I think my 95 4runner with an automatic transmission and 3vze engine has broken the world record for worst gas mileage. My last tank got 4.5 mpg!

I recently bought the vehicle used with a blown head gasket and I just got her running again. She starts right up, idles smooth, has as much power as can be expected from this vehicle, the air filter is clean, no cylinder misfires, no engine codes, no nothing. The injectors have been flow tested and cleaned within the past ~20,000 miles, the valve clearances are great, and the valves themselves are good. I had an emissions test half way through that tank of fuel and it actually came back as running a touch lean, but still passed with ease. I have checked for fuel leaks and have found nothing, I can't smell any fuel, and the fuel lines on the underside of the vehicle look good. Tomorrow I will make sure the fuel rail is holding pressure and hence not leaking, but I don't think that's the issue. My 4runner is basically bone stock and has what I believe are 29 inch tires, I haven't been driving very hard, and the RPMs typically sit around 2,000 give or take during city driving.

Does anyone know what's going on? Most people complain about 9mpg, but 4.5mpg is at a whole new level and my 4runner doesn't have any other symptoms.

 

A couple thoughts come to mind.
You should be seeing signs of richness somewhere. Read the plugs for consistency. Then compression test. A bad exhaust valve would let unburnt fuel escape. The only time i’ve seen good emissions when a car was running poorly was a plugged catalytic converter.
Even more simply, how are you calculating mpg’s?

 

Your logic is good, but the test won't show what you want. The spec for pressure-holding is "above 21psi for 5 minutes." http://web.archive.org/web/201003261...87fuelpump.pdf On mine, it drops to that in only 3 minutes, probably due to a weak check valve in the pump (my mileage is fine). So if yours drops like mine, it doesn't necessarily mean you have a leaking fuel rail.

Check the Fuel Pressure Damper (at the back end of the right fuel rail). At your vintage the rubber diaphragm can fail, leaking fuel. Since it's warm back there, the fuel evaporates quickly. (The FPD is hard to get to; I use a brown paper towel to see if it's "wetted.") I'd be surprised, though, if you were leaking so much fuel it pulls your mileage down like that, AND you can't smell it.

Confirm your ignition timing is correct. Your passing emission test should rule out a lot of things, but that fuel is going somewhere. Was your emission test on a dyno? (You may have excellent numbers at low throttle, but start mis-firing under load.)

 

I'm pretty sure the exhaust valves are good(they where less than 70 miles ago when I put the engine back together, but I will do a compression check. The really weird part is that I am not seeing signs of richness anywhere. The plugs look good, and the emissions test shows that the overall engine is actually a bit lean. On the other hand I feel like a plugged catalytic converter, ignition timing being off, or a low enough compression to kill mpg like this would show massive performance loss with various other symptoms which I'm not feeling.

As for calculating mpg, I topped off the tank(added about 15 gallons), I don't remember the exact numbers anymore, but after running roughly 55 miles I added over 12 gallons more to the tank. When I did the math it came out to about 4.5mpg.

 

I don't believe the emissions test was on a dyno. So it could possibly be misfiring or running rich under load. The engine sounds good, feels smooth, and pulls as strong as a 3vze can be expected through the entire power range so I don't suspect that's the issue, but I will look into it and check the timing.

I also forgot to mention, but the 4runner coasts just fine. The bad MPG isn't being caused by dragging brakes or anything. I also double checked and the 4wd system is properly disengage.

 

No codes? And it passed smog? Odd. Someone stealing gas?

 

Is your evap canister hooked up correctly (including the line through the TVV to the throttle body)? I'd be surprised you'd lose that much gas without it, but it is intended to suck up vapors on a hot day when you're parked, and then "reuse" them once the engine starts and warms up. If you don't have the line to the throttle body, you'll just be dribbling it out onto the street.