Decat - Lambda Question

[Simon]

My car has two lambda sensors.

One goes before the cat and the other goes after the cat.

I had a devil decat pipe fitted in August 2002.

The Cat Warning light had only ever come on twice and both times were when the car was started from cold and I reved the engine hard at standstill.

Since end of December light has remained on perm.

I know that the ECU continuously monitors oxygen via the lambda sensors and adjust timing/fueling to suit except for wide open throttle which uses maps built into ecu.

I never had any problems when the cats were fitted.

Does anyone have a technical reason as to why the cat warning light is on? assuming that the lambda sensors are ok and all electrical connections to ecu are complete.

 

[scott8629]

When I de-catted My MR2 turbo I asked them to tie wrap the car sensor away.

The cat overheat light came one after a long while but only when I was in traffic, the car ran fine so I would not worry about it. Worst gets to the worst take the bulb out of the dash.

Scott

 

[Simon]

Cheers mate. Ive been reading up on this since I last post and have found out quite a bit... here goes:

The lambda sensors can be damaged by jump starting the car but normally they get damaged when first put in. Normally they dont go wrong.

Scoobys made after 2000 have the same problem. If you fit decat pipes then the warning light comes on even though the lambda sensors are housed. This is because a fault code is produced "Catalyst below efficiency threshold". Scoobysport have produced an electronic fix which fulls the ECU into thinking everything is ok. If another fault code occurs then it is dealt with.

Some cars have a fault code memory, faults are recorded in the vehicles ECU. This fault memory needs to be cleared when the lambda sensor problem is fixed or it will keep showing a lambda sensor fault even though it has been rectified.

So Renault dealer with fault code reader or perhaps a SuperChip which may erase the fault code memory !

The front and rear lambda sensors have different functions as follows:

The front sensors (pre-CAT) monitor the engine mixture and report back to the ECU which then adjusts the mixture accordingly. Cats work best when the mixture varies between weak and rich every few seconds apparently.

The rear most Lambda sensors are not used to adjust the engine mixture but simply to identify the performance of the Catalytic Converters. In ideal conditions they should show a constant output indicating that the Cats are working as intended. Ripple which mirrors the output of the pre-CAT sensors indicates Cats that are starting to fail.

 

[Simon]

So to sum up I reckon there is nothing wrong with my lambda sensors and it is the ECU that thinks the cat is not working. The Clio probably has a fault code memory and based on that it thinks the cat is knackered !

So temporary fix is to clear down the fault code memory - Ive tried disconnecting the battery for over an hour and that didnt work. Looks like either visit to Renault Dealer or perhaps a SuperChip which may erase the fault code memory.

Will check all the connectors underneath just to make sure.

Perm. fix is to either get an electronic gadget that fools the ECU aka scoobysport. Remove the decat pipe - not if I can help it. See if it is possible to repgram the ECU to ignore it - SuperChip?

 

[Jellyhead]

Hi Si,

Ie tried everything (including the scoobysport obd2 "delete" box - which didnt help) and found that it can be done, but you need to control/force which state the ECU is running in (open or closed loop). Once you have hard set the ECU to a state, you can then re-map that area of ROM.

Simply removing (or disconnecting) the downstream lambda doesnt seem to cause an error light on the mk2, but it does cause mixture problems :-( you should have your ECU re-mapped immediately afterwards....

By the time Id decided which solution to use (keep re-mapping the OEM ECU or fit a programmable ECU) Id been tempted away and took delivery of a new Cup.

Removing the cat on the mk2 did improve throttle resonse, but as I now have a Cup Ive sidelined the project. If you want a cat-replacement pipe Van Aaken fabricated mine and have a jig prepared.

If you want any more info, Ive got allsorts including the workshop manual photocopy of lamda connections.

Cheers,

Jelly

 

[Simon]

Cheers Jelly this is really valuable information.

I reckon the only way now to fool the ECU is to get a gadget that fits inline and sends the same varying signal that the last lambda sends thus CPU thinks everything is OK. I know someone that designs and builds analogue electronic stuff so will have a word with them.

Spoken to a well known SuperChip company and the advice is to stick the cats back on the car and get the fault code memory reset by Renault.

The DECAT pipe also makes quite a difference in performance, I know the 172 comes with sport cats but looking at my rolling road reports from BB im seeing through the rev range around 6 ibs/torque at the wheels increase.

Estimated flywheel bhp is 180 which is fecking good for the mods. done and that is without a superchip and remap.

Car is running OK as is but im not 100% sure if ECU makes any other adjustments based on rear lambda sensor reading and like guy said Im assuming that I know the problem. Could be something else but i doubt it ! Best to get it checked just in case !

Anyone with a lambda sensor before the cat wont have a problem so phase 1 172s built before december 2000 should be ok.

 

[Cord-172]

Simon, how does the car drive now with the De-Cat? I was contemplating a de-cat pipe/system or a straight through powerflow system from the manifold (mates uncle can blag the MOT bit for me!)- Is it a good way to get appreciable power gains? (heard that 10% engine efficiency lost in the cat)

 

[Simon]

Currently I have a Devil Decat pipe that runs straight through from the manifold to the magnex exhaust system. The Devil decat is large bore and the noise the whole car makes is loud. It definately turns heads !

Looks like im going to take the Devil Decat off as want car superchipped and they wont touch it with cat warning light on which is understandable.

I would only recommend you fit a decat pipe if you have the single lambda sensor before the cat and you shouldnt get any problems. If you have two then i imagine you will eventually run into same problems as myself.

Before the cat my car was producing an estimated 175 at the flywheel and after the cat an estimated 180 bhp at the flywheel so around 5 bhp gain at top end.

Where it does make a difference is torque is increased throughout the rev range so pickup in every gear is better than std.

In my opinion it is a good mod. for the money - wish I had an earlier 172 now.

Im determined to put it back on in the future once I can fool the 2nd lambda sensor.

What also confuses me is my ECU is apparently the same as the ones that have one lambda sensor. How can this be? My ECU will read the 2nd lambda sensor and therefore the ECU program must know of its existence.

That must mean that the earlier MK1 ECU doesnt read the 2nd lambda sensor as it dont exist !

So maybe I get an ECU from an Earlier MK1 and use that - thus problem solved !!

What do you reckon?



Jellyhead: What year and phase is your 172?

Could you please email me any lambda documentation to:

mailto:simon@cliosport.net">simon@cliosport.net