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

[hil...@emailaccount.com]

I'm trying to troubleshoot an HC emissions failure in a 94 Chevy Suburban 5.7 liter with TBI and one heated O2 sensor. Over several minutes idling in park the long term fuel trim will creep down to around 100 (with 128 being stoichiometric). So this works out to like -22% or so if I'm reading it right, cutting fuel to correct a rich condition. But idling in drive it's always around 128. Scope traces look almost identical between the 2 cases; 5 seconds or even onger between switching at idle, but switches much faster at higherrpm and while driving. Why would the car run rich in park and OK in drive, both at idle? It's not that big an idle speed difference. O2 sensor is new; acts same as the 1 week old new one I just replaced.

[Steve W.]

What were the HC numbers? At what rpms?
Are these numbers with the engine at operating temp?
Factory air cleaner/intake?
Original cat?

Which scan tool? Most give you a number in percentage form.

Normally fuel trims should be single digit numbers to maintain 14.7:1 to
keep the cat/con happy.

For instance at idle, no load you should see a long term of close to
0.0% and short term might bounce a bit around +/- 1-2% to match
accessory drive loads.

I am thinking it's a bad cat, but you would need to measure the HC while
adding fuel to be sure.

--
Steve W.

[hil...@emailaccount.com]

HC is 340 ppm vs a spec limit of 220, with a new cat and new O2 sensor. This is at idle. At high rpm (2000) it passes the same limit. In this model year/PCM, 128 counts is what later came to be called 0%. The scan tool is a Snap-On MT2500. Yes, at temp, 200 F. Factory intake and air cleaner with new filter. New plugs, wires, and ignition module. (These were replaced before the new cat was put in because of bad ignition misfiring, so I don't think I killed the new cat.) Just put all new gaskets, o-rings, and fuel pressure diaphragm in the TBI. It was amazingly clean, for not having been touched in 360K miles. (The long block has only 140K on it and the plugs always look very clean.) Short term trim does bounce around a little as it should, staying close to 128 as the long term trim walks its way down mto keep it so. The curious thing is the difference between LT trim behavior in drive and in park, both at idle. PCM? How would I know?

Thanks for the input. Unfortuantely I don't have a gas analyzer. If I did, I would have tried to see if I could measure HC before and after the cat to see if it's bad, no?

[Scott Dorsey]

So, my question is this: what the hell is different in the engine control
system when the transmission is put into park?
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Steve W.]

Yeah had to go back into the dark recesses of pre-OBDII.

The change in trim between drive/park is due to the engine load
expectancy. In park the ECM knows you are not going to be moving so it
uses a different block of data based on possible engine loads from
accessories only. When you shift into drive the trans comes online and
changes the ECM into a more active mode to compensate for probable
motion. This is also why the O2 sensors get polled differently.

Does this engine have Air injection to the cat?
I know the pumps liked to fail due to moisture but I'm wondering if the
bypass has fail open and it's dumping air into the exhaust making the
ECM think it's running lean so it dumps some extra fuel which the cat
isn't handling.

What the MAF doing from idle up to 2K?

--
Steve W.

[hil...@emailaccount.com]

No air injection. No MAF, just a MAP. The MAP seems to respond as I would expectewhen the throttle and rpm are maneuvered.

[Bill Vanek]

On Mon, 3 Mar 2014 12:15:46 -0800 (PST), hil...@emailaccount.com
wrote:

>No air injection. No MAF, just a MAP. The MAP seems to respond as I would expectewhen the throttle and rpm are maneuvered.

What sort of vehicle is this? And have you checked one probably
obvious condition - the vacuum to the MAP sensor? You could tee a
vacuum gauge into the line. Maybe the line is cracked and leaking - it
doesn't take much to screw everything up. And what are the HC readings
at the various RPM's? You said it passed at 2k, but most modern cars
tend to blow zeroes under almost all conditions. Is there maybe fuel
in the oil? Block off the PCV valve and see if anything changes. And
make sure the MAP line is connected to the correct port (to manifold,
not ported vacuum).

[Bill Vanek]

On Mon, 3 Mar 2014 12:15:46 -0800 (PST), hil...@emailaccount.com
wrote:

>No air injection. No MAF, just a MAP. The MAP seems to respond as I would expectewhen the throttle and rpm are maneuvered.

One more thing I forgot to mention - are you sure the pressure
regulator valve isn't leaking? Take the vacuum line off, and if
there's fuel coming out of the connector, it's bad. If there's not,
you should connect a vacuum pump with a clear line, and pump it to
full vacuum. It might leak then.

If I read your other posts right, high HC means raw fuel in the
exhaust, which is not necessarily a running rich condition.

[hil...@emailaccount.com]

There's no vacuum line on the pressure regulator; the regulator is internal to the throttle body injection unit and has a return line to the tank. I had the TBI apart, cleaned it, and put it back together with new gaskets and o-rings yesterday. (It really wasn't crudded up at all.) Measured fuel pressure at idle and higher rpm and it was a rock steady 12 psi (specs 9-13 psi). I've run the fuel pump for several minutes with the engine off with my face up against the TBI with a flashlight and not a drop of fuel drips out of the injectors.

The reason I say it's rich is that the long term fuel trim in Park at idle drops down to close to 100 over several minutes; the PCM wouldn't do this unless it felt the mixture is rich. I know a lean misfire can make the exhaust rich because of unburnt fuel (how ironic). I don't think I have a vacuum leak leading to a lean misfire because the idle is not high, and the PCM is not jacking up the fuel but rather down.

[Bill Vanek]

On Mon, 3 Mar 2014 20:43:59 -0800 (PST), hil...@emailaccount.com
wrote:

>There's no vacuum line on the pressure regulator; the regulator is internal to the throttle body injection unit and has a return line to the tank. I had the TBI apart, cleaned it, and put it back together with new gaskets and o-rings yesterday. (It really wasn't crudded up at all.) Measured fuel pressure at idle and higher rpm and it was a rock steady 12 psi (specs 9-13 psi). I've run the fuel pump for several minutes with the engine off with my face up against the TBI with a flashlight and not a drop of fuel drips out of the injectors.
>
>The reason I say it's rich is that the long term fuel trim in Park at idle drops down to close to 100 over several minutes; the PCM wouldn't do this unless it felt the mixture is rich. I know a lean misfire can make the exhaust rich because of unburnt fuel (how ironic). I don't think I have a vacuum leak leading to a lean misfire because the idle is not high, and the PCM is not jacking up the fuel but rather down.

What about the vacuum to the MAP sensor? And how is the spray pattern
on the injectors? And is the idle smooth, or is there any roughness at
all?

[hil...@emailaccount.com]

The vac line to the MAP sensor is intact and not plugged. I have not tried to look at the spray pattern of the TBI injectors yet (timing light?), but I hear that the turbulence of the air across the throttle plates and through the intake runners is what does the real atomization anyway. The idle is rougher than I'd like it to be, in drive or in park/neutral.

Thanks.

[Bill Vanek]

On Tue, 4 Mar 2014 06:04:13 -0800 (PST), hil...@emailaccount.com
wrote:

>The vac line to the MAP sensor is intact and not plugged. I have not tried to look at the spray pattern of the TBI injectors yet (timing light?), but I hear that the turbulence of the air across the throttle plates and through the intake runners is what does the real atomization anyway. The idle is rougher than I'd like it to be, in drive or in park/neutral.
>
>Thanks.

I would work on the rough idle. EGR stuck partially open, vacuum leak,
valves, round cam, etc.

[hil...@emailaccount.com]

Thank you. I haven't pulled the EGR off yet, but I will. I also have one of those FirstLook sensors that'll supposedly tell you all kinds of things from the timing and magnitude of pressure/vacuum pulses in the exhaust, intake, or crankcase.

[Steve W.]

Bill, was re-reading these posts. With 140K on the block, could it be
valve stem seals allowing enough oil past at idle to increase the HC at
higher vacuum ? If the ECM is leaning out the mix at idle and the TB has
been rebuilt with no real change.


--
Steve W.

[hil...@emailaccount.com]

Very interesting possibility, Steve W about the valve stem seals. I wonder if the FirstLook could tell me. In the past I have changed oil to 20W-50 right before the emissions test just in case oil was getting past something.

[Bill Vanek]

On Tue, 04 Mar 2014 14:36:49 -0500, "Steve W." <csr...@NOTyahoo.com>
wrote:

I'd like to know the rest of the gas readings, but high HC is usually
an indicator of a misfire - it's raw fuel in the exhaust. He said it's
running rough, so I'd try to find the cause of that first. It could
even be valve guides, but I don't think that's too common anymore. Did
he ever say what year/make vehicle it is? Can TBI regulators leak gas
out the bottom, and into the intake? I just can't remember anything
about them. I do remember that plenty of cars had issues with cam
lobes rounding back in the days of TBI. Or at least I think I remember
that... You wouldn't notice much of a misfire off idle with that
condition. But he also never said what the HC readings were at higher
RPM.

[hil...@emailaccount.com]

94 Chevy Suburban 5.7 liter. At idle: HC 344 ppm vs 220 spec, CO 0.90% vs 1.20, CO2 13.4%, O2 2.1%. High idle: HC 26 ppm, CO 0.02%, CO2 15.2%, O2 0.5%. I did pull the EGR off it and it seems fine; not leaking through (it holds WD40 without draining when there's no vacuum on it), not being opened by its solenoid at idle, solenoid not leaking through. I did get a crankcase pressure reading at idle on a scope, but I have to get some help interpreting the trace. If I have piston ring blowby wouldn't it make more HC at higher rpm?

[Bill Vanek]

On Tue, 4 Mar 2014 20:12:51 -0800 (PST), hil...@emailaccount.com
wrote:

>94 Chevy Suburban 5.7 liter. At idle: HC 344 ppm vs 220 spec, CO 0.90% vs 1.20, CO2 13.4%, O2 2.1%. High idle: HC 26 ppm, CO 0.02%, CO2 15.2%, O2 0.5%. I did pull the EGR off it and it seems fine; not leaking through (it holds WD40 without draining when there's no vacuum on it), not being opened by its solenoid at idle, solenoid not leaking through. I did get a crankcase pressure reading at idle on a scope, but I have to get some help interpreting the trace. If I have piston ring blowby wouldn't it make more HC at higher rpm?

I'm still going with a misfire. Did you do a running compression test
at idle? Even a cranking test might show something. Have you looked at
a vacuum gauge at idle? Are you sure the plugs and wires are good? Are
you sure the O2 sensors and wiring are good? Readings the same left
and right?

And do you need to pass an emissions test, or is this just for the
challenge? I ask because if the converter is bad, a new one might get
you through a test. Do you have a pyrometer to check it? After it gets
good and hot, the temperature should be substantially higher in the
back than the front.

[hil...@emailaccount.com]

Thanks for the continued comments. I have done pressure sensor readings in the exhaust cranking and at idle but haven't lesarned enough yet to make any positive conclusions. I may do an old school compression test with a gauge soon, cylinder by cylinder. I have not looked at a vacuum gauge at idle; will do so. The plugs and wires are new, as is the single O2 sensor (no left and right) before the cat. I have traces of it switching back and forth from a scope, so I'm pretty sure its wiring is good. It switches once a second or so whle driving, but only once every 5-10 seconds idling. This is to pass a state emissions test. The cat is new. I have not checked temp before and after but I could touch a thermocouple to the the pipe to check, though I'm pretty sure it's getting hot since I saw a slight metal discoloration on the body after running it a couple of days.

[Scott Dorsey]

Bill Vanek <bilv...@invalid.com> wrote:
>
>I'd like to know the rest of the gas readings, but high HC is usually
>an indicator of a misfire - it's raw fuel in the exhaust. He said it's
>running rough, so I'd try to find the cause of that first. It could
>even be valve guides, but I don't think that's too common anymore. Did
>he ever say what year/make vehicle it is? Can TBI regulators leak gas
>out the bottom, and into the intake? I just can't remember anything
>about them. I do remember that plenty of cars had issues with cam
>lobes rounding back in the days of TBI. Or at least I think I remember
>that... You wouldn't notice much of a misfire off idle with that
>condition. But he also never said what the HC readings were at higher
>RPM.

I like the valve guide diagnosis.

I'd also check the condition of the plugs and make sure there is a good
consistent spark.

[hil...@emailaccount.com]

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. Looked like there was some black discoloration on part of the idle air control (IAC) pintle, but it didn't look like it had any thickness to it that would 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 through 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 difference 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-resistant 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 all the warnings against this practice on the internet. I very carefully counted the threads sticking out before touching it so I could set it back later.) New Jersey inspection rules say 350 - 1100 rpm is acceptable low idle 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 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, presumably trying to bring the idle down in this manner as well. (The mechanic 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 help. I did not try this.) PCM threw an IAC error code and lit the check engine 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; "Indicator Light N/A". (On an OBD-2 vehicle, for which they don't analyze exhaust 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 years 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 discussions, 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, intake, and crankcase, but at least I'm not under the gun now and can do it at my leisure.

[steve...@msn.com]

[steve...@msn.com]

i have the same issue - but my truck has only 64,480 original miles - i replaced 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 after 4 trips to DMV - i have not been back since the new injectors - i do have a true dual exhaust with high flow cats.. i was thinking of adjusting idle screw too - but did not do so yet - i wanted the truck to pass on its own merits and fix it properly - any advice??

[hil...@emailaccount.com]

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