VW Jetta 2008 starting problem - idle problem

Question

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.

Matt Smith · Answer

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

Daniel · Answer

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!

Answered by Daniel on February 26, 2021

VW Jetta 2008 starting problem - idle problem

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