EGR Vacuum Modulator - Sensitive Functional Bench Test & Alternative Part Numbers

Bambusiero

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I've been getting intermittent P0401 error code (Insufficient EGR Flow), so checking the vacuum modulator using the FSM test procedure.
Apparently, I can't tell the difference between air passing through to the filter side "freely" and "with strong resistance".
So - convinced the modulator was bad (with strong resistance), I bought 3 used units on ebay.
Some have installed alternative part numbers from other Toyota cars, and said it worked fine.

What's the difference between similar looking units used on different cars?
And - what does this little gadget actually do, anyway?
I had to see.

All 3 units, and my original, reacted exactly the same to the FSM "freely" and "with strong resistance" airflow test, sooo...

The FSM test is useless for me.
Needed something definitive - Here it is:

1939590


A picture is worth a thousand words, as they say, so I'll let it.
Made on the fly with cheap common stuff found around the house.

- Before bothering with anything deeper -
Check that the diaphragm is good with a MityVac hand vacuum pump on the bottom Exhaust / Pressure port.
Must hold vacuum.
Easy, definitive, takes a few seconds.

Q: What does the vacuum modulator actually do?
A: It gives feed back information about actual exhaust gas pressure to the EGR system.
Anything above moderate RPMs or throttle position shuts off Exhaust Gas Recirculation.
The shutoff point seems to be 3-5 [inches H2O] exhaust gas pressure. (Way less than 1psi)

The test rig shown above will tell you what exhaust gas pressure causes EGR shutoff, with your particular vacuum modulator.

EGR Vacuum Modulator - Port Description
- Exhaust / Pressure Port (bottom center):
Pressure here pushes the diaphragm up, against spring pressure, isolating P/Q port from R-Port and Vent/Filter.
- P/Q-Port:
Connected to intake manifold vacuum.
Straight pass through to each other.
Open to R-Port and Vent/Filter if exhaust pressure is low.
Closed to R-Port and Vent/Filter if exhaust pressure is high.
- R-Port:
Open to Vent/Filter

Here's the insides of the Camry unit.
I think they're all practically identical, except possible fine tuning the spring / exhaust cutoff pressure.

EGR_VacuumModulator_Insides_Top.JPG


There's also a fiber filter on top (not shown)

EGR_VacuumModulator_Insides_TopClose.JPG


EGR_VacuumModulator_Insides_Bottom.JPG


EGR_VacuumModulator_Insides_BottomClose.JPG
 
Last edited:

Bambusiero

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Joined
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Messages
836
Location
Chandler, AZ
Alternative part numbers:
My original OEM Landcruiser green top unit cuts off at 3.0" H2O, a bit lower than the other 3, but not all that much lower.
How much do they vary unit to unit? Don't know. I only have the one.
The Camry units seem like a good choice.
Always available, inexpensive, not too much higher pressure.
Might keep EGR on until a bit higher RPM - doubt you would notice.

1939315
 
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Joined
Jan 17, 2017
Messages
389
Location
PNW, USA
I've been getting intermittent P0401 error code (Insufficient EGR Flow), so checking the vacuum modulator using the FSM test procedure.
Apparently, I can't tell the difference between air passing through to the filter side "freely" and "with strong resistance".
So - convinced the modulator was bad (with strong resistance), I bought 3 used units on ebay.
Some have installed alternative part numbers from other Toyota cars, and said it worked fine.

What's the difference between similar looking units used on different cars?
And - what does this little gadget actually do, anyway?
I had to see.

All 3 units, and my original, reacted exactly the same to the FSM "freely" and "with strong resistance" airflow test, sooo...

The FSM test is useless for me.
Needed something definitive - Here it is:

View attachment 1939590

A picture is worth a thousand words, as they say, so I'll let it.
Made on the fly with cheap common stuff found around the house.

- Before bothering with anything deeper -
Check that the diaphragm is good with a MityVac hand vacuum pump on the bottom Exhaust / Pressure port.
Must hold vacuum.
Easy, definitive, takes a few seconds.

I tried that test on a brand new unit from Toyota and it slowly lost vacuum - does that mean it is bad? Had vac pump connected to bottom only and pulled 10-15 mmHg. Held but slowly went back to zero.
 

Bambusiero

SILVER Star
Joined
Apr 11, 2005
Messages
836
Location
Chandler, AZ
I tried that test on a brand new unit from Toyota and it slowly lost vacuum - does that mean it is bad? Had vac pump connected to bottom only and pulled 10-15 mmHg. Held but slowly went back to zero.
Well...I think so. If the diaphram has a leak, it cannot function properly the way it was designed to. Play with it until you are sure. double check your vacuum tubing for old brittle cracks, etc. Or, the insides of your vacuum pump. Mine will hold vacuum indefinitely, but I did take it apart, clean & re-grease.
 
Joined
Mar 2, 2019
Messages
19
Location
Nebraska
I've been getting intermittent P0401 error code (Insufficient EGR Flow), so checking the vacuum modulator using the FSM test procedure.
Apparently, I can't tell the difference between air passing through to the filter side "freely" and "with strong resistance".
So - convinced the modulator was bad (with strong resistance), I bought 3 used units on ebay.
Some have installed alternative part numbers from other Toyota cars, and said it worked fine.

What's the difference between similar looking units used on different cars?
And - what does this little gadget actually do, anyway?
I had to see.

All 3 units, and my original, reacted exactly the same to the FSM "freely" and "with strong resistance" airflow test, sooo...

The FSM test is useless for me.
Needed something definitive - Here it is:

View attachment 1939590

A picture is worth a thousand words, as they say, so I'll let it.
Made on the fly with cheap common stuff found around the house.

- Before bothering with anything deeper -
Check that the diaphragm is good with a MityVac hand vacuum pump on the bottom Exhaust / Pressure port.
Must hold vacuum.
Easy, definitive, takes a few seconds.

Q: What does the vacuum modulator actually do?
A: It gives feed back information about actual exhaust gas pressure to the EGR system.
Anything above moderate RPMs or throttle position shuts off Exhaust Gas Recirculation.
The shutoff point seems to be 3-5 [inches H2O] exhaust gas pressure. (Way less than 1psi)

The test rig shown above will tell you what exhaust gas pressure causes EGR shutoff, with your particular vacuum modulator.

EGR Vacuum Modulator - Port Description
- Exhaust / Pressure Port (bottom center):
Pressure here pushes the diaphragm up, against spring pressure, isolating P/Q port from R-Port and Vent/Filter.
- P/Q-Port:
Connected to intake manifold vacuum.
Straight pass through to each other.
Open to R-Port and Vent/Filter if exhaust pressure is low.
Closed to R-Port and Vent/Filter if exhaust pressure is high.
- R-Port:
Open to Vent/Filter

Here's the insides of the Camry unit.
I think they're all practically identical, except possible fine tuning the spring / exhaust cutoff pressure.

View attachment 1939309

There's also a fiber filter on top (not shown)

View attachment 1939310

View attachment 1939311

View attachment 1939312
Thanks for this post, it was very helpful. I agree, the FSM test gives an unclear result. I have the blue top modulator. I couldn't see how to block the vent like you showed on your diagram. When I blocked P, air stopped flowing out of R at about 6" of water.
 
Last edited:

Bambusiero

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Joined
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Messages
836
Location
Chandler, AZ
You block the vent by just putting your finger on this hole (which might not be visible if the cap and a wad of filter is over it).
That leave only the "R" nipple hole to allow air in and out of the closed chamber above the diaphragm, as the diaphragm moves.
Correction: Sorry - I partly misstated that about what the "R" nipple hole (and the vent hole) does. (well - it does do what I said there, but also:
If there is no pressure (or insufficient pressure) into the exhaust pressure port, so that the diaphragm does not push up and close the valve, then the air from the aquarium pump can leak out through The "R" nipple (and/or the vent), instead of going straight through and out the "P" nipple and bubbling your water glass flow indicator.
Can't remember every detail now - been too long - but anyway - I did have to block the vent hole to make this test work.
1639251592457.png
 
Last edited:
Joined
Mar 2, 2019
Messages
19
Location
Nebraska
You block the vent by just putting your finger on this hole (which might not be visible if there is a wad of filter over it).
That leave only the "R" nipple hole to allow air in and out of the closed chamber above the diaphragm, as the diaphragm moves.
View attachment 2862031
I didn’t know you could take the cover off without breaking it. Okay, I’ll give it a try, thanks!
 

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