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This has been an intermittent problem since 2009 at 48K miles. First appeared while driving on flat highway and noticed speed decreasing at steady throttle, got to the point it was almost WOT to maintain 50 MPH. Driving uphill was a problem, on a steep one, the trans would downshift to 3rd and RPM would get up to 4000 to maintain speed, Trans fluid temp would rise since it was out of lock-up. I had a scan tool with me and found no codes, everything seemed to be in spec.
Had the truck to a dealer twice along the way just to see if a Tech 1 would pick up something my OTC would not. At the second dealership I got a tech to ride along with the trailer attached and tried to duplicate the condition, naturally, everything was fine. After driving three weeks and over 4000 miles, I decided the crank sensor might be the problem and caused ignition timing to be off. When I replaced the sensor I found engine oil in the connector, cleaned the connector and the truck ran well for the rest of the trip.
Last year we encountered the same problem on a trip to Kentucky, the hills in Western Maryland were the worst. The engine likes to run around 2300 RPM, if it drops below that, I have trouble climbing a hill. When all is right, it will climb most hills and remain in OD, even accelerate in OD. When to power loss comes on, we are back to downshift and high RPM to maintain speed. After reading about some folks having TPS problems, I replaced mine and it ran well for several hundred miles, then messed up again although not as bad.
I have driven over 30K miles since this firs occurred and have still not whipped it. The problem only happens while towing and the trans in tow mode. When not towing, it runs perfectly. Anyone ever encounter this problem?
Just a guess on where to start, I would monitor live data while this is happening. Particularly TPS voltage and MAF data. With the key on engine off and scan tool reading live data, try to slowly increase throttle all the way to the floor and monitor the voltage on the TPS. It should increase linearly with no dips in voltage or erratic values. Should be something around .9V with foot not on pedal and around 4.5V with the pedal to the floor. This is just a rule of thumb spec though. But when they fail, they usually have "flat spots" with a gradual increase of the throttle opening. So best to do this test slow. Even if you say you replaced it, they can still go bad especially if it was aftermarket parts.
Other things that come to mind is maybe it is a transmission issue. For that I would start with getting resistance specs for the shift solenoids and pressure control solenoid/and other electrical solenoids and consider verifying they are within spec. Unless you have a fancy scan tool that can read transmission codes that you may or may not have, this might come down to dropping the pan and going old school with a ohm meter. For this you may need access to GM databases for these sort of specs, or if you can find a hard copy online somewhere. Maybe you could get lucky and the data for the transmission is online. I'm pretty sure you could buy a day or two pass for a small fee to gain access to service manual material from GM. I am not familiar with your particular vehicle though so its hard to say. Just throwing out ideas.
I have an OTC Genisys scan tool that gives me plenty of data. I checked the TPS a long time ago and have found that using an analog volt meter is better than a digital meter or scan tool, you can see problems more rapidly. The TPS tested fine, just shot in the dark when I replaced it.
During the first trip that we had the problem, I had driven about 2000 miles and stopped in Rapid City for a couple days. I checked the trans fluid and it was burned so I bought three gallons and did a "drain & fill". A couple months after getting home. I pulled the trans and disassembled it to check for burned clutches, all was good, just excessive heat from converter slippage that toasted the ATF.
The only time I have this problem is when towing our 28' travel trailer, that is no time for me to be looking at a scan tool. I did have a dealership tech ride along while towing and he found no problems. all readings were in spec.
If your towing you should be using tow haul mode or drive without overdrive as driving in overdrive will ballon the torque converter and send debris thru trans which gets into valvebody and on solinoids causing shift issues. The other thing to look at is maf and map sensor readings during these episodes as maf sensor signal effects trans shift points to a certain extent. Furthermore if fluid level is low will cause shift issues and burning of trans fluid. Did you check fluid level with engine running and in park when at operating temp? Thats the only way to check level of trans fluid.
I see. Yeah that definitely seems to be pointing in the direction of transmission control which is unfortunate because nobody wants to deal with that. If it were me, after confidently ruling out any other simple or less invasive possibility I would bite the bullet and drop the pan to resistance test the solenoids in the valve body. Its not the best test but usually that's the metric given in service data to condemn these parts.
Could of course be a mechanical issue with the transmission. If you could get your hands on factory service data I would use that to try to read up on the descriptions of operation of the transmission. Specifically how the tow circuit works electrically and mechanically. That may be able to give you a narrowed down clue as to what to focus on.
