Hello all.
Update time. 99 F150, 2WD auto, 174K miles. newer Motorcraft wires & plugs, All minor service done.
When leaving a light or just starting off, Truck really stumbles. Once underway, seems to run OK, but I think something is up.
Replaced a vaccum line that runs under the intake that bends a 90 degree bend up to just aft of the throttle body. It had a pin hole.
Part store ran the codes and said "you need new cats". But, they didn't say why they needed to be replaced? I changed the o2 sesnsors (4!) and cleaned the MAF wire with CRC MAF spray. Throttle plate is clean.

Where do I start with better diagnosis? Replace the MAF? Plugs again? I am just really wondering where to start as no one will really give me a definate reason why it is doing it.

Thanks for any advice, Don

 

If it were me, I'd disconnect the battery and let it sit an hour or so, then reconnect the battery.
The ECU will reset to factory defaults and start relearning when you start driving.

It might be as simple as that.

If not, post back and let us know.

thanks.

 

Thanks, Brian. Will do and report back.

 

if it the scan said "lean on both banks" its a vacuum leak.

 

Well, you may have hit the nail on the head. I started snooping at the lines and the PCV valve on the right (passenger) side has a little elbow going up under the intake just behind the TB. It had a pin hole in it!
So off to Ford I go to replace it and they do not have one and will only sell the entire assembly. Assembly consists of 3 vacuum lines, palstic pipe and PCV valve for 38 dollars. I am sure the all should be replaced. I DID sneak to a local auto junkyard and found two other F-150s that were the same way!
So I guess I will order one and replace the whole thing, remove the battery cables for a bit to reset the brain and then will try again.

Thanks for the replies.

 

Thats usually what "lean on both banks" means, but not always- just like 97% of the time

 

Update!
I replaced the PCV assembley for the passenger bank. the part was ordered and was in my hand the next day.
After staring at it for a good 20 minutes, I took off all the hoses and lines in the way and popped in the new stuff. Hit the key and it ran slightly better. Still have SEL turned on, so off to Auto Zone to get a better scan and reset. Also the Motorcraft plugs have about 25K on them so...I may spring for a new set.

But I think I am on the right track and after I gave it a good shot of throttle, it took off right smartly.

Check back in a day or two....

THANK YOU TO ALL WHO RESPONDED. I wish I had the same reception with a Dodge forum I belong to. (F-150 is my sons, I have a old Dakota I bought from a co-worker. )

 

Thanks for the update.
And thanks for the nice words.

I have been active here at FTE for 10 1/2 years. It has been a great source of info and discussion.

Hope you get it all squared away with your truck.

 

Had same hesitation and after realizing that my heat was luke warm and that my coolant fluid was down over a gallon I changed the thermostat and the coolant temp sensor and topped it off and that solved the problem. The root cause was not solved since I know I have a very small crack in my upper head next to gasket and when truck is shut down it boils over and sprays onto the fender creating a small puddle. Oh my. Looks like I'll be adding

 

Clean the throttle body.I bet that will cure your stumble!

 

Update!
I forgot to post this a few weeks ago. Sorry about that!
Well, I did clean the throttle body and the MAF with the correct MAF sensor cleaner. But the real improvment came with ... drum rool ... New set of Motorcraft plugs.
The 'old' (25K miles) ones looked ok, but it seemed that they were firing off the side of the electrode. 3 of the plugs had one side of the electrode rounded off. Weird.
I unhooked the battery while doing the plugs and the CEL went off. When I tool it to Auto Zone, they hooked it up and said something was wrong. it could not register a connection to the trucks computer. Hmmm. I brought the Dakota there and it was fine. I never heard of the scanner not reading the computer.

But anyway, the truck runs better (for now!)

Thanks to all.

 

Good news. Thanks for sharing. I am a wee bit aggravated at myself for not mentioning that early on.
Seems these motors can do funky stuff when the plugs go south.

Glad you found the problem and got it squared away - and didn't have to spend a lot of $$ in the process!