Stubborn P0401

[agni]

The car is a '96 Camry LE, 5SFE, Federal emissions, 150k miles

For about six weeks now I have been grappling with this code (insufficient EGR flow). I know this is a very common issue with these cars. I have done much searching, testing, and swapping out of parts, but the code remains. Whenever I clear it, it shows up again after 10-15 miles of city driving. However, it does not come on during highway driving - I have driven up to 300 miles on the interstate without a CEL, only to have it show up after exiting for gas.

What I have done so far:
1)Tested VSV - found that valve was sticking/broken, did not engage when voltage was applied.

2)Replaced VSV - original one was still under warranty from NAPA (my original VSV went bad about a year ago). I also tested the replacement before installing it.

3)Temporarily swapped VSV with a tested OEM one from a low-mileage junkyard car.

4)Replaced all EGR vacuum lines

5)Tested EGR valve operation - applied vacuum with engine idling and it quickly stalled. I am confident that the valve and tube are clean because I removed and cleaned them myself only 6 months ago.

6)Tested EGR for vacuum leaks (none)

7)Cleaned vacuum ports on throttle body and checked for presence of vacuum.

8)Swapped out modulator three times with ones from the junkyard.



At this point, the only place I have left to look is the electrical wiring to the VSV, unless someone has other suggestions/observations. My manual does not have the correct wiring diagrams, so I'm not sure how to go about testing the wires and/or ECU outputs. If someone could give me some tips here I would really appreciate it. Thanks for reading!

[JohnGD]

Tee or just connect the hose at the EGR valve to the MityVac. Run it at 1000, 2000, and 3000 rpms. What are your EGR readings with a warm engine and TE1, E1 jumpered (puts into default mode).

Does the vacuum rise to about 5 in/Hg at 3000?

[agni]

Originally Posted by JohnGD

Tee or just connect the hose at the EGR valve to the MityVac. Run it at 1000, 2000, and 3000 rpms. What are your EGR readings with a warm engine and TE1, E1 jumpered (puts into default mode).

Does the vacuum rise to about 5 in/Hg at 3000?

Thanks for the reply. I've been meaning to do this but haven't had a gauge on hand. I'll get one tonight and report back soon.

[agni]

Just got done testing the vacuum. I measured only 2" @ 3000rpm. Connecting the gauge directly to port R of the throttle body, I measured the same 2" @ 3000rpm. The FSM is not specific about the required vacuum here, it just says "low vacuum" . . . but you say 5" is the minimum?

I sprayed plenty of TB cleaner to wash out any possible blockages in the port but the results were the same. Could it be that there is some problem with the throttle body? Or maybe may engine isn't creating enough vacuum? (but wouldn't I at least have a MAP sensor code among others?)

As the FSM suggests, I also connected port R of the modulator directly to the manifold; doing this I measured 8" at 3000 rpm. Again, they are very vague and just say that a "high vacuum" should be observed.

[csaxon]

If the EGR VSV/EGR modulator/EGR valve and all vacuum lines and passages in the throttle body are good, then the MAP sensor would be the next thing to check. The MAP sensor reacts to the pressure drop of the EGR valve opening. If it doesn’t sense the pressure drop it sends the P0401 code,

[agni]

Originally Posted by csaxon

If the EGR VSV/EGR modulator/EGR valve and all vacuum lines and passages in the throttle body are good, then the MAP sensor would be the next thing to check. The MAP sensor reacts to the pressure drop of the EGR valve opening. If it doesn’t sense the pressure drop it sends the P0401 code,

Just tested the MAP sensor (using a vacuum pump and measuring voltage at the ECU) - all voltages readings were in spec according to the FSM.

I also did a vacuum check on the intake manifold. Held steady at 19 in/Hg during idle. Stayed the same throughout slow acceleration up to 2500 rpm, then quickly jumped to 22 and back down to 19 when I let off the throttle. Quick opening and closing of the throttle caused a sharp jump to around 5 then back to 19. So I think my manifold vacuum is fine?

Not really sure where to go from here. What kind of vacuum should I get from the ports on the TB? Also, how do I test the wiring to the VSV? They look fine at the connector, but maybe they are frayed somewhere?