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4.6L General Discussion This section is for non-tech specific information pertaining to 4.6L (Modular) Mustangs built from 1996 to 2004.

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Old 04-15-2011, 04:23 PM   #1
youkairyu32
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Vehicle: 2004 Mustang GT
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Default Smog - CAT and EVAP I/M codes

Hey guys, I have a 2004 mustang GT. I have done a ton of work to it including swapping the automatic transmission to a T-56 Manuel and adding a Vortech supercharger.

To finish things up and get it running right, I had the car dyno tuned by AED (Advanced Engine Development in California). I heard a ton of positive things about the shop and it was highly recommended.

They did a great job and the car runs awesome... the only problem is that I am unable to get the last two I/M codes to clear for smog purposes. (these are the self tests which run when a car has its ECM reprogrammed or the battery goes dead) I can pass smog with 1 code remaining but not 2... so I have to try and get one of these codes to clear.

I am not entirely sure as to what I should do to try and clear these codes. I have looked up articles regarding the driving conditions in which they should clear and tried it, multiple times. After 250 miles the car still hasn't cleared those last two codes. So I began digging trying to figure out what could potentially be wrong with each code.

The CAT code:

The car still has its stock H-Pipe and stock CATs, however I do have an after market cat-back exhaust. The car only has 24k miles on it, so I wouldn't expect the CATs to be bad... but I suppose it is a possibility.

I was told by the tuner at AED that the rear 02 sensors were turned off on the tune but that shouldn't matter for smog. After a little researching I found an article stating that the rear 02 sensors measure the air content after the cats and compare the numbers to the values from the front 02 sensors to tell if the CATs are working or not. With this information I got a tune from AED with the rear 02 sensors turned on anyway.

I have now driven an additional 80 miles on the tune with all the sensors on and it still isn't passing the CAT I/M test.

The EVAP code:

I have not touched the fuel system other then to add new injectors and replace the fuel pump/add a boost a pump.

However, something that I notice is even after the car has been running for a while when I open the gas cap there is no hissing noise like the tank is under pressure. Is this normal for an 04 mustang gt?

It is also notable that the guys at AED modified the fuel basket by adding bigger holes and allowing it to fill faster to keep up with the boosted pump. (not sure if that would matter for EVAP but its information if nothing else)

My only other thought would be if perhaps when the seal was broken at the top of the gas tank for them to modify the basket, the seal wasn't properly made again... since it was just a bunch of black tar stuff instead of an actual gasket.

Any ideas or general information about why the CAT and EVAP I/M tests are not clearing would be greatly appreciated. I have been without registration on it for months...
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Old 04-15-2011, 04:28 PM   #2
Derf00
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How long ago did you have the codes cleared/battery disconnected?

It takes just a couple days of normal driving to get rid of the last indicators and be Smog Ready.
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Old 04-15-2011, 04:48 PM   #3
youkairyu32
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The first time the car was tuned I drove it for a total of 4 days totaling 250 miles of various types of driving. This was before the rear 02 sensor was turned on.

After I got the new tune with the rear 02 sensors on I flashed the ECM and have been driving it again. (this is the last time the car had its ECM reset/lost power) I have now driven on the new tune for 2 days totaling 80 miles. Once again everything has passed except CAT and EVAP.
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Old 04-16-2011, 08:47 AM   #4
cliffyk
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FWIW, here's the I/M monitor drive cycle description from the shop manual.

Here is the OBD system summary which describes the monitors in greater detail (it's the 2003 MY document, your 2004 is the same)
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Old 04-16-2011, 08:47 AM
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