Originally Posted By: OVERKILL
Originally Posted By: austinlunsford
Unplugged o2 sensor, idle smoothed out, and the overt fuel smell went away. Still feels like it's skipping at cruising speeds 45mph and up. I should replace it, I suppose?
When you say "skipping" do you mean like a misfire?
What unplugging the O2 sensor harness does is make it run out of base table and never go into closed loop. The base table values are "correct" for the engine but of course don't account for wear or ambient adaptations, they are basically just (good) guidelines for the ECM that it can improve upon. It then writes its new tables when in closed loop using the feedback mechanisms at its disposal.
When you have something that throws a wrench in the feedback process, you end up with an idle "surge" because the O2 sensor is telling it rich, then LEAN! and then RICH and then LEAN and it's flying back and forth trying to compensate for both of these things, which makes the idle surge up and down.
A misfire can cause the surge because there will be a pocket of "nothing" (or unburned fuel) in the exhaust which, depending on whether it ignites on its way will either trigger a massive rich or massive lean. The other thing is that this all doesn't go out the exhaust, some of the fuel stays in the cylinder and so if the mixture is ignited on the next stroke, that cylinder will be extra rich, further screwing up the feedback loop.
This can also be caused by a bad injector that makes one cylinder rich (or lean).
Also, the ECT (engine coolant temperature) sensor tells the ECM which table to run out of. But usually when this is bad, even in open loop with the O2 harness unplugged it will run poorly.
EEC-IV has the ability to run a cylinder balance test. It shuts off the injector for each cylinder and watches for idle drop based on it. However, since yours is a truck and has the single O2, I believe it is batch fire and won't have that capability
Which sucks, it is a great diagnostic tool.
What's the background on this problem, did it just start one day? Or has it been just building up and getting worse?
Thanks for the explanation. The background story is as goes: I bought the bronco in Febuary for cheap. It had the surging idle, but no misfire. Tinkered with the IAC, didn't fix the idle problem. Gave it a full tune up, changed all fluids. Started to misfire one day in OD, died on me, and then I threw the parts at it. I bought it as a fixer upper toy but [censored], this is getting annoying haha. The truck is nice except for tears in the seats and a little rust on the tailgate. Both are common I hear
