BACKYARD MECHANIC: Engine light an irritating problem

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This article was published 10/11/2011 (5120 days ago), so information in it may no longer be current.

QUESTION: I have a 2002 four-cylinder Honda Accord. About two years ago I got an engine code light (P1457 Evap). The shop replaced a bypass valve and shut valve. The day following this work the engine code light came on again. Back at the shop they told me it was the same code and that I would probably need a new computer as faults such as these can travel up the wire and damage the computer.

I have since learned that if I fill the gas tank above about 7/8 full, the code light will come on and stay on until the gas gauge reads about a 1/4 full. I have had a smoke test to determine where the leak is without success. Other than this irritating problem the car seems fine. I would like to resolve the issue without feeling I have been duped.

ANSWER: Code P1457 is an emissions code that warns of a malfunction in the evaporative emissions system of the vehicle. Evaporative emissions come from oil, fuel and plastics, but the majority comes from fuel evaporating out of the fuel tank. Today’s vehicles have almost zero evaporative emissions because the fuel tank is now sealed and connected to a canister that stores the vapours until the engine starts. Then the vapours are pulled into the engine and burned.

There are actually seven different tests that will set off code P1457. Some of the tests are “large leak” tests, while others detect very small leaks in the system that could let vapours escape to the atmosphere. A test has to fail twice before the warning light on the dash will come on, and pass three times before the warning light turns off again. Your vehicle fails the test when the tank is full and it could use most of the fuel before the test repeats three times. That is why the light goes out after the gauge shows about 1/4 full.

The vehicle’s computer tests the system by turning on and off vent and vacuum solenoids in the system while monitoring the vacuum or pressure inside the fuel tank with a pressure sensor. When the tank is full, there is very little air inside the tank so pressures and vacuum readings should change quickly. Because you have already had the solenoid valves replaced, a small leak is the next place to look. The smoke test should have shown if there was one, but they can be difficult to locate. The most common problem is a faulty gas cap.

It is rare that the computer would be damaged because of a fault in this system, so I would rule that out. Another possibility is a plugged filter for the tank vent system. This happens frequently for vehicles that drive dirt roads, but often overlooked on vehicles driven on pavement. I would change the gas cap and check the vent filter first.

 

QUESTION: I have a ’95 Toyota Corolla, purchased new. It’s been a fine-running vehicle, with one exception. For several years now, when the weather starts to turn colder in the fall, it develops a pinging problem when accelerating from a cold start. In the middle of winter, it never really goes away, even when the engine is up to temperature. I’ve tried premium gas, and the dealer has tried adjusting the timing, all to no avail. Any ideas as to what I should do, other than ignore the problem?

ANSWER: Pinging or pre-ignition of the fuel generally occurs in hot weather or high-engine heat. Too high a cylinder pressure can also cause this. Because your problem occurs during cold weather, I suspect that the cylinder pressures are too high. This can be caused by incorrect valve timing, which could be the result of a worn timing chain or one that has jumped a tooth. The cam timing can be checked without doing any major engine work.

Another possible fault could be with the EGR (exhaust gas recirculation) valve. This valve lets some exhaust gases back into the intake manifold and that reduces the pressure in the cylinder because there is less fuel and air to burn. The valve may be sticking, or the control system may have a drop of moisture in the vacuum hoses that freezes in cold weather, preventing it from operating properly. Removing the hoses, putting a little denatured alcohol in them and blowing them out with air could get any moisture out of the hoses.

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists Association of Canada.

kerr.jim@sasktel.net