Over-fuelling - causes?

[Lanius]

A good friend's GT-X has an intermittant fault where it won't idle properly (and sometimes stalls while idling), and is VERY juddery under boost. When the fault occurs, it blows a lot of dark smoke from the exhaust (i.e. running extremely rich). It doesn't happen very often, but when it does its a right PITA.

From the description of the problem, it can be running perfectly fine from home to shops, then when he gets back to the car after a 10 minute shop, it sometimes won't fire, just turns over and over. Eventually after a lot of coaxing, it will start, but will pour out unburnt fuel-laden exhaust smoke. Other times it will fire up first time and run perfectly.

When the plugs were pulled after such an occasion, they were quite badly fouled, which is why I suspect its running super rich (unless theres another reason someone can suggest for fouled plugs?).

Its got a remote mounted pod filter (inside the front guard), standard turbo, front mounted intercooler, standard head, standard exhaust, HKS BOV, no ECU work done. Its been suggested it could be either an AFM fault or an ECU fault, but the mechanic who did the work wasn't sure, and my friend is hesitant to leave the car with the guy unless he has a better idea of whats going on.

Can anyone suggest a reason for it to be intermittantly running this way? Better still, can anyone suggest an Auckland based mechanic/tuner experienced in working on GT-X Familias?

TIA

[MAGN1T]

Probably worth hooking up a voltmeter to the oxy sensor to see what readings you get.....it might be dead as they have limited life, at least it's common on Mitsis. Some models of Mazda use Mitsi computers in them , don't know about yours though. If that's the case then they crap out the same as Mitsis do, as in the caps in the ECU.
Make sure it's got 98 in the tank.
Check timing with a timing light.

Steve

[strx7]

the oxygen sensor shouldn't cause it to run THAT rich under boost, most factory computers drop out of closed loop oxygen sensor control when they get into boost as they need to run at ratios narrow band oxygen sensors dont have a great deal of precision over. I'd be picking the airflow meter.

[Lloyd]

Is it an overfueling or an undersparking issue? Mazdas of that age had issues with ignitors and have seen a few with water temp sensor issues. Can you get any codes out of it?

[Akane]

Did he do the AFM-after-turbo mod? I've seen a few 323's in my time with that mod and they screw up AFM's.

[MAGN1T]

the oxygen sensor shouldn't cause it to run THAT rich under boost, most factory computers drop out of closed loop oxygen sensor control when they get into boost as they need to run at ratios narrow band oxygen sensors dont have a great deal of precision over. I'd be picking the airflow meter.

That's what you'll read on the internet, as to whether it's true or not is very debatable like anything to do with cars, seeing that a lot of what's on the net is BS..
They DO drop out of closed loop (the sensor is ignored in open loop) but if the sensor is dead it will still affect open loop running.....at least in some cars, so ignoring the sensors output altogether and not checking it would be a bit silly (although an intermittant fault probably wouldn't be caused by it).
It might be picking up knock and getting upset. An ECU reset might change things for a while.

As mentioned above, it could have an igniter fault......more than likely got more than just one thing wrong with it.

Steve

[tsoob]

Is it an overfueling or an undersparking issue? Mazdas of that age had issues with ignitors and have seen a few with water temp sensor issues. Can you get any codes out of it?

both these are def in need of emiminating early on in ur diagnosis

[strx7]

the oxygen sensor shouldn't cause it to run THAT rich under boost, most factory computers drop out of closed loop oxygen sensor control when they get into boost as they need to run at ratios narrow band oxygen sensors dont have a great deal of precision over. I'd be picking the airflow meter.

That's what you'll read on the internet, as to whether it's true or not is very debatable like anything to do with cars, seeing that a lot of what's on the net is BS..
They DO drop out of closed loop (the sensor is ignored in open loop) but if the sensor is dead it will still affect open loop running.....at least in some cars, so ignoring the sensors output altogether and not checking it would be a bit silly (although an intermittant fault probably wouldn't be caused by it).
It might be picking up knock and getting upset. An ECU reset might change things for a while.

As mentioned above, it could have an igniter fault......more than likely got more than just one thing wrong with it.

Steve

Unplug the oxy sensor in any car and go for a drive, they wont overfuel as much as being described above.

[Titties]

this wouldn't happen to be a black Ford Laser GTX would it?

[1hypo3]

the oxygen sensor shouldn't cause it to run THAT rich under boost, most factory computers drop out of closed loop oxygen sensor control when they get into boost as they need to run at ratios narrow band oxygen sensors dont have a great deal of precision over. I'd be picking the airflow meter.

That's what you'll read on the internet, as to whether it's true or not is very debatable like anything to do with cars, seeing that a lot of what's on the net is BS..
They DO drop out of closed loop (the sensor is ignored in open loop) but if the sensor is dead it will still affect open loop running.....at least in some cars, so ignoring the sensors output altogether and not checking it would be a bit silly (although an intermittant fault probably wouldn't be caused by it).
It might be picking up knock and getting upset. An ECU reset might change things for a while.

As mentioned above, it could have an igniter fault......more than likely got more than just one thing wrong with it.

Steve

Unplug the oxy sensor in any car and go for a drive, they wont overfuel as much as being described above.

I agree!



Check for codes first!!! they help alot if none check water temp sensor as mentioned above and airflow meter and could check fuel pressure reg as well, thats the order i would be checking.

[YeMs]

also on those mazdas, infact seeing it in alot of vehicles now the rubber is deteriorating, is the intake hose between the AFM and the throttle body/turbo can have a split in it due to perishing, which will alow un metered air in and causes all sorts of problems.

check the water temp sender to the ecu, they can cause mass over fueling. if you can get a s/h air flow meter try that. it really is a elimination game, cars of that age can have multiple issues.

[Lanius]

Thanks for the replies so far guys

Is it an overfueling or an undersparking issue? Mazdas of that age had issues with ignitors and have seen a few with water temp sensor issues. Can you get any codes out of it?

I'll ask the owner if he's checked codes next time I see him, good point, didn't even think to suggest it When you say "undersparking", is that simply the ignitors crapping out, or would it be the result of sub standard plugs?

MAGN1T - I'm somewhat hesistant to suspect it would be the o2 sensor, as the fault is *very* intermittant in nature. I would have thought that if the o2 sensor was dead, it would be constantly running rich or lean, not running fine for a while then suddenly throwing its toys like it has been. I will mention it to him too however, as I'm no mechanic, and it could well be the sensor causing a fault Would the ECU throw a code to indicate fault with the O2 sensor?

Akane - nope, no AFM-after-turbo mods that I'm aware of.

Titties - nope, white Mazda.

YeMs - sorry, I should have mentioned I've already helped him check the intake piping for leaks, and it all *looks* solid. Will have another look under better lighting next time I go visit the owner

[Akane]

To check for rubber hose leaks you have to take them off and stretch the hose yourself to have a very good inspection.

Had a split in a black hose in the MR2, which is not visible when the car's not running, but will expand and the slit with split open under boost.

[Lloyd]

Yeah by underspark I mean its weak. The ignitors sometimes just completely fail, but often they start to go and are worse when they're hot. Rounded off plugs or leaving ones in with a big gap just make the problem worse.

A temp sensor is usually either reading right or completely failed so you'd expect the problem to be there all the time. Thats the easy and cheap one to replace if you wanna start throwing parts at it.

[MAGN1T]

How about a photo of the sparkplugs?

Steve