure that all hoses and hose clamps are tight and intact so that no air can be introduced into the system through a loose fitting or failing hose.

A restriction on the outlet side of the raw water pump will cause low flow and overheating because the raw water cannot exit the engine fast enough to remove the heat. Therefore the quantity of water moving through the engine is reduced. A common place for this to happen is on the exhaust risers or elbows where deposits in the water passages are restricting flow.

A "no flow" of raw water is generally caused by a failed raw water pump, a bad pump impeller or a major suction leak on the intake side of the impeller pump that is preventing the raw water pump from picking up any water. The symptom of this type of overheating problem is generally a quick spike in temperature where you can actually watch the temperature gauge go up.

2. If you have a closed-circuit cooling system in your engine (your engine requires anti-freeze) then your cooling problem could be caused by lack of flow or no flow in the engine cooling system. Check to make sure that the belts driving the circulation pump are intact and that the circulation pump is not leaking water from the “tattle-tale” hole located on the underside of the pulley shaft assembly. If these items are found to be o.k., then the next most logical place to check is the thermostat. If a thermostat is stuck closed, it will cause a spike in temperature from normal operating range. The engine will act normally until it reaches the proper operating temperature. The gauge will continue to rise quickly beyond that temperature.

Dirty heat exchangers are a common problem on closed circuit cooling systems. Symptoms associated with a dirty heat exchanger are generally a slow overheat that increases with engine RPM. Example: your engine will maintain normal temperature at 2500 RPMs but will begin to slowly overheat at 2800 RPMs. The reason for this is the build up of “sludge” on the heat exchanger which impedes its ability to dissipate heat to the raw water system. Heat exchangers can generally be cleaned by your local radiator shop or by a light solution of muriatic acid, (like that used to clean concrete) available at most hardware stores. Please note that you may not use this method if your heat exchanger is made of aluminum! Use extreme caution with this acid: it is harmful to your eyes, skin and produces strong vapors.

3. Air or compression gases in your cooling system will cause overheating due to the fact that air is a poor conductor of heat. Air in your cooling system can also cause your circulation pump to quit. (Circulating pumps circulate antifreeze.) Centrifugal pumps cannot pump air. Air comes into the cooling system through leaking circulation pumps, bad hoses, loose hose clamps on the suction side of the pump, or even by the engine itself in the form of compression gases. Most people would refer to this as a bad head gasket. Example: a bad head gasket allows compression pressure from the piston cylinder to find its way into the cooling system. These gases are extremely hot and allow air into the system which then causes the engine to overheat. There are various places where engine gases or compression gases can enter the system, the most common being the head gasket. The other places that compression gases can come from are cracked cylinder heads, cracked blocks, or exhaust manifolds. There is a chemical test available to check for compression gases in the cooling system. Contact your local auto parts store for a chemical testing kit.

IMPELLER TALK by Bob Van Brunt

Why did the impeller in my pump go bad?
How many engine hours should it last?

The sea water pump is a positive displacement pump. They have better suction than the centrifugal pumps, and you don’t have to prime them. They are used for all sea water cooling applications. About 500 engine hours is the average life of an impeller. The Volvo Penta impellers are made of neoprene rubber. There are some aftermarket impellers (blue in color) that are designed to run longer dry without damage.

Check your impeller prior to spring commissioning as part of your pre-launch maintenance. Look for cracks at the base of the fins and for missing fins. Any of these defects warrants replacement of your impeller. Loss of cooling water due to a damaged impeller can cause serious and expensive engine damage.

An impeller may go bad early because it is run dry. Either a through-hull sea valve has shut, or a sea strainer has clogged. You should have a hull strainer and a sea strainer with a removable basket on your boat. If they do not get water, they will burn up in about 30 seconds. Make sure your valves are open and that your strainers are clean before you go out.

The circulation pump is a centrifugal pump. It has a metal impeller that pumps cooling fluid (jacket water or anti-freeze) through the engine. If your circulation pump is leaking fluid from the “tattle-tale” hole, located just below the pulley and shaft assembly, you may have a bad seal and the pump needs to be replaced. Another problem that arises with circulation pumps is the failure of shaft bearings, usually evident from a rattle or “squeal” from the water pump area. The pump bearings can be checked with the engine shut off by checking the movement on the pump pulley. There should be very little, if any, up and down movement. Most people replace circulation pumps thinking that it will solve an overheating problem. This a common mistake. If the pump bearings are fine and the seals are not leaking, there is a good chance that the pump is fine.


The purpose of a propeller is two-fold: the first is to move the boat, the second is to properly load the engine. The first, most people understand, the second, a lot of people do not understand.

The maximum obtainable RPMs of a marine engine is basically determined by the size of the propeller. A propeller that is too small will allow an engine to go beyond its rated RPM range. This is not good. A propeller that is too large, will cause the engine to run shy of its rated RPM range and therefore overload the engine. This is also not good. By changing the propeller we change the RPM range of the engine. Prop size is determined by two independent factors: the first is the diameter of the prop. The overall diameter, from blade tip to blade tip is the first number used in sizing a prop. The second factor is pitch. The pitch is the distance that the propeller would travel through the water with no restrictions after one revolution. Example: If a propeller has a 16” diameter and a pitch of 16 (16 X 16) would travel sixteen inches through the water with one revolution. It is important to note that propellers are also available in left or right hand rotation for forward. When adjusting your propeller, changing either the diameter or the pitch will change the RPM range of the engine and the performance of the boat.

Differing compositions can improve the performance of a propeller. A stainless prop is a more efficient (less slippage) than an aluminum propeller. The drawback to a stainless prop is that when you strike an object, there is a greater chance that you will do damage to your outdrive because it is a less forgiving material than aluminum and transfers any shock directly into the drive.

If you have a slightly damaged prop, you can fix small dings with a file. A bend in a blade or a folded blade requires replacement or reconditioning at a propeller shop. In determining the proper size prop for your boat there are four things to consider. The design speed of the hull (a knots or mph rating from the manufacturer), the horsepower of the engine, the RPM rating of the engine and the gear ratio. By calculating these four numbers you can determine the appropriate prop size. Call us for an consultation if you have questions on your prop size.


(Contact information for all these classifieds is available on our website in the classifieds section)


7/5/03 - JUST REMOVED FROM 68FT NELSON SAILBOAT. AM REPLACING IT FOR WORLD CRUISE WITH LARGER ENGINE.ENGINE IS IN EXCELLENT SHAPE. 6 CYLINDER MARINE ENGINE USED WITH 2 BLADE FOLDING PROP, AND USED IN FRESH WATER FOR APPROX 9 YEARS. Comes with velvet drive trans at 2.91/1 ratio.ready for shipment from sarnia ontario canada across from pt huron mich. has approx 2800 hours on engine. price $2900us you pay for shipping e-mail me with add questions and payment options or to see engine.


6/26/03 - Low hour 2000 4B 250 horsepower cummins. Heat exchanger, transcooler, aftercooler, cold weather package, Comes bobtail. Nearly 500 ftpounds of torque. Looks and runs like new. $7,900


6/20/03 - I am looking for a good brass core for an oil cooler for an aq 145 engine 1983 cyl ohc.


6/11/03 - Pair of Detroit 16V149 D-Deck Complete Marine Engines, Low-Houred Running Takeouts, Heat Exchanger Cooled, Water-Cooled, Marine Manifolds, Marine Transmissions, Oil Coolers Attached, Factory Detroit Diesel Hi-Grade Engine Isolation Systems, Marine Gears are twin-disc NICO, 3:1 Ratio, Electric Starters, and includes all Engine Hang-ons, FOB NW Coast. These engines were very well serviced and are in excellent condition. $120,000.00/pair. Please call us at any time for any further information @ 888-208-1110. Thanks! mfs0081

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