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Discussion Starter · #21 ·
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.
I am in northern california sea level, normal weather ranges about 60-100F nothing extreme. It looks like gas here has 10% ethanol content so that could be a contributing factor. I did the entire seafoam treatment, about 10oz to 3 gallons of gas, and another 10oz through vacuum hose of brake booster for intake cleaning, no luck.

I used to have a 5 speed manual and commute about 250 miles freeway each week, and the mileage was way better. This current car is an sportcross auto, so perhaps the extra weight doesnt help. But still, I really think there's a leak somewhere and not related to my engine running conditions, as evident from the mpg numbers from obd2. I will check for cats and and lower the fuel tank for the overfill check valve and report back.
 

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And for fuel filter, there is no fuel filter besides the fuel pump filter sock.
The IS300 does have a filter, but it’s a “lifetime” - type. Whatever that means lol. All the “lifetime” crap triggers me. That’s a tangent I won’t go down right now lol. I didn’t know they had a fuel filter for the longest time. It’s inside the fuel sender. Hell, it’s like half the fuel sender housing. Toyota PN 23300-0A020. I guess it’s cheap enough, and if you’re just ruling everything out to get better mileage, I would replace it.
 

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Bro. If your O2s dip to 0 volts, it’s hosing fuel in there to try and get the signal up. Check your plugs. If they are black replace your O2s. Disregard this professional auto tech advise -
Does the narrowband in the Lexus go to 0v when running richer? I'mpretty sure in most cars the narrowband go to 0v when lean. The band when the o2 switches is also very narrow. 1.018 lambda is only 15.0 AFR, 0.3 points from 14.7. The important thing is that the narrow band switches from either side of 0.5V. So the 0-0.8v the OP saw is a good sign that the car is injecting the right amount of fuel and working in closed loop during cruising. Reading the actual voltage of the narrowband is not very helpful unless you calibrated it to a wideband, even then it’s too sensitive to be very useful.
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I live in NorCal also. Same fuel you are using. Still return 330ish most consistently. If I drive the car hard I might get 290ish or 280 but that’s pretty rare. I would try driving faster than 60mph (I drive around 70 with the cruise control) How many miles are on your car? Cats being slight clogged up seems possible but bigger wheels are probably contributing to the issue as well.
 

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Clean your intake filter, clean the MAF, and change the spark plugs if they haven't been changed and change all fluids (tranny, power steering, all the fun stuff). Also, your wheels definitely doesn't help with the gas mileage. With the increase in wheel size, when you are going 60MPH, it's probably actually traveling at 65mph. Also, the last thing is weight, if you have a bunch of extra weight in the car, that also lowers your MPG. Lastly, don't trust the MPG needle, it's there more for looks than anything...it's highly inaccurate.
 

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Discussion Starter · #28 ·
some updates:
  • changed the fuel tank relief valve, what a pain in the ass but at least no more fumes leaking
  • I am doing more mpg calculations, currently at about 70miles with the gauge 1/4 down, which is actually quite bad for the way I drive the car. I make an effort to drive with extremely light foot that it's even not realistic or normal, always aiming for the mpg needle to be above 20, so I am expecting more.
  • Going to change in tank fuel filter assembly and see if that helps a bit.
  • I notice there is excessive amount of water vapor from exhaust in the morning. It's not coolant or oil, it's condensation. I removed my y-pipe once and there is water dripping out of it. I did not notice anything loose peeking into my cats, but its not easy to spot inside. Anyway to verify if it's cat related?
  • again no CEL or fault codes.
 

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Idk how y’all got that terrible fuel mileage. On the street, yeah it’s terrible but on the highway it’s decent. Years back I drove from Chicago to Detroit which is 300 miles and got there with about a 1/4 tank left. I remember wanting to push it to the 400 miles but I was in an unknown city, didn’t want to chance it so I filled up.
 

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Discussion Starter · #32 ·
Change your O2 sensors yet? Willing to bet dinner on it I’m in the Bay Area
They are all reading normal and no cel though... If my fuel filter change didn't do any good that will be next.. :cautious:

Pull the afr sensors out and see if you have excessive back pressure. Just look at your fuel trims.
my fuel trims are normal and in closed loop.. +/- 2-3% correction consistently..
 

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They are all reading normal and no cel though... If my fuel filter change didn't do any good that will be next.. :cautious:


my fuel trims are normal and in closed loop.. +/- 2-3% correction consistently..
Do you know if your o2 sensors are reading correctly? They could be off and isn’t actually switching at 14.7 AFR (well lambda=1 anyways) . But I don’t know how likely it is for both to be off. Here’s a video on how to test them:

Have you checked your spark plugs yet? Maybe a cylinder isn’t firing under load. Or an injector is spraying too much under load. I’ve had that happen before albeit different vehicle. If a cylinder is running rich, the plugs should look fairly black.

Did you ever log your MAF and injector duty cycle (does the stock car broadcast IDC?)? Those might also help with diagnostics.
 

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You are counting on a worn out, well past service limit (80k) sensor to tell you if it’s in good health or not guy. Maintence first. If you have a vacuum leak that will do it as well, and with how worn out I’ve seen the valves, guides,and valve seats on these old motors that could easily be part of the problem as well. Unmetered air can easily be pulled past any on those problem areas mentioned , but your main source of skewed fuel trim is almost always tired 02s when we start playing with 20 year old cars
 
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