Having trouble passing smog--very high HC at idle (1139 ppm, standard is 175 ppm). History and actions to date below. I do have the 3-volume GM 1995 C/K Service Manual and have read through a lot of emissions-related posts on this site but have not identified the problem(s) yet.

The truck:
1995 Chevy K2500
350 V8 TBI, Automatic Transmission
Stock engine, no mods
Used for street and towing racecar, no offroad use
124,000 miles

Two years (10,000 miles) ago:
HC high (about 220 ppm, standard is 175ppm)
--trouble code 32 = EGR code
--Idle fuel pressure = 13.5 psi
--SeaFoam, oil change, fuel filter change, new 195°F thermostat and new spark plugs did not help
--New GM EGR valve solved problem

Changes since then:
Very slight exhaust manifold leak as left rear manifold bolt fell out
Added free-flow cat-back exhaust

NOW:
HC at idle = 1139 ppm (standard is 175ppm) = FAIL
CO% = 0.71% (standard is 1.00%) = PASS
Dilution check (CO + CO2%) = PASS
Gas Cap Test = PASS
Catalytic Converter Test = PASS

Actions and observations so far:
--no trouble codes
--idle smooth, runs well, not down on power
--Idle fuel pressure = 13.5 psi
--spark plugs look fine (all light brown)
--new plug wires and coil wire
--new cap and rotor
--vacuum hoses seem good (did not use vacuum pump though)
--EGR valve appears to function (hand compression slows idle)
--new EGR solenoid
--Base timing confirmed to be 0° at idle

Things not tried yet:
--compression test
--leakdown test
--Coolant temperature sensor test
--O2 sensor test (how to do?)
--coil output test (how to do?)
--vacuum pump test of EGR system
--replace TBI pressure regulator
--ECM tests (how to do?)
...other??

Given the excessive richness I assume this must be a distinct and well known failure. Any suggestions on how to diagnose and remedy appreciated! Thanks--Fred

 

Does your test shows individual O2, CO2 and NoX readings - this may provide additional clues.

Exhaust leaks play havoc with O2 sensors by introducing extra oxygen into exhaust stream. When O2 sensor reports extra O2 (lower voltage) the ECM compensates by adding more fuel 'thinking' that is running lean. Well, it is not. The extra fuel is not burned fully resulting in higher HC levels.

Recommendation: start fixing exhaust leak.

//RF

 

Unfortunately they do not provide individual gas readings. The only other data provided relates to the dilution test:

Dilution check (CO + CO2%)
Emissions Readings: 13.03
State Standards: 6.00
Emissions Result: PASS

While CO% = 0.71% (standard is 1.00%), I do not know how to read the CO2% from the dilution test.

Thanks for suggestion on exhaust leak. It is very minor. A real pain because I fixed it once before and when I went to put the manifold back on the holes did not line up. When I bought the truck (73,000 miles) it had the manifold leak and I had the dealer fix it before I took delivery. A year later the rear bolts loosened and the gasket burned through a bit. When the holes did not line up (by quite a bit) I wondered if they put the wrong manifold on. I enlarged the holes enough to get the manifold back on. I understand some people use locktight on the manifold bolt. I am used to studs and aluminum heads, I may try to go with studs on the Chevy if I have enough clearance to install the manifold.

Is there any way to check the O2 sensor voltage during idle to see if it is reporting low voltage? If it is not I'd rather not deal with the manifold if I can avoid it.

--Fred

 

Follow-up: Well, it turned out to be something simple. I thought the plugs were good but putting in a new set of plugs and throwing a different cap and plug wires (also new) dropped idle HC and passed smog! Thanks for people's help here. --Fred

 

If the engine was running rich, the CO would have been sky-high instead of the HC.

High HC is typically due to MISFIRE, as you've found out.

 

Yes, indeed. I am now older...and hopefully a bit wiser! --FB