Hc emission Mot fail

[HellRaiser1st 1]

Hello,

I recently had my mot on my 12 plate gt86, it failed the mot only on the emissions hc test with high readings. Toyota said its my o2 sensors or cat, the cat was replaced recently under 500 miles both of them and the o2 sensors dont show a cel light. I had before broken sensors on car and every time i had cel light same for the cat. My problem is that my mpg got really good its about 37-37.5 this is showing in the milage i am doing and how much of my fuel tank is gone and im not driving slow. I get at 80-90 mph a mpg around 40 plus. I am really puzzeld i mean a broken sensor could increase mpg instead of making it worse? I never heard anything like this. Im gona take the sensors off today and give them a petrol bath to clean them over night. If anyone had similar issues and can bring some insight into this it will be much apreciated.

The car has 87k miles, mods include dual cach can (removed at the moment as i tought that is the cause of failure by leaking vacum somehow) and cosworth billet power blocks witch are still on the car. The car is a bit rough at iddle, i had one failed coilpack withlch i replaced., Im planing to replace spark plugs as well. Fresh oil change at 86k millers nanotech fully synth oil same spec as oem one.

[Varelco 211]

Just to clarify as somebody is bound to ask is your exhaust system stock? Whats the coolant temperature like? You have 2 cats in the exhaust system it should pass by a country mile. Seems very odd.

[Lauren 2259]

Oxygen sensor failure always results in worse petrol consumption. That is an odd one. Don't see any point in replacing your plugs if they've been at the 60K service, it's not going to be that. Likewise changing the oil is not going to affect emissions. Not sure what effect the Crawford 'power blocks' make to the emissions. Even with one catalyst you should be fine. I am guessing they are getting the cats up to temperature before running the test? 

[Church 209]

Crawford blocks shouldn't affect economy much imho, if only due subjective feel one may drive in different pattern, eg. change how/when one switches gears and how/when presses accelerator. In controlled environment i'd expect BPB equipped car to show similar fuel mileage. One shouldn't underestimate driving input changes, eg. installing some catback that adds up sound pleasure even if efficiency rises may also mean for driver to wishing more often floor it to hear loud noises more thus resulting in worse economy :), so only some simulated driving scenarious with robotized inputs on some dyno may show real objective difference. Also possible eg. month before installing some part there might be different weather/ambient temps, and different traffic congestion. And many other uncontrollable variables skewing results. So imho comparing mileage in several cases might be moot/placebo thing.

 

[Dave86 33]

Hahaha how do you get that good an mpg. I get 30.4 all the time. Next time you take it in go for a long hard drive and get it red hot and see if they will do emissions as soon as you get there.

[Cerastes 204]

I don't even get 30mpg. 

If I remember correctly mine would have failed emissions last year, I think my map is too rich, sometimes you can even smell it. 

Got an MOT tomorrow so will see how it goes. 

[Varelco 211]

30mpg is pretty low unless you do alot of town driving. I consistently get 37-38mpg, a combination of motorway, b roads and country lanes, its never driven economically either.