172 Decat pipe - The next stage !

[Simon]

Ive now taken the decat pipe to the next level on my 172 - fooling the ECU into thinking that the rear lambda sensor (downstream) is working and I have a cat fitted to the car !

The rear lambda sensor has been removed and the 2nd hole in the decat pipe plugged. A resistor has been placed inline between the ECU and wiring loom to the lambda sensors.

Initially the resistor was going to be placed between rear sensor and wiring loom but this wasnt possible (earth problem or something) - thus sensor removed, etc.

First 24 hours after mod. I had two occasions where I was starting the car from cold and the engine turned over several times before firing - which was rather concerning as it has always done it on first fire.

Since then the engine has fired up as normal on cold. It seems a little lumpy on idle to start with.

Im still in two minds whether to bin the decat pipe off and go back to the cats. Decat gives better mid range pull and car doesnt feel strangled top end.

 

[BenR]

But the lambda gives a reading dependant on o2 level and needs heat to work......if its not in there, then you are giving the ecu a constant reading, no matter what the #1 sensor is telling the ecu.....

Thats the only reason i dont like it.

 

[Simon]

What the cons./dangers of this approach then?

I asked him if the resistor was only effecting downstream sensor and the message I got back was it was effecting both. He did a CO reading and said everything was OK. Im still not 100% convinced !

The alternative to binning the decat is to put the downstream lambda sensor back in the decat pipe, take the resistor out and disconnect the cat warning light. The ECU will still register a fault though and Renault will pick it up if they do a diagnosis.

Or may be get the ECU reprogrammed with the earlier MK1 program so it doesnt car about lambda 2.

Ill probably just stick the cats back on - worth the experiment though to see what is possible. Save everyone else the headache and cash ;-)

 

[BenR]

well, i dont know, since i have not tested....but itll be with emission control..

the 1st lambda is fueling as its a raw rading, the 2nd one is emissions control and comparison......to comply with the euro 3 spec which doent not allow a 5 min warmup perid like euro 2. SO, thats proabaly why you are getting rough starts.

 

[Roamer]

Simon, an alternative to this would be to get a freeer flowing cat (less dense), now whilst I was down at Power Engineering Merv said that they have a cat that they can adapt (prob onto any car) which is free flowing upto 380bhp output (scoob or evo it was tested on) so that would be a possible solution without loosing the cat and possibly screwing emmisions

 

[BenR]

untill you find out the price!!! lol

drill some holes in the cat....haha

 

[Simon]

The Service Manager at the local Renault Dealer told me that they werent going to touch the car since I disconnected the downstream lambda sensor.

He said if I got everything put back then they would look at it. Im concerned if the car is running too lean or over fueling and doing any damage to the engine.

 

[Simon]

Cats are going back on today - shame but want it so car can be seen by aRenault dealer and also not sure if any detremental effect of using a resistor.