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2002 gmc envoy_slt_xl
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Discussion Starter #21
Well, I'm lost again. I swapped the TB and accel. Pedal, still no go. The logger still shows a travel range of about 9% to max 32%. While idling in park, if I hold my foot to the floor, it stumbles for a second, then slowly climbs to 4,000rpm in about 4 or 5 seconds. Only other thing I can try is swap the ECU? Anything else I should try?
 

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Discussion Starter #22
What about the timing chain? Is there any chance something happened with the timing? It has 235k miles on it.
 

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Apologies .... I was pecking with my fat finger on a tiny keyboard and had a lot of typos in the first draft of my response, which I accidentally posted b4 I proofread it. I THINK it's corrected now.

Sorry.

But, in ref to the above response..... IN THE Affirmative...... the post cat O2 sensor is only being watched by the ECM and it has no other purpose but to tell if the cat is good or not. It won't affect your mileage or performance... not in, nor of itself.

It should be very steady output voltage like mentioned above, unless the matrix of the cat has holed, and collapsed or gone incendiary so badly to form a puddle.

Now .... you gas mileage sucks, so you know that a lot of excess fuel is going through combustion, but the horsepower created is not able to overcome whatever is holding your vehicle back.

Running hard can incinerate your cat because .... well, it can ... so there.

This may help you to understand that you have been chasing the effect of some system or part that has failed, and not the reason it's screwed up.

I'm still thinking transmission.

IF the last rebuilder re-used a partially worn out seperator plate, and missed that the 3-4 check ball hole was getting oversized and just slammed it back together .... well a 3-4 steel check ball, blown through or stuck in the divider plate will give you this terrific loss of horsepower and performance because your 3-4 clutch pack is ON when it shouldn't be.

A smart bench monkey will usually use a larger brown hard rubber checkball from a TorqueFlite instead of the GM steelie. On a fairly good used Intermediate plate, the stock steelie will work ..... for a while.... but it eventually will blow through.

Powers of observation is what it takes.... because there are too many ways to bad-build a 4L.
 

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Discussion Starter #25
Apologies .... I was pecking with my fat finger on a tiny keyboard and had a lot of typos in the first draft of my response, which I accidentally posted b4 I proofread it. I THINK it's corrected now.

Sorry.

But, in ref to the above response..... IN THE Affirmative...... the post cat O2 sensor is only being watched by the ECM and it has no other purpose but to tell if the cat is good or not. It won't affect your mileage or performance... not in, nor of itself.

It should be very steady output voltage like mentioned above, unless the matrix of the cat has holed, and collapsed or gone incendiary so badly to form a puddle.

Now .... you gas mileage sucks, so you know that a lot of excess fuel is going through combustion, but the horsepower created is not able to overcome whatever is holding your vehicle back.

Running hard can incinerate your cat because .... well, it can ... so there.

This may help you to understand that you have been chasing the effect of some system or part that has failed, and not the reason it's screwed up.

I'm still thinking transmission.

IF the last rebuilder re-used a partially worn out seperator plate, and missed that the 3-4 check ball hole was getting oversized and just slammed it back together .... well a 3-4 steel check ball, blown through or stuck in the divider plate will give you this terrific loss of horsepower and performance because your 3-4 clutch pack is ON when it shouldn't be.

A smart bench monkey will usually use a larger brown hard rubber checkball from a TorqueFlite instead of the GM steelie. On a fairly good used Intermediate plate, the stock steelie will work ..... for a while.... but it eventually will blow through.

Powers of observation is what it takes.... because there are too many ways to bad-build a 4L.
Thanks for the info. I won't worry so much about the rear 02. I'm fairly positive that the cat is at least partially plugged, probably pretty bad. Removing the front 02 and giving aggressive throttle seemed to have more power. The cat also has over 200,000 miles on it. I also noticed that the calculated engine load on the highway is almost always 100%. This may be normal reading, but I feel like it shouldn't be 100% just cruising a flat highway, but I don't know how it's calculated. That might be more evidence of a plugged cat. I still think I should remove the cat and see where my gas mileage is for a while. Is there a pre cat or a smaller cat upstream, right off the manifold? I'll report back after I remove the cat.

As for the trans. I'm still not ruling it out, I'm the one who rebuilt it. It is entirely possible that I messed something up being my first auto trans. Rebuild, but I tried to check everything that I could find online from what expert rebuilders said to check while I was in there. I'm fairly confident I did a good job. I bought all new bushings, gaskets, seals, clutch packs, manifold parts, incl. The sep. plate. I used steel balls. I'm not even sure how to check for trans. Problems if it ends up being the problem.
 

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Discussion Starter #26
Be aware there is a rev limiter in these for when you are at rest (not rolling down the highway)
update
So I temporarly plugged in the spare accel. pedal and TB last night. It was still only showing 30ish% max throttle while in park, same slow climb to higher rpm. It was late and didn't want to deal with it anymore, so I plugged back in the old accel. Pedal, but left the TB. Skip to this morning, the family and I went to the store. I started the logger and noticed right away that the throttle % was getting over 30%, so I gave more and more acceleration, it's going up to 80% , shifting and driving perfect, lots of power. I feel pretty stupid learning that it's limited in park, this would have saved quite a few hours of troubleshooting, haha, oh well. It makes sense now. Now for the million dollar question, WAS it the TB giving the problems? We will see over time, because with the old original TB, the problem did come and go a few times recently, but time will tell if it's fixed for good.
 

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I have often wondered how many perfectly good parts get replaced and the problem goes away,, not because the part was bad but the harness connection had some schmutz in it!
 

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I know it's going out of fad, but I still smear a little dielectric jell on all my critical electrical connex.
 
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Hey.... an epiphany.....

Did we ascertain that the Speedometer works?

If roadspeed is not fed to ECM/TCB, the restriction of 3K RPM might be invoked.
 

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Discussion Starter #30 (Edited)
I have often wondered how many perfectly good parts get replaced and the problem goes away,, not because the part was bad but the harness connection had some schmutz in it!
I bet it happens a lot. I've had this TB off twice trying to test it and clean it, I don't think it's the case here.

Hey.... an epiphany.....

Did we ascertain that the Speedometer works?

If roadspeed is not fed to ECM/TCB, the restriction of 3K RPM might be invoked.
speedometer reads normal.

One other thought is the transmission switch. Could it be possible that the transmission linkage is just slightly off, sending a false signal to the ECU that the truck is in neutral, even though it's in drive, but still show a line under the D on the console display? Just trying to think of every strange scenario.
At this point though, it does seem to be resolved, so on to the next problem....

It still stumbles, misfires, and stalls sometimes when it is started after being completely cold, like, sitting overnight cold. It will do this for a good minute or so until it clears up. There is still the faintest misfire that I can feel and hear if you listen REAL closely, but it's not bad at all. It's not like if you were to unplug a coil, it's very very faint. Not going to worry much about that though, but the cold start misfire is annoying, and can't use the remote start as it often stalls.
Could a clogged cat (excessive back pressure) possibly do this during cold starts when I'd presume the ECU is pulling a startup map and ignoring the sensors? Maybe this truck is different, but my car is like this.
 

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I had a thought .... but cancelled it.

Instead, can you not get a seat-of-the-pants idea about shift points, PRNDL locations, etc from just the way it feels?

Perhaps try manual shifting through the ranges in forward from 1st to 4th to ascertain if shifting actually happens?

I mean, ya COULD have a ruined converter stator.... but that'd manifest by sluggish (read: stupidly slow) acceleration, but surely not 9MPG.

Right now, you're hearing hoofbeats and thinking you've got a herd of Zebras coming over the hill, when in reality it's just a horse.

Simplify and take a step back and observe without forming opinions or you're chasing rabbits all over the place.
 
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Discussion Starter #32
Hey.... an epiphany.....

Did we ascertain that the Speedometer works?

If roadspeed is not fed to ECM/TCB, the restriction of 3K RPM might be invoked.
I had a thought .... but cancelled it.

Instead, can you not get a seat-of-the-pants idea about shift points, PRNDL locations, etc from just the way it feels?

Perhaps try manual shifting through the ranges in forward from 1st to 4th to ascertain if shifting actually happens?

I mean, ya COULD have a ruined converter stator.... but that'd manifest by sluggish (read: stupidly slow) acceleration, but surely not 9MPG.

Right now, you're hearing hoofbeats and thinking you've got a herd of Zebras coming over the hill, when in reality it's just a horse.

Simplify and take a step back and observe without forming opinions or you're chasing rabbits all over the place.
Yes, I did manually shift through the gears at part throttle in both scenarios, For clarity, the scenarios are one being when the truck is allowing up to 100% throttle and it's actually running, and shifting normal, motor has plenty of power and accelerates as it should. The 2nd scenario is when it is acting up and limiting the throttle to 30%. To clarify, whatever the malfunction ends up being, the 30% throttle limit I'm seeing on the logger ONLY occurs when the truck is acting up. I'm still holding out hope that there was a problem with the original TB and it just didn't throw a CEL, again, only time will tell, still have the replacement TB in and still no problems. If the problem does come back, I will take a step back and try to start over troubleshooting.
 

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I should probably clarify my thinking. Since the MAF or MAP/IAT determines how much air is getting into the engine, a bad sensor(s) will give an erroneous reading to the ECU. For example, a dirty sensor will see less air getting into the engine. In turn, the ECU will lean out the fuel because it thinks it needs less (due to less air). This will cause your oxygen sensor to then read lean because the air/fuel mix is not getting enough fuel for the ACTUAL volume of air present (thus the P0171).

The O2 reading then triggers the ECU to ENRICH the fuel mixture (therefore the excess fuel consumption). And so it goes, back and forth between lean and rich.

At the same time, the lean mix causes your difficulty in acceleration and the rich causes stumbling.
 

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Have you ever checked the fuel rail to be sure it's working correctly for all 6 cylinders? ...since you've already spent so much time on the air + spark.....
 

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The one thing I haven't heard about is swapping out injectors. I figured if it's not plugs and coils, but is misfiring, then besides crappy or no injector flow, I've heard these motors can get certain intake manifold vacuum leaks that can affect only certain cylinders. Hopefully you figure it out. From some threads I've read, I think clogged cats might be an epidemic for these motors, and it could be because they are supposedly 600 cell per inch!?!

Rob in AZ
 
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