Shockingly poor fuel mileage - Any recommendations on what I should look at? (1 Viewer)

[Drebin]

Hi there,

I recently bought at '88 FJ62. 340K miles. Engine and trans were rebuilt at 200K according to mechanic who worked on it.

Solid rig. No rust.

I've had her for about 4 months and been tracking the fuel mileage carefully. Just for fun. I know these things don't get good mileage(11mpg city & 18mpg highway on the best conditions).

Here are some relevant details for this question:
Automatic transmission
265/75 R16 tires. No super aggressive tread but All-Terrain tires nonetheless.
No roof rack
80% city driving, 20% highway.
I do NOT gun it off the line. I'm quite easy with her.
New spark plugs about 5months ago
Regular unleaded

I've been getting about 4mpg. Even for a big truck that seems comically low. I know the bigger tires don't help and the auto transmission just kills mpg.

Any thoughts on what I should ask the mechanic to look at to see that can be improved so that I get back close to 10mpg or so?

Thanks for your thoughts. G

 

[HemiAlex]

I’d run through the fsm and do all the diagnostic stuff you can. Also throw and air fuel wideband in it and see what it’s doing.

You can figure out a lot with an AFR gauge and a vacuum gauge.

 

[lechnito]

Hey there fellow Portlander.

Any chance you're leaking fuel from the lines or out the gas cap?

 

[chichiflys]

Silly question - does the truck seem sluggish at all? Not sure how familiar you are with these trucks - but have you driven other FJ60/62 series trucks to compare. How is your compression? Dragging a brake (does it coast well in neutral?)? That's some seriously low mileage.

 

[unklwedy]

I'd look at Honda Civic.

 

[roadstr6]

4 mpg is seriously low. That's like 95 miles per tank. That is inconceivably low. I'm pretty sure top fuel dragsters get better mileage than that. Is your tank rusty? Does truck smell like gas? It's got to be dumping it because I can't imagine how it can be burning fuel at that rate. If the fuel is indeed going through the engine, I'd be worried about cylinder wash-down, ring wear and fuel in the oil. That is a lot of fuel.

 

[Seth S]

I'd start with compression test just so I feel warm and fuzzy inside knowing that I didnt have a dead cylinder or two. Then I'd pull and inspect all the plugs even though they are not that old. See if they are caked in carbon or if they are clean...give you an idea of your running conditions. Make sure your air filter is clean, make sure you are not dripping fuel from the tank, lines, filter, pump etc. Wouldn't hurt to check your valve clearances and check your timing.

80% citiy driving will certainly impact mileage...are you sitting at stop lights a lot? If you take a weekend trip where you can cruise at 50 mph do you see a notable improvement in mileage?

 

[JToobe]

Is your gas tank locked or do you leave your car unlocked so that someone could open the gas tank? I suspect theft of gas....happened to my son in law on his 62.

 

[Steve83]

How exactly are you calculating MPG? The only time I've seen a vehicle actually getting THAT low, it was a 3/4-ton pickup towing a large travel trailer cross-country, and the fuel filter was loose, so there was a puddle of gas spilling off the intake manifold as it was driving.

This TSB & the ones in the caption come from Ford, but there's nothing brand-specific in them - they apply to all vehicles:



It's a LOT of reading, but it's worth it. Be sure to scroll down below the image (which is large) to read the other TSBs. Read it on a computer with a monitor - not a phone.

 

[lcwizard]

check the oil for fuel. bad rings could let fuel pump into the crankcase. That much fuel isn't being burnt unless you're making 500 hp.
It's going out the tailpipe unburnt, into the crankcase or on the ground...or you've got a sneaky neighbor with a siphon hose

 

[Prairie Swamp]

A guy I work with inherited his grandfather's farm truck. 1974 Ford 1-ton 4X4 460 3-speed maunal. He claims 4 MPG and I believe it - it's a beast. So to pile on with what has been said - unburnt gas is going somewhere.

 

[Drebin]

ahhhh, brilliant everyone. thanks for all the input. You've given me lots of good places to investigate.

To answer some of your questions/suggestions:
- There's no discernible gas leaking on my driveway when it sits for a day or two.
- Gas cap seems tight with the gas cap rubber seal in good condition. I don't think it's a gas line or cap leak.
- My neighbors don't look like the petrol stealing type....I think?
- Sparks plugs and air filter are new as of May this year.
- Compression test was done 4yrs ago and each cylinder was within 5lbs.

One follow-up ?: It seems to run a bit loud, even for a rig. Rumbling noise from under the rig...around the muffler area.
Could a "bad" or broken muffler/exhaust be a cause for this really low mpg?

 

[Prairie Swamp]

I see it's a 62. Is the cold start ejector stuck open mechanically or being told electrically to stay on? Easy to pull and check. Running warm engine there should be no fuel spraying. Then you might have to check the 6 main ejectors. But I would think the exhaust would reek of gas and also your cat convertor would likely have suffered a melt down.

 

[SteveH]

EPA rating was 12 and 14. Worst I have gotten is 9.78 (all offroad) - best was 15 (all highway). Be sure you visit the same gas pump and divide the exact # of miles driven by the exact gallons to refill. Be sure your transmission is full of fluid (per the dipstick, hot) and not slipping at any time, hot or cold. Pull the plugs - with that kind of fuel use, they should not look normal. Check and smell the oil - it should not smell like gasoline.

Steve
'89 FJ62 - 254K miles, orig engine and trans.

 

[Steve83]

You didn't say how you're figuring MPG. Is your speedo calibrated? Have you ever checked it on a long trip against the mile markers or a stopwatch?

No, an exhaust leak (alone) can't cause THAT big a drop in MPG.

 

[workingdog]

Second vote for the putting an O2 sensor in there somewhere and see what says. That will tell you if the gas you are losing is going through the engine or you are losing it somewhere else. You could have a significant gas leak and never see anything on the ground, gas is so volatile.