4.0 crap mileage

[Bunger317]

Bought a 94' 4.0 4x4 with a4ld back in April. It DOES have 33's with stock 3.73's. Truck runs great as far as I'm concerned but I'm only getting 12.5-13.5 mpg and i feel like I'm driving fairly conservatively. I've had 4.0s with 33s in the past and don't remember them being this bad. Is this just par for the course or should I dig deaper? It had a bad t stat and wouldn't come up to temp when I bought it. Replaced that and figured mileage would shoot up but it really didn't change. Any ideas?

 

[RonD]

#1 would be O2 sensors, only sensors that wear out after 12 years or 100k miles, they start to go Lean as they run out of chemicals so computer runs engine Richer than needed
Check spark plug tips
If darker brown then engine is running richer than needed

#2 Odometer
Has it been calibrated for the bigger tires?
Larger tires means lower miles shown on odometer than actual miles travelled
So your 13mpg may actually be 16mpg

Use vacuum gauge to see if engine has good compression, 18-21" at idle

Automatics always get lower MPG because the "clutch" is always slipping at lower speeds
Torque Converter Lockup(TCC) was added to lock the "clutch" at lower RPMs and speeds to get better MPG
Do you "feel" that the torque converter is locked when driving above say 30mph?
i.e. when you press on the gas pedal speed should instantly respond to RPM changes, because engine is locked to transmission
If it slips a bit then its NOT locked

 

[Bunger317]

#1 would be O2 sensors, only sensors that wear out after 12 years or 100k miles, they start to go Lean as they run out of chemicals so computer runs engine Richer than needed
Check spark plug tips
If darker brown then engine is running richer than needed

I recently replaced the plus/wires with new motor craft stuff. The truck has 67k original miles on it. The stock plugs didn't look necessarily rich to me. I hoped new plugs would help with mileage but it didn't.

#2 Odometer
Has it been calibrated for the bigger tires?
Larger tires means lower miles shown on odometer than actual miles travelled
So your 13mpg may actually be 16mpg

I believe it has. That what my first thought. Speedo matches my gps speed almost perfectly.

Use vacuum gauge to see if engine has good compression, 18-21" at idle

I have done this. Always between 19-22"

Automatics always get lower MPG because the "clutch" is always slipping at lower speeds
Torque Converter Lockup(TCC) was added to lock the "clutch" at lower RPMs and speeds to get better MPG
Do you "feel" that the torque converter is locked when driving above say 30mph?
i.e. when you press on the gas pedal speed should instantly respond to RPM changes, because engine is locked to transmission
If it slips a bit then its NOT locked

I don't believe it's slipping. It in fact has the opposite problem. The shift from 1-2 is veryyyy stiff. Almost harsh. Unless I easeeee into it or stand on the throttle from a stop. Under normal driving it's pretty harsh. All other gear are relatively smooth. I think my vacuum modulator may need to be adjusted or replaced.

 

[ekrampitzjr]

If the truck has the original O2 sensors, they're 23 years old and it's past time to replace them. You'll probably have to apply heat to the exhaust to get the old ones out or get a muffler shop involved.

I usually get 20+ miles/US gallon with the 4.0 SOHC, so 12–14 mpg with yours is miserable unless you have a lead foot or regularly haul or pull a lot.

 

[Bunger317]

If the truck has the original O2 sensors, they're 23 years old and it's past time to replace them. You'll probably have to apply heat to the exhaust to get the old ones out or get a muffler shop involved.

I usually get 20+ miles/US gallon with the 4.0 SOHC, so 12–14 mpg with yours is miserable unless you have a lead foot or regularly haul or pull a lot.

Don't tow anything. It's got 33s with stock gears and it's 4x4 so I didn't expect much. But figured better than 13.

I'll be looking into replacing o2 sensors

 

[Bunger317]

Also I noticed my water temp fluctuates a bit. Ac on on a warm day going up a hill it'll get to the M and bounce between that and O. Ac off keeps it a bit more stable. According to my infared temp gun the t stat house never gets over 205-210 which leads me to believe the swing between O to M is only about 10-15 degrees?

 

[rusty ol ranger]

Also I noticed my water temp fluctuates a bit. Ac on on a warm day going up a hill it'll get to the M and bounce between that and O. Ac off keeps it a bit more stable. According to my infared temp gun the t stat house never gets over 205-210 which leads me to believe the swing between O to M is only about 10-15 degrees?

Does that have two seperate sensors for coolant readings? One for the gauge and one for the ECM or does it use one for both?

If it only has 1 for both it could be that sensor is screwy and knocking out your mileage.

 

[RonD]

Yes, that's "normal"

1/2 on Ford gauge is 205degF

Bottom of Radiator could be clogging up a bit, or older water pump, but certainly not an issue at all at this time

 

[Bunger317]

Does that have two seperate sensors for coolant readings? One for the gauge and one for the ECM or does it use one for both?

If it only has 1 for both it could be that sensor is screwy and knocking out your mileage.

It's got one for gauge and one for ecm.

 

[rusty ol ranger]

It's got one for gauge and one for ecm.

Back to the drawing board then lol.

 

[rubydist]

33s with 3.73 gears on this engine will not do much better than you are getting unless you never go over 40 mph. I would put some appropriate gears in it.

 

[gaz]

@Bunger317 ,
(A) switching to an electric fan, throwing that crank driven anchor and clutch in the garbage, will not make more power but it will reduce the amount of power robbed
(B) 3:73 is not enough for 33", I would consider the regear or 28.5" to 31" will make a difference
(C) a good quality valve job could be what the doctor ordered. I've only seen horrible valve seats on the 150k+ mile 4.0's I've seen, let alone a fresh set of valve guides and seals.
(D) the valve train on these is non-adjustable, it takes a beating and will suffer from premature wear. There are several solutions to this issue.

 

[Broosedamoose]

I average about 13.5 MPG with 31x10.5's and 3:73's. I tend to go from stop to go pretty fast though! I'm usually up to speed and then some before merging onto the freeway.

 

[19Walt93]

I assume your 33's are all terrains with aggressive tread, good for off road but not for gas mileage. Unless you do a lot of off roading, I'd replace the tires with smaller diameter all seasons. And keep them inflated properly. If your area is hilly like NH, with 3.73's it will downshift often and that hurts mileage. My 2004 Ranger with 4.10's got better mileage than my 11 with 3.73's. O2 sensors and a fresh set of plugs couldn't hurt anything. Install a vacuum gauge where you can safely keep an eye on it while driving- if the vacuum drops, so does the mileage. Accelerating hard is fun but it's rough on the mpgs.

 

[Bunger317]

@Bunger317 ,
(A) switching to an electric fan, throwing that crank driven anchor and clutch in the garbage, will not make more power but it will reduce the amount of power robbed
(B) 3:73 is not enough for 33", I would consider the regear or 28.5" to 31" will make a difference
(C) a good quality valve job could be what the doctor ordered. I've only seen horrible valve seats on the 150k+ mile 4.0's I've seen, let alone a fresh set of valve guides and seals.
(D) the valve train on these is non-adjustable, it takes a beating and will suffer from premature wear. There are several solutions to this issue.

I am hoping that's not the case. This motor only has 67k miles on it. I agree with the gearing and it's on my to do list.

I having been considering the e-fan. Especiallysince I just bought a 40k trans cooler I'm putting up front also.

I replaced the water pump Friday and flush the radiator and heater core out. Still seeing the same temp higher than I think is normal. With the ac on it swings from O to M quite often. Still only seeing 210 on my infared on the t stat housing. And it has been over 100° where I live this week but still seems odd to me.