Ask Away! with Jeff Smith: Diagnosing Oil Pressure Drops in a Big Block Chevy

I have a big-block Chevy in a ‘69 El Camino that I just purchased. I really don’t know much about the engine other than it is a 454 with an Edelbrock Victor Jr. intake, 850 cfm Holley carb, headers, an aftermarket oil pan, and it idles like it has a cam in it. The previous owner admitted he doesn’t know much about the car. I think he’s a flipper. The car runs real strong and I love it but after driving it for a week or so, I noticed that the engine has good oil pressure at idle with 40 psi. But when I run the engine through the gears over 5,000 rpm the oil pressure drops from 70 to around 40 psi. The first time it happened, I thought for sure I broke something. But at idle it ran fine and when I rev the engine up in neutral, the oil pressure jumps back up to 70 psi. I thought maybe it was just low on oil, so I added a quart but the problem is still there. Do I need a new oil pump? — D.C.

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Jeff Smith: There could be other explanations, so you can’t take this answer as gospel because weird things happen occasionally, but we’ve seen this happen several times especially with big-block Chevys.

The simple answer is the engine has too much oil in the pan.

You mentioned the engine has what we’ll assume is a deep sump oil pan. In the case of an aftermarket oil pan, these pans are rated in terms of additional quarts of oil they carry. As an example, a big-block street/race oil pan for example that has a six or seven quart capacity usually with a sump that measures 7 or 8 inches deep. The manufacturer lists this capacity because everyone thinks a performance engine needs more oil — and with a high volume old pump this might be the case. We’ll save the discussion for why most street engines don’t need a high volume oil pump for a different time.

For the moment, let’s assume that someone has installed a deep sump pan on the engine and then added seven quarts of motor oil to the system because that’s what the oil pan manufacturer mentions in its description.

Now let’s also assume the engine has no windage tray. So we have this large volume of oil in the pan and now we accelerate the car hard and all this oil sloshes to the rear of the pan where it contacts the crankshaft and begins to be whipped up like a blender churning up a 10w40 smoothie.

When oil is whipped like this, it begins to aerate the oil — which turns it to foam. Now what used to be liquid oil is now foam which is drawn into the oil pump. The air in the oil is compressed which reduces the pressure created by the oil pump. The net result is oil pressure drops to roughly half of what it is capable of producing if the oil is not full of air.

Luckily, fixing this problem is very simple.

At first, we mentioned that we’re assuming somebody added too much oil to the engine. You mentioned that you added another quart and the problem didn’t go away. That tells us that there is probably too much oil in the engine. So first drain the quart you added and then drain another half-quart. Now run the engine again up to peak rpm on the road.

If the oil pressure still drops, remove another half-quart and test the engine again carefully watching the oil pressure gauge.

We’re looking for a sign that removing the oil is helping. This may appear as the engine now holding more pressure than before but it’s still dropping, but not as much. This indicates you are moving in the right direction.

Remove another half quart and then evaluate the change. Once the oil pressure stabilizes, this is an indication that at least the foaming has been drastically reduced.

At this point, I would carefully drain the oil and measure the amount in the sump as well as an estimate of the amount in the oil filter. This should be the fill volume of oil when you change oil. Let’s say that this volume is 5 ½ quarts. This is the volume of oil your engine wants. At this point, it would be a good idea to re-mark the dip stick to indicate the proper oil level so there’s no confusion.

My friend Steve Brulé at Westech Performance and I performed a similar test a long time ago on a big-block Chevy and it only took roughly a quart and a half of overfill to see the oil pressure drop off at high rpm. So the lesson here is that too much oil can be bad — it’s better than too little but too much will create your situation. The really great part of all this is the fix is incredibly simple and costs virtually nothing to perform. Just to reinforce this idea, we’ve also seen upwards of 20 hp lost to excessive oil in the engine — so there’s another reason to monitor the oil level carefully.