Diagnosing incomplete oil pressure on a small-block Chevy

Nothing is more important to your engine than the flow of oil. If oil stops coursing through the engine’s passageways, the result will be catastrophic failure. This is why nearly every car has an oil-pressure warning light. If it comes on while you’re driving, stop immediately. You may have only seconds before major damage occurs.

Oil is pushed through the engine under pressure by the oil pump. The pump is driven by a chain from the crankshaft, by a gear on the camshaft, or by the distributor. It sucks oil out of the pan and pushes it into the oil filter. From there, oil goes through internal passageways in the block and crankshaft to lubricate the crank and rod bearings, is sprayed on the cylinder walls to lubricate them, and is sent up to the head to oil the valve train. An oil-pressure sensor sits at the top of the block or in the head to verify that oil is reaching the top of the engine.

So why would the oil-pressure light come on? Internally, oil pumps are simply a pair of intermeshed metal gears, so actual pump failure is uncommon. Oil pumps usually have a release valve (a little piston and a spring) to prevent pressure from getting too high. Some oil filters have an internal bypass valve, and that valve could stick and dump oil back into the pan instead of through the engine. A rod or main bearing could wear out and allow oil to squirt past it, reducing upstream oil pressure. Or a component that carries or distributes the oil could become dislodged, damaged, or installed incorrectly.

When an engine is rebuilt, or a new engine is installed, it’s good practice to prime it with oil and verify the presence of oil pressure before running it. You can do that by disabling the ignition and cranking the starter or, depending on the engine, using a priming tool that directly spins the oil pump. Which brings me to this month’s question, from Hagerty member Scott Sortor:

I purchased a new 350-cubic-inch GM crate motor. No oil gets to the rocker arms on the passenger side. There’s plenty on the driver’s side, and a mechanical oil-pressure gauge shows about 40 psi. I’m using a Speedmaster oil primer and a half-inch corded variable-speed drill with plenty of power to prime it. Could some of the three plugs under the timing cover be missing?

Since this is a crate motor and not a home rebuild, I think the odds that it is missing any gallery plugs, or that an oil passage is plugged with silicone, are slim. On the small-block engine, the distributor is an integral part of the oiling system. The distributor body has a groove around it through which oil flows to both heads. The Speedmaster oil primer (it replaces the distributor and spins the oil pump) also has this groove, and if it’s pushed all the way in, that should allow oil to flow to both heads. However, the engine must be rotated to ensure that the holes in the pushrods through which oil goes to the head line up with internal passages. You could remove the spark plugs and hand-turn the engine with a ratchet a few times while someone else spins the primer with the drill. Some online forums caution that these primers can misshape the hole in the pump and throw shavings into the engine.

If I had the car, I’d put the distributor in, prime it by spinning the starter motor, and see if the right bank gets oil. Or pour oil over the valvetrain to prelube it, run it at idle as the vendor recommends, and see if it starts self-oiling.

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