'94 Grand Am 3.1L with ~ 240k miles. Car was running fine (for a 24 year old car anyway) until immediately after the following general maintenance which happened at one sitting:
Changed engine oil & oil filter
Drained & refilled coolant 6qt
Add cooling system seal tablets
Changed PCV valve
Drained & refilled transmission oil with 6qt ATF & filter
Changed spark plugs – Iridium TT
It backfires, stumbles, & has severe lack of power. It stalled a couple times yesterday. Finally got a scan tool with ALDL adapter (don't have a Tech 1) that would read '94 GM OBD1.5 codes (paper clip won't work on this vehicle unfortunately) & DTC 77 was in the history. This is for EGR solenoid #3 malfunction. I could replace the EGR valve which I did at 186k miles with the same code. However, timing seems to suggest something else potentially.
I swapped back to the previous spark plugs which didn't help.
I've pulled all 3 ignition coils & checked secondary resitance which were ~6k & that's within spec according to this data sheet
It could be the intake plenum is dirty but odd that it would show up right after general maintenance.
Low coolant light is on even though I refilled to full. I've done much more than ever done previously to get out any trapped air if that's an issue.
I would appreciate any insight for what may be causing this problem all of a sudden.