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2008 Colorado WT, 2.9L I4, 4X4, 5-speed manual
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Discussion Starter · #1 ·
Hi folks, I've been reading on these forums since I bought my 2008 Colorado in 2018, and I've finally "hit the wall" and need to reach out to those who are more knowledgeable. Since I'm new here, I'll give a little background first; apologies in advance for being long-winded. In 2018 I bought a 2008 Colorado WT I4 4X4 5-speed manual in a quest to replace my beloved 1997 S-10 as the S-10 was well over 300k miles and quite literally was ready to break in half from rust. The Colorado was purchased with about 165k miles on the odometer. The previous owner had been trying to repair a rough running issue and was at wits end. He had replaced the spark plugs and MAF sensor and the truck still would not idle or run properly under about 2,000 RPM. I limped the truck home and replaced the failed VVT solenoid and the performance problem was immediately solved.

The truck was setting EVAP DTCs and smelled heavily of gasoline; I removed the box and found the top of the fuel pump/sending unit to be rusted through, and there was a pinhole rusted into the steel pressure line which was spraying fuel everywhere. I replaced the fuel sending unit with an OEM one (Delco, I believe), not a cheap one. The tank was clean inside, and the truck ran well after the pump swap.

Less than 1,000 miles later I lost control on a frozen bridge on the interstate a ran into a concrete barrier at 65 MPH, then got hit by another car while I was waiting for the tow truck. I was OK, but my Colorado was totaled. The front end was completely destroyed; frame stubs pointed to the sky, LH frame rail crushed back behind the front wheel. Air bag and seatbelt pretensioners went off, the fan was through the radiator, and the LH end of the front bumper was dragging against the rear of the left front tire. It was pretty bad. Any sane person would have let the truck go to auction, but I really wanted that configuration of truck, and they're fairly hard to find. It was just like my S-10, only with 4X4 and half the miles. Fast forward a year and several trips to the salvage yard: I have the truck straightened out and, from a mechanical/alignment/cosmetic standpoint, the truck is in good shape. It drives nicely, straight, does not pull. It was humming right along until a couple months ago.

Now I have a very obvious yet elusive performance/running issue that, despite my efforts, is getting worse quickly. I noticed the truck would have a "hiccup" every once in a while when I would restart it when it was hot and take off in first gear. While releasing the clutch with the engine around 1,000 RPM it would buck/misfire once, then it would be OK for the rest of the trip. I initially ignored it, writing it off as a quirk of programming and figuring that the heat-soaked IAT sensor was causing the issue. Then, several weeks back, I had to run several errands and did four hot restarts. At every restart, the truck would buck fairly hard under any attempt at acceleration under about 2,500 RPM. After a minute or so of (rough) driving, it would clear up. Live data on my code scanner showed reasonable IAT and ECT values, so I do not believe those sensors to be faulty I cleaned the MAF sensor with MAF cleaner and the problem went away for three days. I thought I had it solved. Then, on a hot restart, it did its bucking stunt again. Backfiring through the air cleaner, diesel knocking, no power under 2,500 RPM. It did not set any DTCs.

I removed and cleaned/inspected the VVT solenoid; it appears to be in good condition and all the screens are present over the three ports. I knew the truck had an exhaust leak near the engine since I bought it and removal of the exhaust manifold heat shield showed the exhaust manifold to be very badly cracked where the four runners join just above the upstream oxygen sensor. I figured this was an obvious issue; diluting the exhaust gasses with fresh air would most certainly cause a performance issue during closed loop operation. I replaced the manifold/converter with an aftermarket unit from eBay and reinstalled the old oxygen sensors. The truck now runs worse than it ever has!

When restarted cold, the truck runs well, and runs well for as long as I drive it without shutting it off. Smooth, no misfiring or bucking. I initially thought the new exhaust manifold solved the issue, then I shut the truck down for 10 minutes and attempted a restart. The engine started, searched for an idle speed between 400 and 1,500 RPM for about a half minute, did some clanking and diesel knocking, then suddenly ran smooth. Loading the engine with the clutch yielded horrendous results: diesel knocking, misfiring/bucking, and ultimately the engine stalled. Restarted and unloaded, the engine will smooth out and run nicely after some spitting and sputtering. It's like it's trying to run with the choke on. It it absolutely undriveable; I managed to get it moving on the road and cannot go above 45 MPH. Above about 1/3 throttle it starts to drag a cylinder, and loses almost all power at WOT. Once again, it did not set any DTCs.

While hot, I forced the ECM out of closed loop by unplugging the upstream oxygen sensor (with the key off) and the truck runs smoothly when restarted hot, however it certainly is not running at full power and it is setting DTCs for the unplugged oxygen sensor. I ordered a new OEM sensor when I ordered the manifold, it just hasn't arrived yet.

I'm looking for your input and suggestions here. Below are the next steps I'm aiming to try and will report back with results, but I'm not confident that there isn't a different issue somewhere else that I'm not picking up on.

-Replace upstream oxygen sensor
-Check fuel pressure (I need to buy a fuel pressure gauge...any suggestions for brand? OTC? Something else?)
-Replace VVT solenoid again, just to be sure
-Junkyard every engine control component, including ECM, and swap
-Remove 2.9L I4 and install Cummins 4BT (because a carbureted 300CI Ford I6 won't fit ;))

If you've read this far, thanks for sticking with me. I figure it's better to have excess information than a shortage, but please let me know if there is anything I can clarify, and thanks in advance for your support of a forum newbie.

Scott
 

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2008 Colorado WT, 2.9L I4, 4X4, 5-speed manual
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Discussion Starter · #2 ·
Update: The genuine GM upstream oxygen sensor I purchased showed up today so I installed it and, on a cold start, drove 12 miles on country roads without a hiccup. Restarting the truck hot after 20 minutes yielded the same poor running issue as before. In short, changing the upstream oxygen sensor made no difference whatsoever.

Scott
 

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Hmmmm... Alright stupid question time, have you ever replaced your MAP sensor? (no not the MAF, near the airbox, the MAP sensor on top of the engine, near the intake manifold)

I will say I experienced unholy idling hell with my old 2006 for years (even after replacing the MAP sensor several times) until the day I finally replaced the MAP sensor's wiring harness. Apparently the wires had deteriorated and frayed enough to where the sensor was not getting the proper voltage. During the hot, humid summers here in Louisiana, the truck would always run like @$$, even after I replaced the MAP.

Finally, as a last act of desperation and option to finally and permanently fix my idling issues, I got a MAP sensor plug and harness through RockAuto I believe and got it installed. After that, I patted myself on the back after confirmation of about over 100 miles of driving, the truck ran and idled better than it ever had in years since I got the truck back in 2009 and it was quickly approaching over 125k on the odometer. Aside from needing to clean the throttle body about every 5000/10,000 miles, I never had a lick of idling issues again after that, even up to the day I traded it in this year, with over 200k on the clock.

This truck's electrical system has given me more fits and mind boggling issues than I have ever seen or owned in a vehicle, period. It's more a computer than it is an actual machine. I would also recommend keeping a OBDII scanner with this truck at all times because of that, it's saved my bacon several times from having my truck towed. Find and fix any electrical issues with this truck and I can almost guarantee you'll solve your idling issue. The grounds, electrical plugs for sensors, ground packs on the fenders inside the engine bay, the megafuse ground near the battery, even the PCM ended up slowly dying on me in my old truck causing that damned PassLock to activate alot before I had the PCM replaced. Check all of it, any electrical diagnostics are you best friend in this area.

Hope this helps, good luck!
 

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x2, my first thoughts also, harness plug/ground issue as Colly mentioned ...
TB cleaned and tested? I think you can ohm the pins out and watch the numbers rise and fall as you open and close it, just not sure which pins to connect to ...
kinda an off the wall suggestion, can you disconnect the battery and pull the pcmb fuse overnight?
 

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kinda an off the wall suggestion, can you disconnect the battery and pull the pcmb fuse overnight?
Well, certainly, I'm not sure it will ultimately fix Scott's problem however, maybe temporarily?

Electrical "gremlins" are a real nightmare sometimes, sorry I can't offer more advice than that.

Again, try replacing the MAP plug harness Scott, (and MAP sensor if you've never replaced it) your issues you're describing almost seem very similar to what I was experiencing.

My truck would be perfectly fine one day, run like crap on the next(muggy, humid, hot day), rough idling was always an issue on those days.

Good luck man.
 
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2008 Colorado WT, 2.9L I4, 4X4, 5-speed manual
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Discussion Starter · #6 ·
Colly_shifter: Thank you for your insight and suggestions. I understand the MAP sensor and where it is; I'll unplug it and carefully inspect the harness and connector/pins to make sure everything there appears to be in working order. I may just take your suggestion and replace the MAP sensor, and if I do that I'll throw a harness on the order as they're probably not that expensive. I'm pretty surprised your 2006 didn't set a DTC for improper MAP voltage/faulty wiring with the bad harness. I just figured that mine is probably working because I can cold start the truck with the MAF sensor unplugged and it will run in open loop solely off the MAP sensor without issue. I guess this doesn't definitively mean the MAP sensor is good, but without it and the MAF sensor, I don't believe the ECM (PCM?) would have any idea how much air is going into the engine and it probably wouldn't even start. I guess I'll have to try it and find out.

I've never done anything with the throttle body in my truck. I've heard all this talk about rough idle issues, and cleaning the throttle body being a common fix. I should clean it just to rule it out, but my issue really involves the electronics having a hard time controlling the engine while under load at lower RPM, as I noted yesterday it's the worst under 1,500 RPM. The issue is with trying to control larger amounts of air/fuel, which is counterintuitive because you'd think the struggle would be with controlling small amounts, like it has to at idle. I should definitely check grounds, like you mentioned, and all the important connections like the megafuse. This is my salt truck, so it definitely has rust all over, and I know the gang of ground wires on the lower left side of the engine block are starting to look a little green.

You mentioned PassLock which reminded me of something that's maybe worth bringing up. As my truck's performance began to decline, so did its ability to start normally with the key. What started happening more and more often is that I will cycle the key to the "start" position and nothing will happen, however I'll hear a relay click. When I release the key to the "run" position the engine will do a CAN start (computer starts the engine). Most, but not all, of the times this happens the security light comes on on the dash and stays on solid. I can drive the vehicle, but the light stays on for the entire drive. I'm using the same key I've always used, which is a copy that is metal only, no plastic, so obviously this truck does not have a transponder in the key. I'm guessing this issue is separate from the performance issue, but thought I would bring it up just in case it's a clue of any kind.

I understand how emissions regulations, demand for higher MPG, safety, and "ease of use" have driven electronics to become so embedded in our vehicles, but I've just never been one to like and appreciate technology on my vehicle that I can't see, touch, feel, and tune with a screwdriver. I thought my 1997 S-10 was complicated with its electronics since, up until buying the Colorado a couple years ago, it was the newest vehicle I owned. After owning this Colorado, I have to say the S-10 seems rather simple: cable throttle, there's no variable position sensor on the clutch (just a safety switch that may have been bypassed), the dome light, radio, and "start" circuit aren't tied into the CAN bus (CAN'T bus?), manual headlight switch, no PassLock and there's no digital message center constantly telling me that the stupid TPMS sensors aren't reading because their batteries are dead. With the S-10 down for rebuild and the Colorado undriveable, I've reverted to driving my two "forever" trucks: a '73 F250 4X4, 300 CI I6 with a single barrel Carter and points in the distributor (don't need an OBD2 scanner to tell me what's going on in there) and the 1987 F-150 2WD, 300 CI I6 I converted from EFI to carb/mechanical ignition, same as the '73 F250. Both those trucks run like a sewing machine, they start every time, and the dome light does exactly what the door switch tells it to do. It doesn't turn on when I key off, then stay on after I close the door with some timer/relay that's going to stick "on" someday and run the battery dead. Nope; open the door, dome light turns on, close door, dome light turns off. Simple, just the way it should be.

If I could put a 1BBL carb and mechanical ignition on this 2.9L Colorado, I'd do it in a heartbeat. Too bad there's no place for a distributor.

Newcoly: I should probably clean the throttle body just to rule that out. I've tried disconnecting the battery for a long period to "reset" the computer to no avail. I was told that the run data stored in there is put into permanent memory that isn't cleared when power is disconnected, so this makes sense.
 
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