Hello,

1997 F150 4.2L

Trying to solve a rough idle / miss, poor gas mileage and occasional shaky start. Been through pretty much everything and I am wondering now if it is the upstream oxygen sensor(s) causing the truck to run rich (plugs were carboned up and tailpipe seems extra sooty).

I replaced the driver side o2 sensor because those plugs seemed extra carboned up and the engine doesn't even burn a 1/4 quart of oil in 3k. Truck seems to run the same.

I tested both oxygen sensors (backprobed sig and grnd) when the engine was running and warm: both fluctuated about 3-4 times per second, mostly between .20 to .78 volts. I also tested them by creating a small vacuum leak thus creating a lean condition.... both sensors read 0.00 and bounced back up to around .88 as soon as the leak was stopped. The only thing I noticed was the passenger side oxygen sensor did drop below .10 volts to around .06 about once every other second while the driver side never went below .20 approx. The specified range is .10 to .90 volts, is the passenger sensor bad since it is below this range sometimes? Any other ideas?

Thanks for your help

 

Sensors sound fine. FWIW, O2 sensors rarely affect idle quality.

I'd be looking at uncommanded EGR flow or, since you have a 4.2, possible inatake /coolant leakage.

Steve

 

I agree 100%! Have you had to add coolant? Is the coolant level low?
Read this;
http://forums.treemedia.com/fb/showthread.php?t=224

 

did you ever figure out problem im haveing same issue

 

Mossy- If you have the shaky start, it is highly likely it is the fuel pump. It may be intermittent because the fuel pump doesn't always act up. I also know someone else with the issue; he replaced the fuel pump and it was a 100% fix. I also think the dirty plugs and poor mileage could be related to the pump. The engine is calling for XX amount of fuel and the injectors are firing but not enough fuel and not a good spray pattern due to low fuel pressure. This causes an incomplete burn and wrong fuel/air ratio. Let me know how you make out. Good Luck!

 

The shop that told me fuel pump was bad I had them replace fuel pump and that was about a month ago, I'm gonna check oxy sensors plugs (harness side) and see what voltage and all that they are pulling