Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
Working on this F150. Customer complained of an intermittent hard start and CEL on. Scanned codes and found rich codes for both banks, fuel trims at -25. Because of the hard start I decided to check the purge valve and sure enough it was leaking, so my theory was that this was allowing vapor into the intake causing both the hard start and rich readings. Replaced the valve and guess what... still way rich. I pulled a spark plug and it was supposed to be a double platinum, however the platinum on the ground strap was completely gone and there was a concave spot worn in its place. So... would be enough to cause the truck to run rich? I pulled a vacuum line and sprayed carb cleaner in the intake and did get the o2s to respond in both directions.
I may be way off here but I'm thinking an ignition problem would cause incomplete combustion. That would result in excess O2 in the exhaust which the O2 sensors would interpret as a lean condition.
Are the MAF readings beleiveable?
And did the purge valve fix the hard start? If not that might be ignition related.
It does has front AFR sensors and those were reading rich as well. I guess I was thinking worn plugs= unburned fuel= rich, but I guess the 02 work off of oxygen not fuel huh...?AF was 4.8 to 5 g/s at idle. Honestly just go the valve in today and haven't had a chance to test the starting issue. Either way we are going to recommend plugs.
Make sure you clear all adaptives and keep alive memory. Sometimes I even disconnect the battery and do a capacitive discharge if fuel trims have been maxed out for a long time.
Sometimes they don't come right back to normal even if you found and fixed the problem.
