I've been dealing with a poor cold idle issue ever since getting this truck ('94 F150 5.8). I've changed every sensor I can on it, had the computer rebuilt, etc. and nothing has availed. I decided to try something a couple weeks ago. I unplugged the O2 sensor, which I replaced 2 years ago with a new OE Bosch sensor. Bam, now the truck idles pretty much as I'd expect it to cold (and warm, but that was never a problem). The interesting thing is, so far it hasn't turned on the CEL. In every other instance that I've unplugged something and ran it, it's turned the light on almost immediately. My questions are: 1. Should the missing O2 signal turn on the light; 2. If so, does it sound like the computer is trashed; and 3. Am I hurting anything by running with the O2 sensor unplugged indefinitely?

 

1. It will eventually.
2. No.
3. No but....
Disconnecting a major sensor like this forces the computer into open loop mode which uses richer fuel maps, that is why you notice a smoother idle. But that suggests the computer is getting bad information from one of the sensors because there are also cold start enrichment routines to mimic what a choke did with the old carb setup, the EFI system should work better than a carb when it's operating as it should.
So is this an SD or MAF truck?
Do you get the all clear code 111 from the computer when everything is connected and the engine is at operating temp?

Your problem suggests a bad temp sensor (IAT or ECT) or a bad MAF meter if so equipped, and in this case "bad" doesn't mean dead it means out of spec. And another possibility is dirty or malfunctioning injectors, are they original or have they been cleaned/serviced or replaced?

 

It's an SD truck. Just replaced the MAP sensor about 2 months ago. Yes, just 111 under normal conditions. I replaced the IAT and ECT sensors about a year ago, with no change of behavior noted. Injectors are all original, but I have run cleaners through the fuel pretty aggressively a few times. The truck only has 81,000 miles on it.

 

OK good info there, the fact it only has 81k on it means it has done more sitting around than driving which means the injectors have moved up the list of suspects. These things are damn near indestructible but they do suffer from varnish contamination particularly when not a daily driver, and cleaning them properly requires hitting them concentrated cleaning solution. 2 ways to do that, remove the injectors for a bench cleaning or disable the fuel pump and connect a can of a cleaner directly to the fuel rail. I had this second procedure done on an '89 I had years ago and it definitely smoothed out the idle and improved throttle response, and I have done a DIY bench cleaning where it was visually obvious how the cleaning restored the injector spray pattern.

Have the plugs been changed recently? I suspect you probably have but what I'm getting here is what color deposits they have on the electrodes, reading the plugs like this can help confirm a potential injector issue. With multi port EFI all the plugs should look the same, the two rear cylinders often get sooty from oil pulled in from the PCV but the rest should be a pretty consistent light tan. If you don't have consistent color then that would seem to confirm the injectors need a good cleaning... or replacing.

 

You're correct. It mostly sat in my grandpa's garage from 2011-2019 while he finished his life in a rest home. I changed plugs when I got it 3 years ago, but got incorrect info and put platinum plugs in. Discovered my mistake about a year and a half ago and got Motorcraft copper back in it. Even at 77,000, the original coppers didn't look that bad. And when I swapped out the platinum plugs, they looked pretty much like you said they should. Also did the plug wires, and followed the correct wire looming procedure. As a side note, as of this summer, I began running exclusively 100% gas, no more E10. It was getting vapor locked in the summer.

 

Never heard of any EFI engine having vapor lock issues. That's an issue associated with carburetors. Sounds like there was something else happening.

 

Hard start when hot can be a leaking injector/s.

 

I did have that problem on my previous 95 F150 years ago and new injectors took care of hard starting when warm, due to being flooded.

 

It only happened when we were having 105+ degree heat this summer, and I'd drive it for a while, park to go in a store, and when I'd come back out it would run absolutely terrible for about a minute. As soon as I stopped using E10 gas it stopped happening. Another question. I was under the impression that the computer doesn't pay attention to any sensors until full operating temp is reached, but is that incorrect? Seems like my unplugging of the O2 sensor would make no difference on cold idle if that were true, but it certainly has made a huge difference. Still no CEL, and I've driven about 100 miles this way now.