Fixing Oil-Pressure Readout Problem

Times Staff Writer

Question: I have a 1982 AMC Jeep Scrambler with a 258-cubic-inch engine. The problem is that the oil-pressure gauge jumps from 10 pounds per square inch (psi) to 60 psi. Even at idle it will go between 5 psi and 40 psi. I always check the oil level, but I am worried that the engine is not getting enough oil. What is causing this problem?--N.C.S.

Answer: The oil-pressure readout is the single most critical bit of information you have on the operation of your engine, so you are right to be concerned about the gauge working properly.

The oil-pressure gauge in most automobiles is controlled by a small pressure sensor that screws into the engine and sends an electrical signal to the dashboard.

The information is usually displayed in a dashboard warning light that signals when the oil pressure is too low. Some cars, including your Jeep, have a gauge that will show the actual amount of oil pressure.

Your problem appears to be in the sensor on the engine. AMC has issued a service bulletin to its dealers warning them that the pressure sensor on your model car sometimes is not accurate.

It's normal for an engine's oil pressure to fluctuate somewhat at different speeds, but it should not gyrate wildly. You could try replacing the sensor, but the new sensor may not be any more reliable than the old one.

If you are the type of driver who likes to have as much mechanical information as possible, one solution would be to install a mechanical oil-pressure gauge.

These gauges operate by running a small copper tube from the pressure-sensor hole to a gauge on or under the dashboard. These mechanical gauges are usually more accurate and reliable than electronic gauges. The mechanical gauges are available at most auto-parts stores and are fairly easy to install. You could have it installed at a garage for a small charge.

Q: We have a 1976 Ford Elite that seems to be losing coolant, but we have never seen water under the car or in the oil. The car has an eight-cylinder, 400-cubic-inch engine. We have to add about a gallon of water every 200 miles. Could you tell us where the water is going?--D.B.

A: Just because you don't find water under the car doesn't mean that you don't have a leak. Possibly the water is flowing over a hot area of the engine and evaporating. Or you may have only a steam leak above the water line of the radiator.

First, you should make certain that the cooling system has a leak. Your mechanic can test for leaks by installing a gauge on the radiator and then pressurizing the system. If the pressure goes down rapidly, you certainly have a leak.

If you are correct that the water is not leaking outside the engine and is not leaking into the oil supply, then you have two other possibilities to check.

The water could be seeping into the transmission oil cooler, which is located at the bottom of the radiator. You can determine that by checking the transmission oil level. If it is higher than normal, then water is sitting at the bottom of the transmission case.

The last possibility is that water is seeping into one of the engine's cylinders through the cylinder head or through the head gasket. Your mechanic should be able to determine if that's the case by inspecting the exhaust system and the spark plugs.

Ralph Vartabedian cannot answer mail personally but will respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, You section, The Times, Times Mirror Square, Los Angeles 90053.

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