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I have a VW Jetta 1.4tsi 122 (2008) and it has a strange behavior.

First of all, a have noticed that it doesn’t keep the same rpm when runs on idle. For example, when I stop at the first traffic light it runs at 600rpm and at the next traffic light it runs at 1000rpm.

Also in the morning when its cold it keeps 600rpm and when the temperature rises to 90deg it climbs to 1000rpm. Before five months when I started it in the morning it was stuck at 3000rpm (nothing was pressing the acceleration pedal) and when I restarted it, it was ok.

Finally yesterday night I couldn’t start it. The starter was working correctly and the car seemed to have “started” but it was running at 200-300rpm and eventually turned off. I replaced the battery with a new one but it did the same thing.

Finally, I started it by pressing the gas pedal and after the engine started the fumes seemed to contained unburned fuel. The same happened before six months but my mechanic insists that the car is ok because there are no error codes.

What could cause the problem? Every time that I had the problems I was refueling from different fuel station so I don’t think that it is a fuel problem.

Edit: about the fume I am not 100% that was containing unburnt fuel. It had a light color but I am not sure about it.

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  • Welcome to the site. When was the last time this car had a full service (tune-up)? Is there any airway blockages, clogged air filter, etc? Have you taken it to a different mechanic?
    – CharlieRB
    Nov 30, 2017 at 14:03
  • It has the service that the manufacturer suggests every year by a authorized mechanic. I suspect that the fuel injectors may not work correctly, as they have never been changed or cleaned. Also I told them to check and clean the mass airflow sensor and the Idle Control Valve but I don’t think that they will take my suggestion into consideration. I don’t think that the problem is caused by the fuel pump because a hear it and clear that sends fuel before the ignition.Although I' ll tell them to check the fuel pressure also. If they fail again to locate the problem i will go to onhter mechanic
    – Mike T
    Nov 30, 2017 at 14:19
  • Forgot to ask, how does it run once the engine gets to normal operating temperature?
    – CharlieRB
    Nov 30, 2017 at 14:41
  • It runs pretty good and at normal temp. But the idle rpm vary from stop to stop. 600-1000. Maybe it feels that it has lost some power but I am not sure...
    – Mike T
    Nov 30, 2017 at 15:14

2 Answers 2

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It sounds like your car is over-fueling, and the mixture of air and fuel is too rich to combust properly. Some things I would check that may be causing this:

  • Spark plugs: worn or overgapped spark plugs may produce too cold of a spark to burn all the fuel properly. Copper spark plugs should be replaced every 10,000km
  • O2 sensor: a bad O2 sensor reading will cause imbalances in your fuel mixture
  • MAF sensor: as you stated in your comment above, a bad reading from the airflow sensor could make the car think it has more air than it is actually able to receive
  • Airflow blockages: check the system for any obvious blocks in the air intake system, dirty filters, etc.

Edit: possibly, although less likely, there could be an issue with the secondary air injection system. This system is designed to take hot air from the crankcase or exhaust manifold and return it back into the air intake line to heat up the air and help burn the extra unburned fuel while the engine is cold. If it is broken and stuck in the open position, it may cause irregular airflow and impact the metered air readings

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  • I got the car back from the mechanic and he insisted that no problem found. Also he said that maybe the battery replacement may have solved the problem. (I am not sure about that because after the replacement it didn't start as I mentioned in my post). Also I told him about the o2 and the MAF sensors and he told me that my car doesn't have a MAF sensor (?). He advised me to take the car for a fast spin. I did it and after that I saw that it was idling better. I will inform you if I have any news
    – Mike T
    Dec 1, 2017 at 14:42
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The 1.4 tsi of that range seem to have problems with the timing chain. Especially when cold or standing for a longer period this problem gets more apparent. Have it checked by a competent mechanic, as if that´s the case, your motor is in danger!

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  • Could this explain the stuck throttle during the start?
    – Mike T
    Nov 30, 2017 at 15:23
  • If the timing is slightly off, the motor will not run or idle well. If it is a bit more off this means fatal damage with this motor.
    – Daniel
    Nov 30, 2017 at 15:25
  • I've never heard of a timing chain causing idle issues. The tensioners and guide rails fail and can make noise, but either the chain skips a tooth and you have tons of problems, or it stays on track and everything is ok (in the short term).
    – JPhi1618
    Nov 30, 2017 at 15:49
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    Problem is in the chain tensioner. It is fed by oil pressure. If the motor is standing longer, the tensioner "drains" and the problem gets worse. The are some parts in this motor that have been changed by upgraded replacements by VW after a lot of problems. The camshaft adjustment is also part of that problem apparently.
    – Daniel
    Nov 30, 2017 at 16:00
  • Friend of mine did have a Polo 1.4 Tsi that had that problem and did not idle well.
    – Daniel
    Nov 30, 2017 at 16:02

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