Hi

Just got a 8 a few weeks ago, go to get it checked for road safety. Turns out the emissions are way to high.....changed the CAT on the car but still the same. When the car is idle the rev goes between 500rpm and 1000rpm...up and down up and down.

Anyone know what problems i might have?

 

Moved to trouble shooting.

If your idle is staying below 1,000rpm, but shifting around, this is likely a vacuum leak, a dirty or failing MAF sensor, or an ignition problem. Given that your emissions rates are up, I'd be thinking that it's an ignition problem, and the unburnt fuel is spiking your emissions test results.

Did they give you measures of what emissions points you failed on?

 

Sounds counter-intuitive, but a small air leak can make the car run horribly rich.

The O2 sensor sees the extra oxygen and adds unneeded fuel to compensate.

 

The CO levels are at 1.5 should be 0.3 and the HC is at 999 should be 100, was thinking it might be a leak of some sort as well, the mazda shop wants to do an engine check but seems to me thats not the issue, it runs really well and smooth otherwise

 

Hydrocarbons = unburnt fuel, and you are 10 times the acceptable limits
CO = excess oxygen, and you are 5 times the acceptable limits.

You are shoveling air and fuel through your exhaust rather than burning it.

Vacuum leak would be the first guess since it runs "fine" otherwise, then ignition.

Many methods of finding a vacuum leak, and most shops have reasonable prices for that if you aren't inclined to find it yourself.

 

Thanks alot, would do it myself but i lack the knowhow for it . But its in the shop now so will tell them to check that out, thanks again

 

Also worth noting is that it has smelled fuel from the rear, no smell under the hood tho

 

^ which fits.

You can disconnect the MAF, making the ECU run full rich based on RPM, and flood enough fuel into the engine by 3,000rpm that it will be puddling under the tail pipes.

 

Take out the spark plugs to see how they look.

 

The spark plugs were replaced some 1000 miles ago tho

 

Al was referring to how running very rich will leave tell-tale signs on your spark plugs, and you could be able to confirm that status by checking the tips.

But, Al might have missed how your car is already in the hands of a mechanic.

 

Well, the smell of fuel out the tailpipe means that it isn't getting burned by the ignition process or the cat. That's a lot of unburned fuel. I'm surprised a Mazda mechanic can't get a handle on the problem.

 

No one said it was being worked on by a Mazda mechanic.

I vote bad ignition coils.

BC.

 

^ v


Although I admit that it could be a Mazda monkey working there, not a mechanic.

And with the rest of the description, ignition just doesn't seem likely. Though certainly possible. My money is still on a vacuum leak.

 

Its at a certified Mazda workshop actually, but doubt they worked on many 8s since they are bit rare in Sweden, wouldnt even trust them to do an oil change without telling them which oil to put in . Also got new coils as well. Will tell them what to look for tomorrow, they wanted to do an engine check on it......but think their time would be better suited finding the leak tbh as you said Riwwp,

 

If they are actually listening to what you tell them, then let them know that they should be able to verify a vacuum leak by looking at the live engine data when it's idling.

Idling at the 750-800 rpm standard, the engine should be pulling about 5.5g/s of airflow past the MAF (they would need the decimal, rounding isn't clear enough). A tiny vacuum leak can easily drop this to 4.5-5.0 g/s, and drop it farther as the leak gets bigger. The STFT (short term fuel trim) should be just about 0%, maybe hovering within 1%. A vacuum leak will result in the STFT swinging much farther off of this. I've seen as high as ~8% from a small vacuum leak. Sounds like yours might be even farther.

Still a matter of finding the vacuum leak, but if you aren't seeing the right air flow and STFT, then you at least know that the vacuum leak diagnosis is correct (or if you see it normal, won't go chasing a non-existent leak).

 

Thanks alot for the help man

 

if "Mazda tech" can do their job PROPERLY ... *Sigh*

 

Took it away from Mazda now, gonna let another mechanic look at it, they seem to be rather lost at Mazda