TRY MECHANICAL GAUGES

QUESTION: I have a Thundercraft Magnum Express 290 with two 1989 Mercruiser 4.3 LX engines. Since last year I have had a low oil pressure problem on one engine. The oil pressure on one engine falls between zero and 10 psi on the gauge.

I have checked the oil pressure with a mechanical gauge. The pressure was between 35 and 60 psi. Since then, I changed the sending unit. After that everything was OK until recently. After running it for a few hours, the pressure reading became very low. What should I do?

ANSWER: If you were actually running your engine under load with zero to 10 pounds of oil pressure, engine damage and possible catastrophic failure would have most likely been the result. I believe that your problem is most likely related to an unreliable electric monitoring system that is now making you crazy. So, change it.

I would install mechanical oil pressure gauges. Mechanical gauges do not lie. You will need to route pressure hoses from the place where the oil pressure senders are currently located on the engines to the helm position. I recommend not using the typical 1/8-inch plastic tubing with the compression fittings. Instead, use a good quality -4 hose and push-on swivel hose ends.

I use Aeroquip FC647-4 hoses with the push-on brass hose ends (Aeroquip Part No. 4797B) that do not require hose clamps. You will also need to use -4 x 1/8-inch pipe adapters on the engine (Part No. 2021-4), and adapters on the back of the gauges (Part No. 2022-4 with female pipe thread). The FC647 hose is rated for the pressure and has a fabric exterior jacket, which allows the hose to be pulled through rigging tubes without catching. This hose is capable of making sharp bends without kinking.

CHECK THE TACHS

QUESTION: My boat has twin 5.7-liter Volvo Pentas with the dual outdrives. My question is: How do I get the engines to run the same rpm at idle, and at top speed? The throttles are nowhere near each other. Is there some computer chip or something that would make them run close to the same rpm?

ANSWER: I would first verify that your tachometers are reading accurately. Connect a test tachometer to the terminals on each of the tachometers in the boat and compare the reading. The fact that one tachometer reads lower in all ranges, including at idle speed, makes me suspicious of their accuracy.

If the tachometers are found to both be reading accurately, then check the throttle linkage and cables to make sure they are both connected in an identical manner. Most helm-mounted throttle assemblies have at least two different locations in the cable lever where the cable can be attached. If one of the cables was connected at a closer "hole" to the pivot point than the other, the throttle rate through the range would not match. Perform the same check at the carburetor or throttle-body linkage lever to make sure the cables are attached the same.

It is common for propellers to be unmatched to some degree. The difference from side to side is not usually more than a couple hundred rpm, though. If the drives have the same gear ratio, the propellers are the same pitch, both drives are being trimmed equally, and both engines are in reasonably good condition, the rpm from side to side should be very close.

—Need some guidance from Bob Teague, Powerboat's lead test driver and owner of Teague Custom Marine? To send him an email click here.

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