I recently purchased a 2004 Discovery HSE, 113k miles. I had head gaskets replaced, entire cooling system replaced, new plugs, new wires, both new coils, 8 new fuel injectors, plus an air idle control valve. Upon reassembly, I have a flashing check engine light, obvious misfires or lost engine performance. Random misfire codes, misfire on 4 and 7 I believe. Mechanic who did all the work thinks it's exhaust system related now and my muffler shop guy briefly looked at the catalytic converter and said it was obviously clogged. What are my chances of having a good running truck if I replace the cat? Any other suggestions? I'm running out of money...

 

If the cats are clogged then that will kill power and cause misses.

One easy way to find out is to just cut the cats out and run the truck. It'll be loud, but if you get power back and it stops missing you'll know that's your problem. Cut as close to the weld on either end as you can. That way you have as much material as possible to have new cats welded in at your muffler shop.

 

Appears you've been in the hands of goodwrench types and already spent some bucks. I'd press the HG mech to help out on paying for the additional repairs if you were told his work was going to cure all your problems. Unless he's willing to cut the cats or you want to get dirty and do same as suggested you may have to fork out some more bucks to see if problem resolved.

Others may have additional helpful tips or better plans for action.
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The greatest shame in the US is they enforce the 'emissions' junk on all cars to the 'enth degree including O2 sensors which have an ongoing derogatory effect on the power and mpg's and cost big $$$. I got rid of my EGR, DPF and have now installed a straight through exhaust pipe in place of the centre muffler but retained the final resonator exhaust box, the spooling time of the turbo is far quicker and the mpg's greater, it goes to show just how much the centre exhaust muffler restricts the power and for not much more additional noise even externally, maybe <5dba.

 

Your codes are limited to misfires. Are you getting catalytic codes P0420 or P0430? How did the muffler shop guy determine its clogged by briefly looking at it? I would verify the correct codes and go from there.

 

The misfires you state, if correct, are on different cylinder banks. Each cylinder bank has its own cat. It's possible both are bad; I don't know how probable or improbable that is.

Cylinders 4 and 7 are sparked from the same coil pack, although they aren't paired. I know swapping wires around isn't fun on a DII, but you could try swapping the wires for 3 and 4 and see if the misfire follows. They're the same length. Same with 7 and 8.

I'd also double check that the wires are firmly seated on both the coils and the plugs. I replaced the wires on my '99 DII and getting the terminals to seat on the coil took a little extra effort in a couple cases.

 

Can I chime in with clean your gas cap and spout, reset codes, go for a drive to see if the codes come back?

 

Been thru this a while back.

Here is my 2 cents:

LandRover