Can a bad 02 sensor cause a car to run rich without throwing a code

 

Technically, anything's possible. Your local auto parts store should have an 02 sensor test tool if you suspect yours is faulty. :thumbs:

 

Yes, I had this happen to me.

After I installed my headers I retained my one wire, non heated O2 sensors. That worked great until the winter when it got cold. It got so cold that my O2 sensors stayed cool. Since they stayed cool, they always read lean. The car would then richen up the mixture. A rich mixture is a cooler mixture, so it would read even leaner.

The end result is the car running pig rich and running quite terribly. If I could stay off of idle long enough it would heat up and work just fine.

Meanwhile it never set a code. You could have a "lazy" o2 sensor that isn't reading properly, causing it to run rich.

 

The reason I say this is my vette is running rich and the gas milage has dropped a bout 3 mpg. Yet the car seems to idle fine

 

hmm...mine is acting similar to that...i have noticed some carbon buildup on the tips too...i have been considering replacing the o2's just to rule them out...

 

:iagree:
I am experiencing the same thing. I have replaced the driver side O2 sensor and my fuel economy has gone up some. I still have the passenger side to do. The passenger side on the LT1 motors is a little harder to get out. I did not have a short enough 7/8 wrench. I just picked up a short one from Sears. Now I will have the room to take it out and replace it. :smash:

 

Buy the O2 sensor tool for use with your ratchet.

 

Other than a scanner, never heard of an oxygen sensor test tool. Absent a scanner, you can see if the engine is in closed loop and operating rich or lean by grounding the diagnostic test terminals. Once in closed loop, it should flash on and off every other second if the ECM thinks the fuel ratio is normal. If it's on more than it's off, it's running rich. If it's off, more than it's on, it's lean. Extremely cold weather can cause an engine to go open loop at extended idle. If the engine is running rich, I'd do the simple stuff first like checking the air filter, and from there, plug the canister purge line to the throttle body. After that, start pulling plugs to see if they're all fouled or if it's just one, suspect a leaking injector. Finally, hookup a fuel pressure gauge and check for about 42 psi with the key on, engine off; a few lbs less at idle. Otherwise, invest in a scanner and monitor Block Learn or if you really suspect the O2 is out of whack, swap it out - the 1 wires aren't that expensive.

 

I did a tune up a bout 3 weeks ago and every plug was wet and foweld? :confused:

 

Make sure there's no code 45. Did you change the air filter when you did the tune up? Shouldn't be anything blocking the air inlet, but it doesn't hurt to check and make sure a big wad of leaves or pine needles haven't gotten sucked under the cover. Pressure regulator could be at fault or the return line could be restricted, but you need to hook up a pressure gauge and see what the pressure is. Doubt it's one injector since all the plugs are fouled, unless it's the cold start injector, and someone else will have to tell you how to check that. I'd still check the purge canister by disconnecting the line and plugging the throttle body and the line. You don't want to do this for too long and the better way to test it is to monitor the Integrator and Block Learn after you've plugged the lines. If there is any raw fuel in the line to the throttle body, the canister is satuated and needs to be replaced. Also, if you have a vacuum pump, the line should be able to hold about 10 inches of vacuum with the engine at idle. If it doesn't, the solenoid is bad and is allowing vapors to enter the plenum at idle enrichening the mixture. Also check the Coolant Temp Sensor. At a coolant temp of 210 degrees, it should be about 185 ohms across the terminals. If the engine is cold, and it's about 70 degrees out, it should be about 3400 ohms. Another check would be to read resistance at the manifold air temp sensor and compare it with the reading at the coolant temp. On an engine that's sat overnight, they should be about the same. Unfornately, you'll have to remove the plenum to access the air temp sensor - it's underneath the plenum; the coolant sensor is in the manifold, just below the throttle body. In any event, high resistance at high temps means the sensor is out of range; the ECM thinks the engine is cold and adds fuel. Finally, change the oil. All that unburned fuel will build up in the crankcase and get sucked back into the intake through the PCV circuit compounding the problem.

 

MAD for one sells the O2 sensor test tool, among others.
http://www.madvet.com/shop?store=1&frame=882 :thumbs:

 

This isn't so much a "TEST TOOL" as it is a harness that allows you to connect a meter in parallel with the O2 signal. The meter is really the test tool and you could do this without the harness by backprobing. A Scanner is by far the best method.

 

I had a problem with my O2 sensor that was actually in the ECM ('90). A scanner indicated the ECM was getting a voltage but is was not correct and did not vary as it is supposed to. The engine ran quite well but rarely went into closed loop mode. Occasionally, the check engine light came on. I replaced the ECM and the problem was solved.