EGR Vacuum Modulator - Sensitive Functional Bench Test & Alternative Part Numbers (1 Viewer)


Apr 11, 2005
Chandler, AZ
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:


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.


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



Last edited:


Apr 11, 2005
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.

Last edited:

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top Bottom