Truth in numbers

| June 01, 2006

“Running down the highway, I’m right at about 55 pounds of oil pressure,” says owner-operator Gray, who has a 3406E Caterpillar engine. “If the oil pressure drops much below that, I know something excruciatingly wrong has either happened or is about to happen.”

Like most newer engines, Gray’s 3406E has sensors that shut it down if its oil pressure drops. “If the oil pressure or water temperature go bad, then the engine will go into automatic shut-off mode,” Gray says. “It usually lasts about 10 seconds: long enough to get the truck off the road.” Then the engine shuts itself down. To protect itself from abusive drivers, it won’t start again until the problem is solved.

Correct operating and idling oil pressure varies according to the engine. Gray’s CAT is happy with 55 psi of oil pressure, but Lawrence’s Detroit Diesel Series 60 likes it between 40 and 50 psi.

“If it gets below 40 psi, I’m either low on oil or about to lose an oil pump,” Lawrence says.

But big truck engines aren’t always at highway speeds, and an engine low on oil is more likely to get inadequate lubrication when idling. “It should stay at about 30 psi at an idle,” says Lawrence of his Series 60.

Faithful to the last, big diesels without automatic shutdowns will keep idling and slowly destroy themselves if they don’t get enough oil. So drivers should know what a good idling oil pressure level is, too.

Finding out isn’t hard.

“Look in the owner’s manual,” Lawrence says. “It will show you every gauge on the instrument panel and tell you what it’s for and what it should read. They also have an 800 number to call if you have questions.”

For some, a trusted mechanic is a better source. “If one of my gauges isn’t reading what it should and I don’t know what to do about it, I’ll call my local CAT representative,” Gray says. “We go through a list of possible scenarios. He can tell me if the truck is safe to drive or if I need to have it towed in.”

If owner’s manual and trusted mechanic aren’t available, contact your employer’s maintenance, road service and/or safety departments.

Truck drivers necessarily believe they know everything. If they don’t know, some will make up something that sounds true instead of just asking. Some questions, such as, “What should the oil pressure be at idle?” are hard to ask, but they’re not nearly so hard as telling your employer he has to spend $20,000 for a new engine because you ignored or didn’t understand the oil pressure gauge’s reading.

Besides, trucking company owners smile when their drivers take care of the company’s costly tools, so asking that seemingly dumb question will probably make points with your boss.

For example, what temperature is too hot for engine coolant? At idle? Running down the highway?

“Mine runs at about 175 degrees,” Gray says. “That’s about normal for my engine, but it varies from engine to engine.”

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