Well, 2 days' work and a big anticlimax!

Just replaced the cat and sensor with an aftermarket straight thru s/s pipe and new oem o2 sensor (new old stock off ebay, mind).

The original cat was truely madly deeply connected to its inlet, and after a 22 year affair they took some heavy duty work to separate. I had to fabricate new corners on the inlet flange, but it all fitted fine. I disconnected the battery for the welding.

I also replaced both lower valve covers.

Now, she'll turn over and nearly run, but always dying, and then a pop of overfuelling.

Suspects:
1. Plug wires not seated (unlikely, I have pushed them on has hard as I can) or the single plug I removed for inspection back in wrong? Although one plug down wouldnt stop it running..

2. Something to do with the o2 sensor - but she wont start with it disconnected either (and she would run with the old one disconnected)

3. Something to do with the cat bypass...can't think why.

Any suggestions lads?

 

Sounds to me like multiple plug wires on wrong. Are you sure that you didn't may e get a few of them crossed. I'd triple check that. Hope you get it sorted. I'm doing the same work on my car right now along with new plugs, caps, rotors and a valve adjustment. Keep us updated.

 

I can't weld but I know that my P-Cars specialist unlplugs the DME and ABS controller before doing any welding on mine, to make sure they don't get fried during the work.

but hopefully Merle is right that it's just the plug wires that have been put on the wrong plugs.

 

What kind of welding ? Oxi/act , stick , etc ?
I once saw a police device , it put high voltage / amps into a car to kill the computer .
It was like a taser gun for cars .

 

Thanks for the replies!

I use MIG welding. I have swapped my DME relay for my spare but still no action.

I havent had the chance to check the plug wires, but they are pretty hard to mix up because of their lengths - anyone who has done this will know that there isnt much opportunity for error. Its hard to confuse which plug goes in which hole.

Got an early morning bike ride lined up, but should have some time tomorrow after that, will let you know.....

 

Distributer belt?

 

Super remote possibility but plausible: any chance of an obstruction in your new bypass pipe? I only say this because I know if you block the exhaust pipe, the engine would act like yours...won't start. So did you by any chance stick something in the exhaust manifold or the primary muffler to keep stuff out, and then forgot to remove it when you installed the bypass pipe? Just throwing you a bone, hoping that it's THE bone.

And yeah, the car can run without the 02 sensor connected.

 

Mig = oops , mig is a high amp / voltage welding . No computer should be hooked up when mig welding . When I weld I take the computer out of the car .

The DME relay has no computer in it , it does not care about the mig welding .

I don't think its a plug wire problem .

Do you know anyone who has a computer (DME) that you can try ?

A few weeks ago I watched a show about two guys riding bmws around the world , one of the bikes had a cracked frame out in the middle of nowhere , it might have been Mongolia , they got it to a guy who could weld it , they showed the guy stick welding it . When they got back on the bike the abs was no good , the bike would go right to lock up . The welding no doubt toasted the abs computer .
They said they could not ride the bike . They ditched the big BMW and picked up a very cheap new Russian two stroke , the guy riding it said he was so happy to be on the Russian two stroke , it was so much easier .

"A replacement was found locally (nicknamed 'The Red Devil'), a new Russian-made red IZh Planeta 5, purchased for about US$1,000. This cycle later developed gearbox problems. Two Mongolian sheep herders happened to pass by and quickly repaired the cycle, laughing at the fact that McGregor and Boorman had such a variety of high-quality tools and no idea how to fix the bike."
http://archive.izhmoto.ru/eng/planet

http://longwayround.com/journeys_long-way-round.htm

Its worth watching , just to spot all the mistakes .

 

This is looking bad, everything looks in place, so I can find no other reason for this than a fried ECU.

The car is just back from having some metalwork at a classic car body shop, and I bet they didnt disconnect the DME or ABS..
Reading around, there is a suggestion that leaving the battery connected might actually help prevent this by damping out voltage spikes - so I probably made it worse :-(. My earth clamp was right next to where I was working but it did arc off the brake disc once, maybe that killed it.

OK, >sigh<, here we go. I don't have any local friends with spare DME's, so I guess I'll send it for testing.

Thanks for helping me figure it out.

 

Do you have any other cars you might be able to drive in the meanwhile ?

 

I always clamp a surge protector to the battery when welding. Never had a problem.
Hope you get it sorted and hope its something cheap and simple.
Maybe it's something completely different and it's just coincidence.

 

Ha ha...

I can't think of any other coincidences that might be at play here. I found this place
http://www.the-ecu-doctor.co.uk/ which looks pretty good. If I can get it rebuilt for £150 it won't be the end of the world...

 

That looks like a handy find! Good luck with getting it fixed.

 

Hi Johnny

I've just done the decat and O2 sensor on mine. I've also done lots of MIG welding on my car.

I've never had any problems after welding. I always keep the earth clamp as close to the weld site
as possible and disconnect the cars battery. I note your comment regarding the arc to the brake disk...
I guess that would create quite a long circuit around the car, but I still struggle to believe that fried
the ECU.

Have you tried disconnecting the O2 sensor? Mine started fine even without it attached.
Check that when you reconnected the battery it's making a good connection.

Keep us posted.

 

"Q: Can welding on a vehicle damage its electronics? I have heard that I should disconnect the battery, computer, alternator and other electrical components before welding. This was the policy of a company I used to work for that made and mounted truck bodies. But many companies, including the one I work for now, never disconnect anything. After all, ships and steel-frame buildings with grounded frame systems don't require it. I have done structural work, such as extending truck frames and welding on tractors and on farm machinery connected to tractors, with no ill effects.

A: The sensors for your car's engine management computer can operate at millivolt voltages and at milliamp currents. A MIG or stick welder can reach 50 volts and hundreds of amps. Most of the time, the computer harness is insulated enough to prevent damage. A poor ground can cause eddy currents powerful enough to fry the electronics. If I need to weld on a car, I remove the computer and disconnect the battery ground cable to isolate the car's electrical system."