Bought 1999 RX - Previous Owner Used 87 - I use 91 - MPG Plummet!
 

Hey All,

I bought a 1999 RX300 with 110,000 miles (for $6K - yes!). Spark plugs, timing belt and water pump were done at 89K. Has new tires (at 32 psi), brakes and battery. Put in a new power steering pump and steering rack. When I drove it home the computer read 18.9 mpg.

The previous owner ran 87 octane fuel, and probably filled it up anywhere. I fill with 91 at Shell and after two full tanks my MPG dropped to a nader of 12.8 mpg (oh the humanity!).

I (sorta) cleaned the IACV, stripped one screw so I couldn't remove the plate, just sprayed some stuff in, seemed to help with idling.

I just dumped in a can of BG44K and filled it up with 87, the MPGs are already climbing.

Did my MPGs drop because the computer was trying to burn 87 when it actually had 91 in the tank?

Can I fill up with 91 and remove the battery cable to reset the ECU to fix this on my next fill up?

Losing 6 MPGs is very depressing, please help!

-EZGoRX

You have no clue when that guy reset the mpg. (He could havewhile coasting down the hill to his house making it look much higher) Your octane is not going to make that kind of dif. I'm sure I'll get lots of disagreement from some on this but higher octane does not equal more power and/or effencency. Its a measure of knock resistance. Your engine will make the most power and run the most effeciently with the least octane it can run without detonation. Modern cars often have a sensor that can detect the higher octane gas and will modify the ignition timing to give your some more power. I'm not aware if these have those sensors but I imagine they dont. I'm sure someone can chime in and let you know either way.

Higher octane does make a difference in the RX. Mostly because it is recommended for Premium, and it will adjust the timing/fuel ratio etc. Thus giving the engine more power and better fuel economy. However, if you use Premium in a vehicle specked for regular fuel, you may/may not get any benefits from it. I personally recommend using the Premium fuel. It will reduce your head and exhaust temp`s, And the engine will live a longer happier life. Especially the Catalytic converters and air fuel ratio sensors. Just clear the ecm, (do you really need to do this?..no) but it will start all over again. Speeds up the process a little going from the 87 to 91/93.

I hope you were not an engineering student a Virgina Tech since there is no information that would support conclusions such as lower head and exhaust temps. Please explain that one. If you were an engineering student I will keep it quiet since I live in Roanoke and Tech would not like to hear an alumnus making such an unsupported conclusion. Go Hokies!

It`s quite simply actually. when using a lower octane fuel than the motor prefer`s and with the over reactive knock sensor`s you have on the first generation RX. It will retard the ignition timing, richen up the fuel mixture all in an attempt to keep the engine from pinging. And, what happens when you run a motor on retard timing? the head & exhaust temp`s rise because of higher combustion temp`s. Oh yeah, GO HOAKIES!!!! :D Also, with the higher combustion temp`s on lower octane (for the reason I explained already) along with the already hot spotted heads and smaller oil return holes on the 1MZ-FE. this is just NOT a good combination. Ask Lexmex about the first generation heads if you dont believe me. Did I explain this grey area re; head/exhaust temps okay? :) I`ve been around motor`s pretty much my whole life (my grandfather was a mechanic) so I have experimented plenty with this sort of thing. I would personally never run a motor on retard timing for any reason. if it pinged, get to the source ( the problem/reason for it) Unfortunately, with the motor`s we have today, it`s hard to keep them from trying to remedy the situation on their own.

Not to highjack the thread the OP wants to know what difference in the two octanes. When I bought my RX300 in 01 I asked the Lexus dealer what fuel they ran in their own cars and all of them said in the 300s like RX and ES it was regular. A different octane will not make the kind of difference in the miles per gallon the OP is describing. There is no way changing octane rating makes 6 miles per gallon. Especially since you are going up in octane not down. Also, I am looking at the octane spec for my 2000 and it says and I quote, Select octane rating 87 or higher. Lexus does not recommend premium in this vehicle.

As an aside: lexus114 If you are really from VA Tech you would never spell Hokies with an a in the middle.

Thats correct, it will not make that big of a difference in mpg`s (maybe .5 to 1.5 mpg`s difference) And your also correct re; I`m not a Virginia tech Alumni. I never said I was, However they are my college football team that I support. And I also like Bama too. :D

Did you stay at a holiday inn express last night?

They re worded it in my 2001 RX manual (my previous RX model year) It stated; "select 87 octane ", but then went on to say: "the use of 91 octane or higher is recommended"

Yeah I know right?

My 2000 RX manual says that too.

Also, the RX300 has a high compression motor so it WILL benefit from higher octane.

So, lexus114 is correct!

May all be true, but when I got the 2000 both the salesman and the service manager at Flow Lexus in NC said to use regular and I did on both the RX and ES and I average about 19 MPG on the RX and 22 on the ES (before I sold it in 09) I'll take that any day. There is no way you will convince me that using a higher octane in these engines will make any difference at all and Lexus seems to agree. As the kid states, if the engine is not knocking using higher octane is no benefit.

Thats right Hyper.

The problem could be with the data.

If you are noticing a change in the average, that means there is very little history (too few a data points) ... this means what you saw could have been down-hill run etc (what ever there is that favors the higher display.

Continuing the same reasoning, any idle (stops) etc you do will drop your average a whole lot (again as there is very little run data) and the 0mpg kills the average.

It would be lot better if you look at the instantaneous display. For average I would say, let it accumulate to a steady number (2-3 weeks of driving without doing a reset).

Salim

Sigh....it`s not pinging because the ecm is making the adjustments to keep that from happening. (if your using reg. fuel) But the side affects to this is why I use Premium fuel. obviously there is no convincing you re; this.