I have a 1995 Chevy S-10 Blazer SUV. Under normal driving, the oil pressure gauge needle hovers around 60 PSI. However, if I make a quick stop or in going down a short steep incline brake sharply at the bottom, the needle bottoms back to zero for a few seconds and the “check gauges” light comes on.

The needle returns to normal immediately, and there doesn’t seem to be any adverse effect on the drivability of the SUV.

I am concerned that harm is being done to the vehicle, and one day I’ll be in for a big repair cost.

Clyde Bell, Seattle

My first thought when reading the symptoms you describe is that there may be insufficient oil in the crankcase, and the oil pump is occasionally starving for oil. But, of course, you have checked that.

Is there any chance the oil dipstick is incorrect for the engine, and the crankcase contains less oil than indicated?

Immediately after an oil change, when the prescribed quantity of oil has been added, does it read correctly?

If you’re sure of the correct oil quantity, try connecting a mechanical oil pressure gauge to the engine for a few days, and see if the symptom still exists. This can be a professional tool or an auto parts store cheapie connected to the engine in lieu of the original gauge sending unit. The mechanical gauge’s slender plastic line can be carefully slipped through the hood/cowl intersection, and the gauge head temporarily taped to the base of the windshield.

If the mechanical gauge reads properly – at least 10 to 20 pounds of pressure during hot idle and 30 to 60 pounds during hot high-speed driving – the engine is fine and the fault lies in the Blazer’s gauge circuit.

If the mechanical gauge mimics the occasional drop-out of the electric gauge, there’s a potentially serious internal engine fault, such as a cracked oil pump pickup tube.

My first check of the gauge circuit would be of the tan wire leading from the oil pressure sending unit, as far as you can follow it. On the 2.2-liter engine this part is located midway on the right side of the engine block. On the optional 4.3-liter engine, it’s at the upper rear of the engine, near the distributor.

Could the wire be chafeing against a metal component as the engine shifts slightly in the chassis during braking? This would cause the gauge needle to swing to zero, as the sending unit performs a similar grounding process whenever pressure falls away.

It also is possible the oil pressure sending unit is erratic. Try disconnecting the tan wire from the sending unit and putting the Blazer through its paces. If the gauge needle remains pegged high when braking hard and on steep inclines, this proves the fault is not in the gauge wiring or instrument panel, leaving the sending unit as the likely culprit.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.

E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.)

—

(c) 2005, San Jose Mercury News (San Jose, Calif.).

Visit MercuryNews.com, the World Wide Web site of the Mercury News, at http://www.mercurynews.com.

Distributed by Knight Ridder/Tribune Information Services.

AP-NY-02-04-05 0622EST