[Ciulster]

Hey all. Long time lurker here finally coming out of the shadows to ask a few questions. I have a new-to-me 2002 Dakota Club Cab 4X4 with a 3.9L and a 5 speed stick in it.

I have plenty of Dakota experience with the 5.2, 5.9, 4.7 but this is my first 3.9 and my main concern is fuel economy and highway hill climbing performance. This thing is averaging 15-16mpg on the highway at 65MPH and I usually have to downshift to go up most hills otherwise it progressively loses speed, some hills I have to drop it to 3rd and rev it right out.

I know these six pots are known to be slugs but it just feels like its really held back for some reason. Wheel bearings checked out okay but it has 3.55 gearing and slightly oversize (and very camber worn due to bad ball joints - both to be rectified soon here) tires on it that I doubt are helping. The muffler is also very blown out and maybe that's why the engine sounds like its working so hard up hills

Realistically it is probably normal to need to downshift the truck for hills to get it to 2200RPM or more as it cruises at highway in 5th at like 1600RPM and this gasser probably makes no torque there at all, I am used to my 1st gen Cummins where I can just add a little consumption pedal in 5th at that rpm and it lights up and goes, just wanted to confirm with some other drivers with the same setup that this is normal and I shouldn't be chasing down problems that aren't there ( i.e. plugged cats) I've heard the 3.55 gearing is sub-optimal for the 3.9L, especially in the manual trucks.

I am hoping that proper tires and a tune up kit are going to bring mileage back up to the 18-20mpg I was expecting. Otherwise the engine runs flawlessly and doesn't leak/burn a drop of oil and seems overall to be really healthy for the mileage on it.

 

[Zaan]

I own a 93 with the 3.9. Yes they are a dog on the highway especially if your hauling anything. Being as they are under powered your mileage won't get much better (I get 12). The v8s always got better mileage. I have however installed the V8 intake and A high flow exhaust so it doesn't do to bad at 65, too bad the limit here is 80 it struggles past 65. I did have my cats plug at about 100k its a common issue. These engines are loud underpowered beasts that will go 300k+ as long as you change the oil semi regularly.

 

[andyvh1959]

My 99 Dak had the 3.9/Auto/RWD 3:55, and I did all the common mods; more open exhaust, open cold air intake, relocated the IAT sensor to the air box (not on the intake manifold), electric fan to get rid of the belt driven fan, tweaked the auto kickdown cable to get it to shift more crisp. At best I might have gotten 19mpg at steady state/level ground 65 mph and 2,000 rpm. The 3.9 is just working hard too push that much truck.

My current 2001 Dak has the 4.7V8/5-spd manual/3:55 RWD, I did the usual, cold air intake, Flowmaster exhasut, removed the belt drive fan. If I drive about 60 mph in 5th gear it keeps the revs down to just under 2,000 and I can get just over 20 mpg. As soon as the engine spins more than 2200 rpm the mpg drops way off to maybe 17 to 18 mpg highway. Daks are simply fuel hogs for their size. And anyone that says "the overhead console in the cab shows I'm getting 23 to 24 mpg." Nope. Those overhead console readouts are widely inaccurate. The only way to check true mileage; full the tank up, drive steady state on the highway in top gear (whatever speed you prefer) and after 100 miles stop and fill up again. Do that a number of times and you ger very accurate MPG results.

3rd Gen Daks up to 2003 had shitty single piston calipers on the front, that slide on cast iron "rails". If the rails are regularly greased and have not developed notches, the calipers mistly fully retract. But dry rails create a notch where the caliper only retracts so much and it increases pad drag on the rotors and increases pad/rotor wear. Stock front disc brakes on the 3rd Gen Dak are crap; undersized, under powered, too much drag, too little feel, high pad wear, high rotor wear. Just, crappy all around.

 

[Ciulster]

Wow I'm starting to think the oversize tires are not helping me at all then. They are 16" rims and I think the tire size is just a hair over 33" when converting from their metric sizing. (They rub on the fenders something fierce in parking lots) I guess with 3.55 gears plus the tall gearing of the NV3500 it's just way out of its torque range. 60-65MPH it's pushing 1500-1600rpm. If I tried to do 2200 I'd more than likely be doing 85+ on the speedo and it's likely reading low due to the tires. Hoping smaller tires make things at least a bit more manageable, I ordered the smaller size for the 16"s that come on the RWD trucks. 255/60R16 I believe. I had a 4.7 with the 45RFE auto and 3.92's that averaged 18-19 at 60-65MPH spinning 2200rpm and my buddy has a 5.9 with the 46RE and 3.92's that averages about 16-17 spinning at like closer to 2500rpm (despite the overhead saying like 24 like you said haha) but they have only ever run the 31's on the 15" rims. I'm new to this 16" rim nonsense. Finding tires for them where I am is a pain.

 

[Zaan]

Wow I'm starting to think the oversize tires are not helping me at all then. They are 16" rims and I think the tire size is just a hair over 33" when converting from their metric sizing. (They rub on the fenders something fierce in parking lots) I guess with 3.55 gears plus the tall gearing of the NV3500 it's just way out of its torque range. 60-65MPH it's pushing 1500-1600rpm. If I tried to do 2200 I'd more than likely be doing 85+ on the speedo and it's likely reading low due to the tires. Hoping smaller tires make things at least a bit more manageable, I ordered the smaller size for the 16"s that come on the RWD trucks. 255/60R16 I believe. I had a 4.7 with the 45RFE auto and 3.92's that averaged 18-19 at 60-65MPH spinning 2200rpm and my buddy has a 5.9 with the 46RE and 3.92's that averages about 16-17 spinning at like closer to 2500rpm (despite the overhead saying like 24 like you said haha) but they have only ever run the 31's on the 15" rims. I'm new to this 16" rim nonsense. Finding tires for them where I am is a pain.

UGH your running 33s?? My 31s rub bad enough (93 smaller wheel wells) . If it wasn't my off-road toy I'd have the stock 235/75/15s on it which are about a 29" I'd say with those 33s its definitely struggling hard, and unless you plan on doing some pretty decent off road trails not really worth it.

 

[Ciulster]

Yep truck came like that from the previous owner, and they are very camber worn to boot. I took one look at em and decided they were the first thing to go. The 5.9 is our bush beater hunting truck and we have a set of 33's for it. It has the torque for em and the 6" lift eliminated all rubbing issues but my god does it handle like trash on the pavement with them. I don't like 33's at all on these trucks. 31's were a nice balance on the 2000 and up trucks for light forestry roads but I definitely liked the way my 29" winter tires accelerated on my 4.7 much better

 

[CMax26mo]

Honestly, iv been working with all variaties of the 95’ through 2010’s Chrysler V6’s and V8’s and none of them get much better than 18-20mpg; and thats the high end of the range. Most common is 13-16. If you wanted mpg, you have the wrong truck/engine. Its commonly accepted that these trucks dont get very good mileage.

Its not realistic to compare any of these to your cummins.

iv got a 2002 4.7 auto with 3.92’s and only 59k original miles. Perfect compression, no leakdown, no blowby, no oil burn and it gets 14.5 average.

As for the 3.9, the harsh reality is your driving a heavy, all cast iron, pushrod engine designed in the early 80’s and updated in the late 80’s (actually a cut-down version of the 5.2 design that is much older) that remained largely unchanged until it was phased out by the 3.7/4.7 engine family mid 2000’s. And, that motor is matted with a heavy, mid-sized pickup.

we could guess all day at what could hurt mpg, lots of things can. Hung calipers (use an infared heat gun on your hubs to determine overheating), clogged cat (again, infared heatgun), plugged muffler (same heatgun), needs a tune up, timing, carbed up intake, compression issues, blowby, and a thing the 3.9/5.2/5.9 engine family is known for… blown intake/plenum/valley pan gaskets.

Chrysler has a TSB out there for this issue with instructions on how to modify the intakes’ valley pan surfaces to hold the gasket better. This failure makes the engine burn oil.

Here’s an easy test: pull the oil cap or valve cover breather off. With the engine running, cover the opening with the palm of your hand; if suction builds up in the next 15 seconds, the intake/plenum/valley pan gaskets are gone. Pulling the whole intake is the needed repair.

 

[vonagustus]

Hey all. Long time lurker here finally coming out of the shadows to ask a few questions. I have a new-to-me 2002 Dakota Club Cab 4X4 with a 3.9L and a 5 speed stick in it.

I have plenty of Dakota experience with the 5.2, 5.9, 4.7 but this is my first 3.9 and my main concern is fuel economy and highway hill climbing performance. This thing is averaging 15-16mpg on the highway at 65MPH and I usually have to downshift to go up most hills otherwise it progressively loses speed, some hills I have to drop it to 3rd and rev it right out.

I know these six pots are known to be slugs but it just feels like its really held back for some reason. Wheel bearings checked out okay but it has 3.55 gearing and slightly oversize (and very camber worn due to bad ball joints - both to be rectified soon here) tires on it that I doubt are helping. The muffler is also very blown out and maybe that's why the engine sounds like its working so hard up hills

Realistically it is probably normal to need to downshift the truck for hills to get it to 2200RPM or more as it cruises at highway in 5th at like 1600RPM and this gasser probably makes no torque there at all, I am used to my 1st gen Cummins where I can just add a little consumption pedal in 5th at that rpm and it lights up and goes, just wanted to confirm with some other drivers with the same setup that this is normal and I shouldn't be chasing down problems that aren't there ( i.e. plugged cats) I've heard the 3.55 gearing is sub-optimal for the 3.9L, especially in the manual trucks.

I am hoping that proper tires and a tune up kit are going to bring mileage back up to the 18-20mpg I was expecting. Otherwise the engine runs flawlessly and doesn't leak/burn a drop of oil and seems overall to be really healthy for the mileage on it.

 

[vonagustus]

Hi,I have a 91 sport,3.9,a500,3.90 .Under the holley tbi is a factory h- rise.I load the bed with fire wood and pull a trailer with another 1/2 cord this truck pulls.The 6 throw crank has low end but revs also.91 was only year this set up was used.Getting hard to get parts for some parts not to be found .Good luck with your dakota