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Under the Hood: It’s not the oil pressure, it’s the gauge

What's harming my engine?


Q: I have a 1997 Dolphin motor home with a 454 Chevy engine, and just recently I started it up to go on a three-day trip with a group of other RVs. I did not notice it right away, but the oil-gauge needle went almost up to the peg. After I had driven it for awhile, it moved off the peg a little, but when I had to slow down it dropped to around half. When I got back on the highway and got up to 55 mph again, it went back up to about 1/8 inch from the peg. Is this something that could or would harm the engine? Do you have any idea what is going on? I change the oil and filter every 3,000 miles or as close to it as I can. I have around 87,000 miles on the rig. We travel with a group of RVs about every other month or so, no more then 100 miles one-way.

––Walter Terry

A: Walter, I'll take a high oil pressure concern over a low one anytime! Since the oil pressure gauge does operate in a logical way relative to changes in oil viscosity (thinner oil, lower pressure when hot) and engine speed (higher pressure at higher speed) it's unlikely the fault lies with the gauge or a shorted gauge circuit. It's also really rare for a mechanical engine fault to cause high oil pressure, so that leaves us with a likely culprit; the gauge sending unit.

With the engine cover removed, you should find this gadget at the top rear of the engine. It's black, pickle size, and has three wires plugged into it. A good option, particularly when low oil pressure is seen, is to purchase an inexpensive mechanical oil pressure gauge, temporarily install it after removing the original electric gauge sender, and check for normal and abnormal gauge readings. In your case, I'd shortcut and simply replace the gauge sending unit, a $30 part.


Q: When I see vehicles advertised in TV commercials, I hear the spokesman say such and such vehicle comes with a 1.8 liter, so many horsepower engine, or 2.8 liter, 6.2 liter and so on. What does the liter size have to do with an engine? I ask because it seems that sometimes a smaller liter engine can produce more horsepower than a larger liter size. I've asked a few mechanic friends of mine but none could provide an answer.


A: Engine displacement, listed in liters— or, in the old days, cubic inches –– is the major factor determining the power output of an engine. Basically, the more air and fuel an engine can burn, the more power is generated. A typical low- to mid-tech engine might develop about 50 horsepower per liter (there are exceptions to this). You are correct that some recent smaller engines can more than double this figure through the use of variable valve operation, direct fuel injection, and turbocharger use, among other technologies. A mind-bending example is the 2.0 liter engine in the Mercedes CLA 45 AMG, which produces 360 horsepower and 331 pound-feet of torque. Smaller high-tech engines are more fuel-efficient and weigh less than larger old-school engines and are becoming popular in cars and light trucks. I'm a bit leery of long term durability and repair costs, but wow! Are they fun to drive!



Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood(at); he cannot make personal replies.


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