Abnormal Instrument Readings

stations or between engines. Often, one gauge set will read differently from another—not by much, but different nonetheless. The obvious question is, is this a problem? The answer is no. I have wasted hours trying to match gauge readings on boats with duplicate instruments. Unless I was very lucky or was working on exceptionally high-quality instruments, my results were less than satisfactory. The best gauges have calibration screws that enable you to fine-tune the actual readings, but don’t count on finding these expensive gauges on a typical production powerboat.

Some manufacturers (VDO and MerCruiser, for example) offer sending units designed for two gauges for use on dual-helm boats. Any discrepancy between the two gauges working on a common sending unit is caused by voltage drop, the phenomenon mentioned throughout this book. Since you have read this far, you know that due to the increased resistance to electrical flow in a long wire, the length of a wire has a direct effect on voltage drop. Well, this extra resistance directly affects the instrument reading at your console, because most instruments rely on variable resistance created by an engine-mounted sensor to give you readings.

Differences in the lengths of connecting wires on instruments at upper and lower stations, combined with manufacturing tolerances in gauges and sensors, ultimately control the actual reading you get at your instruments. Don’t panic if you see slight variations from one gauge to another. This is a normal condition, and repeated trips to your mechanic or boat dealer to solve the problem will almost always prove futile. Remember to look for relative changes in your normal instrument readings; that’s what’s important.

At some point your instruments will indicate that a condition other than the norm exists. It might be high engine temperature, low oil pressure, erratic rpm, or dozens of other deviations from your normal readings. Often when there is a change in one instrument there is a corresponding change in others. Falling oil pressure might be accompanied by rising engine temperature and falling rpm, for example. When this occurs, you must respond quickly and verify that a problem does in fact exist. Ask yourself: Is there really a problem, or is the gauge just acting up? Initially, you must assume that a problem does exist and immediately shut down your engine until you can verify that it’s OK.

Verification of engine condition requires some mechanical expertise on your part, and if you’re in doubt, you may need to consult an experienced mechanic. This works fine at the marina, but offshore it won’t be possible to call a mechanic, so some basic tips are in order. If your boat has an outboard engine, my book Outboard Engines: Maintenance, Troubleshooting, and Repair will surely help you solve most common problems. If yours has an IO or inboard engine, the following steps should point you in the right direction.

Low Oil Pressure

When your oil-pressure gauge gives you a low-oilpressure reading, immediately shut down the engine and check the engine oil level. Refer to your owner’s manual to determine the correct level, if you don’t already know it. While you’re at it, look for any signs of leaking oil.

If the oil level is correct, an oil-pressure problem is quite unlikely unless your engine has many hours on it (over 1,000 hours for gas engines and even more for diesels). Excess engine noise, such as ticking sounds coming from the top of the engine block, is a sure indication that a problem exists, and you may not be able to get the boat home without assistance. If in doubt, radio for help; the chance of causing extreme engine damage here is great and not worth the risk.

High Engine Temperature

If excessively high engine temperature is indicated by your temperature gauge and the rise occurred suddenly, you should back off the throttle to an idle and see if the temperature reading begins to drop. Often it will, and it’s preferable to let the temperature stabilize before shutting off the engine to scope out the problem. Suddenly shutting down an overheated engine can cause extreme shock to the engine’s pistons,