Problem with EGR System?

Been having issues with diagnosing a trouble code involving the EGR System on my 97 Previa, and wondering if anyone out there has had a similar issure or any ideas. The DTC is #P0401, "Exhaust Gas Recirculation Flow Insufficien Detected".

It first came on last fall during a road trip in the mountains, and the mechanic i took it to thought it might be caused by the high elevation. They reset it, and it stayed off until early this summer. Since it came on again, i've been intermittently trying to fix it. Seems to be running fine, and the gas milage is as good as its ever been ( i've read that low mpg's would be a symptom?).

My first step in diagnosing it was the following the system check in my Chilton's manual on pg. 4-13. I checked the Vacuum Modulator filter and it was filthy, so i replaced the whole part. Reset the code (by disconnecting the battery), but the check engine light came on again within an hour of driving.

Next i checked the egr parts using a vacuum gauge as outlined in the system check ( same as the one in the Toyota Repair manual on pg # EG-125). However i dont have a ODBII scan tool or a tachometer, so i guestamated the rpms through the process. They all seemed to be fine until checking the EGR Valve. The engine was supposed to die after bridging the EGR valve to the intake manifold, but after a couple minutes was still running fine. I then took the valve off, cleaned it, reinstalled it, and drove it for a while, but the code came up again. I thought i found the problem and bought a new EGR valve for about $150. Reset the code, drove it, but it still came up again.

Since then i bought the toyota repair manual and have been going through the steps outlined for the P0401 code on pg. EG-279. Checked the gas temp. sensor while installed, took it off and did the heated oil tests and the resistance values were pretty close.the harness was also good. Connection of the EGR vacuum hoses also checked out. Next i looked at the VSV for the egr. Didn't mess with its operation thru the ECM ( was warned its very easy to break), but followed the steps on pg. EG-127. It checked out good for both an open circuit and ground. Blowing thru pipe E I couldn't really feel it at the air filter, but could hear air escaping somewhere. Next i hooked it up to the van's battery. i couldn't get it to blow any air out, it would just make a clicking sound when the wires touched its terminals. So now i'm figuring that the VSV is bad? Checked with my local Toyota dealership and they can get a VSV for about $140ish.

So before I buy another part i don't need, I'm wondering if anyone thinks my diagnosis is correct or if i'm missing something here. Also, wondering if there's any possible bad results from continuing to drive it while the problem exist? And sorry for such a long question, thanks!
:cnfsd:

It sounds to me like your are pretty thorough in your evaluation of the issue. As you may already know, VSV's block or allow air flow depending on if they are powered or not. Some block flow when powered but others allow flow. Some allow air (or vacuum) to pass through or vent to atmosphere. I'm not familiar with the particular VSV you are dealing with, but when powered it should do the exact opposite of what it does when not powered. The clicking when powered means it's good electrically, but it could still have a mechanical issue or might be plugged up.

When dealing with this sort of issue I find salvage yards (particularly u-pull-it's) to be my best friend. Why pay $140 for that VSV when you could most likely get a good used one for $5? Sometimes when dealing with difficult diagnosis I'll just pull every part I think it could be and change it all. Yes it's sloppy diagnosis, and yes it could be considered cheating, but sometimes it's the easiest and cheapest way to go.

Another thing that can cause EGR faults is a build up of crud blocking passages. I haven't dealt with this on a Previa yet, so I don't know where the most likely spot would be. Just make sure these are all clear. Good luck. Tim

Thanks for the info Timsrv!
At first i had thought the VSV was actually supposed to be pushing air out when powered up. But after reading your response and re-reading the manual, your right its just redirecting the incoming airflow. After retesting it, the VSV looks like its working fine. So now i'm thinking i'm gona re-examine the gas temp. sensor, try to find one at the junkyard, replace it, and see what happens. And also do a more thorough inspection on all the vaacum hoses.

I'd suggest pulling the throttle body. If I recall, there is a passage for the EGR system that gets blocked with crud (as Tim defined).

Good call Tombat, thanks! Been without internet for a while, so havn't been able to update this thread for a while. Got the throttlebody cleaned out (was filthy) & the vacuum hoses to it, and that seems to have solved the problem. The trouble code cleared itself after a few days of driving & havn't seen the check-engine light since!
:thmbup:

I realize this is an old thread but you stated you purchased a new egr modulator. Do you happen to know where you got it and the make/part number? The aftermarket ones specifically for a toyota van are nearly $100 but other ones that look similar and are for Toyotas, cost about $15. Obviously, I would rather pay 15 but am not sure they will work the same.

Flippo, although principals stay the same, Previa parts are typically quite different from van parts. In some cases the same (or interchangeable) parts may be used, but in most cases the Previas are quite different from the vans. I saw your other posts in the van section and I did a few checks, but cannot confirm or deny the other similar looking EGR modulator you found would interchange (even if diameter and configuration of ports are the same, function of ports may be rearranged). I guess for $15 it might be worth picking one up and comparing. Who knows, perhaps it's similar enough to do the job. Tim

I cleaned and repaired my old one and it seems to work fine. I occluded the two ports, blew air into the single port and it came out of the bottom. That's the test the manual says will show if it is good or not. I did buy a cheaper one. Ill keep it as a spare since it is a little smaller. Now I just need to find a replacement filter or make one. Do you happen to know where an ERG modulater filter can be purchased or what it is made of cut a new one myself? I also cleaned the ERG valve and it seems to be functioning correctly. It is holding a vacuum. Is the only purpose of the modulator to be the filter for the ERG?

Good to hear the old egr moduater is working fine, its been a while & not sure where I bought the new modulator from now. If I didn't get it from one of the local Pick-n-Pull junk yards, it was probably from either RockAuto.com or Toyomotorparts.com, both of which ive got a lot of stuff for the previa from. They also might carry the egr filter your looking for?