Howdy. Kind of dark out, I hooked up jumper cables backwards last week. Helper car (my Villager) would not start afterward. Taking a guess and going by how the engine almost started the first time cranking, I replaced fuel pump relay by taking one from the Air Cond relay. That got 'er to start.
But now the idle is erratic and she takes much longer than usual to warm up. Plus after getting almost to temp, the engine frequently (not always) surges to about 1500 rpm and stays there for 2-3 minutes before bringing itself back down. I did a lot of bi-directional tests thanks to having purchased a used Snap-On MTG-2500 a while back. Nothing showing wrong. No DTCs either. Care to take an educated guess and maybe end up hitting the right solution? I am boy... if that is acceptable.
1. Starting cold, the temp increase is uneven and slow. e.g: 101, 104, 102, 104, 106, 108, 110.
2. Idle initially hunts a bit after starting, which is unusual. Goes away quickly.
3. A few blocks later, still only at 158 - 161, then slowly higher.
4. No cabin heat yet. It should have begun blowing warmer air by then.
5. If going more than a couple miles, temp will rise to 198, then go a little bit higher than usual, 204, back down to 198 again.
6. Cabin heater will be working by that time.
7. This is when idle surge was occurring perhaps 4 out of the 12 times running since the reversed cable event.
8. Using a spare Villager, I replaced all relays under the hood, 10 of them. No improvement.
9. Checked most of the fuses that look in any way related, 5 under the dash, about 8 under the hood. There could be more elsewhere.
Being so erratic, I would lodge an amateur guess of ETC or the PCM. Might be way off though.