Hi all I'm new here and just wanted to say hello ive been reading these forums for quite awhile and finally decided to make an account because ive had this problem with my 420a that no one else has semmed to have. It all started when i had a high flow cat welded on my exhaust and the dipsh*t welder didnt ground somthing out right and ended up frying my ecu.. After replacing a bunch of emmisions parts and other things i thought cuda been the problem i had it diagnosed at a local shop, finnaly realizing he had fried my ecu i put another one in from an older model(mines a 99). My car finally was running again but only on 2 cylinders, so i took it back to the shop and they still said i had the wrong ecu after searching for hours on forums I realized the older models had different ecu wiring harnesses so i switched a few wires around and it runs great now except now it smokes out the exhaust and has a bad smell. If anyone could help me with this I would greatly appreciate it. Thank You

ps. sorry if i posted this in the wrong section

Welcome to the forums, but I have to ask, how did welding on the exhaust fry the ecu?

that is a great question i have no idea, he didnt unhook the battery i noticed. Could that have been it? idk
But the fact that it smokes really bugs me as the car has less than 100k on it. i did run it on the 2 cylinders for a little while tryin to figure out the problem. Maybe i messed somthing up running it like that?

I guess I am just confused how melting metal to metal somehow causes an electrical surge to try a computer is where I am coming from.

Welcome to the boards.

Change the oil asap as the unburnt fuel will have slipped past the rings and into the oil.

With that said also do a compression check and see what that looks like.

If the person doing the welding grounded the welder to the frame and then welded on the exhaust that is mounted with rubber isolators and bolted to the engine with rusty bolts, the only reliable return path would be through the O2 sensor and the ECU.

The battery connected "can" help reduce voltage spikes when welding is preformed on cars but the welder needs to make sure that they have a true solid ground close the where they are working. If you disconnect the negative cable it is a good idea to jumper it to the positive terminal to prevent the main line from just floating.