Jeep WJ gets increasingly worse gas mileage?

02 Grand Cherokee, 4.0, Laredo, tow pack (basically 3.73s and a mech fan), has about 150k on it.

I know these were never good on fuel, and the gears likely don't help, but even with my mother driving it (she usually beats old EPA numbers) it was averaging 18 (all highway/back roads), now it's gone down to 16 over the last year or so. These numbers are all coming from the truck, I have no reason to doubt them.

No CEL, haven't checked for codes, it's got a bad ABS sensor but that shouldn't effect anything.

I do notice, ever since we've had it, that the temp gauge seems to stay well below 210 (middle), gauge acts like it's got a 190 degree stat in it, could that be part of the problem? It does desperately need a cooling system flush anyways.

This thing never idled as smooth as I thought a EFI I6 should, even with fresh plugs in it, it had a set of Bosch fine wires thrown at it about 2 years ago. Are these just really plug hungry/picky? What do they like? It's also always felt kinda sluggish, even with the 3.73 gears, though I always attributed this to the 195hp engine in a ~4000lb truck.

Fan clutch was recently changed, I've considered removing the mech fan as it never tows more than a fairly aerodynamic (big pop up camper) 4000lbs or so and it's only 4 bolts (leave the clutch on) if it turns out to be that important.

Trans behaves properly. Front axle was recently sucked out and filled with 75w140 as specified, rear still needs serviced (75w140 is expensive) but I can't see that causing a problem. Engine gets a change or Rotella T6 5w40 and a Wix filter every 7500.

Tires are inflated (40psi IIRC), alignment isn't too far out of whack, all that obvious stuff I can think of is in order.

Thoughts?

How old is the air filter?

I spent years at a Jeep dealer as a tech. Only use Champion plugs in a Jeep motor. I can't count the number of drivability problems that were cured by removing more expensive plugs and spinning in Champions. I doubt it's your only issue, but it'll help.

I think the temp "problem" is your culprit. Colder motors use more fuel.
Flush the cooling system while changing the thermostat.

I'll bet this starts a hell of an argument, but a dirty air filter has absolutely no effect on gas mileage. Just think of it as less throttle opening.

As to the original question, I'd throw an oxygen sensor at it. A proper, OE equivalent, not the cheapest one on Rock Auto.

The jeep 4.0 likes it hot, but I'd bet this is a plugged cat, sensor or plug degradation problem all mixed together.

I'll bet this starts a hell of an argument, but a dirty air filter has absolutely no effect on gas mileage.

No argument here. Gas mileage is a thing that only happens when you're only using 20% of the air filter's flow capacity. So unless your filter is 80% BLOCKED you probably wouldnt notice a difference in cruise mpg. Even if you needed more throttle opening to make the same power, more throttle opening is LESS pumping loss on the intake stroke, so unless your throttle opening is triggering a richer AFR, even that would not drop cruise mpg.

I might take the bait re: air filter and gas mileage.

What about the tps? Doesn't the computer us that as one of the main inputs for fuel control? Especially during warm up, before the oxygen sensors kick on?

Seems to me if you don't drive much or have a lot of short trips, a dirty air filter could really bring you down overall. Now, hot engine, steady state cruise, I think you have a point, but that is not the only driving condition that might affect average mpg.

Checked air filter, it's far from new, but still passes a lot of light, and is huge for a 4 liter engine that doesn't spin very fast, so that should be fine.

On plugs, just plain old copper plugs?

I'll throw some plugs and/or a thermostat(and flush) at it and report back.

Just plain ol Champions.

In reply to logdog:

DOH! I forgot to check the ion beam alignment! No wonder it's been acting up.

My brothers wj also runs colder like yours, ive swapped 3 different tstats and still no change including a factory one its weird.. dont take the engine fan off, if anything replace it with an electric one, it helped the mpg a bit in my xj.

In reply to chiodos:

I'm 99% sure the WJ tow pack adds a mech fan on top of the existing electric.

Dunno for certain about that. But most cars with a supplemental electric fan come on way hotter than needed. They are more like an oh E36 M3 fan than a safe cooling fan.

But hey ive been wrong plenty times before

Streetwiseguy wrote: As to the original question, I'd throw an oxygen sensor at it. A proper, OE equivalent, not the cheapest one on Rock Auto.

That was my first thought as well. O2 sensors start reporting lean as they age, and the PCM responds by delivering more fuel. It's a gradual thing.

Are there any ways of testing the O2 sensors without dropping $63 (per rockauto for OE NTKs) for the upstream pair?

chiodos wrote: My brothers wj also runs colder like yours, ive swapped 3 different tstats and still no change including a factory one its weird.. dont take the engine fan off, if anything replace it with an electric one, it helped the mpg a bit in my xj.

Probably a stupid question, but I'll ask anyway:

Is that based on the factory gauge or external reading?

Factory gauges are not the most accurate, and some are just warning lights with numbers.

Kenny_McCormic wrote: Are there any ways of testing the O2 sensors without dropping $63 (per rockauto for OE NTKs) for the upstream pair?

Get an OBD-2 reader and check the live readings and the long term trims. If the ltt are heavily positive, the pcm is compensating for something.

The dirty air filter thing dates back to carburetor days when it may have had a "choking" effect.

On EFI, the air flow is measured after the filter, so the only restriction would be at max power.

My 2002 KJ with tow package came with two fans. Clutch and electric. I removed the clutch fan and never ha heating problems even towing 3900 lbs. in 90F. temps.

Many will laugh but I gained app. 2 mpg by changing all lubricants to AMSOIL.

In reply to Keith Tanner:

Noted, will dig out the bluetooth scanner and have a crack at it.

Ok, long term trims, taken at idle, warmed up in park.

Bank 1: 10.16% Bank 2: 7.03%

So something is definitely making it run rich, correct? Air and coolant temps sounded believable. No codes, all monitors were go. It's gonna go see a better code reader and an alignment rack Wednesday to make sure nothing else is amiss. Then new O2s I guess.

The ECU thinks the car is running lean, so it's adding a bunch more fuel to compensate. You've definitely got something going on.

If it really IS running lean, you're looking at things like low fuel pressure or clogged injectors. Potentially also MAF problems. Or a surprise turbocharger, but you probably would have noticed that.

But it's using a bunch more fuel. So it's probably not running lean, it just thinks it is. O2 sensors, maybe an air leak in the exhaust.

OK, changed plugs today, champion copper indicated on hood sticker went in, gap set at .035 per hood sticker.

Old plugs were Bosch fine tip platinum, #1, 4, and 5 looked good in terms of wear, #2, 3, and 6 had the fine tips burned off opening the gap to 0.080 or so. Any thoughts on that? Color looked a bit rich as suspected, about right on one side of the ceramic (hint of color), the darker tan you shoot for when jetting a 2 stroke dirt bike on the safe side of ideal on the other.