Any help is appreciated;

1991 GMC Vandura, 5.7L

High Speed Low Speed

HC 438 (220 standard) 423 (220 standard)

CO 4.32 (1.2 standard) 4.29 (1.2 standard)


any suggestions as what to check? Of course the place says is a dirty fuel system and wanted to sell me the $50 carb cleaner kit. (Valvoline fuel injector cleaner, etc.).

This looks to me like a rich mixture. If there is no O2 sensor, I would look into adjusting the idle mixture, adjusting for best lean idle on tach. If you have an exhaust gas analyzer, adjust for lowest HC readings. This should yield about .3% CO and about 200 ppm HC until the convertor lights. When that happens, the emissions should approach zero. If there is an O2 sensor in the loop, look for voltages close to 1 volt in this condition. If the voltage is much less, it is likely due to a defective sensor. If the voltage is close to 1 volt, look for codes enriching the mixtures. Ray