1997 Tahoe 98K miles in California
(new factory plugs, originals looked good.)

Smog Test Results (upper 15mph/lower25mph)

Speed CO2% O2%
15mph 14.6 0.5
25mph 15.0 0.1

HCppm
103 (10) 79 *note: high side
92 (8) 25

CO%
0.58 (0.02) 0.53 *note: high side but matches O2%
1.02 (0.02) 0.12

NOppm
986 (103) 933 *note: high side
846 (97) 192

= Max. CA Allowable (avg. for like vehicles) Measured

So it did pass but barely at 15mph. I have started to check each of the sensors. I'm not the type to throw parts at it hoping to fix it.

I'm also having to slightly touch the throttle when doing cold start. This is new, usually starts with a flick of the key. Cleaned Air Idle Control (AIC), initially helped but not on second cleaning.

I have checked the ECT (engine coolant temp) sensor and it's OK. The MAP (manifold pressure) appears ok reading 1.2V at idle then 3+ reving. Measure resistance on both AIC coils 50ohms each. IAT (incoming air temp sensor) reads 1.8K ohms (my Chiltons doesn't say so I'll get shop manual at Library). Measured TPS (throttle position) and looks OK.


Looking for O2 sensors. I don't see on manifold, maybe on block. The O2 sensors are not likely the start-up problem as they appear out of loop until up to temp.

Any smog expert got ideas ?

I am aware of the CSFI injector clogging problem and have used Techron to keep under control. Vehicle now at 117K. No ECU codes.

The high NOx can be 1) EGR 2) lean 3) engine running hot 4) timing too advanced 5) high compression or engine problem

The hard starting cold could be EGR or the solonoid if it is opening cold. EGR only operates when warm and at part throttle.

What is the fuel pressure at idle? If the pressure is low, it can cause a lean condition.

//2000CAYukon

You need to talk to fireplug.Hes into this stuff