O2 sensor a bad guess for loss of power

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This article was published 10/10/2008 (6206 days ago), so information in it may no longer be current.

11The check engine light rarely comes on and if it does it goes off within a day or so. The one code that seems to show indicates O2 sensor high voltage. I had the plugs and wires changed, fuel pump checked (good pressure), and sensors played with to check for any loose connections. I have not checked the fuel filter yet. I am basically trying to troubleshoot before getting into costly repairs. Dry or wet weather doesn’t seem to matter. Any ideas? Could it be the O2 sensor? Fuel economy seems good. I thought economy dropped drastically if that was the issue?

ANSWER — My first inclination would be to look at the throttle position sensor voltage. These sensors are a variable resistor with moveable parts inside. Because most drivers tend to operate their vehicle at the same speeds for most of their driving, the throttle position sensor will wear in a very narrow range. Usually the voltage signal drops out for a fraction of a second and the engine hesitates, but if there is a bad internal or external wiring connection, it can operate erratically, such as increasing engine r.p.m. (not usual) or acting like there is no throttle.

With a sudden drop in voltage, the fuel is cut off and the engine may surge or even stall. This can also cause the oxygen sensor to set a code for lean air-fuel ratio (high voltage). I would check wiring and voltage signals for the throttle position sensor first.

Next, I would look at the crank sensor inputs to the ignition module (engine computer). If a bad sensor or connection between the sensor and computer is intermittent, it will cause the same symptoms.

QUESTION — I have a Chev S-10 2.2 L with auto transmission. It has 105,000 km on it. I have been having lots of problems with the MIL (service engine) light coming on and setting code P0436, indicating the fuel tank vent solenoid. I replaced the vent solenoid, then drove about 30 km starting with a cold engine two times and the MIL went out.

After about two weeks the MIL came on again, setting code P0440, which is an evaporative emissions system fault. I only have a code reader, so I have to determine which sensor is setting the code. The fuel cap is tight and I checked for vacuum leaks.

Should I have it scanned by someone who is familiar with the scan tool and the system or is there something I could do before I take it to a professional?.

ANSWER — The P0440 code will set if there is a leak in the sealed ventilation system for the fuel tank. It could be caused by a bad vent solenoid or loose hose connection but most often it is caused by a gas cap that has not been tightened properly.

Dealerships will have a tool to check the gas cap for an internal leak but many aftermarket repair shops don’t have this specialized equipment. Instead, they replace the cap to see if it cures the problem.

Another possible leak location is the rubber seal around the fuel sender unit where it fits into the top of the tank. Because this is almost impossible to see on most vehicles, many shops use a smoke generator to find the leak. Smoke (the ones I have use use a citrus-based oil in the unit) is forced into the fuel tank and lines under a slight pressure. Watch for leaking smoke to pinpoint the air leak. Because of the equipment used, this may be best left to a repair shop.

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

kerr.jim@sasktel.net