My 2004 4.4 v8 exhaust/CAT is popping/blowing a bit when cold. when it warms up it stops. I need to do a little bit of Mig welding.

I realise I need to disconnect the battery before starting to weld. Is there any thing else that needs doing? unplug any ECU's maybe?

Thank you. Gone:

Golf GT TDI 150- Still got.
BMW 325TD
Range Rover Classic
Defender 90

On cars, especially with ECUs, it is best to remove the piece that needs welding completely. The risk of high amperage currents straying the wrong way is quite high and they fry electronics in milliseconds (believe me I have seen it on automated MIG welding machines, all the way through to the control cabinet !!).

If you "can't" remove the offending piece (bodywork for example), remove the battery leads (both to be safe) from the battery. Removing all the ground wires on the car is a very big job but it would be reduce the risk even further.

Put the ground clamp as close as you can get it to the weld area on the piece that is to be welded. Make sure the contact area of the ground clamp is clean of rust etc. preferably power wire brush it to blank metal. This way the risk that the electrons go astray is minimized.

Still, if you can remove the exhaust part to be welded from the car completely, that would be ideal. MY 2010 5.0 SC Galway green and sand interior!!
Have the Faultmate MSV2 Extreme to be tinkering with the settings etc. !!

You could isolate the exhaust from the car by removing the engine earth strap, the exhaust is then on isolating rubber mountings.
I would check doing this isolates the exhaust by using a multimeter before starting the welding.

thank you.
once I have disconnected the earth strap, how do I check with a multi-meter? Gone:

Golf GT TDI 150- Still got.
BMW 325TD
Range Rover Classic
Defender 90

Set multimeter to the resistance scale.
One terminal of the multimeter to the exhaust, a cleaned bit, one terminal of the multimeter to the car body, again a cleaned bit.
The resistance reading should be infinity.

Last edited by nicedayforit on 11th May 2013 2:15pm. Edited 1 time in total

On my meter, the reading should be infinity for an open circuit - zero means it's a perfect connection......

Exhaust removal (well on TD6 anyway) is pretty straight forward, so would make working on it much easier to do this.  .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

Previous: L322 Range Rover TDV8 3.6 2008; L322 Range Rover TD6 3.0 2002; P38A Range Rover V8 1999

On a 4.4 the exhaust is all one piece and has pre and post cat sensors, which talk to the main engine ECU. You'll need to disconnect those or the mig will fry them and your ECU. If the hole isn't too large I'd probably just get some Loctite High Temp silicone sealant on it. Otherwise I'm sure a garage will weld it up for not too much. An hours labour is cheaper than a load of new ECU's. 
07 Supercharged - Still breaking her in!

Boot soundproofing part 1 - http://www.fullfatrr.com/forum/topic15612.html
Custom exhaust - http://www.fullfatrr.com/forum/topic15296.html
18" wheels for Brembo equipped cars - http://www.fullfatrr.com/forum/topic15052.html

Disconnecting earth straps and relying on rubber mounts to insulate away from a circuit path is fraught with danger, once you find out you have fried an ecu it's too late. Gas welding is your friend in this situation. Agueroooooooooo. 93-20
I swear you will never see anything like this ever again....watch it..drink it in.

Unfortunately gas welding is a skill that seems to become lost. My dad (7 can still do it (he did many exhaust and frame repairs on my old bangers) but I doubt whether he will want to fly over to the UK to do it

The oxygen sensors will definitely allow the current to run into the electronics, not only potentially frying the Engine Management ECU but also other ECUs.

Best bet is complete removal or isolation (disconnect the exhaust electrically including the sensors).

I would not take the risk, cost and work of fixing ECUs is just not worth it. MY 2010 5.0 SC Galway green and sand interior!!
Have the Faultmate MSV2 Extreme to be tinkering with the settings etc. !!

Thank you for all the replies.

Last time I used some Quick Steel. That worked well to fill the crack, so I may try a bit more. Taking the whole exhaust off seems a big job, but from all of the replies I don't think I will risk welding.

thank you. Gone:

Golf GT TDI 150- Still got.
BMW 325TD
Range Rover Classic
Defender 90

Informative thread

I'm assuming the ECUs are small boxes with circuits in them. Could they easily be unplugged and removed leaving just "dumb metal" for the electrons to wiz around. (Yes, I realize removing the exhaust would be easier in this case. I was wondering in general.)

ECU = Electronic Control Unit. They are little computers that do stuff like make the spark plugs spark at differing times, or the injectors change the fuelling depending on information they receive from other places (o2 / knock sensors, throttle pedal, air temp sensor). They control almost everything electrical on the car and are vital to it's operation.

I actually had to do the gaskets on my exhaust today and there's no way you'll be able to separate it on the stock exhaust. There are 4 lambda sensors (2 pre, 2 post-cat) and you'd need a ramp to get the stock exhaust off in 1 piece.

I used Loctite 5920 Silicon Copper sealant to sort the gaskets out, which only cost £5.49 from Halfords. I then covered it over with some clear silicon sealant (£4.99) to help protect it from road debris. Unfortunately it takes about 18-24 hours to cure fully with the thickness I have used, so won't be able to report how successful it was until probably tomorrow or Monday.

However in the past I've used the same approach on high powered turbo cars and they never managed to burn the gaskets out either. Although with this car I am a little worried as I've managed to blue the stainless steel nearly all the way back to the mufflers at the exit, which indicates a ridiculously high temperature exhaust :/ 
07 Supercharged - Still breaking her in!

Boot soundproofing part 1 - http://www.fullfatrr.com/forum/topic15612.html
Custom exhaust - http://www.fullfatrr.com/forum/topic15296.html
18" wheels for Brembo equipped cars - http://www.fullfatrr.com/forum/topic15052.html

You are quite right about unplugging and leaving dumb metal but how do you prove everything is isolated before applying a current in the form of welding? Sometimes it's worse to apply a voltage to ones side of a circuit because there is no complete circuit to control the voltage, no reference if you like makes a voltage unstable and can result in damage to electronic circuitry even at low voltages. Better safe than costly. Agueroooooooooo. 93-20
I swear you will never see anything like this ever again....watch it..drink it in.

nicedayforit wrote:

You could isolate the exhaust from the car by removing the engine earth strap, the exhaust is then on isolating rubber mountings.
I would check doing this isolates the exhaust by using a multimeter before starting the welding.

On my own 4.4 there's an earth strap on each exhaust rubber mounting as well. So they would all have to come off.