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Discussion Starter #1
:dunno: I have been trying to solve this problem since Early March, and now all I can do is scratch my head... hoping someone here can offer some advice.

Car is my 1990 Oldsmobile Trofeo, Buick 3800 LN3 motor (vin c).

I drive the car daily and every morning it starts up great, and idles normally (625 RPMS once warm). However, sometimes later in the day, the car will start and my RPMS will be at 1500 at idle. When this happens, the car still runs good, but just a little rougher. The biggest problem with it is that the car does not downshift at the correct points since the shift points are driven by the Throttle Position, and when coming to a stop the car jerks from second down to first at the last minute. If I have my foot OFF the gas, the car will just cruise as if Im at partial throttle, even though the TPS is reading closed throttle. The next morning, start the car and shes back to normal.

If I unplug the battery for 15 mins for the ECM reset, the issue will go away for a couple of days, but it always comes back.. Ive noticed it comes back moreso when moisture is involved (car wash, rain).

In my endeavor to solve, I have replaced the following, in this order:
  • Delphi TPS, adjusted within spec
  • PCV Valve
  • Bosch Oxygen Sensor
  • Fuel Filter
  • Brand New MAF
  • TPS Pig-tail connector
  • AC Delco Oxygen Sensor (was hoping the Bosch was bad)
  • Throttle Body gasket and EGR Adapter Gasket (to fix vac leak)
Im really at a loss as to where I should go next. Im thinking my ECM may be bad. My INT fuel readings are 128, so my vacuum leak/lean issue is pretty much fixed.

I also have access to live ECM data with this car, and can take an ECM snapshot at speeds on the highway. If any ECM data will help diagnose the issue, please let me know and I can provide. One thing I can note is that, while my TPS usually shows .44 volts at closed throttle, sometimes I will notice it down at .42, and sometimes itll even fluctuate between .40 and .42. Wondering if maybe my TPS is faulty, or even just loose. Once the ECM reads the .40, that then would become the baseline for closed throttle. WHen it adjusts up to .44, it thinks the throttle open????
 

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8th Gen Antagonist
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I would triple check the TPS, the readings should be consistent. However I would try to compare the ECM TPS readings against actual voltage at the TPS if possible, if the TPS itself is consistent the issue is in the ECM or in the wiring leading to it.
 

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Discussion Starter #4
^Oh yeah, because after reading these forums its apparent that those cars NEVER have problems...

Thanks King Ill check the voltage at the pigtail, and Ill trace the wires to the ECM to make sure there arent any breaks.

Is it sounding like its TPS driven to you? My readings show that my IAC is in control of the idle speed so I dont think that is the issue.
 

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Discussion Starter #6
^LMAO... right.

The trofeo is only pushing 155 horses, and the styling isnt quite as nice as the BB Impala... although the amenities, and suspension, and ride quality far surpass any Impala of any generation.
 

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^^ I was just responding to intellectual commentary with like commentary. Remember if we are splitting hairs, there is the whole FWD RWD difference I guess too.
 

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Discussion Starter #8
yeah I hear ya Todd... some people, you know? LOL

But for real, I have a legitimate issue with my car and thats the advice this guy can give me? Is it good advice? HELL YEAH! Does it help me? not even a little bit. If I had $7000 to blow Id have the B-Body in my driveway as we speak... in fact that will be my next purchase. Illwill is a smart guy, read my damn mind.

Until then, Im looking to fix the car I currently have. Going to look into the things King suggested and Ill post up some results. Im afraid Ill have to replace the ECM... I can get a brand new AC Delco one for <$100, so thats no the issue, I just have no clue how to install.. I do not have special anti-static shit so Im afraid to even touch the damn thing. How much to shops usually charge for an ECM install? Is there special programming involved?

On a side note... Illwill. you have 9 posts, and 33% of them are merely bashing other cars ... "get a 94-96 Impala"..... "you wouldnt have that problem if you had a RWD Impala"... "My ugly ass wheels are better than yours"..... why exactly are you even here? Do you have anything to offer?
 

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8th Gen Antagonist
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The only reason I suggested the tps was because of the fluctuating readings you mentioned, the baseline shouldn't change. As for the ECM, those older ones have nothing to program, they just run off of a ROM chip. I wouldn't worry too much about anti static gear, just use care and common sense and you won't zap it. Discharge yourself on a ground source before handling (not like that either, sicko) and there should be no worries.
 

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Discussion Starter #10
Cool, thanks again for the advice. Will be working on it this week Ill let you know how I make out.
 

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I have no room to go "bashing" anyones car, my 96 is in pretty rough shape. I don't consider recommending a b body Impala to someone "bashing" but take it how you want to. Certainly didn't mean to ruffle your feathers like I did so I apologize. Good luck with your Trofeo.
 

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Discussion Starter #12
No worries man... I was just kind of scratching my head as to how the hell that was supposed to solve my problem, thats all.
 
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