Electrical fire ruins person’s ‘thing’

Dear Mike: I had a very small electrical fire in the engine compartment of my SUV. The only real damage to anything, other than some wires, was this “thing” — I don’t know what else to call it — that attaches to my air cleaner. My mechanic doesn’t know what it’s for. There were no other parts or anything inside it, and the car seems to run OK without it.

The dealership wants nearly a hundred bucks for a new one, and the parts manager there says that I absolutely must have this thing installed or I risk engine damage. What to do?

He can’t get one for almost a month, by the way, and I need my car.

— S.T., Encino, Calif.

A: That empty piece of plastic is called a Helmholtz resonator. Really. And it is indeed empty.

Actually it isn’t — it’s full of air. It’s attached to the ducting between your air cleaner and the intake manifold and, although it looks like it can’t have any real purpose, it does. It’s there to reduce the noise made by your engine’s intake duct.

The resonator has an interior volume with a very specific resonant frequency, like an organ pipe. Pressure pulses traveling back and forth in the duct have a resonant frequency as well. A pressure pulse whipping past the entrance to the resonator, at the speed of sound, partially enters its cavity. The pulse then bounces off the far end of the resonator and re-enters the duct a very short time later.

By now the pressure pulse has traveled a half-wavelength further along the duct. The re-entering pressure pulse cancels out at least part of the negative pulse at that point. The net result is reduced noise, at least at or near the resonant frequency of the resonator.

Many car manufacturers use this technique to reduce intake-tract noise. Some vehicles have several of these devices in different places along the duct.

Why should you care? This device should have no effect whatsoever on the performance of your engine, regardless of what your parts manager claims. However, the duct is designed to pull clean air from the air cleaner into your engine. A resonator with a hole in it would allow unfiltered air into the works, which obviously would be a bad thing.

Patch the resonator, if there’s anything left of it, or remove it and cover the hole in the duct. Duct tape won’t last very long at all in the heat under your hood, so I’d try a piece of sheet aluminum and some high-temperature RTV silicone. Be sure to use low-volatility silicone rated for engine-gasket use. It should say “safe for O2 sensors” on the label.

Check the rest of the duct for leaks as well, seeing as how you already know there’s damage from the fire.

Dear Mike: I have a Ford Crown Vic, year 2000. If I drive it around town on short trips at 55 or slower, it never uses any oil, even if I go 3,000 miles between changes. But if I take it out of town on a 3,000-mile trip, it uses a whole quart. The service center says that everything is OK.

— C.C., Harpers Ferry, Iowa

A: Let’s assume that the diagnosis by your local technician is correct: There is no real oil leak, the rings and valve seals are all good and your engine isn’t burning oil. Also the positive-crankcase-ventilation or PCV system is working correctly.

Given that, here’s my educated guess: Your short-trip driving cycle means that the engine rarely gets a chance to warm up fully. By “fully” I mean a warm-up that takes a good 15 miles, or even longer in colder weather — and it can get plenty cold in Iowa. Water from moisture condensing inside the engine, raw fuel from cold starts and partially burned hydrocarbons combine to dilute the engine oil, increasing its volume. This dilution offsets the normal rate of oil consumption, so you see little difference in the oil level on the dipstick between changes.

Then you take your trip. The engine comes up to normal operating temperature, and the raw fuel, water and other volatiles boil off and work their way out of the crankcase through the PCV system, which is exactly what is supposed to happen. Within a few hours the oil volume sinks and your dipstick says “Add,” even though the actual consumption of oil past the rings and valve-stem seals remains unchanged.

Actually, I often hear this complaint about vehicles with this type of driving pattern. I would check the thermostat to be sure that the engine is warming up promptly. Mechanics and car owners who are used to the 180-degree thermostats of an earlier era have a habit of replacing today’s customary 195-degree thermostats with the older type, in the mistaken belief that it’s better to make the car run cooler. Today’s cars and trucks are engineered to run plenty hot for better fuel economy.

Dear Mike: I have a 1995 Chevy Lumina van. The fuse for the mirrors, locks and interior lights blew out today. I replaced the fuse, and then smoke started coming from the dash on the driver’s side. I had to take the fuse out to stop the smoke.

Can you help me?

— H.S., via e-mail

A: You had a short somewhere in the wiring that services these devices. The cause is a wire carrying 12 volts from the fuse to the devices controlled by that fuse touched some grounded metal somewhere along its path. The current surge was sufficient to blow the fuse — which, of course, is what’s supposed to happen.

You replaced the fuse with one that was of higher amperage than the one that blew. That smoke was caused by the insulation on the wires getting too hot between the fuse and the short. Now you probably have many more shorts in addition to the original, caused by the wires touching each other when the insulation melted.

Where was your original short? The insulation damage will stop at the short, because current wasn’t flowing beyond that point.

A mechanic with good electrical-diagnostic skills will need to sort this out.