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My 99 mustang 3.8l is running but after awhile the engine…

My 99 mustang 3.8l is...
My 99 mustang 3.8l is running but after awhile the engine shake, the rpm goes down like it wants to shut off , the it goes back up, and it does this a few time. I then looked under the car and I saw some water and sut from the exhaust pipe. I smelled it and it came from the exhaust. just a small amount. I looked on the exhaust pipe and there was a portion of it that was black. Also I'm not sure if it could be the converter either.
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Answered in 5 minutes by:
3/19/2018
Mike V.
Mike V., Auto Service Technician
Category: Ford
Satisfied Customers: 27,801
Experience: 25 years experience on all makes and models, Licensed NYS Inspector.
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My name is ***** ***** give me a moment to review your question Thanks.The site will prompt a call.I will let you know if it's necessary

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Customer reply replied 5 months ago
ok

Is the check engine light on when the car is running?

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Customer reply replied 5 months ago
No it does not come on

Sorry, I am not able to do a call at this time.

How many miles does the car have on it and when was the last time the spark plugs were replaced?

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Customer reply replied 5 months ago
I replace the spark plugs last June

Ok please give me a moment to type out my reply.

Mike V.
Mike V., Auto Service Technician
Category: Ford
Satisfied Customers: 27,801
Experience: 25 years experience on all makes and models, Licensed NYS Inspector.
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AMEDEE
AMEDEE, Automotive Consultant
Category: Ford
Satisfied Customers: 28,012
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Phone call session started

Black soot on the tail pipe usually is an indication of an engine that is running to rich. Since the plugs have been changed fairly recently, it's likely an issue with the mass air flow sensor. This is the largest contributor to the fuel mixture. What you can try is to unplug it, then start the car and see if the idle levels off. If it does, then I would replace the sensor. If there isn't any change, I would check for a vacuum leak. To do this, with the engine running, spray some carb cleaner along the intake manifold and any vacuum lines you can see. If the idle smooths out, or the engien revs up, it indicates there is a leak and the computer is making the fuel mixture rich to try and make up the difference.

Please let me know if there is anything else I can do to help.
Just Answer does not compensate me for my time and resources, you do when you rate my answer by using the stars at the top of the page. Payment for your question includes any follow-up questions you may have. Free trials and subscription questions still require ratings. Your question does not close after you rate. You will still be able to contact me by replying to this post.
Thank you so much!

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Customer reply replied 5 months ago
Hey just tested the MAS and I unplugged it and started the car and it level off and stayed the same

So it ran better? I am a little confused with your reply. It might take a bit for the raw fuel to clear out if it's running rich. But if it leveled off and stayed smooth, you'll likely need a new maf sensor.

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Customer reply replied 5 months ago
Well I unplugged the sensor wire as you said and started the car it idle at 1000 rpm while unplugged and it did not change while running
Customer reply replied 5 months ago
That was the MAS sensor wire

Ok, it usually will idle a bit higher because it's using the factory defaults stored in the computer. Since the idle speed didn't change and held a steady idle, I believe it's causing the problem. I would replace it and drive the car to clear out the exhaust which might have a lot of unburned fuel in it.

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Customer reply replied 5 months ago
Ok thanks a lot

You are very welcome!

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