A couple of days ago I was doing some light maintenance on my new to me F150. On the list was replacing the air filter, however, when I opened up the air box I found this:

It's a tiny little filter that sits underneath the main filter.

Filter by itself:


So, can anyone tell me what it's for? It's quite filthy, so I'd like to replace it. I just don't know what I'm asking for. So you have all the info, this is on a 1992 F150 Custom with the 300 I6 (4.9L).

Thanks in advance for any info y'all can provide!

 

I think that's the intake filter for the PCV system. Mine don't have that separate filter so I'm going by distant memory of having seen one before somewhere.

 

Thank you!

I'm going to really show my ignorance here and ask one more question... What's the PCV system? I've heard it referenced before, but I've never known what it does.

 

Hell that ain't ignorance. Nobody was born knowing these things, just some of us have learned more than others.

PCV is your Positive Crankcase Ventilation. There's a vacuum line running from the intake manifold to your PCV valve which is located on the valve cover. There's also a line that goes from the valve cover to the air filter box somewhere. On your truck it has it's own filter in the air box and on mine it gets filtered thru the main filter instead.

So manifold vacuum pulls fresh air thru the air filter and into the engine, then out thru the PCV valve (which is just a plain 'ol check valve) and into the intake manifold. This keeps the crankcase at a slight negative pressure and helps vent it.

 

Cool! Thank you very much for this information! I've got a lot to learn still, but I'm glad I've got a truck I can work on myself as I learn rather than always having to pay someone else for the work.

Now I need to go get that new filter as this one is discussing!!

 

Just take it to the parts house & tell them where it is located & they should be able to look it up, or check any cross over #s.
But what Dixie 460 told you sounds right.

 

Thank you! That's a good idea too!

 

I just replaced mine. Here's a pic showing part number and filter: