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Hello, I just purchased my 2003 Beetle 2 months ago. It has 161,000 miles I have been driving it around town to and from work for 2 months. Last weekend I took it on a long trip about 215 miles (3 hours of driving) one way. Once I got about 70 miles from my destination it began to misfire at speeds over 75mph+. Once I slowed down, got off the interstate or drove it under hard acceleration it was okay. It has been okay driving to and from work all week. However I am planning on taking another long trip this week for work. Could it just need a tune up or is it something more serious failing. The timing belt was changed before the trip. Thanks
 

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Which engine and transmission is the car; in question? I would scan the car for trouble codes; post them here. The trouble codes; hopefully, will give you some direction in your investigation into the problem. I would have the fuel system tested for correct fuel pressure; to see if it is possibly a fuel delivery problem. Lately, it seems around here; allot of original fuel pumps are failing on therese high mileage, older cars. Seems misfires; come down to either ignition or fuel delivery: with testing, troubleshooting, you need to narrow down the possibilities, through a process of elimination. Give us more info about your car and any trouble codes, we will go from there.
 

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If your car has a turbo engine, good spark plugs and proper plug gap (which is traditionally narrower than non turbo engines) is the first most important part of this equation.

As mentioned, if the SES/CEL/MIL came on, scanning for DTC/Diagnostic Trouble Codes is important for clues. Even if there are not DTC's, often like data like Fuel Trim info can be a good indicator as to what may be going on.

Start with a lower cost OBDII smart phone/tablet app.

Get an OBDII smart phone/tablet App and interface, typically $30 or less. These Apps are usually best for drivability issues and they support Emission Readiness Monitor Status, Freeze Frame and Live/Realtime data. They are also great for Logging data for review after the car has been driven and can really help find unusual problems.

Android - Touch Scan for $3.95 and ELM327 OBDII to Bluetooth interface.

iProduct - OBD Fusion for $9.95 and ELM327 OBDII to Wifi interface.

I usually suggest getting the OBDII interfaces from Amazon if you are located in the US/North America. Make sure you choose an interface that indicates that the order will be "Fulfilled by Amazon" otherwise you may be waiting for WEEKS for an interface to arrive from Asia. Ebay is another option, but pay attention where the interface will be shipped from, again weeks for an interface from Asia, so if you can wait and want to try to save some money, go for it, but if you need an interface soon, choose wisely.

Fuel supply is also important. Fuel filters often have never been changed, fuel pumps are at an age where they are staring to fail, soft failing pumps are hard to pinpoint, because they do not leave you stranded but can cause driveability problems.

Ignition coils and if these coils have coil boots, this can be an issue along with something like a leaking valve cover gasket that can leak oil into the spark plug wells.

Start with spark plug inspection/replacement and reading for trouble codes/monitoring Fuel Trim values.
 
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