failed smog

[stephen p luce]

92 K1500 5.7 TBI failed calif smog test due to high NOX measured 716 at 15mph and 572 at 25mph. All other specs well inside the fail window. Any ideas as where to start? EGR leaking?

 

[RichLo]

Yea, the EGR is the main thing that helps reduce NOx. Since there arent any check engine lights you can try taking it off and cleaning the orfices and hopefully its just a clogged artery.

And I'm sure you know this better than anybody having to go through SMOGs regularly but make sure the truck is completely up to temperature before going in for the test. And if your previous one had some borderline results, run it like John Force immediately before going in for the test.

 

[PlayingWithTBI]

Since there arent any check engine lights you can try taking it off and cleaning the orfices and hopefully its just a clogged artery.

^^^This, worst case scenario, you may have to pull the intake manifold to get the passages cleaned out.

Here's what it looks like, the blue arrows point to access ports to clean the underside of the EGR valve. The red arrow points to the actual passageway the exhaust gasses pass to the EGR.

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I'd pull the EGR valve and inspect them 1st.

 

[Schurkey]

In MY driveway, the inspection would start by verifying the vacuum hoses and solenoid. Have the vacuum hoses fallen off, or split? Connected to the proper ports on the TBI and the solenoid? The solenoid often looks like it has three nipples. It does not. The third "nipple" is supposed to be covered with a little foam air filter, but the foam has long-ago degraded and disappeared. Then someone puts the vacuum hose on the wrong "nipple", and the EGR doesn't work any more.

Does the solenoid get electrical signal to turn "on"? When "on", does it pass vacuum?

Does the vacuum diaphragm of the EGR valve hold vacuum? Reach under it with your fingers, then use a vacuum tester to draw a vacuum on the diaphragm. Can you feel the diaphragm/pintle move upward?

If everything is proven to work, THEN start cleaning passages.

Wouldn't hurt to verify ignition timing, fuel trim numbers/fuel pressure, assure there are no vacuum leaks, and assure that the engine isn't overheating, as well.

 

[GoToGuy]

One year my K2500 350, failed same thing, in my case the vacuum diaphram on the egr has leaking area, small tear. Replaced egr cleaned passage somewhat, recheck passed. That was my smog fail 7 years ago. Thank you calif smog Nazi's.

 

[stephen p luce]

Sorry for the late reply, we've been out of town for a few weeks. Anyway I tested the egr system and everything worked as it should, then checker temp in front and behind the cat ant the temps were the same. Cut the cat out and the honeycomb was breaking apart. New cat fixed it.