I got a 1999 3.0 4x4 manual and I just took it on a road trip and only got about 11 mpg! Mechanic says I may be the aftermarket power steering pump. I have a saginaw pump off an 01 Jeep Liberty because my OE pumps would always scream like a pig. Anyone have any ideas? Not throwing any codes or giving any signs of running rich.

 

O2 sensors are a big part of MPG
O2 sensors are THE ONLY sensors that wear out, 12 years or 100k miles, and they run out of chemicals, so start to do "false" Lean which lowers MPG slowly but surely

So if they haven't been changed change them, 1999 is 24 years old so should be end of life for 2nd set of O2s

Pull out a few spark plugs and check the tips
Light brown is expected
Darker brown and blackish means Rich running

I assume no gasoline smell?
So no leaking fuel lines

Check odometer, use GPS on your phone to make sure its accurate, tire size changes(larger) will make odometer read lower

 

Having technical difficulties with the website so my apologies if this is posted twice. The O2's have not been replaced to my knowledge. I put in new plugs 1000 miles ago so will check them. Would you think the pump could be a culprit like the mechanic thought? I bought a kit from Proquipped recommended by another thread on this forum. Odometer is accurate and tires are stock size. Is there a way to visually tell if the O2's are old or "shot"? Thanks for the help!

 

No, no visual inspection for O2s, but if coated with soot then replace them, lol, from rich running, but in either case I would replace at least the front O2s


No wouldn't be a fuel pump issue or you would have a CEL(check engine light) and Lean or Rich codes along with lower MPG
Just having lower MPG with no codes means Computer doesn't know its using too much fuel
And it would if fuel pressure was low or high, should be 60psi with engine running, 55-65psi is OK

 

It's a power steering pump not a fuel pump. Are there any other indicators it is running rich? Mechanic said (and he was just throwing ideas out there) he thought the aftermarket pump could be causing excess drag on the system and bogging down the motor. I will switch back to OEM pump and see if it fixes the problem as well as the O2's. It's a manual as I mentioned; could there be any issues with the clutch and pp components? I will also pull plugs and get back to you on condition.

 

I wouldn't think power steering pump unless engine is struggling to idle and its making a noise or squealing from slipping belt

The width/ribs on a fan belt and its "wrap"% around the pulley can only impart XX amount of energy to spin a pulley, if, in this case, the pump needed more energy than the old one the belt should slip

 

That makes sense. I'll upload photos of the plugs tomorrow afternoon and see from there. Could there be an issue with the injectors? The part that's really throwing me off is the lack of CEL. Wouldn't any serious air-fuel ratio issue pop up a code? Thank you so much again RonD; you're helping out a young broke college student very much!

 

The way it was explained to me was that it could be like an AC compressor. When the AC is running it's putting an additional load on the engine resulting in lower mpg. Definitely agree it seems a little strang especially considering the drastic impact on mileage.

 

Yes, more load from AC will effect MPG but as you said not that drastically

Check if CEL works
Should come on with Key on, then go off as soon as you start to crank over the engine(it means crank sensor is working, just FYI)
Start engine
Try unplugging the MAF sensor, easy to get at, see if CEL comes on, it should, then plug it back in, CEL should go off, engine may stall, 3.0l were funny about MAF sensors, lol

 

Alright so below I have a few pictures of some findings today. I pulled the plugs and they all looked like a light brown but I'm not 100% what to look for. I also hooked up a cheap little OBD2 scanner and looked at the O2s. Initially the downstream one was reading zero, but after jiggling the wiring it kicked on; definitely going to replace them. My scanner also had a little yellow light on just wondering if anyone knows what that means; no codes spit out. I unplugged MAF and ran for a minute or so then turned off and started again and no CEL turned on and no codes thrown. The light does turn on with key then turns off with the actual startup.

O2 sensor out

After jostling wiring O2 sensor kicks on then does not turn back off


Plug 3

Plug 6

 

Plug 3 looks like they all should

#6 is oil burning most likely not Rich
#3 and #6 on V6 OHV engines often have the darker color because the intake Valve guide seals can leak
#3 and 6# are at the back of the engine and front of engine is higher, so the oil from valve train tends to collect at those rear valve seals, not a big deal and not a reason to rebuild, lol
And not the reason for the low MPG

Clean the plug and swap it with another cylinder

O2 sensors need to be heated above 600degF to work
So they won't start working until engine has warmed up a bit, 3 to 5min at least

O2s are designated this way
Bank 1 is B1 = Passenger side of V engine(only bank on a 4cyl engine)
Bank 2 is B2 = Drivers side of V engine

Sensor 1 is S1 = O2 sensor close to the engine, also know as Upstream O2s
Sensor 2 is S2 = O2 sensor after Cat converter, farthest from the engine, also known as Downstream O2s

O2s responsible for MPG are B1S1 and B2S1
O2s generate their own voltage, 0.1v to 0.9volt
Once warmed up their voltage should jump around a lot between 0.2v and 0.6v, 0.45v being the sweet spot

B1S2 is after Cat converter so usually runs steadier at 0.75 to 0.85v


Your STFTs(short term fuel trims) look good
-5% to +5% is normal
When accelerating it can go as high as +20%

LTFT(long term fuel trim) is also looking good, its the average over a few drive cycles of both side's STFT

So computer "thinks" all is well with fuel trims
And spark plugs don't seem to show Rich running
So the 11MPG may be from a fuel leak

Still change the O2 sensors

And the MAF not setting a code as soon at it was unplugged is very very strange????

 

So would you still replace the O2 senators thinking they’re the culprit? I recently did new cylinder heads and pistons and rings (basically whole rebuild) but it’s had bad gas mileage since before all that work. Any thoughts on the no CEL on with the unplugged MAF or the yellow light on my OBD?

 

O2s last 12years or 100k miles, ONLY sensors that wear out, so if you don't know their age then change them

But 5 of your spark plugs don't show signs of running Rich, so I think you have a fuel leak, or a math error in MPG calculation

Unplugging MAF sensor's 4 wire connector should have set a code instantly, so that is odd

 

How do I find a fuel leak? I would think it would reek of fuel were it leaking. Are there any other tests I can do? I forgot to mention I just installed a new clutch and mechanic said I could have messed that up somehow but I was having trouble before that. It shifts fine so I would be surprised if it was installed improperly.

 

No, I doubt a new clutch would cause MPG change unless you felt it slipping constantly

Facts at hand
Calculated 11MPG
No Rich running seen on spark plugs, and no grey smoke out the tail pipe(I assume this because you would for sure see it)
Vehicle seems to drive fine, Rich running causes sluggish operation
So extra gasoline is not going thru the engine

That leaves 2 options
Error in original MPG calculation, retest, its free, lol
Gasoline leak between tank and engine

A 1999 should set EVAP Code if gas tank has an air leak
Leave gas cap off(or loose) for next day or two of driving and see if you get a CEL
P0442 Evaporative Emission Control System Leak Detected (small leak)
P0455 Evaporative Emission Control System Leak Detected (gross leak)

As weather warms up you should smell a fuel leak after driving and shutting off the engine

 

Alright I will retest that this weekend using my phone to track distance instead of the odometer. Would it be beneficial to buy a kit to test fuel pressure at the rail? Could that tell me if I have a leak? In the mean time I also purchased the OE pump to reinstall and see if that works. I doubt it will but I'm kind of just throwing **** at the wall to see what sticks.

 

Fuel pressure should hold for a few MONTHS with key off, but................often doesn't because fuel leaks back to the tank, not because of an external leak, but possible
If pressure held then no leaks in the lines, but could still be a leak in tank or filler, but would expect those to set EVAP codes

Yes, even 1/2 tank check around town for MPG would tell you if the 11MPG was valid

Only need to do the GPS test for a few miles on a straight road if possible, and at highway speeds if possible, to see if speedo and odo are both correct

 

I would agree on the evap codes but the MAF won’t throw a codes so obviously something is whack. Would an en evap leak cause that drastic of a mileage issue?

 

No, not the EVAP issue itself but a leak in the tank or filler would