WOW... Issues 06 4.6l

I just replaced the intake and valve gaskets on my buds 06 4.6l. Everything went nice and smooth.... until we started it.
It idles from a stall to high rpm then stall to high rpm backfiring a little in between. It will eventually stall... Thought it was a vacuum problem but everything has been triple checked. Any thoughts?

Did it do the same thing before?

No thats the part that sucks.... It was leaking into cyl 2,3,4 wich was wearing the boot out on the plug and causing a miss.
The truck just stalled on him a few times without any hesitation or stumbling. Kinda wondering if I distrurbed some prior problem?

Are you sure the gaskets are seated correctly on the intake? You can't easily find a vacuum leak in the valley of the engine without propane.

Might also want to check all electrical connections to make sure they're all hooked up correctly. Are all the fuel injectors and COPs connected to the appropriate connectors and are the connections tight?

Did you check the boots for any holes/cracks and replace as necessary? Did you thoroughly dry out the COP boots/plug wells and apply a generous amount of dielectric grease to both ends of the boot to keep moisture out?

Lots of questions there so take your time and you'll find what's wrong with it.

Thanks for the reply! I've checked and rehooked up every connection and and done everthing except the propane... Kinda freaked out doin it with the engine backfiring and barking.
I have noticed the brakes arent getting any assist? vacuum??
Ive also heard the iac dont like to be messed with wich freaks me out because I cleaned the throttle body out with inj safe cleaner.

Is the 06 F-150 drive-by-wire or still throttle cable linkage? There shouldn't have been any issues with the cleaning of the IAC/plenum/TB.

If there's no brake boost, than it likely is a vacuum issue. I'd use some brake/carb cleaner or, more preferably, propane around the vacuum lines and in the valley to help identify a vacuum leak. Also make sure the EGR valve isn't stuck open.

Also you might want to go pick up a vacuum gauge if you don't have one. I got one for $20 and comes in handy when diagnosing different things. It should read around 19-22" Hg at idle.

It's cable less... Do i just wave some propane around the vacuum lines and see if it surges more? Thanks I appreciate you taking some time.

Yes, you want to have the propane come out around the vacuum lines and in the valley along the top edges where the manifold meets the head. Also don't forget to try around the fuel injectors, too. If you tore some o-rings that will leak vacuum, too.

You might want to go pick up a vacuum gauge if you don't have one. I got one for $20 at O'Reilly's and comes in handy when diagnosing different things. It should read around 18-22" Hg at idle.

And since it's drive-by-wire, I don't believe there is an IAC valve the computer just opens the throttle plate a certain amount. I may be mistaken as I've never worked on these '05+ DBW models.

Thanks appreciate it... I'll give er a try!

You can also listen to it and see if you hear a giant sucking/hissing noise. The engine should be fairly quiet with the exception of the injectors and maybe some stray intake noise. If you hear one, try and zero in on where it is coming from and you should find your vacuum leak. Injectors are a good place to look if you had those out and didn't use any sort of lube to put em back in their holes. #1 rule of fixing cars. Last thing worked on, first thing checked. I don't know how many times that has saved me boatloads of trouble finding a problem.

I'm an idiot... The plenum gasket had metal spacers on it tha had to come off. Don't know y they were on there for but I know they definetlty have to come off!!
Rookie mistake(That one hurts the ego for sure)... haha. Thanks for your help in narrowing it down, she's purring like a kitten now!

Glad it's working better now!