1990 F150 XLT lariat 2wd e4od 300ci 179,000 miles I have been trying to cure this code since i bought the truck. Both fuel tanks and pumps are new, fuel rail psi is good something like 48 psi. FPR is good. New O2 sensor. Intake is clean down to the metal. Fuel injectors refurbished. EGR compnents all new and in working order. There are only 3 problems left to fix, Tranny (completely trashed), intake airbox, and this code 41. My question has to do with the airbox. It is supposed to be secured shut with 2 bolts i believe, however mine is all cracked up (the bolts cracked off and took some of the plastic with them) and basically sucks in open air since there is no seal from the box being shut. The air isn't even sucked through the filter since the filter sits in the lower part of the airbox. Would this possibly cause the code 41? My thought process is if there is no resistance of incoming air then more is going into the engine...maybe too much air for the computer to compensate? The reason I thought of this is because I heard if you put on too large of a CAI it will sometimes throw a lean code due to the exta air. I will also recheck my vacuum lines for leaks since I am not 100% sure of myself on my last checking of the vacuum lines. I have checked everything else I could think of, and also everything you guys could think of from previous posts. This just hit me today so i figured i would see what ya'll think.

 

No that won't cause the system to go lean, the air still has to go through the TB so it's still metered. The area of control for the EFI system in your case is everything between the throttlebody and the O2 sensor. So if there is air getting into the motor through a cracked or broken vacuum line, gasket leak, rusted or broken thermactor(air injection) system or EGR, or any other exhaust leak between the O2 sensor and the motor, you could get code 41.

 

You need to double check all of the vacuum lines plus all of the PCV system.

In the past, I replaced many intake gaskets on the 4.9 due to leaks. You might want to carefully spray some brake cleaner (make sure it isn't the non-flammable kind) around the intake gaskets to check them.

 

what will happen if there is a leak? the idle will surge?

 

Yes, the speed will increase for a brief time.

Needless to say, (?) keep flammable stuff away from the distributor, wires, alternator, etc. Use a straw on the can and just dribble it around the gasket areas.

 

umm, if You use non flammable, it won't burn/cause the engine to change idle.