My 99 4.2 v6 recently got codes for running lean in bank one and bank two. Usually at idle my truck runs fine maybe a little shaking but other days my truck goes down below 500rpm and shakes real bad trying to shut off. I’ve tried to check for a vacuum leak but I haven’t been able to find one yet. I am new to the ford trucks so I don’t know too much about them so I came to here hoping for a little help. It is also making a hissing noise under the hood and has no muffler if that helps.

 

Could be many things. Since it's on both banks look for common denominators for air and fuel problems. Check the freeze frame data from your scanner to identify what was happening when the codes were set. That can help identify fuel and vacuum clues.

I always recommend checking the MAF before chasing vacuum leaks. Especially on Fords. You can disconnect the sensor and see if the engine runs better or worse. Disconnecting the sensor will cause the computer to default to standard tables for air/fuel mixture. If the engine runs better then you likely have bad MAF. Voltage checks can also be done. Videos are available online for instructions.

When checking for vacuum leaks I recommend a smoke test as leaks can be hard to find. A common problem is the intake manifold gasket is leaking. People replace the isolator bolts along with the gasket. There are a couple of other common leak points including a rubber elbow near the throttle body.

Another possibility is fuel pump problems.

 

If I was a betting man, you’re probably fixing to do the aforementioned intake mani gasket and isolator bolt job.

That is a 10th Gen F150 4.2 engine rite of passage.

You won’t find a vacuum leak because everything is tucked into that cute looking plastic upper intake.

 

Could it be bad plugs?

 

That’s some coincidence to have multiple plugs fail at the same time but not throw a misfire code. I would bet it is the isolator bolts. If it is the gaskets leaking, go ahead and replace the 90 degree coolant pipe on top of the lower intake and the intake manifold runner while you have it taken apart. I had replaced the intake gaskets a couple years ago and now have that pipe leaking and from what I can tell, have to remove the upper intake to replace it.

 

I had the same problem with my 2002 F150 4,2L V6. There is a Ford Service Bulletin about the upper Intake manifold hold down bolt grommets failing and causing the right and left bank lean DTC fault codes. Its been 8 years since I did my repair. I followed a tutorial on one of these Ford F150 sites not sure maybe it was you tube on how to go about this fix yourself repair. I will share what I did if you cannot find any info about the repair. First is go to Rockauto.com and order a set of 6 ea. 1) Intake Manifold Isolator Bolts. They come with the new grommets. You can just order the grommets your call. 2) upper intake plenum Gasket kit. 3) Intake Manifold Vacuum retainer bushing. These little plastic clips are not the problem but a preventive maintenance thing. For $3 bucks your call. You will need about 8 to 10 spray cans of barryman intake/carb cleaner or equivalent. The Intake cleaner is to remove all of the carbon deposits on the secondary intake butterfly type valves. six little secondary intake valves. The EGR valve at the front of the intake plenum is responsible for the carbon buildup. Your truck will run like new so don't skip the cleaning. I had to remove all six spark plugs due to the Intake cleaner flowing down into the cylinders causing a liquid lock. Best to turn the crank by hand 2 times to remove all Intake cleaner this is easy with no compression. If you use the starter you will wish you had not. The Crank will rotate very fast and the pistons will force and splatter the black nasty Intake cleaner all inside the engine compartment staining any and all material under the hood. Washing your Engine with Degreaser after will not remove the stains. I used Gunk Engin Detailer to hide the stains. Having to remove the spark plugs add 6 new sparkplugs from Rockauto.com to your order to go back with. You should do an oil change to. A special over the top engine creeper would come in handy to remove the 20 something 10 mm head bolts from around the intake plenum parameter. 8 years and 35,000 miles later no Lean or any fault codes. My truck ran like it was brand new. It is all most worth removing the upper intake plenum just to clean the carbon buildup off of the secondary intake valves. Removing the EGR valve and adding Seafoam with engine running could help remove some of the Carbon build up.