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Discussion Starter #1
Hi all, new here trying to diagnose a problem.

2012 Mazda3 Skyactiv 2.0L vin 7, bought brand new December 2012.

Started running a little rough around 75k miles and dropping mileage. Changed plugs to NGK Iridiums around 80k miles. Mechanic told me that one plug was so shot, it was barely even sparking. Engine seemed to even out for a bit, then started running rough again. More noticeable when stopped at idle in drive, shift to neutral and it evened out. Thought it might be a mount gone bad.

Roughness began increasing and I noticed a hard miss at idle, but only when the car had been running for a long time. Mechanic told me to check the coils and see if one appears to have shorted the boot, but I haven’t had time. The misfire continued until Saturday, the 22nd. I noticed it was gone. No hard miss, no rough idle but mileage is garbage. 184 miles to a 1/2 tank.

Here’s the kicker for all this. No codes. No CEL. Nothing to indicate a problem. Tried with my Bosch Bluetooth obd and the Bosch 1350 unit at work. Nothing in the computer.

The coil or boot couldn’t have completely failed and not thrown a code, could it? I should get SOMETHING for misfire, bad coil, egr, vacuum, o2 sensors. Some indication of where to start looking.

This is the first major issue I’ve had that effects the engine performance. Need help.

I will start another thread on the headlight issue .....


2 Posts
Are you in possesion of an OBD scanner? Even a simple one with torque app will do.
If so, check your long and short term fuel trims. If the car is running very rich (which is probably the case when the car is using this amount of fuel), you should be able to see this in the fuel trims.
If not so, then you have a leak in the fuel system :)

It sounds like it could be a couple of things:
1. Ignition coil faillure
2. Fuel system faillure (injectors clogged, fuel pump, clogging, etc.)
3. Compression loss (Check with a compression gage)
These cars are direct injection, so the intake valves can be caked with old oil

1 Posts
Hi all, new here trying to diagnose a problem.

2012 Mazda3 Skyactiv 2.0L vin 7, bought brand new December 2012.
I found your post when googled. I have Mazda 6 2.0 (GJ December 2012) with similar problem. Did you find solution ?
My problem started after 60-70k kilometers.
Symptoms are:
- rough idle
- noisy engine
- hesitation
- less power
- car seems very heavy when accelerate
- sometimes problems magically disappears for few hours or even for few days but always come back
- no trouble codes visible thru ODBII
- KAM reset solves problem for a while
- tried different fuels from many stations (95 and 98 octane) – the same problems

Roller performance tester shows my engine gives 140hp and 188 Nn torqe (should be 265hp and 211 Nn). I did 3 checks and each time power and torque curve looks different. After KAM reset test shows 171hp and 211 Nm :)

My findings with ODBII plays:
1. Engine LOAD
- at idle is about 25% (should be approx 17%) - planned future investigation
- at 2k rpm is about 13-14% which looks ok

2. Short fuel trim
- at idle -1,5% to +3% which looks ok
- at 2k rpm 0 to +2% - ok

3. Long fuel trim
- at idle +5% to +7% which suspicious (should be approx -2,5%)
- at 2kprm +8% to +11% which suspicious (should be approx -0,8%)

4. MAF sensor:
- at ilde 1,2g/s (550rpm) – look little bit to low

5. KNOCKR (ignition delay due to engine knock)
- at low rpms 0%
- during heavy accelerate jumps to max 1% which looks ok I think

6. Engine misfires - about 10-15 misfires for 1 hour city trip in random combustion chamber. I don’t know if it’s ok or not – planing future investigation.

Replacements/fixes I did till now:

- replace knock sensor (one crazy mechanic told me that 1% ignition delay causes problem)
- spark plugs replacement
- intake manifold and intake valves carbon cleanup
- throttle body replacement
- MAF sensor cleanup
- many magic fuel additives
- coolant CO2 test (suspected head gasket problems) - negative

Planned next steps:
- compression inspection
- MAF sensor replacement

1 Posts
My 2013 3 used to have similar problems. I took it to the dealer and they updated the computers on it and fixed any idle issues/delayed acceleration problems I had.
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