Hi everybody. New to this forum but was kindly referred here by the "archive forum" over at trailvoy. Regardless, I've found your forums very helpful and interesting! Thank you for all the effort & hard work that undoubtedly goes into making them great! REALLY REALLY nice landing/home page too BTW 
Now for some head scratching...
Never really tried to “haul” anything with my truck until now, but I more or less sought out a V8 (which is quite hard to find in a mid-size SUV!) solely for the purpose hauling my car to the track and back. I'm new to US vehicles and trucks/SUV's in particular, so pardon any noobish questions, but did some searching in the forums regarding my situation only to find that each similar issue I came across was actually slightly different… So perhaps to add to the confusion, but hopefully not, here it goes:
TRUCK: 2008 Envoy Denali 5.3L 4WD with 3.42 rear end. 120xxx miles with new spark plugs and complete trans fluid change out 3k ago. Never had a problem with her till now
SITUATION: Hauling 4600lbs load on double axle trailer. 2400lbs car on a 2200lbs inertia brake trailer across very flat land.
HOW IT HAPPENED:
Loaded car up and headed out of town. Truck pulled car no problem, brakes seemed sufficient. Made it about 2 miles from home @ 35mph (1 mile flat followed by 1 mile slight downhill grade) to stop light. Suddenly truck began running really rough and lacked power. YIKES! Turned around and went back home. About 3 miles into my 4 mile round trip, the CEL popped on. Few moments later it started blinking. Engine still spitting and sputtering but pulled trailer okay. Drove home slowly @ ~25mph. Despite CEL blinking, no messages on digital odo readout. I even searched through all the menus looking for some sort of indication about what was up.
Backed truck into driveway, shut down and rechecked all fluids—everything normal
Started truck again and miraculously it ran fine!
No CEL or anything. WEIRD…” Bad gas?” I thought. I was almost empty anyway so I filled her up with full tank of premium 93 octane two blocks from home and hit the road.
Almost EXACTLY 2 miles out the whole episode repeats.
Pulled over, shut down and restarted but engine still sputtering and CEL still blinking. Shut down for 10 minutes. Try again, no luck.
Limped back home and let her sit overnight and all next day, but still ran like crap. Disconnected battery the next day and she started up and ran fine—even pulled 2200lbs empty trailer back to UHaul (6 mile round trip) without missing a beat. No more CEL.
Drove mixed city/highway next day for about 30 miles. Suddenly CEL comes on with a P0420 code. I figured probably related to previous misfiring and just now registering since I only drove about 10 minutes and 5 miles while misfiring.
Despite P0420 car is running great. Came home today and disconnected battery and cleaned up TB. (It didn’t really seem very dirty, but it’s sparkling and shiny now) Drove another 30 miles of mixed city/highway to give truck time to cycle through system checks etc. Drives great, no CEL.
NoObish? QUESTIONS:
1. Will the truck acknowledge “limp mode” in the digital odo readout somewhere? I cant tell if I was in limp mode or misfiring. I have no idea what limp mode looks/feels/sounds like, but there was no SES or any other message displayed.
2. What happens if one tries to tow too much weight? At 4700lbs total, my truck should have been able to pull this setup no problem, but this was the first real load I ever put on her.
3. What other ignition issues are typical for this truck? Misfiring is generally related to coil packs and spark plugs, but I’ve never heard of a CP failing terribly (truck ran like it was more than just 1 misfiring cylinder) only to then just return to normal simply by disconnecting & reconnecting the battery.
4. Will the engine management system throw a code and then lock itself into that mode? Meaning, if a misfire is detected, does it just quit trying to fire that coil pack or something?
5. What’s a LEAN engine run, sound and smell like? I haven’t changed fuel filter yet. It’s next on my list, but I doubt I was running lean as exhaust was heavy and very rich smelling.
6. What else am I missing?
FINAL THOUGHTS: Perhaps it was “purely coincidental” that I was hauling a heavy load when this happened, but I doubt it. I’ve never had a single problem with this truck otherwise. Including hauling a popup camper, jet skis, and utility trailer…. Though I’ve only had the truck since July and have only put about 5k miles since then. Having never owned an American vehicle before, I really wanted to give GM a chance. Granted 2008 was the height of “The Dark Years” for GM, but it’s my understanding that this 5.3L and 4L60 transmission are supposed to be a pretty solid, tried & true combo.
. I only bought this truck b/c I wanted to haul my car to the track. I can live with 16mpg if she’s up for the car-hauling task, but if this thing’s only good for a kittern fart with of torque, I’m going to be really, really upset. 
I do like my Envoy, so plz dont tell me I should've just bought a 4Runner... I get enough of that as it is from my euro/jap loving family
Now for some head scratching...
Never really tried to “haul” anything with my truck until now, but I more or less sought out a V8 (which is quite hard to find in a mid-size SUV!) solely for the purpose hauling my car to the track and back. I'm new to US vehicles and trucks/SUV's in particular, so pardon any noobish questions, but did some searching in the forums regarding my situation only to find that each similar issue I came across was actually slightly different… So perhaps to add to the confusion, but hopefully not, here it goes:
TRUCK: 2008 Envoy Denali 5.3L 4WD with 3.42 rear end. 120xxx miles with new spark plugs and complete trans fluid change out 3k ago. Never had a problem with her till now
SITUATION: Hauling 4600lbs load on double axle trailer. 2400lbs car on a 2200lbs inertia brake trailer across very flat land.
HOW IT HAPPENED:
Loaded car up and headed out of town. Truck pulled car no problem, brakes seemed sufficient. Made it about 2 miles from home @ 35mph (1 mile flat followed by 1 mile slight downhill grade) to stop light. Suddenly truck began running really rough and lacked power. YIKES! Turned around and went back home. About 3 miles into my 4 mile round trip, the CEL popped on. Few moments later it started blinking. Engine still spitting and sputtering but pulled trailer okay. Drove home slowly @ ~25mph. Despite CEL blinking, no messages on digital odo readout. I even searched through all the menus looking for some sort of indication about what was up.
Backed truck into driveway, shut down and rechecked all fluids—everything normal
Started truck again and miraculously it ran fine!
No CEL or anything. WEIRD…” Bad gas?” I thought. I was almost empty anyway so I filled her up with full tank of premium 93 octane two blocks from home and hit the road. Almost EXACTLY 2 miles out the whole episode repeats.
Pulled over, shut down and restarted but engine still sputtering and CEL still blinking. Shut down for 10 minutes. Try again, no luck.Limped back home and let her sit overnight and all next day, but still ran like crap. Disconnected battery the next day and she started up and ran fine—even pulled 2200lbs empty trailer back to UHaul (6 mile round trip) without missing a beat. No more CEL.
Drove mixed city/highway next day for about 30 miles. Suddenly CEL comes on with a P0420 code. I figured probably related to previous misfiring and just now registering since I only drove about 10 minutes and 5 miles while misfiring.
Despite P0420 car is running great. Came home today and disconnected battery and cleaned up TB. (It didn’t really seem very dirty, but it’s sparkling and shiny now) Drove another 30 miles of mixed city/highway to give truck time to cycle through system checks etc. Drives great, no CEL.
NoObish? QUESTIONS:1. Will the truck acknowledge “limp mode” in the digital odo readout somewhere? I cant tell if I was in limp mode or misfiring. I have no idea what limp mode looks/feels/sounds like, but there was no SES or any other message displayed.
2. What happens if one tries to tow too much weight? At 4700lbs total, my truck should have been able to pull this setup no problem, but this was the first real load I ever put on her.
3. What other ignition issues are typical for this truck? Misfiring is generally related to coil packs and spark plugs, but I’ve never heard of a CP failing terribly (truck ran like it was more than just 1 misfiring cylinder) only to then just return to normal simply by disconnecting & reconnecting the battery.
4. Will the engine management system throw a code and then lock itself into that mode? Meaning, if a misfire is detected, does it just quit trying to fire that coil pack or something?
5. What’s a LEAN engine run, sound and smell like? I haven’t changed fuel filter yet. It’s next on my list, but I doubt I was running lean as exhaust was heavy and very rich smelling.
6. What else am I missing?
FINAL THOUGHTS: Perhaps it was “purely coincidental” that I was hauling a heavy load when this happened, but I doubt it. I’ve never had a single problem with this truck otherwise. Including hauling a popup camper, jet skis, and utility trailer…. Though I’ve only had the truck since July and have only put about 5k miles since then. Having never owned an American vehicle before, I really wanted to give GM a chance. Granted 2008 was the height of “The Dark Years” for GM, but it’s my understanding that this 5.3L and 4L60 transmission are supposed to be a pretty solid, tried & true combo.
. I only bought this truck b/c I wanted to haul my car to the track. I can live with 16mpg if she’s up for the car-hauling task, but if this thing’s only good for a kittern fart with of torque, I’m going to be really, really upset. I do like my Envoy, so plz dont tell me I should've just bought a 4Runner... I get enough of that as it is from my euro/jap loving family
