How to test the EGR Modulator valve,  or "EGR Transducer"  as the 96' FSM, calls it.

This valve modulates the vacuum , passed on to the EGR main valve, USING Cat back pressure to measure engine load.
The path is Throttle body, ported vacuum (4 hole), then to the EGR-VSV, and then MOD valve, and last to the EGR MAIN.
On 1994 and newer cars, you must move the car greater than, >  0 MPH, to get the VSV to open. (the FSM covers this , well) (factory service manual)
My video of  bench testing this device, is here.
This device, converts CAT Converter , natural back pressure to levels of Vacuum to port Q.  ( about 0.5 psi? at 2500 RPM)
The allows the EGR exhaust flow to vary by true engine load. Max HP is max flow. (not wide open throttle)

If there is no back pressure to bark  nipple Exhaust, then the Modvalve will be dead.  See Graphic 1 below for why!
That large nipple below , is  connected to the Exhaust #4 port, via a long path.

The "MOD valve", for short:  Below photo is inverted.       
Click photo to see FSM page on same.  (I also call port E , a  B nipple, for BARK the  tube)
On the Sidekick we think the P and Q are interchangeable, but on Toyota's one side has a restriction Orifice, don't tempt fate, use P to the Plenum or Ported vacuum and Q to MAIN.
If you remove the filter cap on top, the P/Q Directions are lost, it sees to work , no matter what on G16 engines.
Photo 1:

See page 4 , Here....
FSM  snippet. (Factory Service Manual)

Modulator valve testing:

A table x2 showing both OBD2 tests, in full spec. a .zip file)

Begin by marking the P an Q nipples on the main body. (on top above) (error in FSM or you lose the P/Q indexing in step 2)
Some mod valves have an orifice on the P nipple . My 4 do not....

  1.  Remove the modulator from the  EGR body clips on the engine, and the 3 tubes. (tag them) P, Q and B. Make sure hoses are not cracked , melted or clogged.
  2.  Remove the Mod valve Cap and blow out the dust in there.  (cap has P/Q label sticker)  (if filter is clogged replace it Scotch bright pad, cut to fit)
  3.  Make sure the vents hole to the filter is not blocked, seen in below photo3 of MOD, in red color left side.
  4.  Put back the  cap and blow air into either SIDE nipples,  It must be free to blow from both sides, or its clogged up. clean that if found clogged.
  5. While other side nipple is blocked with finger,  and you blow , the is air will pass to the vent orifice and filter, as you blow. if not, clean that path now.
  6.  Do the latch up test. last. next.
6: (details)
This valve latches up, if pressure is held on Nipple Exhuaust (B) the bark hose.
Let me explain.
Block nipple Q with finger tip.
Then you put the hand pump tool (see video?) to the OTHER side tube. P SIDE (pressure side)
Now, pump the hand pump, IT MUST NOT pull a vacuum NOW, if it does?, then the valve is stuck closed and is bad. (99% THE RED VENT IS BOCKED, JUST CLEAN IT)
 The FSM tests are poor , there are many tests and corrective measures,most of which are easy.

Last,  attach a hose to the bottom bark tube.(show up in above photo)  nipple , E (B) (use orig. hose is Ok, but a fresh new hose, is cleaner for lips.  Benzene is not good to eat... )
Now blow gently, on bottom tube E and at the same time you will feel the diaphragm , move up and this closes the valve, and while pumping vac tools (same time) it now shows a huge vacuum reading. it will approach 26 inches or near and hold and latch up, Going over 8 "inches HG is not needed.
It will even hold this vacuum while you are ONLY blowing very gently on the bottom tube. 
At only about 1 PSI blown (little) and valve stays closed holding the above vacuum.
 Just as the pressure you are blowing, drops to 0 PSI, the valve will drop open, (vacuum drops to 0)  Even the slightest blowing will hold it closed.
The Diaphragm is very sensitive. Its very thin orange, silicon rubber, and should last a life time (engines not yours) if cleaned.

The devil is in the details.

After doing all this and witnessing a real working mod valve.
You can fully understand what the exhaust back pressure must do.

If the exhaust never reaches this pressure level (up to 5 psi pulses) , if the Bark pulse is dead, the MOD is dead and so is the whole system.

Test 5:  (Leak down) The Classic leak down test to the Diaphragm.
I also check the bottom tube diaphragm for leaks, do a bottom tube leak down test.
Just like you do all diaphragms.
I pull 8 inches of VAC. , stop , make sure my VAC. Gauge needle don't drop in 15 seconds. (my hand pump has a built in vacuum gauge,)

This completes the MOD valve test.

If you remove your CAT converter this MOD valve will no longer work correctly  Code 51 / P0400 will result.  (OBD1 / OBD2)

This valve decides how much EGR exhaust to induct , based on the  exhaust back pressure ( true engine load).
The Valve actually wiggles at the rate of the #4 exhaust port.  This action actually modulates the EGR flow.

Phases: (operation on car) This is the bark tube theory of operation.  Note the large bark pressures. (they are instant not average)
  • The Exhaust pulse at A ,  causes  the diaphragm in Mod valve to close  (see graphic 1 above) gating EGR flow full on.
  • The MOD valve actually latches up with the vacuum in P/Q line even when exh pressure drops to zero. (keeping the EGR flow going)
  • Next, the Exhausts goes Negative ( that is why exh leaks cause 02 sensor to fail,  so easy to see why? below )
  • This Negative pressure B ,sucks the  diaphragm in the Mod valve off its seat , killing all EGR flow. (see atmosphere vent)
  • This repeats for every cylinder firing and resultant PUTTs.  The diaphragm is a very good silicon rubber.
The A pressure is the exhaust valve opening and the PUTT (pressure wave) hitting the CAT.  ( the B wave is caused  by violent drop of exhaust temperatures  (mass/temperature)
Photo 2:

I'm calling  exhaust port (bark tube port)  , B below in  Photo 3:

The vent in red must not be blocked, (left side)
If the vent clogs, (easy) the diaphagm will stick up ,freeze there (moving ) and cause excess EGR main flow. (no more modulation) and the throttle will hesitated.
If the Cat is missing or rodded out, the MOD will be dead. (the bark tube will be way too weak ,to do anything)

More photos:  60B00 is the Suzuki suffix of part number , real.
Do not do this , below, its a destructive test, for illustrative purposes only,  Photo 4:

End Thesis on mod valves.  It is a tricky critter, no lie.

For on car tests, hit the back or home button above and select your cars GENERATION number, or year.

BTW: (by the way ) the new 99+ cars use software, Not needing any sort of MOD valve.  (seen on 1996 Sports 1.8L)


 2010 , Version 3. ( Ver.5 more edits, more links, more facts, more photo, and a video.  8/2014)