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Dick Hussey's Tech Corner
Some (Inexpensive Ways to Fix) Causes of Low Oil Pressure
The first time your low oil pressure warning light comes on, it can be unnerving. Often this will be while you are stopped, with the engine idling. The first thing everyone does is to pull over and check for the proper amount of oil inside the crankcase. If the level is low, the first thing to check for is a big leak. Once in a while, an oil cooling hose will rupture. This can dump a lot of oil quickly. Another possibility is running over road debris punching a hole in the oil pan. Fortunately, Allanté oil pans are very strong, and there is not much likelihood
of one getting pierced.
This first thing to check is whether or not the oil pressure warning light goes off when the engine speed is increased. If the light goes off and oil pressure gauge reading increases above 1/4 of the scale, it is safe to
drive the car. If the light stays on and there is no reading on the gauge, the car should be shut off and towed, not driven.
In addition to the obvious possibility of oil pump failure, there are a few other possible causes of no or low oil pressure at idle.
One of these is an incorrect oil pressure sensor. This problem is beginning to show up more frequently than not, especially on 1993 Allantés. The reason is that the original oil pressure sensor which was GM part number 3532953 has been discontinued and is no longer available. When my local Delco distributor did not have the Delco part in stock, I accepted a Standard Motor Products part, which looked identical. The connector was the same, the threads were the same, and the size was the same. After installing it, all seemed OK. But, within a few days I noticed that from time to time I would get a low oil pressure warning light at idle. The first thought was to increase the idle speed. That would put the light off right away. After living with the problem a couple weeks, I knew something was going on that needed to be addressed.
The on-board diagnostics give you the capability of seeing values that are coming into the car's computers. I checked to see if the signal coming to the computer matches what is being displayed on the instrument cluster. Sometimes a reading will jump all over the place. If the input to the computer is steady and doesn't match what's showing, then there is a problem between the computer and the cluster. If the parameter value is also bouncing all over the place, then the signal from the computer to the cluster is probably OK, just displaying the result of the bad input signal.
With the Standard Motor Products sensor in, I entered the on-board diagnostics at parameter BD71 to watch the oil pressure reading. As the engine speed decreased, the oil pressure would drop down, which is normal. But what was not normal was that the pressure would drop .. . 20 -19.5 -19.0 -18.5 then immediately to zero. When it dropped to zero, I heard a click. Since the oil pressure had been dropping gradually and slowly, then immediately and consistently went from 18.5 lbs to zero, I was confident that the oil pressure was actually there in the engine and that all I had was a measurement problem.
I knew I had recently replaced the oil pressure sensor, so that was the first place to go. At that time, the correct Delco sensor was still available, so I special ordered one. After installing it, the problem went away completely. When I entered diagnostics to watch BD7 with the new correct Delco oil pressure sensor, the consistent slow drop in oil pressure occurred below 18.5 lbs which it would not do with the aftermarket sensor.
We have seen some low oil pressure problems on earlier Allantés also. The causes were completely different. Revving the engine somewhat would shut off the low oil pressure warning light and restore some bars in the gauge reading. In one case, a stalling problem was encountered. The cause was found to be a defective idle speed control motor. Once the ISC motor was replaced, the low oil pressure at idle problem never reappeared. Apparently in that case the improper 400 RPM idle speed was the cause of that low oil pressure problem.
When this type of problem appeared again recently, replacing the idle speed control motor and the oil pressure sensor had no affect. The low oil pressure light and warning message still came on at idle when the engine was at operating temperature. The oil pressure really did appear to be low. That raises the specter of oil pump replacement or worn bearings and lots of $$$. But fortunately in this case we also found a relatively simple and inexpensive fix. There is a pressure relief valve inside the oil filter adapter, and this proved to be the culprit. The oil pressure was in fact low at idle. Increasing engine RPMs would bring it up. Some of the oil pressure was being bled through the pressure relief, thus reducing the oil pressure available to the engine. Replacing the oil pressure adapter corrected this low oil pressure problem.
In summary, if your low oil pressure warning light comes on, don't assume the worst. A little troubleshooting can save a lot of money.
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