Justin954

I have a 2015 Chevy Silverado 1500 LT with a 5.3L V8 and 6L80 Automatic Transmission with 110,000 miles on it. Around 105,000 miles I started noticing that when the transmission gets up to temperature it starting to shift later (roughly 1,000 RPM higher). I don't usually drive long distances so I don't notice it much. But when I drive for longer periods of time and on the highway is when I start to notice the problem. It doesn't seem to act up if I'm just driving on surface streets but I do notice it after driving on the highway for 10 or more miles and then get off the highway and drive on surface streets. After getting off the highway its more sluggish. The RPMs will ramp up to around 3,000 RPM before it'll start to shift and when it's operating normally it shifts around 1,500-2,000 RPM depending on how hard I get on it. I just recently changed the transmission fluid and filter hoping it would improve but I drove it for the first long distance since changing the fluid and filter and monitored the temperatures to see if maybe it would run cooler since this was the first fluid and filter change I've done on the transmission. But it's still acting the same. When I dropped the pan the fluid has black and there was some metal dust on the magnetic but not more than you'd think to see with 110,000 miles on it. The transmission runs great until it get around 170 degrees then I notice the shift points start to get later until it starts to not want to shift until around 3,000-3,500 RPM. I have to take it easy on the throttle to get it to shift better. I just ordered a thermostat bypass kit to see if it'll atleast run cooler so I won't have any issues. Twice in the past few months I've got P700 (Transmission Control System Malfunction) and P741 (Torque Converter Clutch Circuit Performance or Stuck Off) while driving on the highway but haven't gotten them again since January. I'm suspecting I have TCM problem or Torque Converter problem. But before I go spending hundreds of dollars on things that may fix the problem I'd like to be sure. If anyone has any ideas or had similar problem any input would be greatly appreciated.

dpieroni3

When warmed up, monitor TCC slip speed. You may find it excessive. I see this fairly regular, and the failing torque converter scores the oil pump and then usually the 456 clutch is the first to go. A bit more diag maybe needed to confirm, but from what you said this may be a direction to look. Hope this is helpful.

Justin954

How can i monitor the TCC slip speed? This weekend I was driving down the highway and I kind of got on it a bit to go around some slow moving traffic and I heard like a snapping sound which I immediately looked at my RPMs and they were around 4,500 RPM then it immediately dropped down to around 2,000 RPM followed by a flashing check engine light. The truck continued to run normally after that. But then on Monday I noticed while driving it makes kind of a knocking or skipping gear noise every so often. Best way I can describe it is like a rod knock. I had my dad drive with me last night so he could hear it to see what he thinks and that what he thought it sounds like. But it doesn't do it sitting at a stop light only does it while accelerating and when the transmission is in I believe 4th gear but it's not all the time. It's every couple of minutes it will do it for a second or two then be alright. I've tried holding the brake and throttling it but doesn't do it. I've put it in neutral and throttled it and doesn't do it. I've turned off the overdrive just to see if it still does it and it does. I'm going to try manually shiftting it to see if I can determine exactly which gears its doing it in. But I'm pretty sure it starts to do it in 3rd or 4th gear. it kind of seems like a gear isn't engaging/disengaging intermittently. I'm pretty sure it's the transmission doing it and not the engine as it seems to stop once it shifts to the next gear. I'm thinking of buying a VCX Nano to see if I can monitor or record when it happens to try to determine what may have broke or is failing.

jfmorris

This sounds similar to the failure I had on my 2003 Yukon when it had about 130,000 miles on it. It was still running and moving, but shifting wrong, and making noises you don't want to hear. Cost me about $2200 (10% cash discount) out the door at a place called Mr. Transmission to get it fixed, and it ran fine the next 70,000 miles before we sold it. They had to replace a couple of gears and a solenoid or two as well.

I advice not continuing to drive it around, but to get it into a 3rd party transmission shop (you don't want the dealer - they will likely want you to pay for a whole new transmission), and get it diagnosed and repaired, before its too torn up to be rebuilt/fixed.

Justin954

Last night i figured out what the knocking sound was. On my way home from work last night I decided to video my dash while driving so I can see what my gauges do when it happens and noticed it only does it when the AFM (Active Fuel Management) System is active and running on only 4 cylinders. After doing some research most people say it's due to a failed/failing AFM lifter. They said if you put the truck in Manual 5th gear it'll disable the AFM System so I tried that last night and no problems. Knocking went away. I've ordered a AFM Disabler that comes in today that I'm going to plug in to disable the AFM System permenantly until I can tear into my engine to see what failed. I have a feeling it is one of the AFM lifters. Back before I flushed my transmission I noticed it would make a knocking sound when I'd start my truck up for a minute or so then go away and be fine the rest of the time. A co-worker said he thought it sounds like a leaking hydraulic lifter that when the truck is off the oil leaks out causing it to go flat but once you start the truck up it'll knock until it fills back up with oil. When I did flush my transmission we were looking at the engine and found several of the exhaust manifold bolts sheard off and after removing the broken bolts and installing all new exhaust manifold bolts that fixed the knocking problem but I have a feeling it only masked the problem. Then the other day when I got on it I think the lifter probably either failed or got stuck and that was that snapping sound I heard and now it's knocking because that lifter is shot. For now it seems to be fine but I'm going to have to pull my right valve cover off and check all my rockers to see if any of them have any play in them. Also not sure if it's just a coincidence but so far it's also shifting better now that I have temporarily disabled the AFM System. But it's only been a few test drives so we'll see if that continues to be alright. But yeah I have a feeling in my near future I'm going to be doing a AFM deactivation kit with a new cam and oil pump and possibly a transmission rebuild and Torque Converter upgrade. But that'll have to wait until I can get my car up and running so I have another vehicle to drive. I'm starting to regret buying this truck. I also thought Chevy/GM vehicles were reliable but after doing alot of research on these issues these trucks are garbage and fail around 100K miles. Should have just but an old pick up truck and fixed it up. Atleast it would be more reliable. Cars and trucks today just aren't built to last and it's really sad. How GM/Chevy hasn't been sued into oblivion for engineering such a lemon is beyond me. I'm surprised there isn't a Class Action Lawsuit over these ****ty lifters and transmissions they installed in these vehicles. My car I did an engine swap and installed a used C6 Transmission and torque converter with over 300K miles on it and it still runs like a champ. Maybe I should look into an engine/transmission swap for this truck and find something more reliable to put in it.

jfmorris

Glad you have some idea of what is going on now.

Be careful with an engine/transmission swap, as with modern vehicles, EVERYTHING is integrated with the computer system, and also tied to the VIN of the vehicle. Swapping means swapping the engine *AND* possibly the computer and the wiring harness, and having the capability to reprogram that computer to work in your vehicle. Look through the forum here and you will see people discussing this - I think you have to disable the theft system for example if there is a VIN mismatch between the computer/engine/dash.

I agree that in many ways the older GM trucks were better made. My 1996 C1500 most definitely has thicker metal in the body panels and such than my 2006 Silverado. I can't say that the 1996 has been problem free, although it is running well on the original engine and transmission at 201,750 miles. I just replaced the original starter last week, and noted the mileage, after having to get towed back to the house when the truck wouldn't start. And I've replaced the intake manifold gasket twice in 24 years of owning that 28 year old truck.