cat removal dilemma

fenton · Post by **fenton** » Fri Jul 25, 2014 5:57 am

Hi everyone
I have a 98 discovery v8 import with 50K on the clock. I use for on road and off.
I have had a misfire which seems to have blown a cat.
Sorted the misfire now. But not sure about the cats. Mot testing is not an issue.
The cats have two sensors per side.
Iwas thinking of removing the cats and senors and running the 470ohm tune resistor. But i dont want to increase fuel consumption. Can i remove cats and keep sensors and stay on same tune resistor.
I have been offered a seconhand pair of cats but i could have the cats removed for the same money and never have a problem again.
Engine is standard but the ecu is in a aluminium cast box certainly looks like it came out of the factory with it anyone else seen one of these. Is it maybe specific to the imports

stevieturbo · Post by **stevieturbo** » Fri Jul 25, 2014 7:54 am

The rear sensor is purely to measure cat efficiency.

Most cat delete jobbies only deal with this rear sensor

So front sensor operates as normal, engine should run as normal with no change to fuel use etc

Obviously factor in whether or not your vehicle needs cats to pass an MOT.

fenton · Post by **fenton** » Fri Jul 25, 2014 8:22 am

Cheers for that.
Are you meaning if I remove the cats that I should keep the front sensor and not change the tune resistor.

stevieturbo · Post by **stevieturbo** » Fri Jul 25, 2014 9:17 am

fenton wrote:Cheers for that.
Are you meaning if I remove the cats that I should keep the front sensor and not change the tune resistor.

Typically any resistors or cheats are to make the rear reading acceptable to the ecu
The fronts would operate as normal.

fenton · Post by **fenton** » Fri Jul 25, 2014 9:32 am

So remove cats and rear sensors keep the front sensors and change to 470 ohm resistor for no cats

stevieturbo · Post by **stevieturbo** » Fri Jul 25, 2014 9:35 am

If that's the resistor people are using, then ok.

Ive no idea what cheats people are using these days. Most modern cars they'd just flash the ecu and turn off any codes relating to the rear sensors.

fenton · Post by **fenton** » Fri Jul 25, 2014 9:51 am

Thank you very much for your help.

DaveEFI · Post by **DaveEFI** » Fri Jul 25, 2014 12:15 pm

Talk of the tuning resistor suggests it's Lucas hotwire? If so, versions of that didn't use a cat. You only need a narrow band sensor when you have to keep the mixture at stoich. to avoid damage to the cat. With no cat, it could be run a lot weaker at cruise and improve the economy. And perhaps richer at full power to improve performance. But I dunno exactly what this would involve with the hotwire.

fenton · Post by **fenton** » Fri Jul 25, 2014 12:58 pm

Yep lucas hotwire with serpentine belt 1998 model. Two very large cats two sensors each side one in front one behind cats. I thought all the serp 3.9 v8 had cats

DaveEFI · Post by **DaveEFI** » Fri Jul 25, 2014 2:34 pm

fenton wrote:Yep lucas hotwire with serpentine belt 1998 model. Two very large cats two sensors each side one in front one behind cats. I thought all the serp 3.9 v8 had cats

The serps may well all have had cats, but the same(ish) engine with the same(ish) injection on earlier ones didn't.

fenton · Post by **fenton** » Tue Jul 29, 2014 12:06 am

Well the cats are coming off tomorrow and stainless pipe going in its place. I was going to ask them too keep the sensors in situ just in case they were ever needed.
I have already swapped the tune resistor for a potentiometer set to 470 ohms. Which should have selected the no cats fuel map. First thing I have noticed is that it's hunting on idle in park but I suppose that could be due to a partially blocked cat.

DaveEFI · Post by **DaveEFI** » Tue Jul 29, 2014 8:33 am

If you've got a handy bung, well worth fitting a wideband O2 with reader so you can find out for certain what the mixture's doing.

fenton · Post by **fenton** » Wed Jul 30, 2014 10:43 pm

Well cats off and sensors left in situ.
470 ohm tune resistor in loom. All ok apart from idle is hunting when it's in park. And a little when it's in drive ???.
Any ideas. I don't have ecu mate to be able to check it's recognised the resistor correctly.