Q. I just knocked off the plastic flap that hangs down beneath the front of my car. It was dangling precariously so I removed it the rest of the way. I’ve been told I must reinstall it so the car won’t overheat. So far I haven’t noticed a problem, although it has only been a few days. Is this true?

Jason Alexander, Seattle

A. You really should repair or renew the air dam and reinstall it. Many newer cars and trucks have very small or no grille openings to provide airflow for the radiator and air conditioning condenser. Instead they rely on an air dam beneath the vehicle nose to divert airflow upward and through these heat-exchanging devices.

You may not yet have encountered a high-load, moderate-to-high-speed situation where the air dam is effective and needed. At low speed, the radiator/condenser fan(s) provide the necessary airflow.

Depending on the mounting method and the degree of damage, it may be possible to reinstall the original air dam using fender washers (with small center holes and a large outside diameter) to bridge torn-away fastener holes. Sometimes it’s difficult to remember the typical speed bump and parking stop were designed for foot-off-the-ground Galaxies and Impalas, rather than today’s aerodynamic ground huggers.

Q. My “87 Nissan Stanza has been running poorly and failed its smog test for excessive hydrocarbons. When it’s cold and at low speeds it runs perfectly, but after it warms up and you increase speed it begins to sputter and loses power. Do you have any suggestions? I replaced the spark plugs and spark plug wires a while back. Funds are tight right now.

Jacob,

New Almaden, Calif.

A. Good job on the symptom description, Jacob. This smells of an ignition system fault, even though you’ve already renewed some of the suspect parts.

Here’s why: Because the car runs well cold we can back-burner most engine mechanical and fuel system possibilities. My hunch is your ignition system is leaking spark, due to a cracked spark plug or distributor cap, a burnt-through rotor or a loose/injured spark plug wire.

Ignition spark is a funny creature. It tries to sneak anywhere it can to ground rather than correctly jump the spark plug air gap. Higher temperature usually worsens the insulation fault, bringing on a misfire.

I’d begin by removing and inspecting the distributor cap and rotor for the slightest imperfections. Renew both if any cracks or discoloration are seen. Next, do the same with the spark plugs. It’s possible a plug insulator might have become cracked during installation, particularly one of the more difficult to access rear plugs. Next, check both ignition coils for possible cracking near the coil wire connection. Wrap up by inspecting the plug wire boots for splits and ensure they are fully seated upon installation.

If this fails to do the trick, a professional diagnosis will be needed.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.

E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.)

—

(c) 2004, San Jose Mercury News (San Jose, Calif.).

Visit MercuryNews.com, the World Wide Web site of the Mercury News, at http://www.mercurynews.com.

Distributed by Knight Ridder/Tribune Information Services.

AP-NY-07-16-04 0618EDT