I bought a used 2004 Madza3 with about 130k. It ran fine for a while, until it developped some stuttering. It happens only when accelerating, not at idle, and only when moving ( I.e. if I step on the gas in neutral the engine revs up fine without stuttering). I also get a P0121 once in a while. Looking at the obd II parameters, I see a high long term fuel trim (between +15% and most often +20%). Nothing else stands out. Any idea?

Thanks

PS I replaced the sparks (no change). And did 2 rounds of injector cleaners (very little change)

 

Found 2 links: OBD II Code P0121 : RepairPal
OBD-II Trouble Code: P0121 Throttle Position Sensor/Switch A Circuit Range/Performance Problem
Throttle Position Sensor

 

P0121 is a TPS (Throttle Position Sensor) code, this one is specific for either: 1.Throttle position sensor harness is open or shorted.
2.Throttle position sensor circuit poor electrical connection.
3.Faulty throttle position sensor.

The high fuel trim is from the ECU seeing from the O2s the engine is running lean so it adjusts the fuel trim to compensate, as I said though that is what the ECU SEES maybe not what is actually measuring as in the O2 sensor could be bad, there is a vacuum leak or plugged injectors. Those are some possibilities that could cause a high fuel trim but if its not tripping the check engine light your still within specs on fuel trim.

As for the stutter you will notice a misfire much more when under load than at idle, you might still have the stutter in neutral its just harder to detect. Do you change the spark plug wires (if it has them) when you did plugs? Is the misfire at a certain throttle position? Certain RPM?

 

Trouble Code P0121 - YouTube

darn good video on the subject!

 

Wow, thanks for all the replies. Quite a few interesting links, including that great video about replacing the TPS, which I hope not to have to do.

To answer a few questions, I did not replace the wires : there are coils on plugs and figured there were no wires to change (I have the 2.3L, in the hatch).
The misfire/ stumbles are definitely only when accelerating, and only when moving the car. Stepping on the gas in neutral does not trigger them. There are no specific rpms it happens at. Sometimes though, it's there even at mild acceleration, some day it's on hard accelerations only. Not specific pattern as to why.

 

Ah ok, my 3 has wires on it to the plugs so didnt know if yours did also.

 

If you haven't changed your ign coils on plugs yet, change them and put in new plugs again as well. (mileage @ 130,000!)

 

ok, I just noticed something in the engine bay today. I knew the previous owner had put a high performance air filter (phoyo1), but never noticed that disconnected tubing (photo2) which hooks up in the back of the engine. What is it?

 

It looks like its either the fresh air entrance for the PCV system or its the PCV valve if your talking about the hose with the blue clip on the inside. If that is disconnected you will be sucking in air that is not metered by the engine resulting in your lean burn condition.

 

I am talking about the hose that connects in the back of the engine on the valve cover. It seems that it is the hose for the recirculating of exhaust from the PCV. Would that trigger the lean condition?