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Discussion Starter #1
Hi everyone,
I have a 2005 ford mustang GT (4.6L V8, 300hp) that has back fired on very few occasions upon de-acceleration or when letting off the accelerator, it has done this at both high and low speeds but just very occasionally. I had input that it may have something to do with the secondary air injection system, injection pump or check valve. OH, also there where absolutely no codes or issues found when I took it into the ford dealership so they sent me home without an answer. I can't find this secondary air injection device using a chiltons manual, and do I even have one and could this be the culprit. Thanks for all's advice, Gary
 

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Does it sound like it's coming from under the car?
 

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Discussion Starter #3
Yes it's coming right below the five speed shifter/center console area from underneath of the car
 

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Sounds like un-ignited gas in the cats. Did they check the plugs, cops? Does it seem to happen mostly when the car is cold?

No code seems kind of strange though.
 

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The ford dealership did every possible diagonostics check available, no codes in computer and this included checking spark plugs, they sent me home without a explanation and a $500.00 bill. A suggestion was made by a 23 tech veteran that it's possible it could be the secondary air injection system, check valve sticking or moisture has got internally on electrical connections, he said he has experience this a few times and to check it. I just can't find the secondary air injection pump and check valve he is talking about. He said it's inside of the drive side front fender, can't fine anything in the chilton's manual that talks about this either. Not sure what to look for without a picture or better explanation. Thanks, Gary
 

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Discussion Starter #7
You may be correct, but it has a code for this in the chiltons manual with no other information but the code #P0491 Secondary Air Injection system, insufficient flow (Bank 1). If you could tell me for sure I would start looking else where. The reply from the tech read as follows, Your mustang has a secondary air injection system. The pump is electric and is located in the left front fender area. If you have a bad check valve (or anything causing a rich mixture), you will inject air at the wrong time, causing the back fire. We have had a couple of these in and found the system has water in it and the electrical connector was corroded. This is something you can check yourself by pulling off the rubber hoses and inspecting.

If there is no secondary air system please give me other possible causes, thanks Gary
 

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The evap system is on the drivers side of the engine compartment mounted on the strut tower. I wonder if he's talking about that?
 

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Discussion Starter #9
I found the evap system and I could check into that if that is a possibility. Also the car has been well warmed up when this back fire occurrs. It's happened 3-times twice in town coming to a stop at low speeds and once on the highway slowing down coming up on traffic, this back fire was loud. I'll check into the evap system and read, any other suggestions would greatly be appreciated, thanks Gary
 

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Pull the plugs and see if they're wet with gas. Also have you noticed a change in your exhaust tone recently? That could signify an exhaust leak and would definitely cause a backfire if it's up stream of the cats. What would happen is unburned fuel enters the exhaust system and when you let off the gas, your exhaust backpressure turns into a vacuum, sucking air in. Once you have unburned fuel and oxygen in the exhaust, it flows into the cats which are pretty dog gone hot. It's optimial for a backfire at that point.
 

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Pull the plugs and see if they're wet with gas. Also have you noticed a change in your exhaust tone recently? That could signify an exhaust leak and would definitely cause a backfire if it's up stream of the cats. What would happen is unburned fuel enters the exhaust system and when you let off the gas, your exhaust backpressure turns into a vacuum, sucking air in. Once you have unburned fuel and oxygen in the exhaust, it flows into the cats which are pretty dog gone hot. It's optimial for a backfire at that point.
Agree... The plugs will tell a great deal. If it is dumping gas have those cats may be toast..
 

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Discussion Starter #12
Thanks for the input, I will look over my exhaust system well and the cats to see if damage is apparent and pull plugs for evidence of unburned fuel. thanks again for the input, Gary
 
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