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Thread: Vacuum Modulator, Exhaust Gas Recirculation, Previa

  1. #1
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    Vacuum Modulator, Exhaust Gas Recirculation, Previa

    The Vacuum Modulator on my 91 has really been giving me fits, so I decided to cut an old bad one apart and try to determine why it failed.

    This is what your vacuum modulator looks like dissected...

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    Clockwise from bottom... clamp, bottom housing half, diaphragm, spring, top housing half, filter, cap.

    The bottom housing half has one passage which goes directly to the exhaust pressure chamber. Exhaust pressure from this pipe acts on the diaphragm. The diaphragm should completely seal off this lower portion from the upper portion.

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    Diaphragm - pretty obvious what was wrong...

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    Upper housing half top view... the small hole is there to allow atmospheric pressure into the upper chamber at all times.

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    Upper housing half bottom view... the atmosphere hole is on the left, the central nipple goes to both ports P and Q, and the square hole on the right is the opening for port R. The P and R ports variously pull the diaphragm up enough to seal off the nipple and thus close off atmospheric pressure to port Q which goes to the EGR valve itself. When the nipple is sealed off, vacuum at port P (from the throttle body) supplies vacuum at port Q (the two ports are joined) and causes the EGR valve to open.

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    Here is the top half sectioned to show the design of the P and Q ports... that piece of wire is .009" in diameter and is there to show the tiny passageway connecting port P to port Q and the upper chamber. Two pieces of wire would not fit.

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    So this is how the system works.

    The deciding factor as to whether the main EGR valve is operational is whether the diaphragm inside the Modulator is pushed all the way up against the central nipple, sealing off the Q port from atmospheric pressure in the upper chamber. In this position vacuum at port P (from the throttle body) will transfer to the EGR valve via port Q, and open it.

    The lower chamber function seems pretty simple - exhaust pressure is always acting on the lower chamber whenever the engine is running, hot or cold, pushing the diaphragm up, with pressure increasing as load increases.

    The upper chamber is the more complex one.
    It is vented to atmosphere via the small hole in the pics above, so the diaphragm will only close against the nipple when the vacuum in the upper chamber is large enough to overcome atmospheric pressure through this hole. The P and R ports are responsible for supplying this vacuum.
    The P port is the one with the tiny passageway into the upper chamber (left port with wire in the above pic), is connected to the throttle body via the BVSV (to prevent operation entirely when the engine is cold) and is only active in assisting with producing vacuum after the throttle has opened modestly. If the vacuum from port P is enough to suck the diaphragm up all the way, atmospheric pressure from the vent hole will push it back down again because the vacuum effect of port P is disabled as soon as the diaphragm contacts the nipple (the surface area for the vacuum to act on has all but disappeared). The effect is to turn the EGR valve on and off with the throttle in a gently open position and effectively pass a small amount of recirculated gas into the intake manifold. The R port is connected directly to the throttle body and is only active in assisting with vacuum whenever the throttle is substantially open. The R port produces strong suction and can continue to act on the diaphragm even when the diaphragm is up against the nipple because it is sucking on the whole surface area of the diaphragm from its own hole independent of the nipple. In this situation vacuum at port Q is continuous and a larger amount of gas is recirculated. When the engine is cold the system is not operational because even if exhaust pressure from the lower chamber and suction from port R cause the diaphragm to close the nipple port, the BVSV is closed and there is no vacuum at port P to transfer to port Q.

    This post has been long enough. Hopefully it is mostly error free.

    Edit: just found this, check it out - it's a great write-up of system operation. http://www.lovehorsepower.com/ToyotaPDFs/61.PDF
    The Port E that they are talking about is the port on the throttle body with a cast E next to it. This connects to Port P on the Modulator. The port marked P on the throttle body on our Previas goes somewhere else entirely. Go figure.
    Last edited by Jonny; 09-16-2018 at 09:59 PM.

  2. #2
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    Re: Vacuum Modulator, Exhaust Gas Recirculation, Previa

    Some observations.
    It is interesting that the design hinges around vacuum supplied to the EGR valve via the P port... that orifice joining P to Q is only about .015" in diameter!
    Since the only entry point for gas in the bottom chamber of the Modulator is through the exhaust tube, you can test whether the diaphragm is torn by blowing into this orifice. You shouldn't be able to. If there is any leakage whatsoever you have a problem. This test isn't in the manual.
    If you do have a manual, and it is like my 91 manual for my 91 van, the drawings of the Modulator and vacuum operation are incorrect. The restriction is on the P port which goes to the BVSV. The Q port going to the EGR valve is unrestricted. In addition, and even more confusingly, Toyota has drawn the diagram with the P port (going to the BVSV) by itself, whereas in reality it is the Q port which is lonely. It is possible they've fixed this in later editions.

    Edit: just found this PDF from Toyota. It has better drawings, and a great explanation of system function. http://www.lovehorsepower.com/ToyotaPDFs/61.PDF

    Some questions....
    I replaced my bad modulator (with the torn diaphragm - the one I dissected above) with a scrap yard working modulator. It was good for about 800 miles. Now the code is back and I can blow through the bottom exhaust tube port so I think the current diaphragm has now torn also. So, it could be that this diaphragm has also coincidentally suffered from old age, or it could be something to do with my vehicle. I have read that clogged cats can cause the modulator to overheat. My van runs fine, gets good mileage and there is no visible heat damage to my old modulator. Is that tear in the diaphragm (3rd pic in post above) possibly caused by excess pressure or heat? It doesn't look like heat to me upon close inspection, but it could be too much pressure. I pulled on other parts of my bad diaphragm and it seems to tear pretty easily though! I'm tempted to buy a new modulator, but if that too is going to fail in 6 months I'm loath to part with $100. Any suggestions?
    Last edited by Jonny; 09-16-2018 at 10:06 PM.

  3. #3
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    Re: Vacuum Modulator, Exhaust Gas Recirculation, Previa

    This is an FYI mainly to give you guys a good laugh, but the modulator looks like it can be repaired...

    Sure enough, my "good" scrap yard modulator developed 2 tears in the diaphragm. I fixed it with clear RTV silicone. We'll see how long it lasts.

    Careful when cutting the ring clamp off, you can only go about 1mm into the plastic parts before you'll damage them too much
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    Professional silicone smearing job
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    A 60mm wire hose clamp would fit but the local shop didn't have one so baling wire works
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    If you decide to do a repair like this, make sure you do these two things in addition to the silicone diaphragm fix...
    i) get a piece of wire or something about .010" in diameter (I used some wire from an electrical cable) and push it all the way through from the P port to the Q port. Once the diaphragm is torn soot from the exhaust will most likely have clogged the tiny passage inside the P port, and it is the P port which transfers vacuum to the Q port for EGR valve opening.
    ii) use a thin smear of gasket maker on both sides of the diaphragm when reassembling - the ring clamp doesn't do such a good job once it has been cut.

    Several freeway trips later and no code yet. I'll keep this post edited with info as anything relevant occurs.

    Over my extensive dealings with my modulator-induced EGR system failure I can say this with conviction....
    * If you have coffee-grounds-like stuff under the filter of the modulator you most likely have a tear in the diaphragm... it is from the exhaust pipe and through the diaphragm is the only path by which this stuff can get there.
    * A modulator will still work with a tiny tear, but fails when it gets large enough.
    * If you can blow any air through the big bottom tube your diaphragm is probably torn.
    * If your diaphragm is torn, even if the system still works somehow, you are getting hot moist gases into the spider pipes and they will eventually rust closed.
    * Once the system stops functioning for any length of time, the egr valve no longer opens, exhaust gas no longer flows into the throttle body, and carbon builds up around the throttle body port eventually completely clogging it up. If this happens, even if you fix the problem with the Modulator (and/or the rusted spider pipes) and the EGR valve functions, no gas flows into the throttle body because of the clog and you still have a code.
    Last edited by Jonny; 09-16-2018 at 10:22 PM.

  4. #4
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    Re: Vacuum Modulator, Exhaust Gas Recirculation, Previa

    Ha I love the post for repairing the diaphragm! ^._ .^ thats fun and totally could totally save peoples wallets or be faster when they need it now! or anyone whom is truly in an bind because of where they live in the world, shipping issue etc.

    They charge so much for this thing, I talked to guy a whom replaced the filter with an pice foam of similar density.

    Soo is it holding up do you think? ~ ; ~

  5. #5
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    Re: Vacuum Modulator, Exhaust Gas Recirculation, Previa

    2500 miles later and everything is still fine.

    Here is some further information....
    It seems like vacuum modulators of this type were phased out of Toyota vehicles in the early 2000s. Camrys are plentiful in scrap yards and the '96 - '00 model seems to be the last one with the same style modulator. I bought one from a '00 model to check it out (I think it is part #25870-74090) ...
    Previa on the left, Camry on the right (R port on Camry is broken off).

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    The only difference seems to be the bottom half of the housing - the bottom pipe on the Previa modulator comes off at a different angle to avoid interference, so the Camry one isn't going to be plug and play.

    However, looking at the mounting bracket/spider pipes component, if the two rivets were ground off the modulator holder, the holder could be reattached at an angle and then I think the Camry modulator would fit. Or maybe there's a trick and robust way to make the Camry's bottom pipe take a turn. Some enterprising soul needs to test this out for us and report back!

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