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

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 r ight, cutting fuel to correct a rich condition. But idling in drive it's a lways 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 s ensor is new; acts same as the 1 week old new one I just replaced.
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snipped-for-privacy@emailaccount.com wrote:

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.

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HC is 340 ppm vs a spec limit of 220, with a new cat and new O2 sensor. Th is is at idle. High rpm (2000) it passes. In this 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 t he 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. ) Thanks for the input. Unfortuantely I don't have a gas analyzer. If I d id, I would have tried to see if I could measure HC before and after the ca t to see if it's bad, no?
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HC is 340 ppm vs a spec limit of 220, with a new cat and new O2 sensor. Th is 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 wit h 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 bee n 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?
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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."

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snipped-for-privacy@emailaccount.com wrote:

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.

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No air injection. No MAF, just a MAP. The MAP seems to respond as I would expectewhen the throttle and rpm are maneuvered.
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On Mon, 3 Mar 2014 12:15:46 -0800 (PST), snipped-for-privacy@emailaccount.com wrote:

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

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.
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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 h ad 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 p ressure 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 m y face up against the TBI with a flashlight and not a drop of fuel drips ou t 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 u nless it felt the mixture is rich. I know a lean misfire can make the exha ust rich because of unburnt fuel (how ironic). I don't think I have a vacu um leak leading to a lean misfire because the idle is not high, and the PCM is not jacking up the fuel but rather down.
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On Mon, 3 Mar 2014 20:43:59 -0800 (PST), snipped-for-privacy@emailaccount.com wrote:

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?
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Bill Vanek wrote:

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.

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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.
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I like the valve guide diagnosis.
I'd also check the condition of the plugs and make sure there is a good consistent spark. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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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 throu gh 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.
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On Tue, 4 Mar 2014 06:04:13 -0800 (PST), snipped-for-privacy@emailaccount.com wrote:

I would work on the rough idle. EGR stuck partially open, vacuum leak, valves, round cam, etc.
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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.
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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.
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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 h olds 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 crankca se pressure reading at idle on a scope, but I have to get some help interpr eting the trace. If I have piston ring blowby wouldn't it make more HC at higher rpm?
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On Tue, 4 Mar 2014 20:12:51 -0800 (PST), snipped-for-privacy@emailaccount.com wrote:

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.
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