MPGs are dropping .. a mystery - Page 2 - Mazda 6 Forums : Mazda 6 Forum / Mazda Atenza Forum
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post #11 of 14 (permalink) Old 08-06-2017, 09:53 AM
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My mileage was still relatively normal, I did not get a drop in MPG. I want to stress that I changed my precats (they were the real issue on my car) just as the breakdown process was starting inside them.

The main symptom for my car was horrible rotten egg smells any time I really revved it up for more than a few minutes. The smell would linger for 10-15 minutes in the car before it dissipated.

I also want to add that the 06 has a revised exhaust routing so the same hand grenade issue that happens on the 03-05 is not present on the 06, but clogged cats are never good. Nearly any shop should have a pressure gauge to test the backpressure, that would be my first step if I were you.

The only issue I have with the Torque app is that it is using the generic definitions for the OBD II readings and codes. It is still good information and it is still very useful in helping troubleshoot your car. Many of the tests required to properly troubleshoot cars require a two way scan tool which can perform various tests and cycle solenoids and so forth. No generic OBD II tool can do that.

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post #12 of 14 (permalink) Old 08-13-2017, 09:53 PM Thread Starter
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Thanks, b1lk1, this helps a lot in giving points for comparison. With MPGs still dropping but no rotten egg smell, my guess is that the pre-cats might be okay, but the main cat’ is still suspicious.

Its a huge relief to hear that this ’06 shouldn't be at risk for a big melt-down.

I agree, taking it to a shop for a true back-pressure test is the next move. The other day I stopped in at Mazda (its on the way home from work), and I asked them about what they had for specific cat’ testing. I had to laugh because their suggestion was to go to a dedicated exhaust shop?!? Good to see honesty I guess [shrug].

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post #13 of 14 (permalink) Old 01-22-2018, 11:03 PM
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Quote:
Originally Posted by 06_V6_ m6 View Post
Some DIY testing I have found involved checking back pressure by looking at engine vacuum :

A normal gauge reading should be between 16 and 21 inches of vacuum.

Have a helper snap the throttle (push down quickly and then release). There should be a brief drop in vacuum as the throttle is opened but it should return to nearly the baseline pressure you had at idle within a few seconds. If this reading takes longer than a few seconds the exhaust system is plugged.

An additional test is to keep the engine at about 2,500 RPM and watch the vacuum gauge. If the reading is low or continues to drop this is a good indication there is blockage in the exhaust system causing backpressure to build. The most likely problem is a bad converter and replacement is required.

I used an OBD II scanner and free app to do these checks of vacuum. This also worked to check voltages on the oxygen sensors above and below the pre-cats ... but as b1lk1 suggests, it is maybe a gamble to trust the engineís health to these relatively basic tools.


b1lk1,
if you don't mind my asking, how was the gas mileage on yours when the cat(s) were going out?
Did the main cat need replacement or was it just the pre-cats?
This is actually a really cool way of doing it!
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post #14 of 14 (permalink) Old 07-14-2018, 03:34 AM
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When my cats were clogging for the second time I attempted to apply this cheque and was unsuccessful in the results. If you had a very good reading and vacuum response before the clogging happened then maybe you would have a decent chance. one thing I did notice was the rotten egg smell with very little accelerator pedal effort. The last thing was not being able to accelerate beyond 45mph on the highway. Grades i would do in 5th (ATX) I couldn't do in 3rd.
You could pull the upstream O2 sensors out (tank mode) and see if there is an increase in power.
Something else that you haven't mentioned is fuel injectors. When was the last time they were serviced or cleaned? I use a bottle of Chevron with techron every oil change. I also pulled and cleaned my cam position and crank position sensors. That seemed to help me for about 1 mile per gallon. Other things I tend to think about with gas mileage dropping on cars that are not being driven very much would be coolant operating temperature. Perhaps your thermostat is opening too much or staying to open? Do your cooling fans kick on after running for awhile? Another thing I may have missed is how significant is your gas mileage drop?
I always drop about 1 to 2 miles per gallon when using winter blend fuel. And as fuel gets older it will become less efficient.
Anyhow,just some thougts.


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