Nunyabiz

In the past few months my 1999 ES300 (but has a year 2000 engine from factory) which used to get between 21-24mpg around town and up to 31 on the highway is now getting 16-17 mpg around town.
I have just over 100,000 miles on it.

Just 2 days ago I replaced the spark plugs with Denso Iridium IK20 plugs.
The plugs that were in it for 50,000 miles were NGK Iridiums which looked perfect no problems burning great.

Mechanic checked the codes etc and found no problems at all in fact said everything looked virtually perfect, no problems with O2 sensors etc.
Throttle body was cleaned thoroughly about 3 months ago, one O2 sensors replaced at same time rest are fine.

This last mileage check I was very light on the gas pedal also trying to get as much as I could out of it and it made no difference.
Got 16.72mpg.
Only thing I did that dropped it a bit was idling for about 5 mins a day waiting for my wife other than that just very easy driving no lead footing at all.

I find it hard to believe that I would lose 5-8mpg for no apparent reason.

also is there any such thing as a "chip" for better gas mileage?

any ideas?

Lexusfreak

Anything else changed or inspected other than the spark plugs? IE: Air filter, fuel filter & PCV valve? Tire pressures are spot on?

I'd shut the car off when waiting for the Wife for those 5 minutes too....your getting 0 MPG & wasting fuel.

GSteg

iTrader: (15)

Those 5 minutes idle doesn't seem much but over a period of a tank, that's a lot of fuel. If you're going to be parked for more than a minute or two, turn off the engine.

Nunyabiz

PVC valve was changed also forgot that.

I have not checked tires & air filter yet will do those today when I have time.
I have a K&N air filter that I am sure needs cleaning and re-oiling, tires should be pretty close.
I have them filled with nitrogen from Costco.

I agree I should shut off car when waiting however it was 100 degrees and didn't feel like dying.
I cant imagine idling for about 20 mins total made all that much difference though.
Maybe 17.5mpg instead of 16.7 but still its not at the 22+ I was always getting which was including idling.

GSteg

iTrader: (15)

Idling with AC makes an even bigger difference since you have more parasitic loss due to the engine driving the compressor. If you noticed, the engine's idle is higher with the AC on than off.

GSteg

iTrader: (15)

Another thing I'd recommend is to get your fuel injectors cleaned. It made a difference on my GS400 in both city and highway mileage. My favorite bottle to use is Lucas (full quart for a tank). I use to get get 31mpg on my V8 before traffic came along, and that dropped me down to 23mpg or so.

llcoolpass

How are you calculating fuel economy? If you're not doing it, but relying on a computer to do it in the car, then the computer may have a problem [not unheard of on the lexus]. Do it manually, and compare. If you don't know [which is incredible, but not unheard of in usa today], I will tell you.
You take the odomoter reading when you go the gas station and are filling it up,. righti t down. then dirve around a lot as you usually do. then, when the gas light illuyminates, go to the same gas station and if possible the same pump [just for super accuracy], and have them fill up again. write down the odmoter reading.

subtract the first odo from the second odo. that result is the miles travelled.

take the gallons they added that second gas fill up. that is your fuel consumption.

milestravelled divided by gallons is the number. compare to the one the lexus mpg computer determined.

remember, 3.. 5 ...6... mpg lower than some other point in time is not relevant without relevant facts. if your car got 996 mpg, and suddenly got 991, would you care?

% different from normal needs to be evaluated on the basis of trip lengths, loads, air conditioning being on, and how high the a/c is used, humiditiy, tire pressures, raining outside or not, if you're driving behind large vehicles at high speeds, etc et ceetc

i could go on an on. i did a loto f research into fuel consumption four years ago so i can say im knowledgable on the subject.

Nunyabiz

Yeah, I know how to check my gas mileage but thank you.

I don't have a computer gas mileage checker in my Lexus, I simply fill it to the brim until I can't squeeze another tablespoon in the tank, run it, then fill it back up the exact same way and check miles driven vs gallons used.
and yes dropping from 22-24 down to 16-17 I care, that is like a 30% drop in fuel economy.

I have Lucas oil treatment in the tank right now.

I removed my K&N yesterday and it was dirty , cleaned it with the K&N recharge kit and re-oiled it.

Had the tires checked at Costco, 3 were at 28lb and one was at 29lb so pressure was also a bit low.

I have ran nothing but 0W-20 100% synthetic Amsoil in the car since we bought it 4 years ago.
I am thinking it is highly unlikely that I have much carbon build up since when I had the cars codes checked it showed that everything was running virtually perfectly plus the old spark plugs I just replaced with 50,000 miles on them showed they were burning perfectly, no carbon, not even any dark spots on any plug, they were almost white.
No pinging, no or very very little smoke, good acceleration etc.

I have taken it out a couple of times and just slammed it pedal to the metal a few times just in case because we do use this car almost exclusively around town.

Maybe the proper air pressure in tires and cleaning of the air filter have managed to bring it back up some.
Next time I check mileage will report back.

Thanks for the responses.

GSteg

iTrader: (15)

Lucas not only makes oil treatment but fuel injector cleaner too.

My car has always seen Mobile 1 by the previous owner and has always used 91 octane fuel. It was maintained by the dealer throughout and even so, the injectors were clogged/dirty. On the toyota techstream scanner, it shows that everything was working like it should. But it wasn't until I cleaned the injectors where it made a difference. The engine's ECU cannot detect a slightly dirty injector unless it's so clogged up that your engine is running lean.

Tire pressure being 4-5psi lower than recommended is considered very low, at least to me considering what Lexus recommends is a blend for fuel economy and ride comfort.

Good luck, and report back if you can!

tromly

Lucas products are good, BG are great, good recommendation to clean those injectors

Nunyabiz

Meant to say Lucas GAS treatment not oil treatment.
No need for oil treatment with good 100% synthetic.

I will run a couple tankfuls of Lucas through it, maybe even a tank with Seafoam.
I am hoping that proper tire pressure, clean air filter and a couple tanks of injector cleaner will do the trick, I need to get at least 20+mpg.

The next couple of fill ups wont tell me much because I have been lead footing it and hitting the highway a bit to clean out what little carbon might be there.
Probably know in a week or so if any of this is making any difference.
I suspect the tire pressure & air filter will make a difference.

Nunyabiz

OK, I have ran a couple tanks of Lucas GAS Treatment through it.
The tires are properly inflated, the air filter is clean, the throttle body was cleaned squeaky clean just a few months ago. the spark plugs have been replaced (Denso Iridium's, old ones looked great) the computer says everything is running perfectly.
The car sounds and feels like its running fine, smooth as silk.

I am still getting just over 16MPG around town.

I guess I can try going to another gas station, currently get 99% of my gas at Costco since its 12-15cents cheaper than any place in town.

Other than that I guess my 99' ES300 now gets the same MPG as my damn 95' Jeep Wrangler and that is a sad statement.

QuachmanES

iTrader: (4)

I'm getting around the same gas mileage as you are, with about 70% city driving, and the rest on the highway. I believe that the window sticker on the car rated the mpg at 17 city, 24 highway when new.

GEORGE_JET

There are several possible problems that you need to check. Does your mechanic have access to a scanner? If so the first and easiest thing is the coolant temp sensor. Do not assume that since the temp gauge in the instrument pannel is working, that the ECM coolant temp sensor is working. These are two different sensors. The ECM will generate a code for the coolant temp sensor only when the sensor gets so far out of calb, that the temp cannot possibly be real. While this is easy to check, it is not the most likely.

Leaking injectors. I realize that you have used cleaners, but that does not preclude the possibility that a injector is still leaking. You need a gauge to check this, so your mechanic will need to hook up a gauge and verify that the leakdown is good.

The most likely cause is the is the O2 sensors. toyota calls Bank 1 sensor 1, and bank 2 sensor 1, air fuel sensors. These sensors of the ones that control the mixture. You stated that your mechanic check the sensors and found no problems. Interesting choice of words. Did he check crosscouts? Does he know what the peak to peak voltage swing is? A O2 sensor has a life span of a little over 50K miles. After that they start to get lazy, the cross counts will drop, and the peak to peak voltage will decrease. Most people will notice a drop in mileage long before the sensor will get bad enough to generate a code, (this of course does not include codes set for O2 heaters).

So my recomendations at this point, (since you have allready tried the things that should be checked first). Would be to replace the two air fuel sensors, unless you have the tools to check the first two items that I mentioned.

Trexus

Don't forget to remove the cables on the battery terminals to reset the CPU as you've done much to your car. Good luck and hope you get back to your original miles per gallons...