93 4Runner SR5 V6 Fail on emissions test @ NO(PPM) 15mph. Pass @25mph
Max Avg Meas
356 113 747 @15mph. FAIL
533 150 280 @25mph. PASS
Timing @ 11
any info at this time would help

 

NOx is controlled by the EGR system. That's what it does. That's all it does.

NOx is slightly affected by the Catalytic converter, so look at HC and CO while you're at it. If they'reboth low, then concentrate on the EGR.

In addition to the testing in the manual, http://personal.utulsa.edu/~nathan-b...77exhaustg.pdf, you should consider that your plenum (upper intake manifold) may have so much "gunk" in it that it is constricting the tube into the plenum from the EGR. If completely blocked the EGR temp sensor should throw a code, but if just restricted could easily cause your problem. In my case, it was difficult to pull the tube out of the plenum because of the build up (should be about 2mm clearance all-around).

 

Yep! All other emissions are way low. Recently replaced both head gaskets a few months back after blowing the RS HG. Could not figure out how to get the sensor you mentioned out of the Intake Manifold. What do you suggest? and yes that sensor looked gunked!

 

It's not a sensor. The part I'm referring to is part of the assembly labelled "EGR valve" here: http://personal.utulsa.edu/~nathan-b...68cylinder.pdf The part that extends into the plenum has a "union" connecting it to the valve. Once you unbolt enough to get some wiggle room, you can remove that union and the nuts holding the pipe to the plenum. It should come right out, but if there is enough gunk on it you may have to slide it back and forth until you knock off enough.

If you have that much, you'll need to remove the plenum to clean it. I suggest removing the "freeze plugs" at the ends so you can get inside (I didn't, but I should have).

 

Might be good to also make sure all the vacuum lines for the EGR are hooked up correctly and not cracked or damaged. If they are wrong, basically your EGR isn't recirculating the gases like it should be.

I don't know if this will help either, but if the catalytic converter is cold during the test you'll get higher HC reading since heat helps convert it. If it's hot, you'll get higher NOx readings. Doing the test with the engine cold will get you lower NOx readings (and higher HC) but not that much to fix your current issue.

 

Bad CAT, He mentioned blown HG, Antifreeze messes up CATs.
Suggest new O2 sensor as well.

 

yep its the Cat EGR was clean. The part that goes into the exhaust manifold was caked took a bit to clean out the manifold. gave the engine better response at lower rpm still lacking in power in high rpm. retest came back still high NOx only at 15mph and that's all that was high

 

You might also find this page helpful, taken out of the FSM:

http://personal.utulsa.edu/~nathan-b...72systempu.pdf.

But yeah, if the EGR is good and you're still getting high NOx, the cat isn't doing its job.

 

Is the egr actually working? Did you test the modulator? Did you check the metal vacuum tubes that run under the plenum? Did you test the egr valve with a vacuum pump?