Topic: .060 over, valve job, 4.0 TB, Absolutely TERRIBLE Highway mileage

[Bigboi]

Hi everyone, ive got a 98 dakota 2.5

just bored and rebuilt my 2.5 like 2,000 miles ago, i swapped out a 2.5 TB for the 4.0 TB and i didnt notice much of a mileage difference, but ive been having to drive back n forth about 37 miles each way on a highway and i am chugging gas and im only doing about 75mph

about April of 2013 i went to tampa, FL and riverview, FL from north Orlando, FL and it only consumed about 1/2 a tank of gas, but now im using about $10 for about 40 miles, im gonna check it out today, i may consider throwing on the 2.5 TB and see if that makes a difference

any input is greatly appreciated

[jagular7]

When you used the 4.0l tb, you did transfer the sensors from the 2.5l tb?
IIRC when you swap in the 4.0l tb and use the 2.5l sensors, you also require to swap out the injectors as well.

[sharpxmen]

calculate your exact mpg, it's hard to tell where you're at by approximating in $ or 1/2 tanks. Fill up, note the odo, drive around and fill up again, substract your initial odo reading from the new one and divide the result by the gallons you just put in.

my YJ does about 15mpg on a good day but i don't baby it. at 75mph in your case (since is not lifted or big tires) all hwy i would think it should be around 16-18mpg (just guessing here), if you drop another 10mph should get another 2mpg, something like that.

[jfrabat]

Also check your O2 sensor and vacuum lines; both could kill your MPG rather quickly...

[Danny in Galveston]

At 75mph that 2.5 is under a pretty good strain.  I would not expect it to get good mileage.  My '01 Wrangler will get just over 20mpg at a steady 65 but will drop to maybe 16-17 at 75mph.  Wind resistance kills you the faster you drive.

[Jeffy]

At 75mph that 2.5 is under a pretty good strain.  I would not expect it to get good mileage.  My '01 Wrangler will get just over 20mpg at a steady 65 but will drop to maybe 16-17 at 75mph.  Wind resistance kills you the faster you drive.

Yup, at 75 mph it's a struggle.  I've mentioned it over in the MPG thread.

[Bigboi]

Yup, at 75 mph it's a struggle.  I've mentioned it over in the MPG thread.

before the bore and TB, i drove 260 highway miles on less than a half a tank @ no less than 75mph, i was at 85 for most of the way, i know theres no way i can find out my mpg that day, But i put $40 in that day and i came back home with a lil bit over a half a tank

I am starting to have top end noise from this head i put in a few months ago, so i may just use some valve lapping compound so Cyl #3 and #4 seal up like new again, then slap on my original head

but my truck has always ran Rich, could it be from a bad cat? i am getting a P0420 code but both my NGK o2 sensors are new, i was also told my fuel sync could be off by a few and not cause a code, i was told id need a DRB tool to scan and adjust the fuel sync, i dont know anyone with this tool and ive looked it up and its Expensive so i aint buying it

[grumpygy]

What you have a 36 gal tank.  I've never gotten 260 miles from mine on a full tank.

[Bigboi]

What you have a 36 gal tank.  I've never gotten 260 miles from mine on a full tank.

i should have a 12 gallon tank, $40-45 is about all i can fit in it

I... Believe my exhaust isnt sealing right due to one of the studs for the exhaust bolts missing, which would cause all kind of issues with the 1st o2 sensor but i am not 100% certain, i do know i am gettng a crap load of ticking, whether its related due to the nut for the exhaust missing or due to valve springs/rocker arms/tappets i am not sure at the moment but i am gonna buy some valve lapping compound and refresh my original head because it just has a little build up on the valves which caused #3 and #4 to misfire, my bottom end is fine, so i will be pulling the head off again here soon

[Jeffy]

i should have a 12 gallon tank, $40-45 is about all i can fit in it

I... Believe my exhaust isnt sealing right due to one of the studs for the exhaust bolts missing, which would cause all kind of issues with the 1st o2 sensor but i am not 100% certain, i do know i am gettng a crap load of ticking, whether its related due to the nut for the exhaust missing or due to valve springs/rocker arms/tappets i am not sure at the moment but i am gonna buy some valve lapping compound and refresh my original head because it just has a little build up on the valves which caused #3 and #4 to misfire, my bottom end is fine, so i will be pulling the head off again here soon

The stock tank should be 15 gallons just like with the Jeeps.

[Bigboi]

The stock tank should be 15 gallons just like with the Jeeps.

ive ran it all the way down to just below the red line on the guage and ive never been able to fit 15 gallons thats for certain, but according to Edmunds.com its:

1998 dodge dakota 2.5 4 cylinder 5 speed
Fuel type: gas
Fuel type: regular unleaded
Fuel tank capacity: 15.0 gal.
Range in miles (cty/hwy): 270.0/345.0 mi.
EPA mileage est. (cty/hwy): 18/23 mpg

[sharpxmen]

why don't you just calculate the mpg in a normal fashion, 6gal for 260miles gives you 43mpg, so you're obviously off by a large margin, I mean not even close. Even if you go with 7.5 gallons (15 gal tank) you're at 34, unless you push it won't be anywhere near that. Eyeballing the tank level gauge is really a bad way to calculate it - mine gets about 70mpg if I go by the first 1/4 of the tank according to the gauge.

1. fill the tank, reset the trip odo
2. drive some 100 miles or more (just to make it as accurate as possible, the less you drive the more margin of error)
3. fill the tank again, read the odo
4. divide the odo reading to the gallons you just put in (and include the numbers following the decimal point on the pump display).

EDIT:
official mpg numbers for '98 Dakota 2.5L 2wd
18mpg City
23mpg Hwy

[Bigboi]

After towing a trailer today with a decent size load for the 2.5, i have to say im absolutely sure that theres an exhaust leak before the 1st o2 sensor where the header seals to the head, me and the machinist both missed the fact that theres a stud broken off on the head for the 1st bolt to the exhaust, but like i said im just gonna reuse my original head after i clean up the valves so #3 and #4 seal like new again

[Bigboi]

i popped my hood to take a look and sure enough, 3 bolts were missing from my intake/exhaust, ive heard this is a common problem with this engine, so how do i avoid this issue again? use some kind of thread locker??

[Bounty Hunter]

i popped my hood to take a look and sure enough, 3 bolts were missing from my intake/exhaust, ive heard this is a common problem with this engine, so how do i avoid this issue again? use some kind of thread locker??

The front and rear-most bolts are actually studs and commonly break.  Chrysler makes a replacement kit that is less likely to break.