Very Rough Idle

[metalbeast]

My 93 Geo Metro has a very hard time at idle. It nearly wants to go dead. I had to replace the timing cover dust seal so I checked the timing belt. Looked great. The guy I just bought the car from said it was throwing an engine code that indicated that the O2 sensor was malfuntioning but I don't think that would be an issue since It does this when it is first cranked and has not warmed up enough to heat up the O2 sensor & go into closed loop. Its got new plugs & plug wires.

Any thoughts on other things I can check? Thanks.

[flashlight]

Did he turn the idle speed screw down so much that it is causing the engine to idle super rough?

[metalbeast]

I'll check that. Any other suggestions?

[GM Line Rat]

The idle speed screw should not be adjusted! It's set at the factory and the idle speed is controled by the ECM (Onboard computer under the DS dash). So what you saying is "The car idles rough when it's cold, then idles properly when it warms up".......Yes or No? If your getting a code 13 for the O2 Sensor, you should be addressing it and replacing the bad sensor. The O2 Sensor directly relates to the air/fuel mixture. On a 93 Metro, the O2 Sensor is mounted in the exhaust manifold next to the engine block at the front of the car. Disconnect the 1 wire connection, remove the sensor and replace it.


Oxygen Sensors Are a Critical Key

Oxygen Sensors Don't Last Forever
Here's What Happens As They Age
As an oxygen sensor ages, contaminants from normal combustion and oil ash accumulate on the sensing element. This reduces the sensor's ability to respond quickly to changes in the air/fuel mixture. The sensor slows down and becomes "sluggish".

At the same time, the sensor's output voltage may not be as high as it once was, giving the false impression that the air/fuel mixture is leaner than it actually is. The result can be a richer-than-normal air/fuel mixture under various operating conditions that causes fuel consumption and emissions to rise.

The problem may not be noticed right away because the change in performance occurs gradually. But, over time, the situation will get worse, ultimately requiring the sensor to be replaced to restore peak engine performance.

Oxygen Sensor Failures Can Mean
Big $$ In Repairs If Not Replaced.
The normal aging process will eventually cause the oxygen sensor to fail. However, the sensor may also fail prematurely if it becomes contaminated with lead from leaded gasoline, phosphorus from excessive oil consumption or silicone from internal coolant leaks or using silicone sprays or gasket sealers on the engine. Environmental factors such as road splash, salt, oil and dirt can also cause a sensor to fail, as can mechanical stress or mishandling.

A dead sensor will prevent the onboard computer from making the necessary air/fuel corrections, causing the air/fuel mixture to run rich in the "open loop" mode of operation, resulting in much higher fuel consumption and emissions..

An additional consequence of any oxygen sensor failure may be damage to the catalytic converter. A rich operating condition causes the converter to run hotter than normal. If the converter gets hot enough, the catalyst substrate inside may actually melt forming a partial or complete blockage. The result can be a drastic drop in highway performance or stalling because of a buildup of backpressure in the exhaust system.

[metalbeast]

Thanks for the detailed reply. It idles rough even when warmed up. Just replaced the O2 sensor. Instead of a plug in connection I found that they had cut & twisted the wires together. I used a splice connector to repair that. Like I said though I would thing that since it does this when it is cold, the O2 should not be a factor since the system should still be in open loop. No visible smoke of any color during acceleration.

Originally Posted by GM Line Rat

The idle speed screw should not be adjusted! It's set at the factory and the idle speed is controled by the ECM (Onboard computer under the DS dash). So what you saying is "The car idles rough when it's cold, then idles properly when it warms up".......Yes or No? If your getting a code 13 for the O2 Sensor, you should be addressing it and replacing the bad sensor. The O2 Sensor directly relates to the air/fuel mixture. On a 93 Metro, the O2 Sensor is mounted in the exhaust manifold next to the engine block at the front of the car. Disconnect the 1 wire connection, remove the sensor and replace it.


Oxygen Sensors Are a Critical Key

Oxygen Sensors Don't Last Forever
Here's What Happens As They Age
As an oxygen sensor ages, contaminants from normal combustion and oil ash accumulate on the sensing element. This reduces the sensor's ability to respond quickly to changes in the air/fuel mixture. The sensor slows down and becomes "sluggish".

At the same time, the sensor's output voltage may not be as high as it once was, giving the false impression that the air/fuel mixture is leaner than it actually is. The result can be a richer-than-normal air/fuel mixture under various operating conditions that causes fuel consumption and emissions to rise.

The problem may not be noticed right away because the change in performance occurs gradually. But, over time, the situation will get worse, ultimately requiring the sensor to be replaced to restore peak engine performance.

Oxygen Sensor Failures Can Mean
Big $$ In Repairs If Not Replaced.
The normal aging process will eventually cause the oxygen sensor to fail. However, the sensor may also fail prematurely if it becomes contaminated with lead from leaded gasoline, phosphorus from excessive oil consumption or silicone from internal coolant leaks or using silicone sprays or gasket sealers on the engine. Environmental factors such as road splash, salt, oil and dirt can also cause a sensor to fail, as can mechanical stress or mishandling.

A dead sensor will prevent the onboard computer from making the necessary air/fuel corrections, causing the air/fuel mixture to run rich in the "open loop" mode of operation, resulting in much higher fuel consumption and emissions..

An additional consequence of any oxygen sensor failure may be damage to the catalytic converter. A rich operating condition causes the converter to run hotter than normal. If the converter gets hot enough, the catalyst substrate inside may actually melt forming a partial or complete blockage. The result can be a drastic drop in highway performance or stalling because of a buildup of backpressure in the exhaust system.

[Woodie83]

Once the computer decides that the O2 sensor is bad, it stores that code and doesn't clear it until it sees two complete driving cycles (cold start to completely warm, then the test is run) with proper operation. The fact that the light is on when the engine is cold means nothing.