High Octane Fuel

Hi Steven,
Thank-you for quoting my comments from the Yahoo group. ;-)).

There are three revisions of the V6 Petrol 90/95 series Prado.

The first revision was built in 96/97, the second revision was built in 98/99 and the third revision was built 2000 to 2002.

There are some cosmetic differences such as inside door latches on the tail gate, 2nd row split seats, clip out third row seats, "fog" lights in the bumper bar etc.

In the engine department, the first two revisions did not have to meet the same emission rules as normal sedans. 4WD were exempt until 2000.

As such you will not find in the first two revisions,
a) A Catalytic Converter,
b) An Oxygen Sensor in the Exhaust.

Tuning, is carried out by the adjustment of the CO adjuster. (Term from the Workshop manual).

In summary terms, (ignoring the other sensors such as engine speed, air temp, air density, vacuum,throttle position etc) vehicles that are designed to take advantage of varying levels of octane fuel, the ECU needs to adjust the Air / Fuel mixture. The ECU does this by dynamically sensing the amount of oxygen in the exhaust gases. It adjusts the fuel / air ratios to obtain the maximum oxygen burn without over fuelling.

The first two revisions of Prado do not have the O2 sensor so logic suggests that the ECU must have a given fuel / air ratio that is mapped for the particular fuel octane rating.

It is my assessment (based on experience) that higher octane fuels in the first two revisions of 90 series Prado give very little advantage as a result. I did notice slightly smoother running, that is all.

Ethanol, adds another dimension though. Ethanol is often used as an octane booster. Ethanol in petrol alters the fuel density so what happens in a vehicle without Exhaust Oxygen feedback to the ECU, the ECU injects the amount of fuel it expects based on the normal petrol mapping, but because the ethanol mix is denser the engine effectively runs too rich, and uses more fuel. This certainly was my experience.

So unless you have a 2000 or later revision of Prado, I certainly wouldn't use ethanol based petrol in the version 1 & 2 Prados.

Conventional high octane fuel is less of a problem, because the density is about the same as normal unleaded. I doubt however you will get much advantage.

Cheers,
Mal58

(Wow, what a long post.)

Hi Steve,
No offence taken. We don't know until we ask. I think some-one on this site has a quote like the "only dumb question, is one that is not asked."

Cheers,
Mal