Hi guys. I have a bone stock 2001 V6 Dakota. Before I went on deployment I was getting around 19-20mpg. I had the vehicle "tuned-up" just before I left last february and it didn't seem to run the same. Since I have come home I have made sure the oil and filter are fresh and that the air filter is clean. I'm now only getting around 15mpg and it really seems to be down on power. I hear sort of a hissing sound that I do not remember having before. Where should I start as far as trying to figure out what is causing my mpg and power issue? thanks.

 

Lots of things there to check. The hissing sound could be a broken or cracked vacuum line somewhere. Dakota's are sort of known for that. Check all the vacuum lines and replace the PCV to start. Don't forget to check the vacuum lines under the driver's side of the truck too. Get a cheap vacuum gauge from Harbor Freight or a parts store and pull the cover plug off the passenger side of the manifold and check the vacuum, you should have 20hG vacuum at idle. If not, there is a leak somewhere.

Maybe the tune up shop used the wrong plugs or did not gap them correctly. Pull a plug or two out and see. 3.9's run best with plain old copper core Autolite 3923's, I think the stock gap is .040, check the emission sticker under the hood to be sure. It could also be the shop used and inferior quality distributor cap and rotor button-for as little as they cost, I would say replace them both and the plug wires too. The Belden premium wire set from Napa works very well, and for about $35.00 depending on where you live. The Napa better quality cap and rotor work great in the 3.9 too, part #'s MO 26 and MO 28.

Check the center wire on the MAP sensor connector and if you don't have at least 1.5 volts on it at idle, replace it. In my own truck, replacing a lazy MAP sensor made a world of difference in the MPG and the power increased too. Remove the throttle body and clean it and reinstall it with a new gasket. Clean the IAC pintle end very carefully with a rag soaked in carb cleaner. All you need to do is clean the black crusty crud off the end of it. Don't drop it or handle it roughly or you will need a new one anyway.

While the TB is off the manifold have a look down in the manifold and if you have oil pooling around the edges you could have a blown belly pan gasket. That can affect MPG and power, but I don't think it was as much of an issue on 2001 and up 3.9's. While you are under the truck, hit the catalytic convertors with a rubber hammer or your fist, but not too hard. If it rattles when you hit it, the cat is bad and needs replacing. Clogged cats can really affect MPG and power.

Other than that, check for stored trouble codes from the PCM by turning the ignition key to run then off 3 times, on the 3rd time leave it in run and if you have any codes stored they will display in the odometer. If you have any codes they will help you fix the truck back like it was. You could also have a lazy O2 sensor. The ones in front of the cats can get slow and not read right to the PCM even if there is no O2 code strored. Depending on the mileage, you might need to replace them too, use the OE type from NTK, you can get a better price by buying them online from www.sparkplug.com or ntk.com. Some say O2's go bad from 85K on up. I replaced mine as a maintenance item at 105K. You genereally don't need to replace the ones after the cats, they don't do much.

Try these few things and let us know how it turns out.

 

Wow, thats quite a lot to check. I appreciate all of that info and as soon as I can I will begin looking into the problem.

As far as the tune up guys go, it didn't really start this stuff until they worked on it. I know they put ngk iridium plugs in there. are they good? I know they claim to be the best but I don't believe it.

 

NGK makes a good plug but there is no need for iridium plugs in a 3.9. Even the Dodge engineers themselves did a study and found that the Magnum engines work best with copper core plugs. They also found that the 3.9 makes more horsepower and is more efficient with a 180 degree thermostat. They had to use a 195 degree thermostat though, to meet the Feds' emissions requirements. Some say changing to the 180 thermostat caused their MPG to drop, but in my case it didn't. It stayed about the same and actually improved when I put on a V8 throttle body.

You might pull all the plugs and just check the gap on each, but for the price it may be better to just put in a set of 3923's, gapped at the factory gap setting. You can buy them for less than $2.00 each at Walmart, or a few cents more per plug at Napa or Advance. Just be sure to ask for the plain 3923, not the AP3923 or any other variation on the part number. Maybe take a look at the bill from the tune up place and see what other parts they replaced, it could be they used cheaper ignition parts that are not doing such a good job now. If the truck ran well before the tune up, there has to be some problem related to what they did (or maybe did not do) which is now causing it to not run right.

Jimmy

 

Well I haven't gotten to change the plugs yet but I just popped a CEL code.

P0455 (M) Evap Leak Monitor Large Leak Detected A large leak has been detected in the evaporative sysem

I'm not exactly sure how to find and fix that leak or really what the evaporative system is in the first place.

 

start with replacing your gas cap

 

Yes replace the gas cap first. Then check under the driver's side of the truck, I believe that's where the evaporative emissions canister is located. It's a common problem for the rubber hoses there to crack and break apart. You may be able to just cut off the broken end and have enough slack there to reattach the hose(s). After that, do a hard reset on the PCM by disconnecting the negative battery cable and holding the ignition key in the start position for 30 seconds then reconnect the battery negative cable. The code should clear, but personally I don't think an emissions code is causing your lack of power and loss of MPG. The evap system is the system of hoses and emissions components like the evap canister that funnel emissions gases from the exhaust back into the engine to be burned off, rather than spewed out into the environment. It's all due to Federal emissions regulations to help control air pollution. It sounds like you may have a loose, disconnected or broken vacuum line somewhere that might be causing the power loss. That would explain the hissing sound from the engine. It could also be that your truck just does not run well with an Iridium plug, or the plugs the tune up shop used are not gapped correctly. Others here have had similar problems with spark plugs, and most of us agree the 3923 works best in a 3.9 with a stock ignition.

Jimmy

 

well it turns out my gas cap was just loose but here's my new problem. I disconnected the negative terminal and held the key on start for a few seconds to clear the CEL but now the truck will not run unless I hold the gas down. It seems to run just fine while I push the gas pedal but as soon as I let off the truck stops.

 

i got a feeling your IAC solenoid crapped out on ya.

 

It could be the IAC, usually when the IAC is bad you will have a rough idle and stall condition present. A bad IAC can also cause the O2 sensor to go bad or read incorrecly. I have no idea if a bad IAC will set a code but if the IAC is bad it also could cause the hissing noise you hear.

This could also be the Throttle Position Sensor is bad. If the only way the truck will start is when the throttle is floored, that can be an indication of a bad TPS. Check the 2 outside wires at the TPS. There should be about 5 volts there. With the key on and the throttle at rest, there should be about .35 to .90 volts on the center wire. As you slowly open the throttle, the voltage on the center wire should slowly increase until it reads about 4.5 volts with the throttle wide open.

Jimmy