I've learned about the PCV finally. :) (often called PCV valve, or positive crankcase ventilation valve).
If you're getting black on your air filters, maybe all in one area? Perhaps near where the PCV connects via a hose to your air filter? This means your PCV is fouled or "broken" and is being a negative flow instead. I've seen that a lot in Toyota Corollas or the Geo/Chevy Prizms**, more than just mine, so I thought to pass that on.*
...I tried cleaning mine, if the labor wasn't free buying a new one is cheaper. 45 minutes of banging on the PCV valve (with my hands only) against rags, using break cleaner on it, etc., I got the entire life of the car's cake of oil and carbon out.
Next time, I'd leave it to soak in gasoline over night, probably would have been easier.
I failed to do one test so I need to get back in there. If it rattles when you shake it, you should be good. (this was probably possible earlier in the cleaning, but I thought it best to have less goop to prevent a faster re-clog.)
Finding your PCV valve...unless you want to YouTube it ;) there's a new channel on YouTube that I'm fairly unimpressed with that helped me find it (so sorry, I don't think I'll have luck finding the video to give credit; kind of rude of me maybe, but sometimes helpful information isn't in a glamorous place). There are two tubes off your valve cover. One that comes at you if your at the front of the car, and one that points to the driverside. The driverside one has is your PCV valve.
If it has it's original clamps it's easy to pinch it and pull the hose off. If it's hard or cracked it should be replaced. The PCV valve can then be unscrewed with a 17mm deep socket or box handle wrench. Be sure to have some teflon tape to put back on the threads before restoring it in the car.
A quality Toyota PCV is all metal, and if you're willing to clean it, I suspect it's possible to be a forever part. If you chose just to replace it, at least see if it rattles first, it's fine, don't replace it. I've seen plastic ones for sale for our car on eBay that cost about what the metal one would, aftermarket ones are fine, just don't go plastic.
The PCV threads in, but has multiple pieces of rubber around it even where you thread it into the valve cover; replace any that are bad. I spotted everything for sure that could be bad on ebay for cheap, except maybe the tube that goes from the PCV to the throttle body. (take it to a local parts store to match the INNER diameter if not).
*Ooops, I think that means our PCV system can't foul the air filter...maybe I need to look at my brothers 7th generation Corolla/Geo Prizm again**. This part of my information is more general to cars maybe than just to ours. I learned most of what I did about PCV valves from Briansmobile1 channel on YouTube.
Saved for last an explanation of what it does? lol, you can watch Brians video on it, or read my regurgitation:
See, engines sometimes have vacuum on your PCV valve and sometimes it has pressure. When it has pressure, unburnt gases from combustion sneak up under your valve cover and modern cars for better emissions send that through the PCV valve back to the intake so your engine can burn it off (arguably more efficient on a car's MPG, especially on bigger engines). Older designs took care of this pressure problem with a filtered cap that doubled as your oil cap.
When the "pressure" or vacuum is present in an the valve cover the PCV prevents air from coming from your intake into this part of the engine, which is then a vacuum leak (definitely reduces your car's MPG and deserves it's own article). Without this valve functioning correctly, the pressure build and builds, and will help even more unburnt fuel and exhaust past the piston rings and down into your crank case. Piston ring blow by reduces the viscosity of your oil over time. And that can be really bad news for the longevity of your engine. If this is in addition to worn piston rings and/or the bore, then your engine can really be worn faster than it needs to be.
(Severe blow by can push out the oil dipstick, if you have the regular habit of checking your oil like you should, and it keeps being pushed out, uh oh. Even if it doesn't push the dipstick off, this is a good way to find a worn engine to not buy a used car because you know now about problems with lots of blow by.) Check out my article about Synthetic Oil Controversy.
**Note:
1998 to 2002 is the 8th generation Toyota Corolla/ Chevy Prizm
93? to 96 is the 7th generation Toyota Corolla/Geo Prizm
by AutoBravado
