'99 XJ: Low MPG and leaking valve cover ... ??

[xdarrows]

So, my '99 XJ (4.0 with NP242) has reached it nearly final state (waiting on rock sliders from JCR), to include regearing. I'm getting less than 13mpg.

I'm running BFG AT 31x10.50 tires with 4" of lift, SYE, and regeared to 4.10 plus various armor and upgrades.

The only engine problems that I've ID'd are weeping freeze plugs in the back of the block and lots of leakage around the valve cover ... would any of this lead to my sub-13mpg mileage?

Would it be worth getting a valve job done?

 

[xdarrows]

NOTE 1: only have about 132,000 on it.

NOTE 2: at some point (not very soon), I'll drop in a Golen stroker.

 

[RCP Phx]

If your freeze plugs are leaking they can cause lower mpg's.

 

[Tim_MN]

Are all the tune-up parts fresh ? Do you have the correct speedo gear in the t-case, or are you calculating mpg's using uncorrected odometer data ? What diagnostic testing of the engine sensors have you performed ? Does diagnostic testing confirm that there are issues with the valves ? Do you drive like a 16 year and/or routinely exceed 70 mph on the highways ?

My 00 has 5.5 of lift, 31's and stock 3.55 gears. The corrected combined city/hwy gas mpg's are 17.4 with lots of miles at +70 mph.

The primary suspects for low mpg's are an exhaust leak before the O2 sensors, followed by faulty O2 sensors. Most 99 and older XJ's likely have a cracked exhaust manifold. Obviously, you should test the O2 sensors, inspect their fuses, and test/inspect their wire harnesses for damaged/shorted wires, and their wire plugs for corrosion and bent or pushed back wire plug pins.

 

[75SV1]

As mentioned the Speedo gear, O2 sensors, I'd add look at your spark plugs, spark plug wires, cap and rotor. The 4.10's should be right for 31's. Is the 13 mpg hiway or around town? I'm on 31's (metric though) and 3.73's. Highway, I'm between 17.4 to 18.3 mpg. Daily commute, so open roads at 55 mph, country roads, some stop lights at probably 14 mpg.
I don't know how much a leaking valve cover will affect mpg. It does mess with the CCV system. It might allow more oil into the airbox. Oil into the Fuel/air mixture would result in loss of power. Still, a bit of a strech.

 

[xdarrows]

The tune-up parts could use a refresh. I just had the correctly calculated speedo gear installed. I drive conservatively about 80%-90% of the time. I don't have any diagnostic gear.

What kind of prices am I looking at for head, valve, and freeze plug work on top of the easily replaceable bits?

 

[Unclewolverine]

Does your cooling fan run a lot? Just noticed here lately that if I run my aux fan all day I lose about 2mpg.

 

[Evan03]

Shit id like to get as good as 13.

If you don't know the last time it had a tune up then its time now. Service the transmission to.

Plugs, wires, muffler fluid etc

I do it ounce a year regardless of miles driven. Around 5000 miles for me

Tune up parts are consumables (O2 sensor) oil leaks don't normally affect fuel mileage

Sent from my SM-G960U using Tapatalk

 

[Unclewolverine]

Now wait, you change your muffler fluid once every 5000 miles?! How can you afford that?!

 

[xdarrows]

Does your cooling fan run a lot? Just noticed here lately that if I run my aux fan all day I lose about 2mpg.

No, it doesn't.

I found what seems to be a good engine shop; I'll take it by next week and see if they can look at it.