High idle, poor gas mileage

Tiny
BCOUNTRY699
  • MEMBER
  • 1997 CHEVROLET SILVERADO
I have a 97 chevy k1500 with a 5.7 litre vortec. I have had the truck for 5 years and it has never had a cat converter since I have had it. It has always ran fine with no service engine light on. Recently the light came on and ever since it will not shift into overdrive and when it is warm and I go to start it, the engine will idle high, around 2000 rps, if I shift into drive it will die. If I wait till the idle slowly drops to about 9 or 800 rpms then it will go and I can drive it. However gas mileage is completely gone! Horrible gas mileage. I took it to a parts store and they said it was Ho2 sensor on bank 2 sensor 2. However I was told by a mechanic that I trust to be very knowledeable that an o2 sensor is going to be a waste of money without a catalytic converter. He said it could be a map sensor. Can you help me diagnose this so I dont have to take it in or throw away lots of money on parts I dont need please?
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Friday, April 26th, 2013 AT 12:16 PM

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Tiny
CARADIODOC
  • EXPERT
Diagnostic fault codes never say to replace parts. The guys at auto parts stores are in the business of selling parts and that's what they jump on first. In fact, those codes only indicate the circuit or system that needs further diagnosis. The place to start is by finding out the exact code description or number. There are about a half dozen different codes related to any one oxygen sensor and they mean entirely different things. The downstream sensors report on how well the catalytic converters are doing their job. You don't change the messenger if you don't like the message. The code will tell you if there is a problem with the operation of the sensor or whether it's simply reporting an unacceptable condition.

The fuel / air mixture is adjusted by the Engine Computer based on readings from the front oxygen sensors. The mixture can only be adjusted up or down about ten percent over pre-programmed values. If your fuel mileage is down considerably you have some other problem, including a vacuum leak, and the oxygen sensor is picking that up.
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Friday, April 26th, 2013 AT 12:29 PM
Tiny
BCOUNTRY699
  • MEMBER
I went to oreilly auto parts and they plugged into the truck to see what codes it was throwing. The trouble code is P0161. Code defintion says. Heated oxygen sensor (bank 2 sensor 2) - Circuit malfunction. However as I said.I asked a mechanic who I have past experience with that has never steered me wrong and he says that without a catalytic converter it doesnt matter. However like I said the engine light was never on before and the converter has been out of there for years and it ran fine. Hope you can help.
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Friday, April 26th, 2013 AT 12:53 PM
Tiny
CARADIODOC
  • EXPERT
You mean the catalytic converters are physically removed from the exhaust system? Who did that job?

The way the system works is the Engine Computer makes the fuel / air mixture too rich, then too lean. It switches back and forth about two times per second with the average being perfect. During the lean periods the unburned oxygen is stored in the catalyst, then during the rich times the unburned fuel is mixed with the oxygen and burned. The front oxygen sensor reports on how well the mixture is responding to the computer's control.

When the converter is working correctly the exhaust gas coming out of it will be a little lean for a long time, then a little rich for a long time. The switching rate is very slow, as in perhaps once a minute or two. When the catalyst starts to lose its efficiency the mixture takes less time to switch from rich to lean so the switching rate increases to maybe twice a minute. As the efficiency decreases even more, the switching rate speeds up more. When there's no change taking place in the composition of the exhaust gas the switching rates of the front and rear O2 sensors will be the same. That switching rate of the downstream sensor is what the computer looks at to determine how well the converter is working and when to set a fault code. The converters have nothing to do with engine performance unless they become plugged. Too much raw fuel going into the exhaust will overheat them and melt the material into a glob. The Check Engine light will flash when too much fuel is going into the converter. That's your warning to stop the engine right away before that damage occurs.

If you don't have the converter in the system the computer is going to set the efficiency code. That will cancel some of the other self tests performed by the computer so there can be a pile of other problems that won't be detected. The computer initiates things like momentarily going too lean to see if the converter responds as expected. When it has a fault code in memory, it knows it can't rely on that sensor as an accurate reference, so those tests won't run.

Code 161 is not an efficiency code. It means there's a problem with the heater circuit for that sensor. Usually that means a cut wire or corroded terminal in the connector. The heater could be burned out too but that is not very common.

All '96 and newer vehicles have this "OBD2" (on-board diagnostics, version 2) emissions system with upstream and downstream oxygen sensors, and catalytic converters between them, but that applies to cars and trucks sold in the U.S. If you're in a different country you may have the older emissions system but then you wouldn't have a downstream sensor. Your mechanic can connect a scanner to view live data that will show the switching rates of the converters. If the downstream sensors are switching very slowly, you have catalytic converters in the system and just didn't recognize them.

If someone removed them, the only thing I can suggest is to install replacements, check the wiring to the rear sensor on the passenger side, erase the stored fault codes, then see what new codes are set. There are well over 1000 potential fault codes. About half of them have to do with something that could potentially cause an increase in emissions, and those are the ones that must turn on the Check Engine light. About half of THOSE will result in high fuel consumption. Without a code to direct you to the circuit or system that's causing the excessive fuel usage you don't know if that is the cause, as in a leaky injector, or if it's the result, as in a vacuum leak that the computer is trying to correct for extra unburned oxygen in the exhaust. Without a code to get you started there are way too many things to check. Even on older vehicles with only a few dozen potential codes you could spend countless hours testing everything.
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Friday, April 26th, 2013 AT 4:59 PM

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