Teague On Tech: Q & A with Bob Teague September/October 2021

As seen in Speedboat Magazine’s
September/October 2021 Issue.

Low Oil Pressure

Dear Bob:

I have a 2000 Mercury carbureted HP500 with an M3 ProCharger in my boat. Last winter, after 470 hours of operation, I had the engine rebuilt. I had asked the builder to install a high flow oil pump; however, I believe he installed a standard Melling M77 oil pump. Prior to the rebuild, my engine had oil pressure of 70 to 75 PSI at WOT (5,000 rpm). Now, the highest oil pressure is 45 psi.

Above Left: Melling 10778 Oil Pump and Pickup. Above Center: Measuring crankshaft for rod bearing clearance. Above Right: Measuring main bearing installed in block.

I am running Valvoline VR1 50 racing oil and the idle cold oil pressure is about 40 psi, which drops to about 24 psi when the engine is hot. The oil pressure comes up to 45 psi pretty quickly off idle when hot but will not go any higher. I have confirmed and duplicated the dash gauge pressures with a mechanical gauge.

Above: Torqueing rod bolt to spec with bearing installed.

I have received opinions at both ends of the spectrum. The builder felt that the 45 psi was sufficient and offered to provide a different pump at no charge. Of course, that’s the easy part compared to pulling the motor again. I ran the boat last season with no apparent issues, but the oil pressure makes me nervous as I know the rule of thumb of 10 psi / 1,000 rpm. I am well short of that standard. I would appreciate your input.

I also wonder if you would offer your opinion on what fuel pressure I should be running at WOT and 8 psi of boost.

Michael Cosgrove
Louisville, KY

Above Left: Measuring rod bearing to compare clearance. Above Right: Final assembly with proper rod bearings.

I share your anxiety regarding the substandard oil pressure in your motor. If the engine were a stock 260 hp small block, or a stock 300 hp big block Chevy, I would not be so concerned. Being that your engine is unnaturally aspirated, and capable of turning in excess of 5,000 rpm, 45 psi of oil pressure with straight grade 50 weight oil makes me nervous too.

It is important to realize what establishes the oil pressure in the engine. When your engine was stock from Mercury, it did not have a high-volume oil pump. But with the standard pump, it had decent oil pressure. The point is that the oil pump is only part of the equation. The oil pump does have a pressure relief valve built into the cap. The fact that your oil pressure immediately comes up to 45 psi (and no further) could indicate that the oil pump that the builder used has a relief valve spring that is too weak for the application or the relief valve plunger is stuck in an open position. If a Melling 77HV is used, sometimes it is necessary to shim the spring to raise the relief setting. I recommend that you use a Melling part number 10778 pump which has chrome-moly shafts. It also has the drive and idler shafts that have been extended into the cover for additional support, which eliminates possible deflection inherent during higher RPM operation.

In order to verify if your oil pump is the problem, I would perform a comparative test. Obtain a pre-oiling device. You can make one by modifying an old distributor. Remove the distributor, install your mechanical oil pressure gauge, and “pre-oil” the engine using a substantial 1/2 inch electric drill motor. Then, remove the oil pan, install the new Melling 10778 oil pump, replace the pan (without sealing it for the test), and repeat the test with the new pump. If that resolves the pressure problem, put the pan on for real and you are good to go. If it does not, the following are other things to consider.

Above: Rotating assembly complete.

Engine bearing clearances are one of the most significant components of the oil pressure equation. If the rod and main bearing clearances are too great, the increased flow results in lower pressure, with all other things being equal. Proper rebuilding of a high performance engine requires that the crankshaft be precisely measured, and compared to the measurement of the bearings installed into the torqued main caps and connecting rods. This operation requires special inside micrometers that are designed to measure engine bearings. This is because the bearings are not perfectly round when installed. The measurement is less from top to bottom, compared to side to side. The crankshaft is measured with a standard micrometer. It is also important that all the measuring instruments are calibrated to each other. The block, crank, rods, and measuring micrometers must all be the same temperature to achieve accuracy. Once the clearances are measured, using a combination of “standard,” .001” under, or .002” under or .001” over (HX) bearings are used to establish the correct bearing clearances. For high performance big block Chevys, the main bearing clearance should be about .003” on the number 1 through 4 mains, and about .0034” on the rear main. Connecting rod bearing clearance should be between .0027” and .003”. It is possible that your builder simply used “standard” bearings because the crankshaft was “standard.” If that is the case, it is also possible that the clearances are excessive which would result in lower oil pressure.

Above: Results from inadequate oil pressure.

Other things that can cause low oil pressure include internal oil leaks in the engine. This includes the accidental omission of a galley plug, or a bypass check valve installed incorrectly. You likely have a Generation V or VI GM block. I have seen re-builders forget to re-install the 1/8 inch pipe plugs in the lifter galley which are not present on the Mark IV blocks. Omitting the lifter galley plugs (under the timing gear) will also result in low oil pressure but usually causes the lifters to be noisy.

If the low oil pressure problem is due to excessive engine clearances on the crankshaft or lifter bores, the result is that excessive oil is added to the windage in the engine. Excessive oil that is exposed to the rotating assembly creates additional windage problems, low sump levels, aerated oil, and increased oil temperature.

Above: Crankshaft damage from spun bearings.

Oil is also heated excessively by heat that is absorbed from the underside of the pistons which is increased if your engine is running lean causing higher combustion temperatures. Low fuel pressure or flow restrictions can lead to lean conditions. Your carburetor is enclosed in a “box” which is pressurized by the centrifugal compressor. Your fuel pump has a boost reference hose that causes the fuel pressure to increase as the boost increases. If your carburetor is a Holley dual feed design, it is desirable to be able to maintain about 8 psi fuel pressure under full constant load in a normal performance environment. Being that your carburetor is in a pressurized environment, the fuel pressure must be increased pound for pound as the boost increases. In other words, if you are running 8 psi of boost, the fuel pressure should increase to about 16 psi. If the fuel pressure does not increase as the boost increases, the fuel flow will stop or be hampered significantly as the boost pressure reaches a pressure equal to the fuel pressure. I assume you have the set-up where the boost reference hose connects to the tube on the mechanical pump, resulting in additional pressure on the diaphragm. Alternatively, you can use a higher-pressure mechanical or electric pump with a boost referenced regulator.

Ultimately, if you want to know if the engine is too lean (or rich), you will need to install bungs in your tailpipes, and test the boat with air/fuel ratio meters attached. AEM (Advanced Engine Management Inc.) has a nice set up that works well and is available at a decent price.

More Oil Pressure Problems

Dear Bob:

I am experiencing oil pressure conditions, but with a different set of circumstances. I have an original MerCruiser 454 multi-port injection (MPI) Magnum motor in my 1998 Formula 252LS. I had an unfortunate incident while underway and running at about 2,500 to 2,800 rpm; my oil pressure sensor was rusted without my knowledge and it was pumping oil out of the engine into the bilge. It ran so low that I observed the oil pressure gauge drop to zero and then started hearing clicking in the engine. I guess the noise was coming from the valve train. I shut it down right away and got towed in. I later put in 6 quarts of oil, which means I probably pumped just about all the engine oil out. It seemed to run fine in my driveway with the garden hose connected to the drive with my flushing device, but once I put it in the water, oil pressure dropped from 40 psi down to about 10 PSI and I again heard which seemed to be a valve train clicking or tapping. I limped it back in and now I’m wondering if I ruined the engine. Do you have any suggestions?

John Campione
Phoenix, AZ

Referring to the previous question and answer, I think that your low oil pressure situation is being caused by excessive bearing clearances created by wear to your main and rod bearing due to lack of proper lubrication. As I stated in the previous answer, excessive engine bearing clearances are one of the primary causes of low oil pressure. You may even have a bearing or two that has spun in the block or in a connecting rod. Sometimes, rod bearing halves will stack themselves on each other.

The reason that the oil pressure started out at 40 PSI and then dropped significantly is likely because the oil temperature was increased causing the viscosity to be lower. It is also possible that your oil filter is clogged with engine bearing debris. If you did not change the filter, this could be the case. One way to determine the extent of your engine damage is to remove the oil filter and cut it open to look for engine bearing material. If there is bearing material present, you have some level of engine damage. It is also possible that the oil pump relief valve is stuck in an open position by debris. The same situation can occur with the oil thermostat, if your engine has one, causing it to be stuck in a restrictive position.

I think you should consider that your engine has serious damage and a rebuild or a new long block is in your future.

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