Ford Super Duty (1999-2016) :: 2001 F250 5.4L - Smoke From Exhaust Pipe

2001 F250 5.4L - I recently had my muffler changed and the pipe flange on the tailpipe up by the exhaust manifold welded back on. I haven't driven my truck much this past summer so when I took it in to get the muffler fixed the front caliper got temporarily stuck like it tends to do. It eventually freed up but in the meantime I noticed a hot oil-like smell coming from my truck from time to time. I chalked it up to either the new muffler or maybe it just smelled hot and it was instead the front brake caliper still being stuck. One day I'm driving down the interstate and I start uphill. I noticed smoke behind me and pulled over. The brake was not overheating so I got underneath the truck.

The smoke was rolling off of the exhaust pipe all the way down it all the way up near the manifold but mostly around the muffler. The smoke quickly went away while I was under the truck. I noticed it doing this only while the engine is under a load like going uphill. Just driving around town nothing is noticeable. When it does smoke I lose power. Once I slow down for awhile and then gradually speed back up things seem okay. I checked the oil.

Normally I don't lose a drop but this time I was a quart low but the oil on the stick was clean. It's been awhile since I had my oil changed but very little mileage on this change (my truck has sat awhile). My truck doesn't leave a drop of oil under it when it sits. From what I read you can remove the radiator cap and start the truck to see if it's a blown head gasket. At least that one of the things I can check easily without actually taking the plugs out even though the oil looks clean. The radiator has not lost any fluid. The transmission fluid is also good. It's been doing this for about 3 weeks.

I still drive it to and from work (about 15 miles round trip) everyday but I take it easy on the acceleration and stay on city streets. I looked into the EGR system online for my truck and instead found an EVAP purge solenoid/valve in its place on the passenger side. I don't have any service lights on the dash. I went to Autozone and had them scan it but nothing came up. I read somewhere that either the purge solenoid assembly or EGR (I can't remember) can burn up the board if it goes bad resulting in no code showing up.

I recently changed a vacuum valve out that went bad that controlled the vacuum chamber used to operate the hubs on the 4WD. The 4WD is now operational again. Assuming the head gasket is not blown, what to check for next?