diagnosing cause of excessive fuel consumption (standard fd auto)

[Juz]

i\'m getting significantly poorer fuel consumption from my current fd than my previous ones (all standard twin turbos running factory ecu, though this one is an auto and others were manual). i\'ve always only ever used v power / ultimate / tesco 99 in them all.

my previous cars returned 15mpg with a mix of fast and gentle driving, to 20mpg when driven just gently.

this one is giving me 7mpg - 10mpg driven slowly and changing gear at 3-4k revs (auto box in regular drive mode, only giving it gentle throttle).

with it having standard ecu i\'m presuming the fuel map can\'t have been reprogrammed? when it\'s running at operating temp the emissions do smell quite strongly of petrol.

what\'s my best course of action to diagnose what\'s causing the this? any particular components i should check out first? or is it best just taking it to an electrical mechanic with a fault diagnostics reader?

[JBerg]

prime the fuel system with the diagnostics box.
 
then smell for fuel around the uim.
 
could be  some fuel leaking from the fpd or broken seals around the injectors.
 
otherwise I would check all the sensors out...its clearly pumping excess fuel into the engine

[johnm]

I have an auto also, when I picked it up I drove from Brighton to Aberdeen and averaged 35 mpg.  Driving around normally I get 20 to 25 mpg.

Start with the basics - air filter is this clean
Plugs how old, remove and inspect
Exhaust and cat should be in good condition

Fuel mixture is a function of closed loop control using O2 sensor and Mazda mapping.  Low rpm being closed loop control, this can be checked with a voltmeter, paper-clip, 10mm socket and screw driver.

I was using more fuel than normal and just had trouble getting through an MOT, found the O2 sensor needed to be replaced, then the ecu was not going in to closed loop control.  Easiest way I found to check this system out was disconnect O2 sensor, located next to/behind rear engine lifting lug and check the voltage, it should be around 0.96 volts at idle.  Once disconnected (O2 sensor) the ecu will start compensating/controlling and the voltage will drop over the next couple of minutes (getting richer).  Reconnect and leave for a couple of minutes, recheck volts and the volts should have gone up.  If this is the case you are in closed loop control and your O2 sensor is working.  No voltage could indicate the O2 sensor needs replacement (£155 from Mazda - £35 else where) I also found the throttle plates were not closing; this could have been causing my issues also.  Remove the plastic bend leading to the throttle body (4 x 10mm head nuts) ensure there is no carbon build-up, if there is clean with carb cleaner.  Then ensure the throttle plates close.  There is an adjustment screw on the forward side of the throttle body, back this off until you see a small gap.  Once sorted check the throttle positioner voltages and ensure this is set correctly.  Also if you need to adjust idle speed there is an adjuster on the bottom of the throttle body, which allows air to by-pass the throttle plates.

The above is a starter for ten, to ensure the basics are in place.

There is a couple of threads to show you how to also down load faults codes, if your car is a uk spec, you should have a light on the dash to watch, if JDM you will not and have to set up a light on the diagnostic plug.

John

[Juz]

thankyou both very much i will try those suggestions, seems i have plenty of things to be checking...

sorry what is the fpd?

also another thing i noticed is that a couple of times (not very often, only 3 times in around 300 miles) after coming to idle at a roundabout/junction there was black smoke coming from the exhaust. after then setting off again and coming back to a standstill it was completely clear again. from searching i understand this black smoke reinforces that i have a fuel issue. with it only happening so rarely could this indicate anything in particular?

[johnm]

Black smoke is typical of a really rich mixture - it maybe wind condition and various speeds you don\'t notice the smoke.

[pdtaylor18]

pop your car down your local garage and get the emissions tested,see if it really is running rich. Whats your mileage? has it got a CAT? As with any car and the rotary certainly no exception the basic service is a must and as mentioned above.  That sort of MPG is pretty poor and will soon destroy your cat if its running way rich so its important to varify that your engine is in tip top condition with everything working as it should.  Take the plunge and go for a PFC or something similar and then you will be able to monitor whats going on and also get a better map for your car. You will gain more BHP but at the same time quite often, increase your MPG.