What causes codes to read lean?

[Brian T]

Hey all,

Engine light came on recently and intermittently (comes on more when off the gas and less if I'm on it).

Before clearing codes:
KOEO 172 176 181 189
KOER 136

After clearing codes:
Same KOEO
KOER 129 167 225 536 632 (?)

The cps, plug wires, timing belt and cam ps have been done. Forget if o2 sensors were done on this car. Car overheated last summer and shotimes says this would result in rich running thereafter, although I don't know if this is the case.

The rough running is not quite like it was with bad plug wires, and the exhaust coming out is like rapid misfiring sound. Sometimes at starup it runs fine for a little then settles into the funk.

It feels like it's running on 4 or 5, and it even seems to shift a little later.

Also, when the heat is on I can smell coolant, which may be unrelated heater core problem....

Mechanic says prolly o2 sensors, but I'd rather not just replace stuff without kowing.

Any ideas?

Thanks a lot.

Brian

 

[Yamaha V6]

The 172 & 176 is 99.9% O2 sensor, especially with the throttle on / throttle off situations.

 

[sdpatt]

You have a either a vacuum leak or engine control system malfunction (that is not setting a code - not likely) causing a lean condition. If both oxygen sensors are showing the same condition, it is a good indication that the sensors are working properly and measuring a true condition. Your engine is running lean due to unmetered air being allowed into the intake manifold.

Other than the O2 sensor codes, the remaining codes are indicative of an improperly run KOER test. Just learn how to do the test correctly to alleviate those codes. Look here for the test instructions and here or here for the code descriptions.

Since you said that the CHECK ENGINE light comes on more when you lift off the gas pedal, that also points to a vacuum leak due to the higher vacuum in the manifold at closed throttle. The good running at startup, when the engine is running rich and the control system is not in closed loop mode, also points to a vacuum leak and reduces the possibility that a sensor is the cause.

The overheating you experienced could be a real problem and may have warped the aluminum heads. I would first check for cracked or loose vacuum and PCV hoses and then inspect the spark plugs for evidence of coolant getting into the cylinders. Before you start repalcing parts, you need to figure out what is wrong.

 

[SLOSHO89]

O2's reading lean usually mean, either bad O2's or air getting into the exhaust system between the head and the O2 sensor(vacume leak).

If the rear bank is running lean then one posibility for the vacume leak is the EGR tube that goes between the rear exhaust manifold and the EGR valve. I believe all ATX's came with EGR(someone correct me if I'm wrong).

 

[sdpatt]

Both the front and back O2 sensors are reading lean. Most likely a truly lean condition.

 

[Larry Bialecki]

I had the 181 and 189 codes show up every once in a while. I changed the O2 sensors as Scott suggested with no improvement. I also has a 3 to 2 down shift problem. All problems were resolved by cleaning the MAF sensor. I took the sensor off and found its elements covered with soot. After cleaning with carb cleaner the whole car ran much better. No shift problem, no 181 and 189 errors, and my MPG went from 21 to 24. I have had no errors for two months now. I suggest you look at the MAF elements. Car has 174K on it now. Its a 93 ATX.

 

[Brian T]

Larry Bialecki:
I had the 181 and 189 codes show up every once in a while. I changed the O2 sensors as Scott suggested with no improvement. I also has a 3 to 2 down shift problem. All problems were resolved by cleaning the MAF sensor. I took the sensor off and found its elements covered with soot. After cleaning with carb cleaner the whole car ran much better. No shift problem, no 181 and 189 errors, and my MPG went from 21 to 24. I have had no errors for two months now. I suggest you look at the MAF elements. Car has 174K on it now. Its a 93 ATX.

 

[Brian T]

Thanks everyone. I'm gonna check the MAF and look for vacuum leaks, and then redo the KOER test.

Brian

 

[Brian T]

An update on my 93 atx.

I cleared all codes and re-ran both tests and got:

KOEO: 189 189
KOER: 136 136

According to these, I have 02 sensor picking up left bank lean, and left bank rich limit is reached.

This seems to suggest maybe not a vacuum leak, but rather left 02 sensor bad? Or could it be a left side vacuum leak?

What else could trigger these codes, and can I assume they're related?

Thanks,

Brian

 

[sdpatt]

Since it is only one O2 sensor showing the lean codes, it may very well be the sensor has outlived its usefulness. It would be wise to replace both sensors together to keep the engine control system at best performance. Your engine uses the Bosch 15716 from AutoZone for $49.00. You may want to shop around for these sensors as someone mentioned finding them at a lower cost.

 

[NotSoSlowSHO]

Napa has O2's for $44, Kragen for $55, and Car-Quest for $50

 

[Porod]

$48 at autozone. There are three different part numbers, but the sensors are all the same, the different part numbers just indicate the length of the wire.

 

[Brian T]

Thanks, fellas.

Brian

 

[Brian T]

Just replaced the left 02 sensor and the problem persists....plus now it's doing that weird bucking it used to do before I changed the plug wires last year, worse in high gear with minimal throttle.

The really distinctive and rapid fire exhaust rythym at idle has got to be a clue of some sort. Anyone heard this before?

Thanks,

Brian