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98 Tacoma 2.7 ltr has P0401 code

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May 31, 2008, 6:39 PM

Post #1 of 7 (5733 views)
98 Tacoma 2.7 ltr has P0401 code Sign In

I've got a code reading of P0401 EGR flow insufficient. I just had a new catalytic converter put on but I think it might be leaking at the weld. Would this cause the fault? If not, does anyone have an idea what it could be? It's a 98 Tacoma 2.7 liter.

(This post was edited by landshark on Jun 1, 2008, 3:18 PM)

Double J
Veteran / Moderator

Jun 1, 2008, 11:38 PM

Post #2 of 7 (5722 views)
Re: 98 Tacoma 2.7 ltr has P0401 code [In reply to] Sign In

After the engine is warmed up, the intake manifold absolute pressure is larger than the value calculated by the ECM while the EGR system is ON (2 trip detection logic).

Possible Problems
EGR valve stuck closed <= Most common Clean EGR valve
EGR Vacuum Switching Valve (VSV)
Open or short in VSV circuit for EGR
EGR valve position sensor open or short circuit
Vacuum or EGR hose disconnected
EGR valve position sensor
Manifold absolute pressure sensor malfunction <=See P0105 ... for testing MAP sensor

The description of the expected voltages for the MAP sensor output (backprobing Terminal 2) in the Haynes manual is incorrect. The voltages listed are not the expected voltages, they are the voltage drops expected from the reference voltage.

With the MAP connector attached and the ignition on and the vacuum line disconnected, measure the reference voltage by backprobing terminals 2 and 1. Measure the voltages at these same connectors while applying different vacuums at the port. If your reference voltage without vacuum is 3 volts (for example), then you should see the following voltages at these vacuums:

3.94 in Hg 2.5-2.7 V [3.0 V (reference voltage) minus 0.5-0.3 V]
7.87 in Hg 2.1-2.3 V (3.0 minus 0.9-0.7 V)
11.81 in Hg 1.7-1.9 V (3.0 minus 1.3-1.1 V)
15.75 in Hg 1.3-1.5 V (3.0 minus 1.7-1.5 V)
19.69 in Hg 0.9-1.1 V (3.0 minus 2.1-1.9 V)

Although your MAP may not exactly match what is listed above, the trend should be the same. I don't think there is anything magical about these absolute numbers, it is having a smooth trend that is important. There is bound to be some variation.

I dont have Toyota this off a website...
Passing it along...
Hope it helps you


Veteran / Moderator

Jun 2, 2008, 2:39 PM

Post #3 of 7 (5711 views)
Re: 98 Tacoma 2.7 ltr has P0401 code [In reply to] Sign In

How bad is this exhaust leak; the replacement converter is it of good quality and proper application?

The EGR system on this uses a vacuum modulator that regulates the vacuum supply to the EGR.
The modulator uses exhaust back-pressure to determine this allowed vacuum.
No or low exhaust back-pressure; no or low vacuum to the EGR valve.

Many of the aftermarket universal converters are a one fits all; not always true or I should say that they fit but are not exactly right. Some converter manufacturers even go as far a stating “free flowing converters” that’s fine if that’s what the application needs.

I cannot say that this is the problem for sure but if it all started after the converter was install; it’s something to think about?


Canadian "EH"


Jun 5, 2008, 11:59 PM

Post #4 of 7 (5703 views)
Re: 98 Tacoma 2.7 ltr has P0401 code [In reply to] Sign In

Another question. The EGR valve moves, but it's kinda stiff. Should it move easily or is it supposed to be a little stiff? Thanks for info.

Veteran / Moderator

Jun 6, 2008, 6:39 AM

Post #5 of 7 (5701 views)
Re: 98 Tacoma 2.7 ltr has P0401 code [In reply to] Sign In

Hard to give an exact answer to that; all depends on what you consider, kind of stiff?
If the valve opens with you applying equal pressure too it and it closes on its own; it’s probably ok?
With the engine running, find a constant vacuum source from one of the vacuum lines. Once you find one, try attaching it to the EGR’s vacuum port; if the valve opens; then for sure it’s ok.
While you’re doing this, you should feel the engine start to misfire and or stall; which is a good thing, because that tells us the EGR and its passageways are clear and exhaust gases are flowing, when the valve is opened.


Canadian "EH"


Jun 6, 2008, 4:11 PM

Post #6 of 7 (5698 views)
Re: 98 Tacoma 2.7 ltr has P0401 code [In reply to] Sign In

"Kind of stiff" to me means I have to use quite a bit of force to move it. In other words, it doesn't move easily. I will try the vacuum test you suggested. Although it seems it would take alot of vacuum to move it. Thanks for your help!

Veteran / Moderator

Jun 7, 2008, 6:55 AM

Post #7 of 7 (5695 views)
Re: 98 Tacoma 2.7 ltr has P0401 code [In reply to] Sign In

By your description it does sound like there might be a problem with the valve but at the same time, it’s amazing how strong vacuum and vacuum motors can be?


Canadian "EH"

98 Tacoma 2.7 ltr has P0401 code

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