I am stumped. I have a 96' 150 with 61k miles, my truck started idleing rough about 1k ago and had a lean trouble code. I have replaced two of the three O2 sensors, replaced the cap, rotor, wires, fuel filter and plugs but the the check engine light continues to shine! I took it into very reputible dealerships to diagnose the problem after it got so bad that my truck will die when the RPMs lower after a stop. They said that I had a lean and a rich trouble code but they could not find the cause. It seems that it would be running rich OR lean, not rich AND lean. As Ive said I repaced the fuel filter, but while browsing my owners manual I saw that on 95'&96' 150s there is a fuel filter in the fuel tank, I dont know if these go bad or if this could even be the cause. I hope and pray that someone has been in a similar situation and can give me some advise.
I also have a buzzing noise for about the first 5 sec. after start up, it is loud and comes from a valve that connects to my smog pump. the vavle has a vacume line that goes to an EGR solenoid, when the line is disattached while the buzzing noise is happening, the noise will stop. Possible EGR solenoid? EGR? Smog pump? I dont know if these two problems are related. I hope that Im not the first person to have these problems, If someone can shed some light I would really apriciate it.

 

Go to a parts store and have THEM pull the codes for you. Most of them do it free now. They'll even look them up for you and clear them from the computer. You can also look them up here.

Just because a code shows up, it does NOT necessarily mean ANY part should be replaced. That's the LAST resort. All the oxygen sensors could be perfectly fine and you ACTUALLY had a lean condition. The HEGOs don't CAUSE that- they just detect it. I hope you still have the old ones because they're probably good.

Diagnose each part separately - the buzzing sounds abnormal, but it may not be related to the codes, HEGOs, or anything else. You can probably find exactly what it is here. It could be the purge solenoid, the EVR solenoid, or one of the smog pump solenoids (TAB or TAD).

Also, you should have the fuel pressure checked, or buy a gauge (~$25 @ Sears) and check it yourself. A bad fuel pump could be the cause.

 

Did you disconnect the battery after replacing the parts? I usually disconnect it while doing the work, then hook it back up afterwards. This resets the computer, and clears the codes.

 

Thanks for the advice. I have cleared the codes and they keep coming back. I believe the site was YMM lookup, I found that very usefull and hopefully I can diagnose the problem over time. I have thought of the fuel pump, but have been trying to avoid it. I did check the fuel PSI before and it was 2 PSI lower than the lower limits, possible problem?
I dont know much about the emisions solenoids and how they can effect the drivablity, hopefully that website will shed some light. I truly appriciate all of the help, Thanks.

 

2 psi low indicates a problem, but it could just be a clogged filter or bad FPR.

A better reference is the Haynes manual.