Originally Posted by NickB47
I'm having an issue related to an o2 sensor and lack of cat too. I bought this truck right after the cat. had been stolen. Then the PO had an exhaust shop just bypass (IE: only a muffler, no cat) and a new O2 sensor. Well, now with the O2 sensor in the truck runs very poorly and when it is unplugged the CEL is on and the ECU (presumably) defaults to a failsafe setting. I'm getting pretty bad gas mileage now and think it is related to this.
I do have the FSM and will check the O2 sensor for proper functionality, but the guy who installed the exhaust said to just leave the O2 sensor unplugged because the fact that there is no cat. will mess up the readings anyway (which doesn't make sense to me as the O2 sensor was upstream of the cat. to begin with).
Any thoughts/advice on this?
I have an '89 truck with no cat and 22RE.
It's an interesting question, about whether no cat could burn out the O2, but I'm guessing it shouldn't have anything to do with that, as Co_94_PU says.
My truck just rolled over 236K miles. The kid I bought it from was a big junkyard parts fan. He had put a new manifold in it before I bought it, but (I assume) the downpipe where the O2 sensor attaches was original. I put a new exhaust on it about 10 months ago right after I got it, and had the cat left off.
Some time ago my CEL started blinking on and then off even as I drove down the road. I pulled codes and saw O2 sensor stuff. Friends told me that new O2 sensors tend to make vehicles run better and get better mileage even if the old one hasn't failed. So I figured I would just replace it. I got one and saw that the mounting bolts were completely rusted.
I took the truck to a guy hoping he'd be able to remove the hardware and install the new sensor. He found that the wires to the sensor had gotten too close to the exhaust and melted. He just fixed the wires and declared it fixed since the CEL would stay off.
Next long trip I took, after about 2 hours on the road the CEL came on and stayed on. During that trip I got amazingly sheety gas mileage. On one windy stretch of NM highway I got LESS THAN 14 MPG.
So I dug into my wallet again and got a new downpipe and sensor.
CEL was off, and the truck ran like gangbusters. But then on the second trip the light came on again!
So I crawled under there and un-taped the splices in my wiring. They were way sloppy. So I stripped the wires and spliced them really clean (I'm generally an idiot with vehicles, but have done phone and computer wiring, so I have some mad skillz in that area).
So all that was about 500 miles ago. The light is off. Mileage has gone from no better than 20 (usually high teens) to solid mid-20s even in the mountains and cold air.
To anyone who has issues with O2 sensors, I would urge you to look at the way the wires are routed to them. Obviously those wires have to be reasonably close to the exhaust (right over the cat if you have one) since that's where O2 sensors live. A couple clips go bad, a ham-fisted mechanic doesn't put the wire back into the clips... whatever. The wires don't have to move very far before they are close to the exhaust.
I fixed my splices. Actually shortened up the wiring because there was too much. Taped it all up with electrician's tape. Then I went to the parts store and got some heat shield tape and wrapped it up with that. Finally I rigged up a good, solid method for zip-tying that sucker up as close to the underside of the body and as far above the pipe as I could.
Will my new O2 sensor die prematurely because there's no cat downstream of it? If it does, you'll hear about it here first!