EGR Insufficient Flow Code and MPG

[96v6wagon]

I've been concerned about poor MPG on my 96 v6 wagon, and idle is a little high, about 850 or maybe a little higher, and not as smooth as it used to be. Granted it is about 160k, but has always been meticulously maintained, since I bought it in 1999. I've suspected O2 sensor, and/or ECT sensor, but mechanic said they would throw a code if they were bad.

So, just tinkering one day, I checked the flow of the EGR Vacuum modulator, by applying a small amount of compressed air, according to the manual instructions, and the flow of the VSVs, also according to the manual. After that, the CEL came on, and mechanic said it was code P-0401 Insufficient EGR flow. He said the EGR vacuum switching valve (VSV) was stuck closed, and appplied 12V to reopen it. Reset the computer. 67 miles later, the CEL came on again. He checked and it was the same code.

He thinks that the EGR may be plugged (maybe because i dislodged some crud by blowing air thru the system, and sugggests cleaning the EGR and ports.

I don't want to start down this bunny trail of checking/cleaning this and that, resetting computer, paying $$, code comes back, and starting over. So, my questions:
1. Do you think i may have caused this problem in the first place, not that it really matters now, I guess.
2. Do you think it is really the VSV that is at fault since it was stuck closed to begin with?
3. According to the manual, there are 3 VSVs on the IMZFE. Is one in particular responsible for the system, or could it be one of the other 2. In other words, which VSV is in charge? Manual, and other things I've read always refer to "the VSV," but there are three!
4. Does the EGR system not functioning properly affect MPG? I've been getting much worse MPG for the last several months. I'd say a drop of about 25%.
5. Would it be ok to let this EGR problem go for a month or so before getting it fixed? It will have to be done eventually to get past Calif. smog test, but would like to put it off for a while to collect $$ for what may be a long trial and error process.
6. Or, why not just bite the bullet and replace the EGR, Vacuum modulator, and whichever VSV is in charge? Might that not be cheaper in the long run, both in time and money?

Thanks everyone; as always, I appreciate your opinions!

[fenixus]

slow o2 sensors will NOT throw a code (they will screw up MPG easily and offset the fuel trims). they must be terribly off the grid to throw a code (slow response code) or be damaged (circuit malfunction code), other than that they can still work and quietly kill the MPG by means of improper feed to ECU which relies on them greatly.

bad ECT for EFI sensor will NOT EVER throw a code, it's one of primary feeds to ECU and is not monitored as far as I know. instead it will be reporting false coolant temperature to the computer thus fooling it.

search for a thread by Nervous, he did some extensive diagnostics on '96 V6 EGR (1mz-fe) which includes troubleshooting and cleaning of stuff.

if you have lots of spare time then go down the precise troubleshooting route and find what fails exactly. it will cost you less this way.
if you want to fix it fast, then just replace the forsaken trio (valve, modulator and vacuum switch valve) and be done with it, just don't forget to check all vacuum tubing for air/vacuum leaks and restrictions too.

[96v6wagon]

Thank you, fenixus. Interesting about the O2 sensor and ECT. I'll bet that's cause of the MPG problem I'm having.

I'll also look up the diagnostics done by Nervous. Thanks for that tip.

Which VCV would I replace? There are three, aren't there? I'm assuming it's the one that's by itself on the rear (towards the rear of the engine compartment) of the "emission control valve rail"?

Do you think I can run the car with this code for a month or so without causing some other problem?

[fenixus]

here's the thread I mentioned, just googled it:
DIY: 1MZ, EGR, code P0401 & P0402 diagnostic & repair

it has all the answers you are looking for.

it should be fairly safe to run it as is for some time, just get it done when ready.