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I have a Mitsubishi Montero sport 2003 with the 6g72 3.0l v6 engine. Recently it started to develop an issue where it idles rough for like 2-3 seconds only on morning cold starts. After that the car just runs and drives perfectly. Couple of months back, a tuneup was done with new plug wires, spark plugs, new top end seals (all genuine) and cleaned intake systems and injectors.

Would be great to know what maybe causing this issue and should I be worried about it.

Thanks alot.

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  • Is the check engine light on? This one should be hooked to a scanner to find data about temperature sensors, fuel trims and misfire data. Otherwise you can spend a lot of time and money chasing the cause of this. If you don't have a scanner or don't want to pay a shop to take care I of it, bring it somewhere like a parts store and have the codes read and post the codes here. Somebody may be able to narrow it down from there. But do not replace parts based solely on codes.
    – Jupiter
    Commented Oct 10, 2023 at 11:43
  • @Jupiter, thanks alot for your response. There is no check engine light. I hooked it up to a scanner and got some readings, and the only one that was off was the baro pressure sensor reading, which was static and wouldn't react to any driving condition changes. Also the fuel filter has never been replaced on this vehicle and its passed 100,000km. Also there is no fuel in the vacuum lines going from the fuel pressure regulator. So the only anomaly is the baro pressure sensor reading and the 20yr old fuel filter :D.. I have ordered one fuel filter cz it has to be changed.
    – Hasitha Shan
    Commented Oct 14, 2023 at 2:09
  • When cold starting, turn the key to the run position (not start position) for 3 seconds, then turn key off. Repeat this 3 or more times. Then start the engine. If it starts right up without the rough idle you may have a restricted fuel filter or an internal leak in your fuel pump. To determine which one, do a fuel pressure test. Hook up a fuel pressure gage. Turn the key on for 3 seconds or more then turn key off. They fuel pressure should not drop more than a couple of pounds in 10 minutes. If it does you need to replace fuel pump.
    – Jupiter
    Commented Oct 14, 2023 at 11:00
  • Thanks alot for your recommendations! the issue is finally fixed. It was fixed and I will post it as an answer
    – Hasitha Shan
    Commented Nov 16, 2023 at 8:57

1 Answer 1

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The issue was fixed after replacing 2 parts. Since there was no error codes thrown, we followed the variables that require when starting up. Fuel, air and ignition. The spark plugs and the plug wires were replaced recently during the periodic tuneup. While doing that, the injectors were checked too. Therefore, those were eliminated and we also knew that the starter and the battery was good working condition.

Next we checked the airflow and its readings which was ok except for the barometric pressure reading. In this particular model, this sensor also resides within the MAF sensor assembly and the reading was stuck at 97. This should change according to the atmospheric changes (so I was told).

And the next culprit we suspected was the fuel filter which was not replaced since new and had about 100,000kms of service.

With these findings we replaced the

  1. MAF sensor (The barometric pressure reading now changes as expected)
  2. Fuel filter (The old fuel filter had a lot of debris collected on the fuel receiving end)

Now the car starts and idles just fine. Hope this helps someone else.

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