Failed CO emissionss--high NOx

[TrickyT]

Just to throw out a few other possibilities: 1) Old vacuum lines tend to crack and leak at their connection points on the manifold. New rubber can be had from CDan and Beno on the cheap. 2) The EGR VSV is notorious for testing good when the engine is cold, but then failing after it heats up. This makes accurate testing a challenge. 3) The vacuum modulator is the "brains" of the EGR system, and basically functions as an analog computer. So look carefully at the PDF Pin_Head posted to understand how the system operates.

 

[TrickyT]

I got another pdf from another thread with testing procedures. It's a 1996 EGR inspection procedures pdf. It includes tests for the operation of the VSV and the modulator. Modulator passed those tests. So with a new EGR and proven modulator I think I've narrowed it down to either the VSV or temp sensor.

There's a connector on the top of the intake manifold that you can use to easily access the electrical connection to the VSV. (The actual VSV is located underneath the intake manifold and much more difficult to get to.) When my EGR VSV failed a few years ago it would test good (i.e., a few ohms continuity) when cold, but open circuit after the engine heated up to normal operating temp.

 

[hadmatter]

Kerrye,
Would you mind if I stopped by some time next week to see what you have done so far? I need to address this same issue on our truck. I've read several threads, but seeing it in person would really help me understand how to get to the problem.

I don't have any technical advice to offer you, but could lend a second set of hands if that would be helpful (and I have a garage in Centennial that can be heated, if you wanted to roll down to the burbs). If you wind up having to remove the intake, I'd love to see how that's done.

 

[kerrye]

You're welcome to stop by. It is much easier once you seen the various parts and how they interrelate. I had seen that thread about how to test the VSV from the connector and I will probably do it. But the next easiest thing to do is to test the temp sender I think. I should have done it when the EGR was off but it's not that difficult to remove with the EGR in place I think. If the temp sender passes the test then it's only the VSV left. My tests so far indicate the VSV is open at idle which is as it should be I think, venting the vacuum to atmosphere. But that interpretation is at odds with the youtube EGR test which implies it should be closed.

 

[Pin_Head]

But that interpretation is at odds with the youtube EGR test which implies it should be closed.

The youtube video is vague as to what line he is applying vacuum to and what else is happening when he pinches off the tube. The FSM is very specific about what you are testing and what the results mean.

 

[kerrye]

Looks to me as if he's applying vacuum to the passenger side nipple on the EGR. It makes the engine stumble. Then he pinches the hose coming off the driver's side nipple on the EGR and the engine stops. I take this to mean that the EGR is moving a little bit with the first step and enough in the second step to stall the engine.
He then pinches the hose under the intake and applies vacuum to the passenger side nipple and the engine stalls again. I think this test is essentially the same as when he pinches the hose above the intake, he's just pinching it further downstream.

 

[86tuning]

Verify that the vsv is operational. You can remove it to test, or actuate it with Toyota techstream or TDT.

A failed vsv will cause egr low flow because it won't allow the egr to be actuated by the engine computer.

 

[kerrye]

Anyone tested the temp sensor while on the engine? Seems possible to me. Test I've seen involves removing it an immersing the probe in boiling fluids. Cold engine it should be high resistance and hot engine it should be low resistance?? Open and it's bad.

 

[Timpanogos Slim]

never tried it but if you have insulated gator clips, why not?

 

[kerrye]

Wife just got home from work. Engine was warm. Disconnected the wire to the temp sensor. Ohmeter showed no connection between the two wire. Temp sensor must be bad.

 

[Pin_Head]

The FSM says that the resistance at 68F is less than 600,000 ohms and after warming and running at 4,000 RPM for 3 minutes the resistance should be 4,300 ohm or less. Depending on your ohm meter and the scale you choose, k ohms may look like no connection, so don't get fooled on that.

 

[kerrye]

I had it set at 200 ohms but I think the important fact was that the screen did nothing when I touched the probes to the pins. It stayed just the way it was when the probes were apart. I'm not that sharp on electricity though, so I could be fooled.

 

[kerrye]

ok, just re-tested it using all the ohm scales. Results were the same. Infinite resistance. The two pins are not connected anymore.

 

[TrickyT]

ok, just re-tested it using all the ohm scales. Results were the same. Infinite resistance. The two pins are not connected anymore.

Certainly sounds like the likely culprit then. I'm not sure which year FSM Pin Head got his resistance values from, but my 1997 manual says between 2K ohms at 302*F and 97K ohms at 122*F.

 

[kerrye]

Here's the other pdf I've been using for diagnostics. I found it in another thread but can't refind that thread again so I'll post it here.

 

[TrickyT]

As I think more about it, it will be interesting to see if a new temperature sensor solves your high NOx problem. I bet it will address the 71 code, but it was my understanding that the sensor was just there to provide feedback to the ECU that the EGR system was functioning correctly. In other words, you were getting a 71 code because the ECU thought the system wasn't working. If you still have high NOx after replacing the EGR valve, I'm not sure the temp sensor will change that.

 

[ToyotaDon]

Auto-ranging Fluke DVOMs are nice, and I've seen people think a part was faulty when testing it with a non-auto-ranging low end meter. What's the highest range on your meter?

X2 on the temp sensor being there to monitor the system. You may still have a faulty VSV as well.

 

[kerrye]

2000k is the highest range. My EGR valve was bad so that easily accounts for the high NOx I think. I haven't got it retested yet so I don't know if replacing the EGR alone fixed the high NOx reading. However, I seem to recall reading that the temp sensor also controlled the operation of the VSV, keeping it open to atmosphere in certain conditions. So I was thinking that if the temp sensor wasn't sending it's signal the VSV might be open to atmosphere under all conditions which would mean the EGR valve wouldn't operate. I could T in a vacuum gauge to the line which controls the EGR and see if it ever reads vacuum. If it does, I think that would prove that a malfunctioning temp sensor wouldn't stop the EGR from operating. But the idea that the temp sensor is just a monitor makes sense in that I have passed multiple emissions tests with a Code 71. This suggest the EGR has actually been working even with the code. So it's quite possible that my EGR valve failed since the last emissions test and that's the only 'real' problem that has caused the high NOx reading. I have 10 to get a free retest so I think I have time to get the temp sensor and install it before retesting.

 

[kerrye]

Just did some more testing. (temp sensor is unplugged since I'm awaiting a new one) T'd in my Mity Vac gauge to the EGR vacuum lines and drove around. No measurable vacuum under any conditions. (just a slight miniscule movement of the gauge when first accelerating). I then pinched off the line going from the EGR to the VSV between the EGR and the intake manifold. Voila! Vacuum on the EGR while driving. Maybe 4 inches max but I think that's enough to activate the EGR.

So, either the VSV needs a temp signal to close or it's stuck open to atmosphere. I'll know which it is when my temp sensor arrives. However, in the meantime, something occurred to me. If I plug the line from the EGR to the VSV with a bb or something so the EGR valve gets vacuum, will it reduce the NOx emissions so it can pass the emissions test? Will a functional EGR without the VSV cause any other combustion weirdness which might cause it to fail the HC part of the emissions test or something? I'm pretty sure this won't get rid of the Check Engine light and the Code 71 but if I clear it, it should stay clear long enough for the test. Has anyone tried this?
I get one free test. I'm pretty sure my temp sensor will get here in time to install before the 10 days are up on the free test, but I won't have a VSV by then. Hence my interest in passing the test without a functioning VSV.

 

[TrickyT]

Can't answer all your questions, but you should be able to test the EGR VSV in situ. There's a connector on the top of the air intake chamber that connects to the VSV. You can disconnect this connector and then apply 12v to the VSV. That should close the VSV and prevent air going through it. You can use your Mity Vac to make sure the VSV isn't leaking when closed.