I got a 98 ford 4.6 4x4 that I believe had a vacuum leak but not sure. It idles fine put it in drive and it will run fine as long as it's getting gas but let off gas like taking a curve or stopping or just too idle rpms start fluctuating until it dies. With the ac on it will run better although rpms still seem low but when you give it throttle ac kicks off starts blowing hot through vents on panel does not switch too defrost. Also when it does this you can pop hood ac clutch is engaged and working properly. Turn the ac too off on dash and back too ac and ac works until you give it gas and it kicks off again, freon is good done checked that. I've checked reservoir behind battery by pulling line off with truck off and it's holding vacuum and I've changed pvc elbow due too it being rotted and having a hole I had these codes p1401 doge sens circuit high volt p0171 and p0174 banks 1&2 too lean. Since I've changed elbow no codes but did disconnect battery. Any help would be appreciated. Thanks

 

Replace the IAC.

 

I'll give that a try was debating it already but ac not coming through vents has had me confused when clutch is engaged.

 

AC not coming through vents is a broken vacuum hard line behind the battery.

 

Everything looked good behind battery ac canister in good shape lines looked good no cracks and are not brittle,

 

i also have code p0171 and p0174 and my truck struggles to start like if it is not getting enough fuel to start up, i have to start it and then let go and try to start it again and it starts fine, i believe it is a vacuum leak just dont know where to start looking for the leak.

 

spray carb cleaner around intake and vacuum lines and see if rpm picks up to find vacuum lines. A smoke machine helps to find vac leaks.

 

Replaced iac idles good in gear with no throttle but busting out like it's not getting enough gas on higher rpms. Hasn't been long since filter was changed but got a new one to try out. But I did a vacuum smoke test couldn't find no leaks dang ac still kicking off on throttle where do I need too check