There'* a few tests on the bottom of this post. https://www.gmforum.com/t284298/

Thanks! I'll test it after work.

Tested the TPS… it'* working just fine.

Colin, please start a new thread for your issues. Although your symptoms are similar to Rick'* he has a P0304 DTC and you have a P1870. Jumping into his thread with your issue is confusing because it is hard to tell which responses are for which questions. Start your own thread and we can focus on the unique elements of your issue.

Let'* get this thread back on track and focus on Meatcircus' issue.

Sorry. Same issue, different cause. I'll chime back in if I think of anything for him to try.

I worked for a Pontiac dealership for thirteen years, and we ran across this many times. I think I know exactly what you are referring to. GM released a bulletin to replace the valve body in the trans with an updated v/b. After a period of driving the valves in the valve body begin to stick in their bores, causing the tcc to lock and unlock, resulting in rpm fluctuation and very light surging.

It'* seeming like my problem is twofold. I replaced the plug and wire after noticing that the 4th cylinder plug and wire had carbon tracing. The problem cleared up a bit, but I still had a noticable surging, especially on my way home from work, after the car had been warmed up.

I'd check with a Dealer for the bulletin regarding the valve body issue.