Question: I have a 1976 Toyota Celica with a five-speed manual transmission. When the engine is cold, the car backfires through the exhaust pipe each time the clutch is disengaged. After the car has been running for a few minutes, the backfiring becomes hardly noticeable. I have had the car tuned twice since the problem started a year ago, but that did not solve it. Could you explain the possible causes of this problem?--X.W.
Answer: Most backfiring occurs because of an explosion inside the exhaust manifold or the exhaust pipe. In your case, the explosion is a result of a malfunction in your emission-control system.
Your Celica has an emission-control system in which air is injected into the exhaust gases to complete the combustion of any partially burned fuel. When the engine is decelerating, the air injection is supposed to be cut off.
In your case, the air injection is not cutting off. When you disengage the clutch, the continued injection of air is causing explosions inside the exhaust manifold, which you hear as a backfire.
The air-injection system is controlled by a sensor on the intake manifold. When the intake-manifold vacuum rises to a certain level, a condition that is triggered by deceleration, the air injection is supposed to be diverted to the air cleaner.
A small filter over the sensor is clogged. Thus, the sensor is not detecting the increase in vacuum. The problem was covered in a technical service bulletin in 1978, which your mechanic may have forgotten about. The new filter should be relatively inexpensive.
Q: I have a 1969 Malibu 350 that uses leaded gas. Can I use unleaded gas? The car runs like new.--E.W.
A: Under the new federal laws that went into effect at the beginning of this year, leaded gas contains .1 grams of lead per gallon. That's a lot less than 1.1 grams of lead added before July 1, 1985.
As long as you're not towing a trailer or driving the car unduly hard, leaded gasoline shouldn't cause any significant short-run problems. I would avoid unleaded gas.
Q: I have a 1985 Toyota Camry. It is generally a fine car. It has the hydrogen sulfide smell common to Toyotas, but it is a different smell that I am writing about.
When running the ventilation system, there is a smell vaguely like stale cat urine. The service manager at the dealership professes not to have encountered this before. I have no animals and the car is parked in a locked garage at night. I'd be grateful to get some information about this problem.--E.M.S.
A: Between the sulfur odor and the cat urine, I'm surprised you can drive straight. At least the cat-urine odor can be fixed.
The ventilation and air-conditioning system has something called an evaporator core underneath the dashboard, which brings cooled refrigerant into the passenger compartment. It looks like a small radiator.
Like any air conditioner, it gathers water condensation when it operates. The humidity causes a fungus growth, which creates the nasty odor that you associate with cat urine.
High humidity in the air and cigarette smoking inside the car make the problem worse.
You can obtain a new evaporator core with a different surface treatment that will retard fungus growth. Depending on your approach, the dealer may replace under warranty.