Failed Emissions, '93 Chevy P/U - Sanity Check


Old 12-05-05, 01:52 PM
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Failed Emissions, '93 Chevy P/U - Sanity Check

Failed latest emissions test - high hydrocarbons - on '93 C1500, 350. History: engine rebuilt, added hypertech chip, headers, TBI spacer, roller rocker arms. Ever since rebuild (25k mis and 5 yrs ago), on hot days at a light, engine temp would increase almost to overheat. Driving away brought engine temp back to normal. Figured I had a bad fan clutch but never did anything about it. Also, since rebuild, idling more than briefly in Park only (idle in gear-no light), check-engine light would come on. Revving engine or driving away turned light off. No other symptoms--until now. Always passed emissions until now.

Checked code: 35, Idle Air Control. Replaced IAC, no change except maybe the light would come on sooner at idle in park. Also lately (before changing the IAC) I noticed the 'overheat on hot days at stoplights' problem went away. Now that weather's gotten cold, noticed temp light indicates engine not warming up. Before putting 2 + 2 together, decided to change the original 02 sensor. Also removed, cleaned, and successfully tested the EGR. Engine ran better, still throwing code 35, though.

Finally woke up and thought, 'thermostat'. Remembered shop that rebuilt the engine said he put in a low-temp ts. Replaced it with a new 160 deg. thermostat with no apparent affect at all. Gauge still shows cold during cold weather driving, and barely any heat from the heater.

Bottom line: did I most likely get another bad thermostat which could be causing a cold engine and failed emission check due to rich mixture? Not sure how the IAC fits in, though. Again, the truck runs better than ever now with more power. Never had idle speed problems, no hesitation, no rough running, etc. If it weren't for the failed emission test, I'd have suspected nothing. I hear newer thermostats fail open, which would make sense that I got another bad one. Anyone?
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Old 12-07-05, 07:44 PM
Join Date: Apr 2002
Location: Pittsburgh,Pa
Posts: 2,417
First of all a 160 degee thermostat shouldn't be in your truck.Put the 195 degree in that belongs in it and see if the heat comes around like it should.I won't cover the emission failure since it has been altered.

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