jared5

Hey guys im new here, i just bought a 1998 lexus LS400 4.0L, My only concern is that the previous owner ran regular unleaded fuel for as many as 2-4 years.. It has been idling badly and i think it is due to this.. im going to take it to my nearest dealership and see what a service tech has to say but what would be the best way to switch back to premium fuel?
Thanks
J

Greg5OH

shoudlnt really effect it. The computer will dial back the timing so nothing will get hurt.
Just performance and mileage will suffer.
Best way to switch back to premium-put premium in it. That simple.
dont let em try to sell you on a flush or any junk like that.
I would simply replace the fuel filter (just for maintenance, nothing to do with reg/prem), run the tank as low as possible, fill with premium and dont worry.
Running rough could be a bunch of stuff, dirty maf, clogged air filter, clogged fuel filter, dirty oil, etc.

Sc0pe

iTrader: (1)

...what? Isn't all fuel technically unleaded? In my opinion, just head out to Chevron and fill it up with the best gas they have. My reason for recommending Chevron is that they mix in a chemical called "Techron" which helps clean out the engine internals, and keep them that way.

fixmiester

There was a time when I used premium fuel in my '99ls. Then I started experimenting with regular. I could tell no difference in driveability or power (but I never did time trials, or anything like that). Now it has 140K miles on it. Still has original NGK iridium plugs (incredibly, they look like new!). It's been my wife's car, but it hauls *** when I punch it. Great acceleration to red line; no pings, etc. Don't worry about the fuel.

dengman

I run midgrade in my 06 mdx and 99 LS400 from time to time. Performance does suffer but fuel in the states have detergents and additives that Im not too worried it would do major damage in the long run. The computers are advanced and they can compensate in timing. LS400 seems to be tuned rich anyways.

LScowboyLS

I get 11-13% better fuel economy using premium, it costs 7% more at Sam's club for 92 octane over standard 87 - and I don't have to worry about any damage from ping (knock, detonation) - it's a no-brainer!!

not to mention the performance hit when the ECU winds back the timing in response to the knock sensors when using regular unleaded

if regular was fine, then Toyota would not tell you to use premium, thereby increasing the apparent cost of ownership (but not really increased, see above)

RA40

iTrader: (6)

Welcome on your first post and the LS. No need to for special testing, just begin filling it with premium.

I'm with Cowboy, my tests using other graded fuels gave similar results over extended drives. The engineers at Toyota calculated what fuel will allow the car to run optimally not marginally.

For more gas discussion, there is this thread:
https://www.clublexus.com/forums/ls4...thread-16.html

PureDrifter

iTrader: (10)

idle isn't gas related, clean the throttle body.

Shmee

iTrader: (3)

The "cleaning" additives in gas do not really clean and switching to a higher grade fuel will not suddenly nor miraculously over time clean your engine. About all they do is resist varnishing better than lower quality fuels. I would less concerned about low octane fuels and more concerned with low quality fuels from sketchy stations.

If you were that concerned about it, change the fuel filter, clean your throttle body and intake and get a proper fuel system service with a MotorVac, TerraClean or equivalent injector flush.

01LEXPL

iTrader: (10)

As mentioned, your ECU retards the timing. There's been a few guys on the IS board running regular in the leased IS250's without issues. Less performance, yes, but it still runs fine.

This.

LScowboyLS

This isn't very smart, the drop in mileage ~11% is more than the higher fuel cost ~7% - so why would anyone lose money, just to have worse performance and potential engine damage if the knock sensors or ECU caps should fail?

Run premium like Toyota recommends, their engineers are infinitely more intelligent than "a few guys on the IS board"

PureDrifter

iTrader: (10)

because everyone gets the same mileage difference as you?

There have been many MANY scientific studies showing that the mileage argument for premium gas is basically moot.

There are definite benefits to premium gas, but mileage is not one of them.

billydpowe

iTrader: (1)

ok,now the old man speaks....... forgot what I was going to say..hehe.. well....my hot rod days are over.. smile
way over... I have two 99LS's and have never run anything but regular the last 3 years... they both run and idle excellent.... mine has 235875k on it and my wifes has 173645....
you have a different problem, not fuel..

RA40

iTrader: (6)

Guy's let's get back to the specific issue of solving his idle problems. Any other gas related talk will be merged into the Mother gas thread.

RXGS

iTrader: (2)

I'd say a simple throttle body cleaning and a MAF sensor cleaning would make a world of difference. I'm not saying its not something else, but these two are extremely cheap to try first. After doing both in our lx470 it idled so much better.