BACKYARD MECHANIC: Lean engine code key to fuel woes

Hey there, time traveller!
This article was published 16/07/2010 (5658 days ago), so information in it may no longer be current.

QUESTION: I have a 2006 Hyundai Sonata and in February my check engine light came on. My mechanic said the code was lean running condition but the car was running fine and he couldn’t find anything wrong. I was on my way to Nashville, so I brought it into my Hyundai dealership. Their code said lean running condition on idle and they couldn’t find anything wrong so they told me I should change my fuel filter. When I crossed the border I noticed my car was burning lots of fuel — it took more than half a tank of gas to go the 140 miles. When I took it to a dealership the next day the smell of gas filled the garage. They told me "it is just your car."

After I got to Nashville my check engine light came on again and there was some hesitation. A garage told me a close valve and the purge control valve needed to be replaced at a cost of $503.80.

I called my good friend who is a mechanic. He said they must have hooked my gas tank up wrong and the gas went into my canister and into my engine. My check engine light came on again as I was driving back to Canada and now I can hardly drive my car because there is an intermittent smell of gas that so bad sometimes you can’t even stand to be in the car — it is especially strong when in idle.

We changed my spark plugs but the code is still there. Can you tell what you think has happened to my car and why my car smells like gas when it idles — especially since the codes have been saying lean on idle all along?

ANSWER: The key to solving your car’s problem is the lean engine code that keeps reoccurring. The computer sets a lean code when the oxygen sensor detects too high a percentage of oxygen in the exhaust gases for several seconds or minutes of operation. Under normal operating conditions, the engine burns the fuel and oxygen in the cylinders.

Only about one per cent of the oxygen in the air is left unburned and passes out the exhaust. The exhaust gases are continuously changing as cylinders fire, so the oxygen sensor should be switching continuously between lean and rich signals. The computer averages these signals to adjust fuel delivery to the engine. If the computer thinks the exhaust gases are too lean, it will increase fuel delivery to all the cylinders.

Your car’s increased fuel consumption is likely because of the increased fuel delivery, and this could also be creating the fuel smell you are experiencing. I would be concerned about the operation of the catalytic converter, as it can overheat and become damaged if too much fuel passes though it. Finding the reason for the lean mixture code should fix everything.

Watching the oxygen sensor signal with a scan tool can help the diagnosis. If it is showing a continual lean signal but the engine runs fine, then the wiring from the oxygen sensor to the computer should be checked to make sure it isn’t rubbed through to a ground connection. If the wiring is good, check for an exhaust leak at the manifold or the exhaust pipe in front of the sensor. A crack in an exhaust manifold may not be obvious but can cause this problem.

QUESTION: Two of our mechanics don’t want to tackle our non-working speedometer on our 2001 Ford Windstar. Can you shed some light on how a speedometer in this vehicle works? Is this a major job to fix? Is there another alternative for checking your speed, ie a GPS system? I would appreciate any information you can provide in this situation.

ANSWER: The instrument cluster gets speed information over a data buss network from the ABS computer. If there was no data getting to the instrument cluster, the ABS light would also be on and the odometer would not display. Therefore, the problem has to be inside the instrument cluster. A new cluster is expensive, but it is possible to get these repaired. Check the phone book or ask at your repair shop for a vehicle cluster repair shop in your area. Repairing it would be a likely be a better alternative than using a GPS system.

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

kerr.jim@sasktel.net