egoidealmusic

so here's the story (sorry if some of this is repeated from an older thread). 1998 Cherokee 4.0. CEL comes on and I get some hesitation in 2nd gear when cold--once warm though, no problems at all as far as I can tell. I check the codes, get 135 and 138. Replace the 02 sensor and have the CEL cleared. She still hesitates and, after 2 days, the light comes back on (same codes). OK, I think, the computer still thinks something is wrong so I need to reboot it. Yesterday, I disconnect the battery for 15 minutes, then reconnect and drive her for 20 miles so the computer can relearn. CEL light doesn't come back on (at least yet), but I'm still getting the hesitation before she's warmed up. Any thoughts/advice?

alpine.adrenaline

A thought... Just getting our first snowfall here in the CO rockies, and before I even backed out of the driveway I let my motor run for ten minutes. I get hesistation and misfires if I start it and immediately go, even if its in the summer time. I think most vehicles should have a 5-10 min warm-up before any strain is put on the engine, this lets the piston rings warm up and seal correctly, the computer can control emissions accurately based on nominal engine temps, oil cycles thoroughly, and its a little more comfortable in the cabin once you start driving. You don't have to do this every time, just after the vehicle has been off for 6 or more hours or however long it takes to cool to ambient temps. Getting in the habit of doing so (I've heard) will greatly extend the life of your engine.

egoidealmusic

as a general rule, I think you're right about letting it warm up. With this, though, it just started when the 02 sensor seemed to fail and continued even though replaced. It's as if I've fixed the cause, but still have the symptom! I suppose it could be a coincidence that it happened, but maybe not. . .