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" Cleaned Out Q: My dashboard Check Engine light came on and a scan tool said Code P0171 was stored on my car&amp;#039;s computer. I checked the O2 sensor, mass airflow sensor (MAF), 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 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&amp;#039;s too far out of range, it sets an error code. Periodically, there&amp;#039;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&amp;#039;re supposed to replace the MAF sensor. According to almost every manufacturer, it&amp;#039;s not recommended or even possible to clean the delicate wires. Of course, they&amp;#039;re selling those sensors, so their motivation might be suspect. Next time, I&amp;#039;d try cleaning it first--what do you have to lose? I used to use aerosol carb cleaner, but CRC MAF Sensor Cleaner is new and less prone to dissolving other components in the MAF sensor. Do yourself a favor and remove the MAF sensor from the vehicle before attempting to clean it. Also, I&amp;#039;d make sure the air filter housing, air filter and all associated ducting are intact, clean inside and not leaking dirt. This will keep you from needing to clean or replace the MAF sensor anytime soon. Repair Solutions Online Every car made since 1996 has an OBD-II (On-Board Diagnostics) connector underdash. It&amp;#039;s where the mechanic plugs in his scan tool to check your car&amp;#039;s health. A pro-grade scan tool can cost thousands. CarMD costs $90, which is close to what a mechanic would charge you to use his scanner to diagnose a problem. The CarMD uses the memory and screen on a Windows-based computer to display the data from your car&amp;#039;s engine management system and to look up information from CarMD&amp;#039;s Web site. There you can find an explanation of what all those trouble codes really mean, and some insight into what might be causing them. carmd.com Power Surge Q: 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&amp;#039;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&amp;#039;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&amp;#039;s electrical system. Free as Air Q: I have a 1994 Lincoln Town Car with 161,000 miles. 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&amp;#039;re driving, but you probably don&amp;#039;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&amp;#039;s just a leak. If you&amp;#039;re lucky, it&amp;#039;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. Does backing up and then moving forward a small distance cause your 2000-03 Mazda MPV to make clunking noises when you touch the brakes? Mazda Technical Service Bulletin (TSB) 04-001/03 specifies a different, revised guide plate for the pads that will eliminate excess clearance and stop the noise. A rumbling noise from the rear axle of a 2004-06 Toyota Sequoia may be caused by damaged bearings--a result of water intrusion in the axle. Corroded bearings soon become noisy bearings. TSB DL001-05 says to replace the bearing and seal assembly with a revised part that is better at keeping water out and lubricant in. Setting Limits Q: I inadvertently redlined the engine on my 4.0-liter six-cylinder 2005 Nissan Frontier for a very short time. Would this have caused engine damage? What would it take to ruin an engine? I have 7800 miles on it. I usually am very careful and don&amp;#039;t drive aggressively. The engine management computer on all current production engines incorporates a rev limiter function to prevent engine damage. If the engine was warmed up and the incident didn&amp;#039;t last long (i.e., a few seconds), there should be no damage. Older engines, ones with carburetors, typically have no such protection. Their upper engine speed is usually limited by valve float, which is when the valve springs can no longer close the valves fast enough. This can result in near-instantaneous damage, as the still-open valves hit the piston as it reaches top dead center. Your engine is probably fine. Just don&amp;#039;t make a habit of holding the throttle wide-open in Neutral. Oil Crisis Q: I was changing my oil and put in a bottle of oil treatment--but I forgot to pour in any oil. I drove about a mile before the truck stopped when the engine seized. I&amp;#039;ve put in some tranny fluid and have been cranking the harmonic balancer clockwise. It now has loosened about halfway, but the other half is still stiff. Would you please give me some advice on anything else that may work? I saw on your Web site something about warm brake fluid. A: First off, I have never recommended putting brake fluid into the crankcase of a motor vehicle. As for your truck: You will never get it to run again by pouring solvents into the engine. I&amp;#039;ve used Marvel Mystery Oil or even ATF to try to free up an engine that had seized from being improperly stored. This is to help break up the corrosion (rust) between the rings and cylinder walls. Usually, they don&amp;#039;t run long or well after this abuse. But, you ran the engine without any oil. What&amp;#039;s keeping it from turning over is the connecting rod shells that have welded themselves to the crank journals. If you&amp;#039;re able to turn it with a wrench, it&amp;#039;s because the rod shells are spinning in the big end of the rods. The crankshaft will have to be reground to a standard undersize--after it&amp;#039;s Magna-fluxed to look for cracks. I&amp;#039;m betting the pistons are scuffed, as well. You&amp;#039;ll probably need to bore the cylinders to a standard oversize and use new pistons. And I won&amp;#039;t even go into the damage in the cylinder head to the valves, valve guides, or the camshaft and lifters. My suggestion: Find a friend with a boat and give him your engine to use as an anchor. Then buy a used engine at a junkyard. "^^<http://www.w3.org/2001/XMLSchema#string>
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