OBD Code 71 EGR Failure? but passed CA SMOG? (1 Viewer)

[FishNinJay]

I have a 1990 FJ62..

Thanks for this! I'll try it.. I'm just wondering how I passed smog in CA with an active OBD Code 71.???
I'm also showing a code 42, which is some kind of speed sensor malfunction. I assume it might be due
to a faulty speedometer cable, which I already know is an issue.

Thanks for posting this.

Having just gone thru this process when my truck failed emissions, I thought I'd write up how I tested my system. I know there are tons of threads on this and the FSM has procedures but this seemed like a simpler process to me. My main testing device was a Mity Vac. It can pull a vacuum and measure vacuum. I turned to this tool because I own Mercedes 123's and virtually everything on them runs via vacuum so I am familiar with the Mity Vac as a testing tool. You will be testing the function of the EGR, the VSV and the modulator.
So if you have a code 71 or p401 follow this sequence.

With the engine idling, remove the vacuum hose from the passenger side nipple on the EGR. Attach and pull a vacuum with the Mity Vac.
Engine should stumble. Squeeze off the hose coming off the driver's side nipple on the EGR. Pull a vacuum. Engine should stall. If it does EGR is fine and passages in intake are probably clear.

If you can't pull a vacuum on the EGR with the driver's side hose squeezed off, diaphragm in EGR has failed and valve needs to be replaced.

Testing VSV--use hose coming off driver's side nipple on EGR. This hose goes THRU the manifold to the VSV. When cold you should not be able to pull a vacuum on this line since VSV should be open to atmosphere. If you can, VSV is stuck closed and should be replaced.
Next step is to T into this same line and run a hose into the cabin so you can monitor vacuum during driving. There should be no vacuum when engine is cold. When engine warms up, you should see vacuum on the gauge as EGR is called for. The conditions for the activation of the EGR vary but typically with a warm engine you should see the gauge move at some point as you accelerate up to speed. If you're not seeing vacuum on the gauge, either the VSV is stuck open or the modulator is bad. so, test the modulator. The amount of vacuum used to operate the EGR system is quite low. Don't expect to see over 5 inches or so.

Modulator test--remove and pull a vacuum on the bottom nipple. It should hold. If it doesn't, it's bad.
Blow into the single nipple on the top side while holding your fingers over the other two nipples. Air should come out thru the center top of the modulator. If it doesn't modulator is bad and needs to be replaced.
If the modulator tests fine and you're still not seeing vacuum while T'd into the line on the driver's side of the EGR, then your VSV is open all the time and needs to be replaced. I used a Dorman 911-850 and relocated it to a more accessible place.

The only other thing, apart from bad wiring, that would cause a code is the temp sensor mounted either in the EGR on early models or in the intake on later models. These rarely fail The tests on the sensor need to be conducted with an ohmeter which looks for high resistance when cold and low resistance when hot. Leave this test until the end. You will likely have found the culprit with the earlier tests.


I'm sure other people have other methods and I know lots of people test the resistance of the VSV. I'm mostly writing this stuff down so I remember what I did if I face the problem again. It took me a while to sort out all the possible tests and come up with what I thought was a simple process. Of course vacuum lines can leak so you need to make sure they are not. Mine were tight.
Chime in if you think I misunderstood or conducted an inadequate test. I don't think I did because I got my system working and no more Code 71.

Here's a link to my original thread where I was trying to figure out the system and how to solve my problem. It has a link to a YouTube video on the EGR valve testing and some pictures of where I installed the Dorman replacement VSV as well as how I temporarily used a BB to in essence close the VSV valve all the time so I could reduce the NOx levels while I awaited the arrival of my new VSV.

https://forum.ih8mud.com/80-series-tech/793426-failed-co-emissionss-high-nox.html

 

[Output Shaft]

Your smog technician didn't follow the rules.

 

[ToyotaDon]

Is the Check Engine light on while the engine is running?