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2nd Gen Dakota 1997 - 2004 Dakota's

3.9 v6 MPG and Check engine light

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Old 12-05-2009, 12:02 PM
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Default 3.9 v6 MPG and Check engine light

Hey, guys. I need to bug y'all on this side of the forum for a bit. My friend asked me to swap out the O2 sensor in his mom's 97 Dakota 3.9 liter V6 2WD. The complaint is that the check engine light is on and the code complains about the oxygen sensor. I'm also told that the truck is sucking gas worse than my Ram. Since the 3.9 in many aspects, is a miniature 5.9 Magnum, I looked at a couple of other things. I found that the cat rattles so I'm sure that's in need of replacement. I looked through the throttle body and saw that the intake had a thick oily residue on the bottom (worse than in mine before I did a hugh's plenum fix. There was also next to no oil in the engine. My not-so-professional opinion is that the plenum gasket has blown, sucking oil, killing fuel efficiency along with the cat and oxygen sensor.

How far off am I?
I just would like a second opinion before I have the old lady buy a bunch of stuff if it just needed the o2 sensor after-all.

On a side note, the truck has had less than stellar levels of maintainance. It has about 125,000 miles on it and judging from how the spark plugs were locked into place, I would say that they have not been changed in a very long time and an oil change is about 6000 miles over-due.
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Old 12-05-2009, 01:43 PM
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Typically old/bad plugs will throw an o2 sensor code stating that the engine is lean. I would start there and let her know about the plenum gasket failure.
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Old 12-05-2009, 02:52 PM
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do the 02 sensors as well... get direct replacement, not the ones where to slice into the harness.. clean the throttle body, possibly replace the map sensor.. theres a big thread on this side, if i remember correctly, about the major increase in mpg replacing the map sensor.. when i repalced mine i gained about 2mpg, but that in turn with a clean throttle body, brand new plugs, wires, cap, and rotor.
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Old 12-05-2009, 04:11 PM
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Sounds like that truck needs some TLC...
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Old 12-06-2009, 11:01 AM
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Sounds like that truck needs some TLC...
No doubt! Thanks for the help, guys.
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Old 12-06-2009, 01:57 PM
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i'd say your diagnosis was very accurate. Its sucking in oil through the bad plenum, fouling spark plugs, fouling the O2'd and probably clogging the cat. The plenum gasket will have to be changed, and the 02's and the cat, and the spark plugs. I'd do a major seafoam treatment right before I changed the oil also. Run it through the vacuum lines, 1/3 of the can in the crankcase, and 1/3 of it in the gas. Just let it run long enough to get hot, then change the oil. Hopefully you can get out some sludge and clean up the combustion chambers a little too.
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Old 12-06-2009, 04:46 PM
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Yeah, I agree with wink. Change the plenum gasket, and while you're at it, clean up the tb good. Run seaform through and change the oil and filter. While you are changing the plugs, a nice tune up would certainly help.

So....
-Change plenum gasket.
-Clean TB
- New o2's
- New cat
- Seaform, then change oil & filter
- New plugs, plug wires, cap, rotor, air filter.
- Clean the map sensor.
- reset the CEL


Do all of that and I think she'll run a TON better...
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Last edited by bpark8824; 12-06-2009 at 04:53 PM..
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