Gryss

It sure isn't going to help.

^.^

MBTC

I'm not a mechanical engineer but I have read that depending on the car, octane that is either below spec or octane that's higher than spec can both contribute to carbon buildup. I've also read that there is absolutely nothing saved by running cheaper gas due to loss of MPG and/or repair expenses that will offset the savings at the pump, and likewise nothing gained by running higher octane gas.

The main reason I posted the question is I'm legitimately curious of how many of those having carbon build up issues are guilty of running octane lower than the car was designed to run (or perhaps having bought a previously owned 2IS that prior owner ran cheap 87 octane in).

Gryss

I run 91 in all my cars not always for the octane rating but for the detergents in the 91 gas. techron, v-power etc.

chikoo

Can you explain how?

Low octane burns faster/quicker than higher octane i.e. is more combustible. Due to higher compression, I can understand it combusting before it needs to, but why would that lead to production of carbon soot that would cake up the valve necks?

lobuxracer

iTrader: (2)

Total BS. Burn speed is independent of octane. Who told you this lie?

Kurtz

iTrader: (1)

All gasoline, of all brands, and all octanes, are required to meet the same federal standards for detergents and other additives. (though of course for something like the IS250 it wouldn't make much difference either way, since fuel never touches the part that's the actual problem- and if it did fuel of any kind would solve it)

Generally speaking you get no benefit whatsoever running a higher octane fuel than the car is designed to use (and some negatives running lower octane than it's designed to use).

the ISx50 is designed to use 91.

chikoo

You are right. I did not intend to refer to burn speed, but the affinity to combust/detonate.
Premium (higher octane) fuels will take more heat & pressure before it will combust (or detonate) than regular gas. But the product of the combustion should still be the same. Hence my question as to how does using regular gas lead to more carbon deposits.

zen

iTrader: (6)

I had to make sure therefore I took in the car for service and I asked the service consultant. He replied the extended warranty would cover and he also states it's a permanent fix after the piston and piston ring replacement. From this point, I'm not sure if I should trust the consultant or the people here about if it's a permanet fix or not. Anyways, I'm covered 7 yrs from this yr and until 125,000 miles, I guess time would tell.

autox250

Chikoo unfortunately youre wrong. Kurtz's post is 100% on. Gas grade may have other effects and consequences but NOTHING to do with the carbon issue. The carbon is being formed from excessive oil vapors from the PCV. With no port injection the oil mist coats the valve turns to carbon and keeps absorbing more oil. Its not caused by the fuel, gas was the preventor all these years

chikoo

I am not wrong autox250.

I am the one asking the question on the relationship of octane and carbon buildup. I don't know the right answer and therefore trying to learn more.

lobuxracer

iTrader: (2)

Running a lower octane will tend to make more carbon deposits because the ignition timing will be retarded by the ECM to compensate for the lower octane. Retarding the timing is an engine saving measure and results in less than ideal combustion, so the products of combustion will not be the same as in a properly running engine. The big HOWEVER is, it won't impact the carbon problems the 4GR is experiencing if what Lexus claims (blowby) is the truth. I'm still very skeptical the primary cause is PCV related.

autox250

Chikoo carbon buildup "in the cOmbustion chamber" will happen with gas grades im not arguing that at all. I am saying that our problem is happening outside the combustion chamber for diff reasons. The cause is blowby, the upper manifold is soaked in oil past what youd expect.. Maybe sometime soon the 4gr will be redesigned

flipmode

iTrader: (1)

Well, my 07 is at the dealer getting the pistons etc. replaced. fingers crossed...

KillaIS250

iTrader: (1)

Let us know how it goes!

is250gurl

I took my 2007 IS 250 in for an oil change on Monday and mentioned that the car is studdering again occasionally at stop lights and asked if Lexus has come up with a more permanent solution as this is ridiculous for this car to be doing this. I've already had the top engine cleaning 20,000 miles ago. They still have the car and are checking it out. We shall see what they want to do next. I've been to the dealer at least 5 times for this problem now. I do have the extended warranty, but beyond that not sure I will keep the car.