'88 silverado fails tx NOx emissions test

[wolcott]

My buddy called me up today from SE Texas. He has a '88 chevy silverado with 133k miles with a 350 throttle-body-injection, smog pump, and headers. He got a rejection sticker for high NOx. Last year he passed easily. Vehicle lateley had an oil change, new filters, new pcv valve, new plugs, and half-ass $40 wires from Autozone. he says it runs good. Low speed limit on NOx was 11,094 and the sniffer said 16,059 PPM. High speed limit was 1098 PPM and the sniffer said 1500 PPM. 1)Anyone have any ideas on what may have caused it to fail this year? He got it nice and hot and put in passing gear a few times before he got to inspection station and let it idle while awaiting his turn. 2)Could it be the EGR valve? 3)A lazy O2 sensor with those inexpensive wires? All opinions appreciated. TIA

[upflying]

High NOX is caused by a bad EGR valve or a vacuum leak. Basically combustion temperatures are too high. EGR cools the air-fuel mixture and a vacuum leak raises temps by adding oxygen. Check the base gasket under the throttle body for leaks by spraying WD-40 on it while it's running. An increase in RPM means you found the leak.

[smiller]

EGR valves can get pretty crusty so a stuck valve is a possibility, but there's often a lot of stuff in the vacuum path to the EGR these days (vacuum switches, modulators, differential pressure sensors, etc.) so a lot of possibilities.

 

A quick way to check the valve is to apply manifold vacuum to it while at idle. If it is working properly the engine should start to run very rough or stall.