different long term fuel trim idling in park vs in drive?

Finally passed HC emissions at idle! (Can't be sure, but I think the idle speed did it; see below.) Here's the whole gory 4 inspections history, in case it can help somebody else. 94 Suburban 5.7 liter has 358K miles on it , long block replaced at 222K. TBI and distributor itself never replaced. No OBD-1 codes. Plugs look good, no visible smoke.

1/31/14 NJ inspection, running rough, Idle - FAIL HC, ppm 322 220 limit CO, % 0.61 1.20 CO2, % 13.8 O2, % 1.0 High Idle - FAIL HC 722 220 limit CO 1.69 1.20 limit CO2 14.2 O2 0.1 Put in new plug wires (one was bad), new ignition module, new plugs. This seemed to fix the occasional backfiring and simultaneous tach leaping about erratically at cruise. 2/15/14 inspection, running back to normal, very slight rough idle (as for years now). Passed high idle HC and CO, but still flunked HC at idle: Idle - FAIL HC, ppm 306 CO, % 0.88 1.20 CO2, % 13.8 O2, % 0.9 High Idle - PASS HC 149 220 limit CO 0.09 1.20 limit CO2 14.8 O2 0.2. I figured I must have toasted the 100K mile old catalytic converter due to the misifiring, so I put in a new cat and O2 sensor. 2/22/14 inspection, running back to normal, very slight rough idle (as for years now). Yet again flunked HC at idle: Idle - FAIL HC, ppm 344 CO, % 0.90 1.20 CO2, % 13.4 O2, % 2.1 High Idle - PASS HC 26 220 limit CO 0.02 1.20 limit CO2 15.2 O2 0.5. Disassembled the throttle body injector and cleaned it, back together with new o-rings and gaskets. Amazingly clean inside for having 358K miles. Lo oked like there was some black discoloration on part of the idle air contro l (IAC) pintle, but it didn't look like it had any thickness to it that wou ld keep it from seating. Before disassembling the TB, I had been able to s ee the IAC pintle moving by looking down into the throttle body at idle, so it wasn't stuck. Poured almost a whole bottle of Seafoam cleaner through the TBI, then stalled the engine by dumping in the last quarter bottle. Le t it sit for 15 minutes and drove it. Giant clouds of white smoke. No dif ference in very slightly rough idle. Based on advice from an old mechanic, and reading a 1977 EPA tech report "Effect of Engine Speed on Undiluted HC and CO Emissions" ( found on the internet), I drilled out the tamper-resi stant plug over the minimum idle speed screw and jacked it up from 650 at i dle to 1000 - 1025 rpm (when warmed up, in neutral). (This in spite of all the warnings against this practice on the internet. I very carefully coun ted the threads sticking out before touching it so I could set it back late r.) New Jersey inspection rules say 350 - 1100 rpm is acceptable low idle for emissions testing. (NJ Motor Vehicle Commission, Private Inspection Fa cility Manual, 2010 edition, found on the internet) No problems driving th e vehicle in this manner for a day. Snap-On scanner showed the IAC control going all the way shut to try to drop the idle, even switching on and off, in vain. PCM also backed off the timing from 20 degrees to about 14, pre sumably trying to bring the idle down in this manner as well. (The mechani c also suggested I might try pulling the set timing connector to drop the t iming and hold it back at the 0 degrees base; he said this might also help. I did not try this.) PCM threw an IAC error code and lit the check engin e light, after coming back down to idle from high speed. Luckily, for OBD- 1 inspection in Jersey, they don't care about the check engine light; "Indi cator Light N/A". (On an OBD-2 vehicle, for which they don't analyze exhau st gas but rather just check the computer, it would flunk immediately for the light.) 3/7/14 inspection, running back to normal, very slight rough idle (as for y ears now). Passed! Idle - PASS HC, ppm 28 CO, % 0.13 CO2, % 14.6 O2, % 0.6 High Idle - PASS HC 97 220 limit CO 0.06 1.20 limit CO2 14.7 O2 0.3.

Now I get 2 more years. Thanks for all the thoughtful and educational disc ussions, guys. I'm still planning to learn how to understand the various t races I've gotten with the scope and pressure sensor from the exhaust, inta ke, and crankcase, but at least I'm not under the gun now and can do it at my leisure.

On Monday, March 10, 2014 8:17:27 PM UTC-4, snipped-for-privacy@emailaccount.com wrote:

e speed did it; see below.) Here's the whole gory 4 inspections history, i n case it can help somebody else. 94 Suburban 5.7 liter has 358K miles on it, long block replaced at 222K. TBI and distributor itself never replaced . No OBD-1 codes. Plugs look good, no visible smoke.

s seemed to fix the occasional backfiring and simultaneous tach leaping abo ut erratically at cruise.

r years now). Passed high idle HC and CO, but still flunked HC at idle:

o the misifiring, so I put in a new cat and O2 sensor.

r years now). Yet again flunked HC at idle:

h new o-rings and gaskets. Amazingly clean inside for having 358K miles. Looked like there was some black discoloration on part of the idle air cont rol (IAC) pintle, but it didn't look like it had any thickness to it that w ould keep it from seating. Before disassembling the TB, I had been able to see the IAC pintle moving by looking down into the throttle body at idle, so it wasn't stuck. Poured almost a whole bottle of Seafoam cleaner throug h the TBI, then stalled the engine by dumping in the last quarter bottle. Let it sit for 15 minutes and drove it. Giant clouds of white smoke. No d ifference in very slightly rough idle. Based on advice from an old mechani c, and reading a 1977 EPA tech report "Effect of Engine Speed on Undiluted HC and CO Emissions" ( found on the internet), I drilled out the tamper-re sistant plug over the minimum idle speed screw and jacked it up from 650 at idle to 1000 - 1025 rpm (when warmed up, in neutral). (This in spite of a ll the warnings against this practice on the internet. I very carefully co unted the threads sticking out before touching it so I could set it back la ter.) New Jersey inspection rules say 350 - 1100 rpm is acceptable low idl e for emissions testing. (NJ Motor Vehicle Commission, Private Inspection Facility Manual, 2010 edition, found on the internet) No problems driving the vehicle in this manner for a day. Snap-On scanner showed the IAC contr ol going all the way shut to try to drop the idle, even switching on and of f, in vain. PCM also backed off the timing from 20 degrees to about 14, p resumably trying to bring the idle down in this manner as well. (The mecha nic also suggested I might try pulling the set timing connector to drop the timing and hold it back at the 0 degrees base; he said this might also hel p. I did not try this.) PCM threw an IAC error code and lit the check eng ine light, after coming back down to idle from high speed. Luckily, for OB D-1 inspection in Jersey, they don't care about the check engine light; "In dicator Light N/A". (On an OBD-2 vehicle, for which they don't analyze exh aust gas but rather just check the computer, it would flunk immediately fo r the light.)

years now). Passed!

scussions, guys. I'm still planning to learn how to understand the various traces I've gotten with the scope and pressure sensor from the exhaust, in take, and crankcase, but at least I'm not under the gun now and can do it a t my leisure.

i have the same issue - but my truck has only 64,480 original miles - i rep laced the 02 sensor, EGR valve, PVC valve, air cleaner, fuel filter, plugs, cap, rotor, ignition wires, Coolant Temperature Sensor, oil, filter, fuel injectors, and have not gone back yet -- almost same results as you had aft er 4 trips to DMV - i have not been back since the new injectors - i do hav e a true dual exhaust with high flow cats.. i was thinking of adjusting id le screw too - but did not do so yet - i wanted the truck to pass on its o wn merits and fix it properly - any advice??

Update 2 years later - same story, fast idle helps a lot with HC emissions.

378K miles now (actually about 160K on engine). New distributor cap and rotor installed before inspection since center button read open on ohmmeter and internal spark terminals had a white corrosion coating. 20W-50 oil installed, old filter remained. Failed inspection: Idle - Fail HC ppm 425 220 standard CO % 0.85 1.20 CO2 % 12.8 O2 % 2.3 Idle was roughly 650 rpm in neutral. High Idle - Pass HC ppm 111 220 CO % 0.81 CO2 % 13.3 O2% 1.6

Went home and simply sped up the idle to 1025 - 1050 rpm.

Passed inspection: Idle - Pass HC ppm 117 220 standard CO % 0.89 1.20 CO2 % 13.5 O2 % 1.3 Idle was 1025 - 1050 rpm in neutral. High Idle - Pass HC ppm 50 220 CO % 0.59 CO2 % 14.3 O2% 0.7