Guys, I have spent the last 3 hours searching posts and looking for a situation like mine to no avail. The truck in a 99 5.4 it has about 150,000 miles on it. About 3 months ago on the way back from the beach the CEL came on but I had no difference in driveability. Got home, scanned it and had 0171, 0174, got online found what they meant, tried the carb cleaner method to find the vacuum leak and couldn't find one. Talked to mechanic that I go to when I can't figure it out and he said the added ethanol in the gas can sometimes cause those codes.

The wierd part was that I actually got better gas mileage (~1.3 mpg) after the CEL came on and since there was no performance problems I just left it alone. I don't drive it everyday anymore so I've only probably put 500 miles on it in the last 3 months since this happened.

Fast forward, this morning I went out to get a Christmas tree started it up and it felt rough and was idleing low, only when in gear, in park or neutral it runs fine. So I suspected a vacuum leak especially since when I got on the road I had no low end power, like when vacuum advance failed on old cars. Dad had my scanner so I took it to auto zone and got it scanned and not in addition to the lean codes it is throwing 305 and 308. The PCV elbow had indeed failed so I replaced that but the rough idle still exists. I ran out of daylight so I will continue tomorrow.

Anyone have any ideas for me to check. Thanks in advance.

 

Misfires can cause lean codes. Vacuum leaks and lean conditions can also cause misfires, so as to which is the cause and which is the effect, hard to say. I would suspect that you have a leaking intake plenum as one possibility, but it could also be an EGR or ignition related problem. I'm not sure what the firing order is on your truck, but I suspect that #5 and #8 may be on the same coil or might have something in common.