Bizarre trans/ECM (?) issue

[Bowtch]

New poster but longtime lurker here.. My 2011 Chevy Malibu (150k miles, LT) has a bizarre issue where if you're driving in slow stop and go traffic it'll lose power after sitting at a stop for a while, but it takes like 15-20 min of driving like that for it to happen. The car doesn't stall, but if you hit the gas it barely revs up and won't shift gears anymore. Putting it in reverse or neutral shifts really hard from drive.

If I'm in drive the car only revs up to maybe 1800 rpm from idling at about 800. If I put it in neutral or reverse the engine will rev like normal. I can get the car going maybe 15 mph, but the trans won't shift up another gear so that's about all I can get. The manual shifting mode also doesn't work while it's happening either.

It only happens in slow traffic, if driving normally it runs fine. Throws a generic transmission code (P0700 I think) when it does happen. If you turn the car off, wait 10-15 sec and turn it back on it'll run normal again for a few min then happen again. Leave the car off for an hour+ and it'll take 15-20 min of driving slow to happen again.

The dealer had the car for a week and couldn't replicate the problem, so I drove it around their parking lot slow stopping a lot and got it to happen, took it right into the garage and they still don't really know. Their best guess is something internal in the trans and want to tear it down, but dropping $4k+ on a fishing expedition is a little out of the question.. Any ideas? I feel like it has to be something with a computer but I'm pretty lost, and seems like the dealer is too.

 

[kornphlake]

I've had issues similar to what you describe when the battery and/or charging system is bad. My guess is the alternator is beginning to fail and doesn't charge the battery at low RPM, as the battery voltage begins to sag the various sensors go out of range and the computers go on the fritz with bad signals. The alternator is probably working at higher RPMs so you don't have problems in normal traffic conditions and the battery voltage recovers somewhat when the car is turned off. Assuming you live in north America it's gotten cold recently, battery problems are not uncommon when the temperature drops. It's easy to test (an auto parts store can load test the battery and alternator in a couple minutes) and a lot less expensive than a new transmission.

If you're not a shade tree mechanic yourself, you should take the car to an independant mechanic for a second opinion. Dealership service departments, in my opinion, are only good for warranty repairs, factory recalls and expensive oil changes. The mechanics at a dealership most often only see new vehicles and only know how to fix problems new vehicles might have. An independant mechanic will be more experienced working on a 9 year old vehicle with 150k miles.

 

[Bowtch]

I've had issues similar to what you describe when the battery and/or charging system is bad. My guess is the alternator is beginning to fail and doesn't charge the battery at low RPM, as the battery voltage begins to sag the various sensors go out of range and the computers go on the fritz with bad signals. The alternator is probably working at higher RPMs so you don't have problems in normal traffic conditions and the battery voltage recovers somewhat when the car is turned off. Assuming you live in north America it's gotten cold recently, battery problems are not uncommon when the temperature drops. It's easy to test (an auto parts store can load test the battery and alternator in a couple minutes) and a lot less expensive than a new transmission.

If you're not a shade tree mechanic yourself, you should take the car to an independant mechanic for a second opinion. Dealership service departments, in my opinion, are only good for warranty repairs, factory recalls and expensive oil changes. The mechanics at a dealership most often only see new vehicles and only know how to fix problems new vehicles might have. An independant mechanic will be more experienced working on a 9 year old vehicle with 150k miles.

Someone on Reddit suggested the car is going into limp mode, https://www.700r4transmissionhq.com/limp-mode-causes-and-solutions/

Now just need to figure why because there's clearly not a major problem. I like your theory, maybe there car is getting a faulty signal from a voltage sag and dumping the car into limp mode. Also gonna check the wiring harness from the ECM to the TCM