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2003 subaru outback: 3.0at highway speeds it has a misfire..000 miles…

Customer Question
I have a 2003 subaru...
I have a 2003 subaru outback H6-3.0
at highway speeds it has a misfire,I tried a set of plugs but it is still there,it has 100,000 miles on it and is in good shape other then this,
Can you help me?
Submitted: 8 years ago.Category: Subaru
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Answered in 1 minute by:
6/14/2010
Subaru Mechanic: Mike V., Subaru Technician replied 8 years ago
Mike V.
Mike V., Subaru Technician
Category: Subaru
Satisfied Customers: 27,779
Experience: Certified Subaru Technician, factory trained by Subaru.
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Greetings Sir or Madam and welcome to JA! I will do my very best to assist you with your problem.

IS the check engine light coming on when it misses?

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Customer reply replied 8 years ago
no check engine light
Subaru Mechanic: Mike V., Subaru Technician replied 8 years ago

Ok, this is likely going to require some tools you may not have. I have personally seen the O2 sensors cause this problem as well as carbon buildup on the valves. You need a scan tool to watch the sensors while the problem is occuring, and a leak down tester to check the valves. A simple compression test won't pick this up because the leak from the carbon on the valve train only creates a very small leak.This however causes the misfire.

I would start with a couple of can of injector cleaner and some higher octane fuel to try and clean the valves. If this doesn't work, then I would have the leak down test done to determine if it is a valve issue or not.If it is, then the car will need a valve job to repair the issue.

Hope this helps!

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Customer reply replied 8 years ago
I am a Corvette mechanic at a Corvette shop, I should have the tools to do it,you dont think that it could be just a bad coil?
Subaru Mechanic: Mike V., Subaru Technician replied 8 years ago

It is not likely without the check engine light. You will normally get a code pretty quick if a coil is shorting out.

You can watch the front a/f sensors then with a scanner and make sure they are functioning properly as well. They don't normally set a code unless it is a lean or rich mixture code.

Mike V.
Mike V., Subaru Technician
Category: Subaru
Satisfied Customers: 27,779
Experience: Certified Subaru Technician, factory trained by Subaru.
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Customer reply replied 8 years ago
ok,thanks for your help,
I will try that tomorrow at the shop and see what I find,
thank you!
Eddie witt
Subaru Mechanic: Mike V., Subaru Technician replied 8 years ago

You are very welcome and thank you for the accept!

Mike

 

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