Q I have a 2000 V6 Ford Explorer with 103,000 miles. Several times a week when I try to start it, the vehicle will turn over strongly but refuse to start. Then, after five to 10 attempts, it kicks in. Today, it turned over and almost started, then wouldn't stay running. It did this three times until it finally started and ran fine. In the past three weeks I have had it tuned up, replaced the plug wires and installed a new, powerful battery. The problem continues to worsen. Any ideas?

A Three likely suspects come to mind: vacuum leaks, low (or no) fuel pressure or a faulty idle air control valve. Attaching a mechanical fuel pressure gauge to the pressure relief valve on the fuel rail for an overnight cold start test may reveal low or no fuel pressure from an inoperative or weak fuel pump. The repeat cycling of the key may eventually cause the pump to start. Pounding on the bottom of the fuel tank with a rubber mallet while cranking the engine may also start the pump -- and confirm a "tired" pump.

My Alldata automotive database pulled up Ford bulletin 03-3-5, outlining a test for checking the duty cycle of the idle air control. If the duty cycle is high, check the electrical connections for the valve. If the duty cycle is low, look for a vacuum leak.

Q My El Camino has an oil pressure gauge that flickers when you first turn on the key. The needle goes to max and then back to zero, where it quivers a bit. Once you start the car, the needle flickers and then settles down at 35 to 40 pounds per square inch. It quivers as I accelerate or decelerate. I think it may be the sending unit, but I'm not sure where it is. Could I remove it and thread in a mechanical oil gauge and see if it flickers?

A Yes. On most older, small-block Chevy engines, the electrical oil sending unit sits just above the oil filter or at the top rear of the block. To test the sending unit, unplug the wire, then turn on the key without starting the engine. With the wire disconnected, the gauge should move all the way in one direction and stay there. Then ground the wire connector to the block; the needle should move all the way in the other direction and stay there. There should be no quivering.

If the gauge reacts properly to this test, the problem is likely the sending unit. If the gauge flickers or doesn't react correctly, it might be the gauge itself.

I like your idea of installing a mechanical oil pressure gauge; no crafty electrons to confuse the issue over oil pressure. In fact, with the correct "T" fitting you can keep both the sending unit and a mechanical gauge.

And finally, make sure there's a solid electrical ground with no resistance between the engine and chassis.

Q A month ago, I had the rear differential fluid in my wife's '09 Acura MDX replaced with "premium" automatic transmission fluid at a drive-through oil-change shop. They also replaced the transfer case fluid with "high performance" 80W-90 gear oil. Am I compromising the integrity of the MDX differential by using "premium" transmission fluid? Should I have it replaced with the Acura brand fluid to maintain lubrication and possibly my warranty? Also, is "high performance" 80W-90 gear oil a proper replacement for the MDX transfer?

A Acura recommends GL4 or GL5 SAE 90 or SAE 80W-90 gear oil for the transfer case, so no issues there. Acura specifically recommends their DPSF all-wheel drive fluid for the differential, but it allows ATF-Z1 for topping up the unit. It's your call, but unless the "premium" ATF meets Acura's specs, I'd stick with the carmaker's fluid.