Crap economy and power after decat

[Flibbib]

Hi all

 

I decatted my 1.6 petrol, and ever since the fuel economy has dropped massively, and it's a little down on power. There's an adapter for the rear lambda sensor to cheat the o2 reading, but apart from that was all done properly, on ramps, all sealed back up properly and I have a cat back system too. It's only slightly louder too which I thought was a bit odd.

 

Any advice? At this rate I'm just gonna put the cat back on cause it's a load of *****. Also pretty certain it's not backpressure as the power loss is higher up the rev band.

[ajzetecs]

maybe the are some dtc codes need wiping off, i had issue with my egr it threw codes up and ones clear car was much better, maybe not that but its an area to tick off

[TomsFocus]

You'd be best off putting it back on tbh, there aren't any benefits of decatting small NA petrols. 

The fuelling is all done by the front lambda so the decat shouldn't really affect the MPG though, unless your putting your foot down more to compensate for the reduced torque I guess.  I'd check the live data for the lambda and MAF/MAP (whatever it is on the 1.6), sounds like one of them may be going out of range towards the top end.

[Flibbib]

Cheers guys. Forgot to mention that there are no DTC's (At least on dash and with wifi OBD reader) but I'll hook proper kit up to it today and have a look at all the modules.

 

The fuel economy is so bad that if I'm rolling in 5th it shows 80 - 90mpg. Before I could roll at 10mph and get 99. Also gal p/h is 6x higher at times on idle.

[TomsFocus]

6x more fuel at idle is worrying tbh.   The lambda reading should be well out so it's odd there's no DTC for it. 

Maybe the 2nd lambda 'fooling' device isn't working, I was under the impression the 2nd one could only reduce fuel if it sensed too much after the cat but maybe I'm wrong there.

 

[BobbyDee]

On a small N/A I don't see a decat making much of a difference. If anything I would expect your fuel economy to stay the same or slightly improve due to the exhaust not being as restrictive. I don't believe the 2nd O2 sensor does much more than monitor the cats efficiency I may be wrong. 

With you fuel economy have you manually checked it as I always take the computer generated ones with a pinch of salt.

 

[south_bound]

42 minutes ago, BobbyDee said:

With you fuel economy have you manually checked it as I always take the computer generated ones with a pinch of salt.

 

I would agree with this comment and do a manual check on the miles done and fuel need to fill the tank - my fuel display is way out since I got the DPF and EGR valve sorted on my diesel, with the dash readings showing it getting worse but the actual fuel going into the tank was really better than it had ever been.

[Flibbib]

Nope, fuel gauge is dropping quicker with steady driving, and with full throttle it's about twice as quick to drain. I mean really quick. I travel 34 miles to work, at a "brisk" pace, i used to use about £5, now it's edging pretty close to £10.

There was a code earlier, P0139 Oxygen Sensor Circuit Slow Response, cleared it and it didn't come back. Also a couple on the HEC but that's just related to me programming new keys.

Took it for a spin, some very light eco driving, then a full pelt 0-70 on a dead road, then a little bit of normal driving whilst hooked up to forscan measuring RPM, Speed, DTC status, MIL status, exhaust bank readings and fuel usage. All looked normal as far as I can tell, nothing out of the ordinary. The rear lambda also increased at full throttle so it's sort of working even with the extension.

Just wierd, fuel economy is ***** now, when I got the cat back it improved, even the air intake improved it too. Ripped the cat out and now it's literally as bad as an ST (I had one before, it is honestly that bad)

[Flibbib]

Just been reading up on o2 sensors and it states that if the first sensor (manifold) fails, then the car can just run a fixed fuelling ratio causing large jumps in emissions and fuel consumption. Is it possible it wasn't screwed back in properly or reconnected properly? I watched the guy do it (£20 labour so thought why not) but I was helping by pulling the cat back to the side when he put it back in so didn't see about the sensors. 

Surely this would throw up engine management if the first sensor had an issue though? Or would it just crack on and run a standard mix, to be fair it wouldn't damage the engine, might hurt the cat but that's what's getting me thinking it wouldn't throw up the MIL.

[Tomson]

When was your fuel filter last changed? I had a similar issue once where I was using mad amounts of fuel but no lights etc - turned out my filter had split pouring out behind the car...

just my two cents but might be something to look for, even check the pipes for tears/splits

[Flibbib]

Never. I was under the belief the filter isn't serviceable on the 1.6 petrol 100hp (Non TI-VCT). oil is changed every 3 - 5k although it's been about 8k due to winter. No leaks from the car at all (just a bit of oil around the gearbox but that's always been there)

[Tdci-Peter]

1 hour ago, Flibbib said:

fuel economy is ***** now

Have you had the ECU re-mapped for the de-cat?

The cat and the two O2 sensors are very much an integrated part of the engine management system on modern cars.

To both reduce NOx & unburnt / CO emissions the ECU constantly adjusts between rich & lean. When lean, CO & unburnt HC are oxidised by the excess O2 in the cat, and some O2 is also stored in the cat. When rich, the NOx is reduced (de-oxidised) to Nitrogen, and the stored O2 in the cat can continue to oxidise CO & HC. The upstream O2 sensor detects and controls this swing cycle, and the downstream sensor should get a more stable signal that can be used to optimise the engine fuel to air ratio. Any deviation from this will probably result in the engine running rich to avoid the risk of damaging detonations (pinking).

What all that techno gibberish means is that removal of the cat without some fairly major re-programming of the ECU will probably result in a loss of power, and worse fuel economy. It is the way things are going these days, with everything from the radio & heater to the engine, cat & dpf (for diesels) all tied up with complex software. Any changes can easily upset the software.

If the ECU is re-mapped, then you are reliant on the re-map experts knowing what they are doing, and doing it well. From the re-map reports I have heard, this can be a bit hit and miss. Often it is ok, but sometimes all goes a bit pear shaped!

 

[Flibbib]

Thanks mate. I'll look into it a bit further. Just seen many people decat OBD-II based vehicles with no issues and no remap, might be that though.

[TomsFocus]

You can get away without remapping after even a petrol decat usually, I've fitted quite a few as my mates like ruining their cars.   Fuel economy always gets slightly worse and they lose a bit of torque but apparently they like the noise.  Having had a decat and a sports cat on the same car, I'd always go for the sports cat myself if I wanted noise without all the other issues.  Your economy is ridiculous though lol, somethings definitely wrong there.

If there's a fault with the front lambda it would flag a code, but if it's just loose or if the join between the cat and mani isn't perfect it could get weird readings (too much oxygen) and increase the fuelling to compensate.  But as there's no actual fault with the sensor it wouldn't flag a code.  So you might be right about it being loose, should be easy enough to feel.

 

[Flibbib]

I'm gonna have a mess about when it's dry. Thinking of giving the o2 sensor heads a clean too, any advice? Also thinking of removing the adaptor on the second sensor, putting it straight into the cat and ignoring engine light. 

[madmole]

Lack of back pressure, so when your injectors fire, most is going straight out the exhaust port (the openings overlap). Its over scavenging. So it detects the loss of powers and overfuels massively to make up for it?