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OK, some quick advice needed.
The Problem:
I took my stage one 33gtr v-spec to Nissan Birstall nr Leeds last week to be undersealed. When it was on the ramp they phoned me saying they had noticed a large crack in the exhaust centre box and so offered to weld it up for a small fee. They showed me before and after pics so I believe them about the crack.

However on picking it up, the car is now misfiring but ONLY ON A TRAILING THROTTLE OR AT IDLE. Under load it drives like its normal 400bhp self. The misfire wasn't there before and I very much doubt undersealing has caused it.
Fuel consumption has also degraded alarmingly and you can almost see the needle move when stood in traffic. I believe with my limited experience that the misfire is 'cos it running very very rich and in some cases too rich to fire, hence the burbly grumbly miss-the-odd-beat-at-idle misfire.

It has new plugs (Trust/Greddy no. 8's) and new Splitfire coils so I am sort of ruling them out (I know you can get a bad plug but it was fine when I drove it to Nissan). Also ruling out inlet manifold leak and dodgy injector until someone tells me my theory below is stupid!

My theory:
Have they connected the earthing electrode of the welder to the lambda sensor (which is about a foot away from where the crack was) when they welded the exhaust and have somehow "blown" it when they started welding. I know they did not disconnect the car battery before welding as my Sony radio did not "lock out" and the clock in the car did not reset. So if the lambda sensor is now not working properly as a result of the welding and telling my ECU to run the car far too rich at low load/idle causing my issue.

I am taking the car back to them on Thursday and would dearly appreciate the advice of a welding or electronics bod on this before I stick 'em with this theory to make them pay for what they've done!

Thanks all in advance and sorry for going on...
 

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Put the car on lift and look at the O2sensor,maybe there are scratches:confused:

Every small child knows that the battery should be disconnected befor welding,but maybe they have a small box that they connected to the battery that is there to save the ecu to get damaged(dont know how the name is in english).

If it wasn´t there befor,it must have something to do with the things they made(and it cant be from the underseal:chuckle: )
 

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If it is misfireing off load it sounds like it may be ignition amp.
I had the same issue with my 33, it was ok on Boost but when the load was off it miss fired.

I can see how the Lamba sensor failure would affect the Fuel burn.
But not causing it to miss when off load.

There is a test where you can measure the out put of the Ignition amp when running and off load the voltage will be low, If I rember is should be 14 Volts when running, but mine was only at 7 volts to the cyl / Coil pack which was misfireing.

Nigel

Nigel
 
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