Changed Air Filter, Spark Plugs, Did Isc Reset & Air Pressure... W

[cduluk]

Hi all,

Lately my Rx400h had been driving like a pig... 2008, approaching 90,000 miles, and in mixed driving i was getting--at best--22mpg.

Since then, i changed the air filter--which was so filthy, it had a full grown MOTH in it,-- I had the spark plugs changed, filled the tires to 34psi, and did the ISC reset procedure.

In the past 25 miles of mixed driving since then, well, take a look:

I know i broke the cardinal rule of the scientific method, but at least we know changing all of these variables can lead to better gas mileage. Going from the low 20s to 30s in mpgs is incredible. I didn't know the Rx400h could even get this good of mileage!

What are your thoughts?

[maseace]

Let's see your mpg after a whole tank of gas!

[lenore]

How much did the plug change cost? I am dreading that task....I have 130k on mine....As for mileage, hey the dirty air filter couldn't help....Have you had the transmission fluid changed?

[RX400h]

A filthy air filter will destroy mileage. You should check the filter after 10,000 miles of use. Your plugs were probably okay.

[maseace]

The latest articles I've read say that for modern cars with computer-controlled air/fuel mixtures, a dirty air filter doesn't affect mpg, because the computer constantly adjusts the mix to not waste fuel. Source: http://finance.yahoo.com/news/debunking-fuel-economy-myths.html

[cduluk]

Let's see your mpg after a whole tank of gas!

50 miles later and i'm down to 29mpg... still not bad!

How much did the plug change cost? I am dreading that task....I have 130k on mine....As for mileage, hey the dirty air filter couldn't help....Have you had the transmission fluid changed?

Well, i bought the plugs ($7 each) and had the "intent" on doing the change myself, however the three in the back are NOT easily accessible. The manifold NEEDS to come out; i don't care what others say. There's no "reaching behind". I changed the three in the front myself, and had to pay a nearby mechanic two hours labor to put in the other three.

And i haven't had the transmission fluid changed yet, but it's very high on my list! Since there's no filler nor accessible drain plug, i'll probably take it to Toyota to have them do it.

A filthy air filter will destroy mileage. You should check the filter after 10,000 miles of use. Your plugs were probably okay.

RX400h, this filter was FILTHY! A moth, bees, seeds, leaves, and oh yeah; dirt! From now on i'll change out the air filter once a year. The plugs (at least the three in the front) looked pretty bad. A lot of carbon, and the gap was off.

The latest articles I've read say that for modern cars with computer-controlled air/fuel mixtures, a dirty air filter doesn't affect mpg, because the computer constantly adjusts the mix to not waste fuel. Source: http://finance.yahoo.com/news/debunking-fuel-economy-myths.html

It could be that the dirty air filter wasn't affecting my mileage (since it's not the only variable i changed) but my mileage now is noticeably better. The engine also "sounds" better at idle. I can tell by the way it's driving now that something before was "off". It could have also been the ISC reset, i really don't know. I'm just glad i had these things done. :)

[RX400h]

The latest articles I've read say that for modern cars with computer-controlled air/fuel mixtures, a dirty air filter doesn't affect mpg, because the computer constantly adjusts the mix to not waste fuel. Source: http://finance.yahoo.com/news/debunking-fuel-economy-myths.html

That article is deceptive. As your intake air flow decreases due to an air filter that is getting more and more clogged, engine power decreases. To accelerate as briskly as you are accustomed to and assuming you did not somehow decrease the weight of your vehicle, a steeper throttle tip-in is required, resulting in GREATER fuel consumption.

A good example is an air compressor. If you slowly restrict intake air flow, output pressure is reduced. Now, if you could increase the motor speed to compensate for the diminished output, power consumption would increase. Anyone who tells you it would not needs to retake physics.

Cduluk, knowing your driving style, I'd say it's a sure thing that replacing your air filter restored power, allowing you to ease up on the gas pedal, thus reducing your gas consumption.

[lenore]

the front transmission does have easy access to drain and fill plugs.

http://us.lexusownersclub.com/forums/topic/68738-cvt-rear-differential-fluid/

[riwyle]

The latest articles I've read say that for modern cars with computer-controlled air/fuel mixtures, a dirty air filter doesn't affect mpg, because the computer constantly adjusts the mix to not waste fuel. Source: http://finance.yahoo.com/news/debunking-fuel-economy-myths.html

That article is deceptive. As your intake air flow decreases due to an air filter that is getting more and more clogged, engine power decreases. To accelerate as briskly as you are accustomed to and assuming you did not somehow decrease the weight of your vehicle, a steeper throttle tip-in is required, resulting in GREATER fuel consumption.

A good example is an air compressor. If you slowly restrict intake air flow, output pressure is reduced. Now, if you could increase the motor speed to compensate for the diminished output, power consumption would increase. Anyone who tells you it would not needs to retake physics.

Cduluk, knowing your driving style, I'd say it's a sure thing that replacing your air filter restored power, allowing you to ease up on the gas pedal, thus reducing your gas consumption.

A dirty air filter only affects performance at wide open throttle. Before that, the engine doesn't care where the air flow restriction is, be it the throttle valve or the air filter- in this case a summation of each. The mass air flow value determines fuel flow, not the position of the throttle vavle.

[RX400h]

The latest articles I've read say that for modern cars with computer-controlled air/fuel mixtures, a dirty air filter doesn't affect mpg, because the computer constantly adjusts the mix to not waste fuel. Source: http://finance.yahoo.com/news/debunking-fuel-economy-myths.html

That article is deceptive. As your intake air flow decreases due to an air filter that is getting more and more clogged, engine power decreases. To accelerate as briskly as you are accustomed to and assuming you did not somehow decrease the weight of your vehicle, a steeper throttle tip-in is required, resulting in GREATER fuel consumption.

A good example is an air compressor. If you slowly restrict intake air flow, output pressure is reduced. Now, if you could increase the motor speed to compensate for the diminished output, power consumption would increase. Anyone who tells you it would not needs to retake physics.

Cduluk, knowing your driving style, I'd say it's a sure thing that replacing your air filter restored power, allowing you to ease up on the gas pedal, thus reducing your gas consumption.

A dirty air filter only affects performance at wide open throttle. Before that, the engine doesn't care where the air flow restriction is, be it the throttle valve or the air filter- in this case a summation of each. The mass air flow value determines fuel flow, not the position of the throttle vavle.

Try this experiment: Restrict your air intake flow by 75% and then regularly drive aggressively, as does Cduluk. Continue this type of driving for one tankful and don't forget those occasional but brief full-throttle runs on hills and on the highway.

Now measure your fuel consumption and report back to us.

I think it's safe to say that you will experience what Cduluk had when his air filter was severely restricted.

[riwyle]

The latest articles I've read say that for modern cars with computer-controlled air/fuel mixtures, a dirty air filter doesn't affect mpg, because the computer constantly adjusts the mix to not waste fuel. Source: http://finance.yahoo.com/news/debunking-fuel-economy-myths.html

That article is deceptive. As your intake air flow decreases due to an air filter that is getting more and more clogged, engine power decreases. To accelerate as briskly as you are accustomed to and assuming you did not somehow decrease the weight of your vehicle, a steeper throttle tip-in is required, resulting in GREATER fuel consumption.

A good example is an air compressor. If you slowly restrict intake air flow, output pressure is reduced. Now, if you could increase the motor speed to compensate for the diminished output, power consumption would increase. Anyone who tells you it would not needs to retake physics.

Cduluk, knowing your driving style, I'd say it's a sure thing that replacing your air filter restored power, allowing you to ease up on the gas pedal, thus reducing your gas consumption.

A dirty air filter only affects performance at wide open throttle. Before that, the engine doesn't care where the air flow restriction is, be it the throttle valve or the air filter- in this case a summation of each. The mass air flow value determines fuel flow, not the position of the throttle vavle.

Try this experiment: Restrict your air intake flow by 75% and then regularly drive aggressively, as does Cduluk. Continue this type of driving for one tankful and don't forget those occasional but brief full-throttle runs on hills and on the highway.

Now measure your fuel consumption and report back to us.

I think it's safe to say that you will experience what Cduluk had when his air filter was severely restricted.

You are correct- if it is a vehicle with a carburated engine. Anything that chokes the intake air, either a manual choke being left partly closed or a very dirty air filter, will increase the fuel/air ratio causing it to run fuel rich. This is because pressure is lowered in the carburator venturi compared to the same unchoked air flow causing the jets to supply an increased fuel flow increasing F/A ratio lowering gas mileage.

But modern cars, in the US since mid eighties, use computer controlled fuel injectors and no longer have the fuel flow or F/A ratio affected by any choking of inlet air due to any reason, be it dirty air filter or closing of the throttle valve. Required fuel flow is determined by mass air flow measured by the mass air flow sensor, trimmed by feedback from the O2 sensors. Therefore extremely blocked air filter can greatly affect power available at WOT but will not affect efficiency (mileage) at any condition. In fact it would force Cduluk to drive more conservatively increasing his fuel mileage.

I realize this may seem counterintuitive, but if you do a complete system analysis using physics (conservation of energy), chemistry(combustion), thermodynamics and feedback control theory you cannot but come to this same conclusion.

Although I don't always believe everything the goverment says, their webside fueleconomy.gov states this same fact.

Also your analogy to a compressor is not valid since the compressor uses energy transferred from a gas engine or electric motor to increase air pressure and if you think it through you will see it is a completely different system than air passing through an air filter, throttle body and throttle valve.

[RX400h]

How can you possibly explain the massive change in fuel economy? If you believe the spark plug change could do this, you would have to assume that the plug gap were substantially larger, but even if this were true, wouldn't the input voltage increase to compensate? It is obvious to me that a computer controlled engine cannot fully compensate for extreme changes, including those of air flow and spark plug electrode degradation.

That being said, I must also add that my experience with late model spark plugs and conversations with dealership service managers has prompted me to believe that many modern spark plugs reach 100,000 miles or more before fuel efficiency is significantly affected.

Back a few years ago, there was a heated discussion about aftermarket (oiled) filters. This was posted by an automotive engineer. See his last paragraph:

I am an engineer, specializing in air and fuel filters, at an American auto manufacturer and as a result must remain officially “off the record”. However, here is the last word on filtration elements (in laymon’s terms).

While we officially say we do not test aftermarket filters, the reality is that we do. We do so to understand the consequences of poor filtration. The filter element (the paper) used in our vehicles is the identical element used in the Ambrams M-1 tank and filters out 99.7% of all airborne particulates.

The purpose of the filter is NOT fuel economy or power, it is engine protection. This is the single most important reason for filtration. The OEM paper easily allows sufficient airflow to the engine required perform at specs. Airflow is a non-issue.

The aftermarket filter makers emphasize power and fuel economy as benefits, and filtration rates of up to 97%. However, none mention that the OEM filer they are replacing has a filtration rate of 99.7%. Basically, you are buying engine damage with all aftermarket filters. The only question is… is the damage sufficient to impact the vehicle to the degree that it is noticable.

As another writer mentioned, the computers monitor airflow and controls combustion, thus, the benefits of improved airflow (fuel economy) are negligible unless you also alter the computer control module. i.e. “chip” the motor. You can get improved fuel economy, combustionability, and power, as a trade off to engine wear or potential engine damage from higher combustion temperatures (and the resultant repair is NOT warrantable).

All test data supports the manufacturer’s interval for changing filters, excepting that local conditions warrant a more aggressive interval (see off road operating conditions, frequent towing, etc. etc). The reason for longer intervals with some aftermarket filters is that the dirt stopped by the OEM is NOT stopped by the aftermarket. Thus, it takes longer to stop up a filter with big holes! Doy!

While I have no date to support increased fuel economy with an aftermarket filter, I do have data that indicates a dirty filter can reduce fuel economy by about 10%. So its a matter of perspective. You can say “improves fuel economy by 10% when compared to a dirty filter” but not a new one.

Hope this is helpful.

[cduluk]

Just to update, I'm at ~200 miles and i've averaged 28mpg, about 6mpg higher than i did before. I've also noticed a significant increase in "power" when i push it a bit. It feels the same at first, but after a few seconds i can "feel" the acceleration increase compared to how it drove before.

There was something very wrong with the way it was driving before. I'm not sure if it was the air filter, but i'm liking how it's driving now! I wish i took a picture of the old air filter- it was disgusting.

[removeum]

Did you have your fuel system flushed? My Rx400 took a hit with gas mileage. I had the fuel system flushed and am back at 28 to 30 mpg.

[cduluk]

Did you have your fuel system flushed? My Rx400 took a hit with gas mileage. I had the fuel system flushed and am back at 28 to 30 mpg.

I've never heard of that being done, actually- but you've got me interested! How do i have this done? I should note i've always (since i've owned it in '09) used 93 from the same Mobile station near my home. It's the most heavily trafficked station in the area, so i figured the fuel is always fresh!

[RX400h]

Consumer Reports agrees with the majority of this article, concerning whether the dealership fuel injector cleaning service is necessary:

http://www.kbb.com/car-advice/articles/do-you-need-an-injector-flush/

I pour a can of concentrated Techron in the tank, every 6 months or so and my gas mileage has stayed rock-steady throughout the years. Of course, I replace my air filters every 12,000 miles, also.

[Jonathantphan@yahoo.com]

How much did the plug change cost? I am dreading that task....I have 130k on mine....As for mileage, hey the dirty air filter couldn't help....Have you had the transmission fluid changed?

Local shop in ca wants $300 for parts and labor