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50 Posts
Discussion Starter #1 (Edited)
Hiya. Colorado here.

Recently obtained a 3.0 auto Daytona to fix up for my gf to use. Installed some new shocks/struts, fixed a few electrical problems, and scheduled and a new windshield install only to fail emissions. It failed on high Hydrocarbons and because of a bad gas cap. It also has high NOx and CO. Results are at the bottom.

The car drives just fine. Seems to shift a little late, but shifts are as smooth as they should be. Could just be the way I drive it - its been a while since I've driven a v6 auto. It sounds like the car might have a minor exhaust leak.

Since the failed test I've done the following:

  • Cleaned EGR valve and verified function by applying vacuum
  • Cleaned throttle body
  • Replaced spark plugs and verified gap (Old plugs look very lean, see pic at bottom)
  • Replaced spark plug wires

I'm planning on doing the following:

  • Replace MAP Sensor (Spectra)
  • Replace Distributor Cap
  • Replace Ignition Coil
  • Replace Gas Cap
  • Replace O2 sensor (DENSO)
  • Run fuel treatment (CRC Guaranteed to Pass)

Unfortunately I do not have the correct tools to test the catalytic converter. I did shake the hell out of the exhaust and didn't hear any rattling noises coming from the cat. No smoke is coming out of the exhaust at idle or while driving. (edit: After replacing the plugs etc some smoke was coming out of the exhaust on first start. After driving for ~5 minutes it went away and no longer smokes at idle) The car has plenty of power and easily gets up to 70+++. I don't know how old the gas in the tank is, but I did add about 5 gallons yesterday.

I'm just looking for tips/advice. I've gotten some great answers on facebook, but they seem to be a bit sporadic. I'm hoping that I don't need a catalytic converter as Colorado adopted a new regulation earlier this year that requires California compliance for replacement converters.



25,144 Posts
HC is unburnt fuel, CO is Ash from incomplete combustion, NOx is a photochemical smog that can only be produced under high heat conditions, O2 and Nitrogen will not combine under 2500 degrees F or under extreme pressure (Nitrous Oxide)

Before you start throwing parts at the problem...
The Map Sensor is the dominant fuel control sensor, the O2 is a fuel trimmer which brings the fuel control into a tighter range and usually only affects CO emissions and fuel economy.

1)Are any fault codes stored in memory?
2)Tap into the signal wire (BK/DG wire on the harness side) of the O2 sensor with a digital voltmeter and watch the O2 voltage.
This is your view of what is happening in the engine as far as fuel control is concerned.
The voltage should cross above/below .450 volts constantly as the controller makes small fuel corrections attempting to keep the A/F at stoichiometric (14.7/1), and should react more quickly at higher rpms since exhaust flow increases.
Preferably you want the O2 crosses to be in a tighter range (.250 - .750 volts), not at .1 - .9 volts which could indicate engine issues or a lazy O2 sensor.
a)What is the voltage doing?
Is it crossing or latched above/below .450 volts?
3)What is engine vacuum reading at a manifold source and at the Map Sensor?
If engine vacuum is low the engine sees this as more air entering the engine so it adds more fuel, since there is not more air the added fuel makes the engine run rich.
Engine vacuum needs to be in the 16"-20" HG range @ idle.
If vacuum is low you will need to determine why.
4))Is ignition timing correctly set?
Advanced ignition timing makes the engine run hotter.
IGNITION-2.2-2.5 Ignition Timing.jpg
5)Do you have the correct heat range spark plugs?
A hotter plug will increase combustion chamber temps.
6))Is the cooling system functioning properly.
Without proper heat dissipation the engine runs hotter.
Backprobe the CTS with a digital voltmeter and read the voltage, the voltage = a specific temp.
Backprobing the CTS.JPG CTS VOLTAGE CHART 2.jpg

If all of that tests OK you have a problem with Cat efficiency.

50 Posts
Discussion Starter #4
Holy detailed response, thank you! I'll try to check as much as I can next time I'm working on the car.

I'm short on space and only have 10 days to get another test so I ordered a bunch of parts already. Only $115 so not too bad.

Does harbor freight have a good vacuum gauge? I'm not sure exactly what tool to use to check vacuum.

As for stores codes - these would show a check engine light correct? The check engine light does function, but it has never come on while the car is running.

Oh yeah and the radiator fan was not running and my temp gauge was not working. I fixed the temp gauge last night and found the radiator fan was unplugged. Engine was too hot to plug it in so I'll do that and test it hopefully today. The oil gauge stopped working after I fixed the temp gauge of course, lol. I also didn't perform an oil change prior to the test so I'll get that done as well.

25,144 Posts
If you do not own a vacuum gauge and since this is an N/A (naturally aspirated, non-turbo) engine, I would suggest if you can only afford one, that you purchase a handheld vacuum pump instead of just a vacuum/pressure gauge, it will allow you to do more.

If you encounter any problems, need more info, have questions, Please feel free to ask.
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