So i was driving my truck to school the other day, no problems, just a normal day. then i get out of school and get into my truck and take off and there was absolutely no power, i had to be floored in second gear to go 25mph. When i got home i checked the oil because i have a valve cover leak and noticed it a half quart low, but when i took the oil cap off it let off pressure! so i started the engine with the cap on, then i took the cap off and the idle dropped down to about 200 rpm, what the heck is going on!?!?!?!?!?!

 

id fix the leak, then buy a new oil cap

 

Really clogged up PCV?

 

i agree- check the pcv.

 

Positive or Negative Pressure ?

 

in most cases itll be a pcv.

 

negative pressure because after i took the cap off and noticed the idle drop i just set the cap back on and the idle picked up again

 

Afm/maf?

 

PCV $11 at toyota

 

That's correct operation, and confirms your PCV(Positive Crankcase Ventilation)valve IS functioning. You'd notice the same thing if you removed the PCV valve hose with the engine running too.

Here's why.

Your PCV valve is a one-way valve in a rubber hose connected to the valve cover at one end and your air intake at the other. O.k., so when your engine is running the intake is sucking on that hose through the PCV valve as it sucks air into your engine. Some air from the intake is also blown simultaneoulsy into the crankcase through another hose creating a positive pressure flow, out of the crankcase, of blow-by gasses through the PCV valve into the intake. Whatever blow-by gasses sucked, through the PCV valve, out of the crankcase get recycled back into the combustion process through the intake.

This system causes a vacuum(negative pressure)under the valve cover due to the constrictions in the flow of the blow-by gasses from the crankcase to the PCV valve. When you remove the oil cap(while the engine is running)and break the valve cover's air-tight seal it prohibits that from occuring. This creates a vacuum leak. Vacuum leaks cause your idle to lower and usually make the engine struggle to run at all.

 

Negative Pressure is good , PCV is working (Positive Crankcase Vent) , the PCV is pulling air/oil vapor from engine from valve cover right next to oil cap on the 3.0's generally - if you have the motor running and pull your oil cap then it is much the same as pulling the PCV loose with oil cap on and then will result in idle change/drop (stumble) , this is quite normal as far as the idle drop with oil cap off with engine running and further indicates that your PCV is probably operating fine .
As for the acceleration problem I would check for codes :

http://www.off-road.com/toyota/tech/codes/

If you have no codes stored in the computer given the acceleration problem assuming it was not missing then if it were me I would imagine I would look for restrictions either in intake air (air filter) or exhaust (catalytic converter stopping up or muffler baffle broken stopping up exhaust maybe) , did you hit anything and bend your exhaust pipe ? or flatten out your tailpipe ? or did one of your buddies stuff a potato in your tailpipe .
Some things to consider for the wide open in 2nd only doing 25 thing , outside of a engine code being generated and giving you a clue look hard at your exhaust and intake air systems to see if you have a blockage in either place causing your power/acceleration problem , maybe someone else will post that has had this problem and give you a more definitive option .

 

lets just say the pcv valve is not working, what effect will it have if the oil cap is removed

 

it will release the excess blowby, thereby reducing crankcase pressure. Don't know but I would assume it has some affect on the EFI (sorry, I am a carb. guy). To make matters simple its a 3 dollar job that will eliminate one of the more simpler solutions.

 

If the PCV was not working then you would not have Crankcase Ventilation -simple as that , as well if it were clogged or removed then you would have positive crankcase pressure (bad for oil seals ) , if you do a ISR and do not tie back in your PCV to the intake tube then you have to install a vent (like the olden days) on your valve cover or your crankcase will in a sense be sealed and blowby etc will have no where to go but by your gaskets and oil seals.
Mine is a '91 and there is no PCV Valve in the tradiitional sense as the hose connects directly from the Air Intake Tube to the Valve Cover , not like most PCV systems where you have the spring loaded valve between the intake and the actual oil vapor/valve cover .

If the PCV is not working/clogged and the Oil Cap is removed (while motor is running) then more than likely no difference would be noticed outside of a small amount of positive crankcase pressure possibly .

 

But if the PCV valve is clogged then there will be built up pressure (postiive) in the crankcase and when you take off the oil cap it will release this pressure and your idle will stumble correct?

 

No. Because the valve is clogged and plugging the vacuum leak at the intake. The positive pressure would be relieved, and probably little else would occur. Untill you unclogged the PCV valve and the intake could draw air through it, too much air if the oil cap is off. Then the idle would be effected.

 

I very much apprieciate the good info, im gonna go through this old pig and do what i can. i pulled the intake tube off the throttle body and it looks like black gunk is built up on the inside walls of the upper plenum (like a 1/4 inch thick!!) so ill take it off and clean it as best i can unless there is a method i dont know about, i dont want to damage anything in there, and at the same time ill replace the valve cover gaskets. Any other info would be very much apprieciated. Thanks!

 

If you have all that in your upper intake then while you are at it clean your throttle body also if you havn't already .

 

for cleaning i would suggest MAF cleaner and TB cleaner used accordingly

 

Yeah , you don't have to take off TPS to clean throttle body and in that case be careful not to use standard carb cleaner / degreaser on your TPS as it could damage it , very good advice to use a plastic safe cleaner / degreaser if you leave the TPS on and get the cleaner on it in the process (and if it checked out good then I would leave it on unless you want some practice setting it hehe).
The main thing you want to clean on the throttle body is the throat of it and spray out the vacumn line ports after you clean the inside to make sure they are clean/clear of junk , then wipe out or spray out inside again and your done .
If you concentrate on the inside and the vacumn ports then the TPS should not see any of the cleaner anyway .