Throttle Body PVC Bypass

(Straight TB shown – 2001)


     When Forced Induction is added to the N/A L36 engine the positive boost pressure from the sc blower unit can pressurize the crankcase if certain precautions and modifications are not performed.  An over pressurized crank case can cause engine seals to fail, oil can be forced out of valve covers, dipstick may shoot out of dipstick tube, etc.  This is not a good condition to say the least.  In order to prevent or reduce the over pressurization of the engine crankcase there are two basic mods.  Both involve the removal and ‘blocking –plugging’ the PCV hole in the throttle body but the two methods are substantially different as to warrant two separate install guides.

    For this project the first PCV TB bypass mod was used.  The optional second method will also be detailed at the end of this page.  The second method requires drilling vent holes in TB. (Developed I believe by ‘riles’ –  member )  The first method utilizes a valve cover breather in order to improve crankcase ventilation.


(Parts required – 3/8” x16 threaded plug, 3/8” x16 tap, RTV Blue or thread lock, valve cover breather, carb cleaner, TB gasket, tools)

NOTE: at time of this writing the 99+ TB gasket was only available from dealership.  The older angled TB gasket is slightly different.


1)    Remove sensor plugs from TB, cover sensor under TB with plastic bag/tape to keep moisture out on electrical connectors.

2)   Remove the three TB bolts/nuts.  (You may wish to jack up the front of the car to reduce coolant loss when TB is removed from manifold)

3)   Gently tap TB with rubber mallet to break it free from manifold.  (I was able to re use gasket as it stayed in one piece on manifold…I just cleaned it up a bit before replacing TB)


5)   See FIG 1 below.  Place the TB MAF side down.  Use a 3/8 tap (16 or 24 threads) and tap the PCV hole as shown.  The TB PCV hole is already almost 3/8” diameter so the threads will be very light.  (Blow out metal shavings with air compressor/carb cleaner)  You need a matching threaded “hex plug” (hardware store).  Suggest using a thin coating of RTV blue or Thread Lock compound on the threads of the hex plug to help with sealing/seating the plug.  Gently screw the plug in until it is flush with TB PCV hole. 

6)   The TB will no longer allow positive boost pressure from the front of TB intake into the crankcase.  Clean all mating surfaces and double check that the TB is clean of any debris/metal shavings, etc and install TB (with gasket) on manifold.  (Suggest spraying TB gasket with Carb Cleaner to ‘swell’ gasket before torqueing down TB.)

7)   Replace all sensor/electrical connections and PROPERLY top off cooling system.

8)   Remove oil filler tube from passenger side valve cover and replace with valve cover breather.  You can also leave the oil filler tube in place and just remove the cap and put the breather on the end of the filler tube.  (Less oil will seep out with the breather mounted on filler tube)  There is a cheap ‘universal’ valve cover breather that sort works…but it does not have threads and likes to pop out…. search for part number 4285 the breather is $11.99.

A much better – threaded and designed for the L37 valve cover breather is available from ZZP   $27.

   (ZZP threaded valve cover breather)





Optional Throttle Body PCV ‘VENTED’ Bypass


Riles has a website detailing this mod.

NOTE: This was performed on an ANGLED Throttle Body and has never been performed by me.  This ‘vented’ mod is rather innovative, however, and deserves mentioning.  Good Job Riles.


In case the above website is down its contents are reprinted below.

Note:  With planning, the vent holes could also be ‘tapped’ for future use of threaded plugs if it is ever needed to return throttle body PCV system to stock performance.


This mod will plug the FRONT of the throttle body PCV hole rather than the rear port.  Plugging the front hole, however, leaves a ‘channel’ in the TB that can be vented allowing crankcase pressure (from the PCV valve) to escape to atmosphere.  You can still use a valve cover breather in addition to the vent holes if you like.  You can also use threaded hex plug as in first mod (FIG 1) for the main PCV plug as well as the optional threaded vent holes.





FIG 2   (front TB w MAF screen removed.  The PCV hole has been plugged – lower left inside TB near butterfly) 



FIG 3    Next, get a Dremel or a drill, and drill vent holes along the tube in the throttle body (visualize where it is using your brain!).  You can do it my way with a bunch of small holes, or you would probably be fine with one big hole as well.


Done! Now your PCV valve is happy, because it can still do its job EXACTLY the same as it was doing it before! The air will vent out of the six little holes just like it used to vent out of the one big one.