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Discussion Starter · #1 ·
Hello guys, I need some help on directions to solve my poor fuel economy issue. A few things:
  • I am getting around 13-15mpg combined, this is me trying to squeeze as much as possible driving around 60mph freeway with very light foot.
  • Car has around 150k miles
  • Idle smooth around 500-700 rpm, no misfire or any weird symptom
  • no CEL code or any logged CEL code with scanner
  • wheels are 19inch Fsport with 225/35 and 245/35, not standard yes but I don't believe this can cause mpg to be 30% less?

Here's what I have checked or replaced:
  • new spark plugs
  • new coil wires
  • new intake air filter
  • MAF and throttle body sparkling clean
  • tire pressure normal around 35psi
  • brakes are not dragging
  • new transmission fluid. Auto trans runs and shifts smooth.

Here're what I gather from OBD2 scanner:
  • Throttle position reading ~14% at idle, ~79% at fully depressed throttle pedal
  • all 4 O2 sensors reading 0-0.7V (fluctuates slowly but I believe this is normal)
  • IAT around 50C at idle (I believe normal)
  • Timing seems to be consistently advanced (+) at idle, as high as ~11deg
  • Fuel trim 1 and 2 both reads consistently near 0, ranges from -4% to +2%

I am really puzzled at this point. Can anyone gather anything from the readings above, or point me towards what I should look for next? Many thanks!
 

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Your wheel and tire height is almost an inch bigger than stock. So it can be an issue. Depending on the terrain you drive. Hills will load the engine more along with accelerations. Also even if your engine is running fine. and has no cel codes. Your cats could have diminished flow. That can cause an issue. But in this case I think it is the wheel and tires. To test this. You could put the stock stuff on for a while.
 

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2002 is300
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There is only a 3% difference between your wheels and factory in terms of height. I think a big contributer could be 1) your larger wheels are going to weigh more which has an exponential affect on fuel ecconomy. 2) wider tires will be more sticky and increase roll resistance, fyi tire pressure should be more like 29-30 psi. I did a blow through exhaust and noticed some decent mpg improvage. Also shimming the trans adding a cooler and switching to full syntetic also made a difference. One thing I have noticed is fuel ecconomy is best around 2500-3k rpm. 60mpg is pretty low rpm mixed with heavy and sticky tires can cause a lot of load. Your easy soltuion might be more speed. I've achived 25mpg in mine. I've also got 12 mpg all depends on how you drive weather idling etc.
 

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Discussion Starter · #4 ·
There is only a 3% difference between your wheels and factory in terms of height. I think a big contributer could be 1) your larger wheels are going to weigh more which has an exponential affect on fuel ecconomy. 2) wider tires will be more sticky and increase roll resistance, fyi tire pressure should be more like 29-30 psi. I did a blow through exhaust and noticed some decent mpg improvage. Also shimming the trans adding a cooler and switching to full syntetic also made a difference. One thing I have noticed is fuel ecconomy is best around 2500-3k rpm. 60mpg is pretty low rpm mixed with heavy and sticky tires can cause a lot of load. Your easy soltuion might be more speed. I've achived 25mpg in mine. I've also got 12 mpg all depends on how you drive weather idling etc.
As you said, I dont believe the small wheel height difference is the cause, similarly for the tire pressure difference. Trans is full synthetic fluid. On the freeway, I was just aiming as high as possible with the little mpg needle, and I found around 50mph-60mph gives the best result. It was an experiment to see how much I can really squeeze out, and it's showing something else is wrong.

I have had the 18s 3IS F-sport wheels on my old IS, they are same width 225/40 and 245/40, and I did not notice any major change with mpg. In addition, these 19s Rays fsport wheels are actually forged and quite light, they are definitely lighter than the 18s 3IS oem 5 spoke fsport wheels.

I'll put my stocks back on and check again. But fundamentally I think something could be wrong with my fuel system. Perhaps the injectors bad spray pattern and needs cleaning?
 

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2001 AR5 Swap, Built NA-T, Standalone
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Very strange, I get around 19mpg and I mostly city drive. I have heavy 3 piece wheels with 225/255 tires. I idle high and have boost. On the other hand my manual transmission has very slightly better gearing, .73 vs .75 on an auto diff. I'd try gutting the cats or getting high flow ones.
 

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Discussion Starter · #7 ·
More puzzling signs.. I noticed a mpg reading from my little Bluetooth obd2 scanner, so I plugged it in during my morning 10miles commute. And it reads around 25mpg! So what could cause me losing gas.. i don't smell any gas around the gas tank area accessed from under backseat, nor in engine bay. Fuel rail and injectors looks dry no signs of leaks!

Speedometer Gauge Vehicle Motor vehicle Automotive lighting
 

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Discussion Starter · #11 ·
its gotta be the wheels and plus size tires you can borrow my stock wheels with all seasons if you wanna data log
Thanks I have a set of stock wheels to try, but I really doubt that's the cause. Anyone know how the mpg reading from OBD2 is calculated? If it's indicating 25mpg and it's actually accurate then it's not related to the actual engine running conditions. So I am thinking somewhere from fuel tank to fuel rail there is a leak.
 

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2002 is300
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25 mpg and the dash mpg is calculated using the engines sensors. If those sensors are off so will the computers mpg. I've noticed short trips are terrible on gas in these cars. Short 5 10 mile trips suck gas. The only was gas can escape is the evap injectors or a leak. Honestly 15-20mpg is typical.
 

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Dash mpg and the OBD2 mpg is probably from injector duty calculations. The fuel level takes in MAF data as well i think. My car when stock gets 19-21mpg and will dip into the 16-18mpg range with rougher driving, though it seems like you’re trying to drive economically. 15mpg is pretty bad even for this car.

And I second the questions regarding where that 25mpg measurement and how the 13-15mpg is taken. I can get a 25-28mpg steady state consumption fairly easily but across a tank, it’s only 16-20mpg. If it’s possible, fill up right before the highway then drive 20miles at a very constant pace. Refill the tank and see how much fuel you’ve used.

The MAF readings might also be able to tell the story but I don’t think there’s a way to log them and comparing it to another is300 might be difficult. If the cats are clogged, the MAF should show a lower reading than typical. You could also get a shop to test if the cats are clogged, or get a exhaust back pressure sensor from eBay/Amazon.
 

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I also would like to know how you’re calculating your current mpg and what type of range you’re getting form a tank of gas?

I don’t calculate anything myself since, well I gotta get gas when it’s empty anyway. I would assume I’m getting around 20-21ish myself. I get around 330-350ish per tank depending. Only mod to my car currently is a DC sports intake. Stock everything else. Car has ~191k miles.
 

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Discussion Starter · #16 ·
I came up with around 13-15mpg by looking at the my odometer driven since last full fill up, and how many gallons of gas it takes to fill it back up. It is definitely accurate, I just drove 100ish miles from my last full fill up and it already used up close to 7 gallons (about 2 ticks above middle of gas gauge).

The obd2 mpg is reading normal as I would expect with how I drive, around 20mpg with my normal driving, and if I try really hard 25mpg for my daily 20 miles of freeway commute. It resets everytime you shut off the car, so I can see how the mpg reading increase from 0 as I start driving, all the way to ~25mpg when I am about 5 miles into that trip on freeway.

Dash mpg and the OBD2 mpg is probably from injector duty calculations. The fuel level takes in MAF data as well i think. My car when stock gets 19-21mpg and will dip into the 16-18mpg range with rougher driving, though it seems like you’re trying to drive economically. 15mpg is pretty bad even for this car.

And I second the questions regarding where that 25mpg measurement and how the 13-15mpg is taken. I can get a 25-28mpg steady state consumption fairly easily but across a tank, it’s only 16-20mpg. If it’s possible, fill up right before the highway then drive 20miles at a very constant pace. Refill the tank and see how much fuel you’ve used.

The MAF readings might also be able to tell the story but I don’t think there’s a way to log them and comparing it to another is300 might be difficult. If the cats are clogged, the MAF should show a lower reading than typical. You could also get a shop to test if the cats are clogged, or get a exhaust back pressure sensor from eBay/Amazon.

I will need to check my cats, but the upstream cats/header have both been replaced with aftermarket recently so they are quite new. O2 reading is normal I believe fluctuates 0-0.8V at idle and no CEL. I can glance over my app on MAF reading when I am on freeway, what would be the expected values? I know at idle it was pulling around 4 g/s.

I dont see any signs of fuel leak at the injectors when engine is running. I also dont see any signs of leak near the charcoal canister area when I crawl under. I am going to check the dreaded the gas tank overfill check valve, I suspect gas could be evaporating from there, although I imagine it would have to be a pretty big leak to cause me loosing this much gas, and it would have triggered an EVAP CEL.

The key to all this mystery is, no CEL! And the car really runs smooth, accelerates well, cold starts right up everytime at around 1.2k rpm and once warmed up down to 500-ish rpm.
 

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If I had to guess, MAF should be ~25 g/s at 60mph. That’s about how much air it takes to make ~25hp with reasonable VE numbers. I don’t actually know since this isn’t exactly something people report on.

Another source of possible gas leak around the fuel pump hanger. Maybe the rubber fuel hose has a tiny leak. You’ve mentioned fuel trims but do you know if short term fuel trims are working (closed loop)? The O2 voltage switching indicates that it’s in closed loop during idle so I’d imagine it does at cruising as well but can’t hurt to check (short term fuel trims). Since the car runs fine otherwise, I can’t think of any obvious problems.
 

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Discussion Starter · #19 ·
How many miles are you getting per tank?
Well, no more than 250 miles per tank, and that is with very gentle driving and 80% freeway miles at ~60mph. I am getting about 30% less miles than what you would typically expect. I


If I had to guess, MAF should be ~25 g/s at 60mph. That’s about how much air it takes to make ~25hp with reasonable VE numbers. I don’t actually know since this isn’t exactly something people report on.

Another source of possible gas leak around the fuel pump hanger. Maybe the rubber fuel hose has a tiny leak. You’ve mentioned fuel trims but do you know if short term fuel trims are working (closed loop)? The O2 voltage switching indicates that it’s in closed loop during idle so I’d imagine it does at cruising as well but can’t hurt to check (short term fuel trims). Since the car runs fine otherwise, I can’t think of any obvious problems.
Pretty sure the short term fuel trim is close loop when I was cruising will double check again. I dont see any signs of leak or smell any gas when I open the gas pump hanger access door, I dont think that's the cause. Someone told me about changing fuel filter, will a clogged fuel filter cause this? I imagine it will be misfiring, or throwing a cel due to low fuel pressure.
 

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Well, no more than 250 miles per tank, and that is with very gentle driving and 80% freeway miles at ~60mph. I am getting about 30% less miles than what you would typically expect.
I would expect 275-300 miles a tank when my car was stock and that was 90-95% highway driving. (At 70mpg which I found to give me better mpg, in my case with how I drive, and I have A/C on at all times. A/C gives 1-2mpg better at highway speeds for me) That was with stock wheels and tire size at sea level (50-300ft) and usually ~15-25C. What are you ambient conditions? Also do you know the ethanol content of your fuel? (Higher ethanol content could definitely swing it down by 1-2 mpg and “E10” ranges from 5-15% where I am)

Another thing to check could be engine compression. Though I would be surprised if compression was a problem on a well maintained car. And for fuel filter, there is no fuel filter besides the fuel pump filter sock. There is no fuel pressure sensor so it won’t throw a CEL for that but fuel pressure problems can show up in the LTFT. Since your LTFT don’t show anything abnormal, I doubt there’s a fuel pressure/injector problem.

You could try throwing in a bottle of Techron and see if that helps to clean anything. I’ve had it help with carbon buildup but it won’t work miracles. Can’t hurt to try.

Edit: I found some old notes where I kept track of my mpg. It’s worst than I remembered. It appears I was recalling the mpg when I was manual swapped rather than fully. 21.3mpg at highway speeds with a manual on stock sized tires. Auto will be worst. 18-19mpg is what I think was normal with 90%+ highway driving with A/C on.
 
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