Air sensor issue confounds

Dear Mike: My dashboard Check Engine light came on and a scan tool said Code P0171 was stored on my car’s computer. I checked the O2 sensor, mass airflow sensor, etc. All voltage and resistances were well within the factory specs. So, I took the car to the dealer, and they replaced the MAF sensor.

Since then, the car has been running fine and the light stays off. Why? Is there any secret?

A: Mass airflow sensors constantly measure the amount of air entering the intake manifold. This lets the engine management computer know how much gasoline to inject into the cylinders for proper combustion. Hot-wire-type sensors — like the one you have — work by keeping a short piece of wire or film at a constant, elevated temperature. An electrical current is applied to the wire to maintain its temperature; the amount of current necessary to maintain the temperature is proportional to the mass of cool air flowing past it. Sounds complicated, but there are no moving parts and the response rate is very fast. So far, so good.

Eventually, dirt and oily film bake onto the wire, insulating it from the cooler air — and when the engine management computer sees a value for air mass that’s too far out of range, it sets an error code.

Periodically, there’s a cleaning cycle during which the wire is heated to a very high temperature right after engine shutdown. This burns off deposits.

When this cleaning cycle fails to remove all of the contaminants, you’re supposed to replace the MAF sensor. According to almost every manufacturer, it’s not recommended or even possible to clean the delicate wires. Of course, they’re selling those sensors, so their motivation might be suspect.

Dear Mike: Can welding on a vehicle damage its electronics? I have heard that I should disconnect the battery, computer, alternator and other electrical components before welding. This was the policy of a company I used to work for that made and mounted truck bodies.

But many companies, including the one I work for now, never disconnect anything. After all, ships and steel-frame buildings with grounded frame systems don’t require it. I have done structural work, such as extending truck frames and welding on tractors and on farm machinery connected to tractors, with no ill effects.

A: The sensors for your car’s engine management computer can operate at millivolt voltages and at milliamp currents. A MIG or stick welder can reach 50 volts and hundreds of amps. Most of the time, the computer harness is insulated enough to prevent damage. A poor ground can cause eddy currents powerful enough to fry the electronics.

If I need to weld on a car, I remove the computer and disconnect the battery ground cable to isolate the car’s electrical system.

Dear Mike: I have a 1994 Lincoln Town Car with 161,000 miles on it. After parking the car I hear air venting from the air suspension system. After I restart the car, the air pump will run for 30 to 60 seconds. This started about three months ago.

Is there an adjustment or an easy fix to correct this problem?

A: The compressor runs after you start the car because the air springs lose pressure and sag. They are just rubber bags, and there is a fair amount of plastic tubing hooking them to the pump. When you restart the car, the ride-height sensor sees the suspension is too low, and the pump runs until it comes back up to the required height. The leak continues while you’re driving, but you probably don’t notice the pump running while the car is running.

There might be a problem with the ride-height sensor — but, more than likely, it’s just a leak. If you’re lucky, it’s a hose damaged by road debris or a loose fitting, which is easy to fix. A leaky air bag will cost a lot more to replace.