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98Dakota
Dodge Dakota
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2/04/2003
18:15:08
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 Subject: Bad 3.9L Fuel Mileage
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I have a 98 dakota with the 3.9L engine. The previous owner had removed the catalytic converter, and has cut the wires to the o2 sensor. As far as i know, this is all that he did to it. It now gets 9 miles to the gallon which absolutely sucks. Any suggestions?
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DakotaDan
*GenIII*
2/04/2003
18:28:15
|  RE: Bad 3.9L Fuel Mileage
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With no o2 sensors & the cat removed your PCM probably never goes into "Closed Loop Operation" The PCM uses the signels from the o2's to adjust the fuel injector pulse once the engine is warmed up.
http://www.speedtweaks.net/tips_tricks_D.html#openclosed
Engine Start-Up Mode (OPEN LOOP)
Battery Voltage
Coolant Temp
Crank Position
Intake Air Temp (IAT)
MAP
TPS
Starter Motor Relay
Cam Position
Engine Warm-Up Mode (OPEN LOOP)
Battery Voltage
Coolant Temp
Crank Position
Intake Air Temp (IAT)
MAP
TPS
Cam Position
Park/Neutral Safety Switch (Automatics Only)
Cruise Mode (CLOSED LOOP)
A/C Select (if equipped)
A/C Request (if equipped)
Battery Voltage
Coolant Temp
Crank Position
Intake Air Temp (IAT)
MAP
TPS
Cam Position
O2 Sensors
Park/Neutral Safety Switch (Automatics Only)
Dan
Next Time You Think You're Perfect
~~~~~Try Walking On Water~~~~~
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98dakota
Dodge Dakota
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2/04/2003
19:27:03
| RE: Bad 3.9L Fuel Mileage
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ok thanks, is there anything else that you can think of that could be causing the problem
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Marvin
Dodge Dakota
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2/04/2003
19:49:34
|  RE: Bad 3.9L Fuel Mileage
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I have a 98 with over 110,000 miles on it and stil get the same excellant gas mileage as when I firs t bought it in 98. You need to put the cat and o2 sensors back on the truck to get the 18 plus miles to gallon. Which I still get with mine.
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00dak
Dodge Dakota
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2/04/2003
19:58:14
| RE: Bad 3.9L Fuel Mileage
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I can't even get that in my 2000 dak, that is unless your talking highway miles..
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Marvin
Dodge Dakota
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2/04/2003
20:05:29
| RE: Bad 3.9L Fuel Mileage
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Most of the 110,000 are highway miles. I get 18+ around town. I get somewhere in the neighborhood of 25 on the highway. The only thing I have done to it since I bought it is changed the plugs, wire, rotor and button.
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DakotaDan
*GenIII*
2/04/2003
20:29:41
|  RE: Bad 3.9L Fuel Mileage
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My truck has been to FL 4 times and I average 23-24 running 70-75 mph and get 17-18 around town
Dan
Next Time You Think You're Perfect
~~~~~Try Walking On Water~~~~~
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Marvin
Dodge Dakota
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2/04/2003
20:32:55
| RE: Bad 3.9L Fuel Mileage
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I go up and down the east coast. Same as you 70 -75 mph, from FL to NY and in between. I am very pleased with the way my truck has been running.
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98dakota
Dodge Dakota
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2/04/2003
20:33:14
| RE: Bad 3.9L Fuel Mileage
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I think i'll reattach the o2 sensor and see what happens. I also read somewhere that i need to reset my PCM, don't know if i should do that or not. I'll leave the cat alone if i can boost it up some by reattaching the o2 sensor.
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Marvin
Dodge Dakota
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2/04/2003
20:35:55
| RE: Bad 3.9L Fuel Mileage
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Hope that works for you. This is really a good truck. Do what you can and keep up with the oil changes. They really do help. That is really all I have done to mine. Nothing major ever.
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98dakota
Dodge Dakota
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2/04/2003
20:45:42
| RE: Bad 3.9L Fuel Mileage
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yeah me too
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DakotaDan
*GenIII*
2/04/2003
20:47:52
|  RE: Bad 3.9L Fuel Mileage
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98dakota
If you surf around here, you will also find that the V6's are sensitive to to big or to open of an exhast system, you loose low end torque !!
Dan
Next Time You Think You're Perfect
~~~~~Try Walking On Water~~~~~
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DakotaDan
*GenIII*
2/04/2003
20:50:16
|  RE: Bad 3.9L Fuel Mileage
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Also is you "check Engine Light" On ???
Dan
Next Time You Think You're Perfect
~~~~~Try Walking On Water~~~~~
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98dakota
Dodge Dakota
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2/04/2003
20:50:29
| RE: Bad 3.9L Fuel Mileage
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that is interesting, what do you think would happen if i put a dual flowmaster 40 series on it?
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Marvin
Dodge Dakota
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2/04/2003
20:51:14
| RE: Bad 3.9L Fuel Mileage
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Message:
Thank you. I want to get dual exhaust put on mine. Just stumbled on this forum. Will stick around and get some ideas on what to have put on my truck. I know it all comes down to what I want. I have a motorcycle and as long as I am happy with the changes it doesn't matter what anyone else thinks.
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98dakota
Dodge Dakota
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2/04/2003
20:55:01
| RE: Bad 3.9L Fuel Mileage
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For some strange reason, no the check engine light is not on, i think that the previous owner must have had it turned off.
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DakotaDan
*GenIII*
2/04/2003
20:59:09
| RE: Bad 3.9L Fuel Mileage
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Let me give you guy's some sites that have a lot of Dakota info.....
http://www.speedtweaks.net/tips_tricks_D.html
http://www.fast4x4.net/
http://www.dakota-durango.com/
http://www.dodgetruckworld.com/
Dan
Next Time You Think You're Perfect
~~~~~Try Walking On Water~~~~~
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DakotaDan
*GenIII*
2/04/2003
21:01:43
| RE: Bad 3.9L Fuel Mileage
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This from SpeedTweaks site
Which exhaust system is better:
Many people have debated this and a lot of it has to do with loyalty to a company label. We have personally run Dynomax, MagnaFlow, Flowmaster, Ravin, JBA mufflers and exhaust systems, and found that the single exit systems were slightly better than the dual exit systems...on the Dyno and at the track. The reason for this is that on a single exit system, the exhaust pulses don't fight each other on the exit.
Dan
Next Time You Think You're Perfect
~~~~~Try Walking On Water~~~~~
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Marvin
Dodge Dakota
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2/04/2003
21:05:57
| RE: Bad 3.9L Fuel Mileage
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Thanks. I'm looking at getting an exhaust system. I can't entirely say I don't want that extra power, but when it comes to the exhaust for the truck I want to hear it and feel it rather than just feel it.
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DakotaDan
*GenIII*
2/04/2003
21:06:34
| RE: Bad 3.9L Fuel Mileage
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98dakota
You might take your truck to AutoZone they will scan your trucks PCM codes for free.
They will probably find codes triped from the o2 sensors.
pick you up a Haynes Repair Manual I think they are around $15 it will be very useful
Dan
Next Time You Think You're Perfect
~~~~~Try Walking On Water~~~~~
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98dakota
Dodge Dakota
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2/04/2003
21:07:52
| RE: Bad 3.9L Fuel Mileage
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Thanks for the advice. Is it possible that the 02 sensor could be bad after the wires just being cut and left there. It was "rednecked" as i like to say
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DakotaDan
*GenIII*
2/04/2003
21:10:17
| RE: Bad 3.9L Fuel Mileage
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Your saying the o2 sensors are still in the pipe and they jumpered the wires some way ????
Dan
Next Time You Think You're Perfect
~~~~~Try Walking On Water~~~~~
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98dakota
Dodge Dakota
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2/04/2003
21:10:32
| RE: Bad 3.9L Fuel Mileage
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So you're sayin with the exhaust that i won't necessarily lose power with dual, i just won't gain as much right?
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98dakota
Dodge Dakota
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2/04/2003
21:11:58
| RE: Bad 3.9L Fuel Mileage
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yeah, the wires are just hangin around not doin much. and the 02 sensor is still in the pipe. Like I said "rednecked"
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Marvin
Dodge Dakota
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2/04/2003
21:16:27
| RE: Bad 3.9L Fuel Mileage
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Dude,
I can't believe the guy before you did that. I'm sorry. I'm not a redneck, but I am still sorry you have this problem. Try reconnecting the wires if things don't improve then you'll now you will have to get new o2 sensors and might have to try and get the wires going up to the engine.
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98dakota
Dodge Dakota
JOIN HERE
2/04/2003
21:18:55
| RE: Bad 3.9L Fuel Mileage
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how many sensors are there. i only see the one near the former home of the catalytic converter.
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DakotaDan
*GenIII*
2/04/2003
21:19:16
| RE: Bad 3.9L Fuel Mileage
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I would get a Repair Manual for your year of truck and see if you can hook them back up and go from there, Your probably going to have to reset the PCM at some time also.
Exhast:
The way your truck is right (no Cats ) now, I woudn't think you could hurt anything with duals
You also might look into getting a High Flow Cat, It is a Fereral fine $2500 per Cat running with out them !!!!
Dan
Next Time You Think You're Perfect
~~~~~Try Walking On Water~~~~~
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DakotaDan
*GenIII*
2/04/2003
21:23:49
| RE: Bad 3.9L Fuel Mileage
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You should have had at least 2 one before the CAT and one after, some trucks have 3 and 4
Mine has 3 o2's and 3 Cats
Dan
Next Time You Think You're Perfect
~~~~~Try Walking On Water~~~~~
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98dakota
Dodge Dakota
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2/04/2003
21:28:26
| RE: Bad 3.9L Fuel Mileage
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Alright, I don't have a clue as to where the other sensor is but it may be cut too so i'll look for it. I just hope that this solves my truck's fuel binge.
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DakotaDan
*GenIII*
2/04/2003
21:34:24
| RE: Bad 3.9L Fuel Mileage
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Like I said pick up a Haynes Manual and you should see what you need to do, lots of photos and very helpfull ;)
Good luck
Dan
Next Time You Think You're Perfect
~~~~~Try Walking On Water~~~~~
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98dakota
Dodge Dakota
JOIN HERE
2/04/2003
21:40:58
| RE: Bad 3.9L Fuel Mileage
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Thanks Man
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