1995 ford F-150 King Cab, 5.0 engine.
Slight skip at idle and going down road at crusing speed. Poor mileage, low on power pulling a trailer. Plugs and manifold color tell me its running very lean and high combustion temps. Checked everything, no codes, all checks ok. I do notice the 02 sensor voltage read out is erratic. Rolls around at .07, .3, .6, .7 over and over every three seconds.
I have ran a wire through the firewall, hooked up to my VOM, so I can watch it while driving. I have noticed when I unplug the o2 sensor, and BEFORE it goes into open loop, because of the unplugged o2 sensor, which is about 5 minutes or so of driving, it has good power and the 02 sensor is reading about .5, .6. It reads .8 or so when im on the gas hard. Runs great, then settles to .5, .6 as it should.
I have check everything that I know of,, and I mean everything. all is within specs.
Any suggestions?

 

Well, I checked a few last things today, I give up, its going to a quality repair shop on friday. I'll let everyone know what they find. I'm pretty sure its a erratic sensor.

 

Have you done a tune up latley? Plugs, wires, cap, rotor, etc. Could have a plug wire arcing or something simple.

 

A normal O2 sensor will jump back and forth between .2vdc and .8vdc, several times a second while driving. Unless your equipment is top of the line, your readings could fall anywhere in that range. The average is what you want to be around .5vdc. Also, if your ECM takes 5 min. to go into open loop with the O2 disconnected, something is wrong. The ECM should respond much faster than that.

I'm curious to hear the results as well

 

Did you check you engine coolant sensor?? And when is the last time you changed your fuel filter. What was your pressuer reading??

 

Actually, its had a light skip for awhile, but its gotten a bit worse. It would not bother most people, but I know how it should run. I bought the truck used and I figured a full tune up would correct it. Well, it didnt. So I went further, checked compression, cat pressure, manifold vacuum, all sensors that I could, fuel rail pressure, etc, etc. all checks ok against specs.
I called one of the best mechanics in town, said I give up, i'll bring it by and you fix it. Described the prob to him. He said, nope, my scanner will prob not pick it up. You need to go to the ford dealer over in the next town, they have a full factory scanner and a really good mechanic there. So, thats the plan.
I do know the ecm is pushing the engine towards lean, no doubt about it. Watching the o2 voltage, it goes up about 0.5 or 0.6 volts, then is slammed down hard, drops down to .005 volts. Thats getting down in the hundreths of a volt. Very very lean. You can hear the engine miss as its heading down. Thats telling me the ECM is pretty much shutting the injector flow off. Unplug the o2 sensor, and it purrs, with gobs of power, until it goes into open loop. Does the same with the o2 sensor that was on it, and the new bosch one I just put on, same readings too. Its a good meter, i make my living with it. Never had a engine whip me yet, and i will admit , its bugging me good on this one. Im betting the ECM has a glitch, or I have missed a loose connection or loose ground to the ECM, or the tranmission is telling the injectors to cut back. I will let you know what they find.

 

get your injectors cleaned, I bet your problem will go away. Get it professionally done with something like a motorvac, it was night and day in my stuff.