So my truck started running extremely rich awhile back. So bad it literally smelled like straight gas coming out of the exhaust rather than gases. It also ran oddly in that it would begin to start stuttering or jumping when in low RPM mainly right when it shiften into final drive. I figured that it was an o2 sensor but just to make sureI went to my local auto parts store and had them use the machine to tell me why the check engine light was on. The results where there was a misfire on the 1st and 3rd cylinder. The employee there said; change the plugs, plug wires, and maybe the coil packs. I then asked if that could cause it to run rich. He said yeah, because the pcm is sending extra gas to the other cylinders to make up for the loss. So I went ahead and bought new plugs, wires, and a new distributor cap. Totalling $116 for the good stuff. I went home and as I was putting them on I began to think that maybe the 02 sensorWAS BADand it was causing these other problems. The problems that caused the CEL to come on. When in reality the whole problem was a bad o2 sensor. And I spent all that money when all I need is a new sensor.

Sorry for the novel.... But could I be accurate? Or could it be totally something different?

 

hard to say. could be in need of tune up, where its not burning a normal amount of gas. could be a bad/dirty o2 sensor telling pcm to send more gas. could be a leaking injector dumping more gas. a tune up usually won't hurt. if that doesn't help, if money is tight, try removing and cleaning pre-cat 02. might help/ might not, won't cost much. if money is not tight, just replace it.

 

Well I put the new plugs, plug wires , and distr cap on, then reset the PCM. Drove it around a little bit last night and it still seems to run just a little bit rich not near as much as it was. Im guessing because I reset the PCM right? The check engine light has not popped back on and the jumping and stuttering went away. So far so good.

When you say pre-cat, thats the one thats in the midpipe , not the one thats in the cat right?

 

good news so far. you are correct. pre-cat 02 is the one just before the cat (engine side). it sniffs the engine exhaust and tells the computer how much oxygen is present, and the computer uses that to fine tune the air/fuel mix. the post-cat 02 is the one just after cat. (muffler side). all it does is provide feedback sthat the pcm compares to the pre-cat readings, which verifies that the cat is doing its job, and turn on a check engine light if its not.

 

I see, so if the pre-cat is defective will it cause the CEL to come on or no?

 

Well ive been driving it with no problems till today the CEL popped back on and its back to the same old story. So should I just go ahead and assume its the 02 sensor that caused the other problems and go buy a new one, or try to figure out if its something out.

 

i don't see your truck year, but if you have the digital odometer you should be able to display the codes by switching the ignition on-off-on-off-on. if that doesn't work or you have an older analog odometer, i'd recommend getting the codes read by a local auotparts store. most will read them for free. write down the code(s) and you can look up and/or post them here. see if its still the same codes. i think (not sure) that when you reset the pcm it will clear the codes. you can do that by disconnecting neg battery cable and turning ignition key to start and holding it for 30 sec.

if money is no issue, replace the o2, as they are supposed to be replaced every 60-100k miles. if money is tight, investigate a little more. it does sound like the o2 may be giving the pcm a false signal, causing it to dump in more gas. it also sounds like it ran pretty good after pcm was reset, and has now, after some miles, decided to adjust/tweak fuel settings into a terribly rich condition.

 

Yeah you pretty much hit the nail on the head.

The check engine light came on, started running extremely rich and started missing out at low RPM. I then had the CEL ran on machine, said misfire on 1st and 3rd cylinder. Employee said change plugs, wires, and possible coil packs.
I changed plugs and wire and distrubutor cap. Reset PCM correctly.
Ran fine for a couple days. Then went right back into the same ole same ole.

I'll the light ran again and go from there I guess. Money isnt bad, just dont want to waste money on wrongful solutions.

 

The problem with these "smart" vehicles is they don't know how dumb they are. The PCM doesn't know the O2S is sending faulty information ... it just adjusts the mixture based on it. When the engine runs crappy, it'll tell you how bad it's running but not really why. It'll tell you the symptom but not the cause.

 

Oh and I forgot to add that it is a 97 so it doesnt have the digital odometer. But I guess that info is notmuch help now. I plan on going ahead and replacing the sensor tommorow. Hopefully that will fix it for good. If not I guess it will be back to the old drawing board.

And thanks for all the help and information.