Rookie

Alright, I have a '99 GMC Sierra with the 5.3. It has 265,000 miles and besides a few tranny's it has been a real good truck for me. But this summer the fuel mileage has made dropped considerably. I replaced 1 bad O2 sensor, Evap valve something or another to clear all the check engine light. New plugs and air filter. Are there any sensors or other things that I can replace to get my mileage back up.

Thanks

Jupiter Sunsation

Check the catalytic converter, if they go bad the mileage plummets.....the honeycomb material plugs the exhaust flow.

aquaforce

Is there any exhaust leaks with all the tranny work done the exhaust can be disturbed. An exhaust leak will hurt mileage.

1 bad sensor???? Just because of a code??

There are several 02 sensors on that truck if I remember right. Can someone scan the running data for you to see if more 02 sensors are lazy. An 02 can be lazy which will hurt mileage but still be inside of a parameter range so a code is not set but a good driveability guy can see the scan data and tell if this senario is happening.

Wally

On my 98 5.7 vortec there are 4 O2 sensors...two before the cats and two after the cats....always replace them in pairs...

Airpacker

Why?

Wally

i have been told that even though the O2 sensors will read within limits it will throw the computer readings off as far as A/F mixtures go due to the wide swings/differences they can have because of their age. So for instance lets say the normal readings are from 5-10v on the sensor....the new sensor is reading in the middle at 7.5v...the old sensor is on the low end right at 5.1v The computer is reading the differential between the two and compensating one side differently.
I dont know enough about EFI systems to know if it makes that big a difference or not...just something i was told my a GM tech a few moons ago

Rookie

I do have a little manifold leak (kinda scared of getting those bolts out of the head after all the years. I only replaced 1 sensor cause that was one of the codes it threw, bank 2 sensor 2 or something like that. I will replace the other 3. What about MAP sensor and other sensors like that. I use to get 18mpg when I was driving back and forth to Kentucky every other week.

Thanks guys, keep them coming.

Airpacker

A manifold leak is about as bad as it gets for an exhaust leak causing a mileage issue. Newtons laws of physics come into play here. "For every action, there is an equal an opposite reaction." Meaning that for every "puff" of exhaust that comes out of the leak, an equal amout of air ( oxygen) puffs back into the exhaust system. That extra air is seen by the O2 sensor as a lean exhaust condition. The PCM adds fuel to compensate for the perceived lean condition even though one does not exist.

Fix the leak FIRST before changing any more parts.

PS, Bank 1 and 2 sensor position "2" has no effect on fuel delivery. Their job is a catalyst monitor sensor only.

Airpacker

Wally, normal operating voltage of a standard O2 sensor is .2 (lean) to .9 (rich) volts dc with a mean average of .45 being the result of a 14.7 to 1 air / fuel ratio.

.45Vdc is actually the target the PCM shoots for in its fueling calculations. The reason the voltage varies is because the PCM just isn't that good of a guesser so it over fules, sees the reaction from the sensor and then under fuels and watches and then over fuels ad infinitum.

O2 sensors rarely degrade at the same rate. Using a digital scope with two channels, its easy to see exactly how two sensors compare to each other. Yes, they will get "lazy" over time BUT, in my experience of fixing this stuff for near 30 yrs now, I can tell you the number one fail is actually the internal heater in the sensor that quits long before the actual sensor fails.

Arbitrarily replacing them in pairs without scoping them in full operation first is a lazy way of doing diagnostic work. Usually found happening in the dealership because the techs get paid squat to do diag work and make better coin just swapping out both sensors quickly.

Most newer vehicles use lean air/fuel sensors and at several hundy a pop, just tossing in a few gets mighty spendy real quick.

Old saying, TEST, then replace as needed.

Rookie

Ok needs some help here. Tried removing 1 manifold bolt, now have 1 bolt missing its head. How do you get the bolts out of the head, what's the trick?