Denali 6.0 bucking under load

catfishblues

catfishblues

Well-Known Member
Joined
Jul 26, 2005
Location
Pfafftown, NC
I've got an 03 Denali XL with the 6.0, 4l65e, and about 180k on the clock. It could use a tune-up and it's throwing a code for the upstream bank 2 O2 sensor heater circuit, but runs well typically. Unloaded, you really don't notice it. Put a ton or so on the hitch, though, and you start to notice a pretty violent buck when pulling a hill or trying to maintain highway speeds. It almost feels like a misfire, but it reminds me of a u-joint going bad when you're pulling away from a stop, kinda like a clunk. When I get time, I'm going to go through all the basics that I know I need to do. Plugs, filters, checking joints, etc. I was just curious if this was a common problem that someone here was aware of. Thanks.
 
Subscribed, I have a 04 Yukon XL Denali with 150k
 
Mac5005 said:
Crank sensor? The one on my 6.0 went out at 230k. I thought the torque converter was locking and unlocking, but it was shutting the ignition off and then back on. Only did it under load.
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Did it throw a code? I've also got a hunting idle when I crank it sometimes, along with a couple other oddball issues, but this one is the killer right now. Overall, though, I love the truck. I don't have any high-zoot diagnostic equipment, but I can run a torquepro scan and do screenshots and whatnot.
 
My 03 Yukon xl does that occasionally, and since I got it.. I also thought it was because its a big dumb slow car :lol:
 
Perhaps the knock sensors are throwing codes and kicking the timing back down? They're notorious for going out.
 
That's an ignition failure I would bet. Start with a quality tune up and clean your throttle body thoroughly with a toothbrush and some aerosol cleaner. My FIL had the same issue on his 04 5.3 with no codes and a good set of wires and plugs stopped the buck and the TB cleaning took care of the hunting idle. I also put a fuel filter on it while it was in the shop but it was in good shape.
 
Infamous1 said:
That's an ignition failure I would bet. Start with a quality tune up and clean your throttle body thoroughly with a toothbrush and some aerosol cleaner. My FIL had the same issue on his 04 5.3 with no codes and a good set of wires and plugs stopped the buck and the TB cleaning took care of the hunting idle. I also put a fuel filter on it while it was in the shop but it was in good shape.
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You're probably right. I need to set aside some time to deal with this.
 
Is the tachometer bouncing a little when engine is bucking? Crank position sensor.
 
catfishblues said:
Did it throw a code? I've also got a hunting idle when I crank it sometimes, along with a couple other oddball issues, but this one is the killer right now. Overall, though, I love the truck. I don't have any high-zoot diagnostic equipment, but I can run a torquepro scan and do screenshots and whatnot.
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No codes at all. Started only at 160 degree engine temp, so was hard to make the problem happen as it was fine below that temp and once warmed up all the way. Steadily got worse to the point it would barely do 50 mph with no load to get home....
 
bad plug wires! But like Infamous1 said do a complete tune-up on it.
 
It's absolutely worth it to get some 8.5mm MSD plug wires for it or any other aftermarket brand for that matter. A lot of the time Summit sells them cheaper than what a parts house wants for stock replacements and they'll handle heat far better. I run either NGK or AC Delco plugs.
 
mysterync said:
Think I'd find a scan tool and do a little diag work instead of guessing and suggesting all the common problems. Just my 2 cents.
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I'm not a parts tosser. I just like to be armed with as much knowledge as possible before I start troubleshooting. Naturally, a good tune-up is a logical starting point. It's due for one anyway, I'm sure. As for the scan tools, I own a decent older OBD2 scanner and a newer bluetooth scanner with the TorquePro app, but it's still somewhat limited in its realtime diagnostics.
 
If you can read the 02 voltages, that will be a dead giveaway for a miss. I had two burnt wires that were causing a loaded misfire in my truck, but no MIL popped up.
 
Croatan_Kid said:
If you can read the 02 voltages, that will be a dead giveaway for a miss. I had two burnt wires that were causing a loaded misfire in my truck, but no MIL popped up.
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I don't know if it's the adapter or the app, but when I use the bluetooth scanner with TorquePro, (which people really seem to like,) the O2 voltages seem all over the place. Rarely steady and often outside what I'd consider acceptable. I don't know whether it's set up wrong or what. I really haven't dug into it that much. I might do some more digging into that tonight. The delay in the readout throws me off when I'm watching it.
 
mysterync said:
Bank 1 sensors should never be steady. You should see fluctuations between 0mv-900mv sub 450mv is lean, above is rich.
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Whyfor? I know the fuel injection is reacting to what the O2 sensors feed it, so I expect them to jump around a little, but I thought the goal was to keep things as close to stoich as possible, and the readout from the sensors should stay close to half a volt. School me. I understand a fair amount about what's supposed to be happening, but troubleshoting FI on this level is something I need to understand more deeply.
 
catfishblues said:
Whyfor? I know the fuel injection is reacting to what the O2 sensors feed it, so I expect them to jump around a little, but I thought the goal was to keep things as close to stoich as possible, and the readout from the sensors should stay close to half a volt. School me. I understand a fair amount about what's supposed to be happening, but troubleshoting FI on this level is something I need to understand more deeply.
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It plays in part to catalyst oxygen storage, and catalyst efficiency.
Check out this video, it'll explain it better than I can!
 
Need a good scan tool that will read the misfire counters while the problem occurs. As you have stated you need a tune up, so doing one really isn't throwing parts at it. The way you are describing the symptoms it sounds to me like a lockup converter shudder or the solenoid giving you a problem. May want to look at the transmission fluid to see if it's burnt.
 
3DCrawler said:
Need a good scan tool that will read the misfire counters while the problem occurs. As you have stated you need a tune up, so doing one really isn't throwing parts at it. The way you are describing the symptoms it sounds to me like a lockup converter shudder or the solenoid giving you a problem. May want to look at the transmission fluid to see if it's burnt.
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Good point. I checked it a month ago and it was starting to turn. Still more red than brown, and smelled fine, but it was about a pint low and I had noticed a slip between 2nd and 3rd in part-throttle acceleration when cold. That seems to have cleared up since I topped it off, but it also could use a service. I just haven't had time to do much more than drive it lately, so I've been trying to keep the miles down until I can spend some time on it.
 
If they're all over the place, that's a misfire. Under normal operation, they'll have a range that they will stay in, but if it's 50mv up to 800mv, that's not normal. I've got some data logs from a buddy's truck that are like that, whereas mine are consistent.
 
Croatan_Kid said:
If they're all over the place, that's a misfire. Under normal operation, they'll have a range that they will stay in, but if it's 50mv up to 800mv, that's not normal. I've got some data logs from a buddy's truck that are like that, whereas mine are consistent.
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Misfires cause abnormal oxygen sensor readings, but they can't be used to determine if in fact it's a misfire. Many different types of misfires, cause any different types of exhaust gas mixtures. If it's a misfire, the misfire counter will show it. If fuel trims are in typical range oxygen sensor switching is useless for misfire diagnostics. I guess what I'm trying to say is that the range (50-800mv) is not indicate of a misfire or not.
 
I knew what you were saying, I was just letting him know that's what I had found out. This was recent actually...tuning my buddy's ProCharged Silverado SS and the tuner looked at the data log and said there's a misfire. Odd thing was, we couldn't feel it while driving the truck.

*if they bounce from one extreme to the other, something is usually amiss. (See what I did there?!):lol:
 
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