Nuts and Bolts with Buffalo Jim

Dear Buffalo Jim,

My 1998 Chevy S-10 Blazer has been running sluggishly for the past month. Last week, the "Check Engine" light came on, and I brought it to a local shop, where the mechanic told me I had a bad mass airflow sensor. He replaced the sensor, but it didn't help. He also replaced the spark plugs, oxygen sensor and distributor cap and rotor, but the truck still died a few blocks from the shop. I took it to a different mechanic, who also said it was the mass airflow sensor. Now I don't know what to do. Both the computer and the mechanics say it is a part that I just replaced, but I don't want to spend any more money fixing a perfectly good sensor. Any ideas?--Henry S.

The first problem, bro, is that people put too much faith in automotive computers. Too often mechanics--especially new ones who aren't used to tinkering under the hood--expect to be able to plug in their test equipment and read the diagnosis right off the screen. The truth is, the only thing computers can tell us is which systems are functioning incorrectly--not necessarily what's wrong with the car. Your case is a perfect example. The computer indicated mass airflow sensor, so you replaced it. That didn't work, so you swapped out a bunch of other parts--most of which were probably still good. Now you're out the time and money, your truck still won't go a mile without dying and that stubborn warning light won't go away.

In order to get to the bottom of the problem, you need to look at what the computer's diagnosis really means. Right now, the computer says that the mass airflow sensor is bad. Since we now know it isn't the sensor itself, we can assume the computer is saying that because there isn't enough air flowing past the sensor before it enters the fuel system. In other words, the computer is telling us that the engine is suffocating before it even has a chance to get going.

One problem might be with the timing chain. If it were to jump from its correct position, it would significantly alter the engine timing, which would restrict the airflow to the engine and cause the car to run sluggishly or not at all. You can test this hypothesis by running a compression check on the engine. If the timing is off, the compression in the cylinders will be significantly lower than the expected 150 pounds per cylinder.

The other possibility is that something is obstructing airflow through the exhaust system. For example, if you had a plugged catalytic converter, that would limit the volume of air passing through the engine and might cause the computer to interpret this as a backup in the air intake. You can test this by disconnecting the exhaust system at the exhaust flange and tying it off to the side. (If you're interested, I described this process in detail in the Nov. 18 Mercury.) It will be loud, but if the engine runs well, you know it was a problem with the exhaust--and with your mechanics.

Tip of the week: Water pumps

It's inevitable: As soon as the weather starts to get warm like this, people start rolling into my shop with leaky water pumps. More often than not, these people have tried all manner of quick-fix, stop-leak additives before finally dropping off their cars for service. I hate to say it, but these products will NEVER work on a faulty water pump. Once the ceramic seal inside the pump starts leaking, the only thing to do is to bring it in and have it replaced. On some new cars, this can be an expensive repair that requires as much as four hours of labor, but I haven't seen a car yet--besides air-cooled Volkswagens, of course--that can survive a Vegas summer without a working water pump.

You do, however, want to be absolutely sure that it's the water pump that's leaking. When the seal on a water pump goes, coolant will leak through a tiny hole in the bottom of the pump. If you get down beneath the car, you should be able to see evidence of a green liquid around this hole. Sometimes, what looks like a water pump leak might just be coming from the hose that leads to the pump. In this case, all that needs replacing is the hose itself--a service that may run you as little as $20.

Buffalo Jim is the owner of Allstate Auto Marine. Despite reports to the contrary, he doesn't want to be the next "American Idol." E-mail him you car questions at buffalojim@lasvegasmercury.com.