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Help (!/?) [UPDATED - AT BOTTOM]

My truck has some issues... BESIDES the fact that it’s a 1st Gen Colorado. Beat you to it. You know who you are. Internet Diagnosis plz? (WebMD says it has cancer.)

(Video for thumbnail purposes as I cannot upload pics anymore)

The Facts:

- 2010 Chevy Colorado, LH9 5.3L V8, ~145,000KMS, has been tuned

- About a year ago I started getting a P0420 code. I just kept clearing it, on the basis that if the cat was fucked I couldn’t be bothered un-fucking it until it was actually causing drivability issues... and if it was the downstream sensor that had issues, I wasn’t terribly worried because the downstream O2 sensor is really only there to see if the cat is working or not. Plus, it only came on every couple weeks or so.

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- Truck has had a slight stutter at 3500-4000 RPM for some time now. Recently this got much worse, but still went away after crossing the 4000RPM mark.

- A few weeks ago I started getting P0420 again. This time it was much more persistent, a couple times a week up to an almost daily CEL.

- Shortly after the P0420 came back, I had a very brief misfire at 5100RPM that set a P0300.

- After the P0300 came on I ran a couple bottles of injector cleaner through for shits and giggles to see if it would do anything at all.

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- On the tail end of the second injector cleaner tank I was accelerating uphill on the highway, AC on, again at roughly 5100RPM, and I got a very definite misfire (I throttled back as much as possible, but had to more or less stay in it as had a person behind me catching up).

- The “person behind” also happened to be in convoy with me... and remarked that the truck smelled like no cats when I was on the gas.

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- Timing the truck 0-60 indicates I’ve lost between 1-1.5 seconds over the last 3 years. (Admittedly with several different timing methods) Also looks like 1 second in the last year. So it definitely was down on power.

- Seemed to me the best thing to do was to change all the easy things that could be suspect... on the basis that for the cost of getting a proper diagnosis, I could install the parts myself. Seemed like I had a good shot at fixing the problem and I’d have new parts where they were probably ready for a change anyway. So, I ordered new O2 sensors, plugs and wires.

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- New plugs and wires seemed to make a slight difference, in that the mid-range chatter subsided a little and the high RPM misfire went from ~5100 to ~5600RPM. Also cleaned the MAF, that seemed to completely erase the mid-range chatter... at least around town.

- Plugs had a little carbon buildup around the flat area at the very end of the threads (facing directly into the cylinder), but the electrodes and the insulator were a beautiful light brown. However the threads on all 8 were wet. *shrug*

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- Next day, get on the highway... no. At highway speeds, it still chattered under load, and while there was definite improvement, it was quite slight. Did seem to edge the 0-60 numbers in the right direction, but the margin of error is so huge I’m not really putting much faith in that.

- Remember that bit about changing stuff myself so I paid for new parts instead of a diagnosis? Yeah... at least a couple O2 sensors were stuck so hard I had to take it to a pro anyway. Oh well. Anyway, part of the reason I bought them was that not only were they a possible cause of the P0420, the voltage output from one of them was erratic enough that I didn’t trust it anyway.

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- New sensors went in this Tuesday. The O2 voltages were slightly different from the old ones, so something was going on for sure. Also, the fact that the engine light hasn’t come back on is a giveaway.

- Old O2 sensors varied from side to side. The Bank 2 sensors had a thin wispy white layer on them, no carbon. The Bank 1 upstream sensor had carbon on the base but the tip was clean. It had been too stuck for me to handle but came out OK for the mechanic. Bank 1 downstream had been totally seized and had shredded it’s threads on the way out. It had more carbon around the base, and evidence of physical damage on the tip. Scratches and debris inside (Could have been from removal, or maybe the mechanic dropped it... but I have an idea this was all there before removal).

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However. Problem NOT solved.

- Midrange chatter is gone or almost gone. So that’s nice.

- Top end(5100RPM+) misfire appears to be drastically lessened, but still present.

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- I haven’t rechecked acceleration yet (which is basically my only measure of power loss).

- Downstream sensors are reading roughly 0.6-0.7v which is pretty much what I’ve always seen when the truck has been running right.

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- Fuel trims are within the +/- 10% range—I haven’t done the proper idle/1500/2500RPM check yet—but LTFT on both banks is generally positive (Bank 1 more so than Bank 2) at idle and dips negative the second any throttle is applied. I found diagnostic charts for other LTFT behaviours, but nothing matching that. STFT I can’t say much about as it adjusts too fast... but I can see that the combined STFT and LTFT combined generally stays well within the +/- 10%. Not sure what any of that means and would be glad of help. Maybe it’s normal. I’ll do a proper test and update this.

- I have not checked into the PCV system, EVAP system or injectors. I do tend to run the tank to the LOW FUEL light and when filling I do two extra clicks, after giving the fuel time to settle between them (as suggested to me by an Oppo as a way to normalize fuel economy numbers between different pumps).

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- There has been a noise at idle on the drivers side (as best as I can tell) for quite some time. I’ve asked the dealer to have a look before and they came up empty. WAG’s about what it may be include an injector (*shrug*), a lifter (I don’t have AFM, but it’s still not unheard of for a 5.3 to have lifter noise), weak valve spring (really doubt it, but...), piston slap (again, typical 5.3 noise apparently), small exhaust manifold crack. Anything from a “tsht tsht tsht” to sounding kinda like a really quiet 7.3 Power Stroke.


My going theory is that a fluffy Bank 1 upstream O2 sensor messed with the mixture long enough to fry the cat, and the downstream sensor caught in the debris as it let go. And now the added restriction is causing the top end misfire to persist. Dunno, I’m not a mechanic. I’ll try to get more info if it’ll help.

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Help please?


- So the P0420 code did come back after about a week, and then every couple days after that. The misfire was nowhere near as bad, but it was still there when I ratted it to 5100+RPM on the highway. All in all I’m convinced the sensors were bad for it to improve that much.

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- Also noticed that at idle, it was bouncing in and out of closed loop. As soon as I moved anywhere, it would switch back to closed loop and stay there until the next stop.

- I got the rear cat removed and the front cats gutted yesterday. Much better throttle response, pulls clean all the way up, doesn’t make a horrible racket either... at WOT.

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- Part throttle is another story. At 1800 to 2500RPM - ie, the exact range I need for DD purposes, it’s obnoxious and sounds like shit. Most likely because of the big hollow shell it get to play in now. So that’s a thing I’ll have to work on. Oh well, I guess #neverlift?

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