Hi,

'97 4.6L, 138K miles. Got this F150 used about a month ago, so I don't know what went into it before then. Anyway, I drove it for a week, and then the check engine light came on. I noticed it came on when the gas tank was down to about 1/4. Autozone pulled the code and it was P401. Then, I filled the tank with fresh premium fuel, and the light went off after driving 1/2 mile. Since it was running great (no misfires, no bad smells, everything perfect), I let it go and cleared the code memory.

Well, another couple of weeks have passed, and the check engine light showed up again yesterday. I noticed that once again, I had about 1/4 tank of gas left when it came on. So, I went to the gas station and filled up again with fresh premium fuel. The light went out before I even pulled out of the gas station.

I haven't had the code from yesterday checked yet.

Is there any connection between the light coming on when the gas tank is low, and then going off almost immediately when it is full?? Could it be that there are some contaminents or other junk settled in the bottom of the tank that set the light off when the gas gets low and the engine starts burning that junk? If that is the case, do I just keep going and try to burn the tank lower and lower each time until it finally cleans itself out? As I mentioned, it runs great - I can't notice any problems at all.

Thanks for your help!

 

a few things,

welcome to the board!

2nd stop using premium fuel it will cause more problems in the long run.

3rd it sounds like you simply need a new gas cap. these trucks are very sensitive to the pressure in the gas tank and will throw a code if you even install the gas cap incorrectly and do not ever run the engine while filling up as you will get a CEL. your truck has a lot of miles on it and the gasket on the gas cap is prolly weak so when the tank gets to 1/4 the pressure is at it's highest and the cap leakes you get the code.

good luck-

 

Hi,

Thanks for the quick reply! The guy at Autozone told me the P401 code, in conjunction with the timing of the light coming on when the tank was low, was probably a gas cap issue. But after researching this forum, I found that 401 is usually an EGR or DPFE issue, so I just thought the autozone guy was wrong. But I will definitely try replacing the cap - sounds like an easy fix!

Also, you mention not to use premium (93 octane) fuel - which octane do you recommned?

Finally, thanks for the advice - this forum is awesome! I have had a few minor problems since getting this truck, and I have fixed all of them myself with help from you guys. I am not the most mechanically-inclined person, but you guys can talk anyone through anything! Thanks so much!

 

no sweat.

your engine only requires 86 octane. The only time you will need premium is if you get a chip. other than that you are just throwing your money away and possibly accelerating carbon build up!

the EGR systems on the 4.6 are famous for getting clogged do not buy a new EGR valve because it rarely is the cause it is almost always the two egr lines running to throttle body and if you still have problems they are also famous for crappy DPFE's