Ok, could it be that my engine light keeps comin on if I fuel up with 87-octane instead of 93-octane (that is whats recommended)???

 

No, 87 Octane fuel will not cause a CEL to be set. It's something else, got to get the codes read.

Boy, I need to get over to the Beacon again one of these days - Still serving Pork-A-Plenty?? With onions??

 

ok thanks, yep they sure do!!! lol

 

Explanation you might find interesting:
The lower octane is faster burning and is more likely to give sprurious end-gas spats, commonly known as "knocking" or "knock-ping". Also sometimes called detonation. It sounds like someone rattling a chain under the bonnet, and usually happens when you've been driving along on the level the encounter an uphill gradient needing a lot more power.

Knock-ping is very much involved with ignition timing, retarded giving less knocking. So modern cars have an acoustic sensor 'listening' for knock-ping, and as soon as it is detected, that digitized acoustic waveform is compared with ones stored in the ECM, if a match is detected, the ECM retards the ignition.

Engine combustion chamber and spark-plug conditions are also involved. A sooty chamber is more liable to knock-ping. Long ago it was discovered hemi-heads were less likely to knock-ping, the worst being L-heads and sidevalves.

The most resistant fuel to knock-ping is propane, where a compression ratio of 12:1 can be employed. So high compression engines are very suitable for propane burning.

Leedsman.

 

Interesting, Leedsman?! That helps alot thanks for the info...

 

Not quite correct. All grades of gasoline burn at the same velocity. Gasoline rated at higher octane levels simply contain additives to suppress spontaneous or 'premature' combustion. High compression engines would tend to cause the air/fuel mix to combust purely from the heat of compression, long before the spark plug actually fires. Once the 'fire is lit' the rate of flame front progression or power output (# of BTUs available) is unrelated to octane rating.

Leedsman is correct that propane has a higher inherent octane level than most gasolines, but it's far from the highest. The much feared and dreaded ethanol has a rating of 129, vs propane at 110. The highest rated common fuel is actually methane at 135, but it's not convenient to carry around a herd of cows in the back seat.

There's lots of myths and misunderstanding on this subject.

 

Re. myths...Mikey is describing a phenomenon called PRE-ignition where burning is initiated before the spark; this is usually caused by glowing elements at the spark plug. Often caused by having a wrong heat-rating for the plug.
Knock-ping or random POST ignition is an effect of fast-burning where the flame front is so fast it causes the acoustic shock-wave ahead of the burning flame-front to randomly initiate end gas 'spats'. Additives were used in the past such as tetra-ethyl lead and iodine compounds to slow burning down to achieve "chemical octanes" by reducing the shock wave ahead of the flame-front. Because tetra-ethyl lead is so incredibly poisonous (one drop of the neat stuff on your skin is reputed to kill you in around 10 seconds) it is now banned in UK and other places. Therefore the industry has to use either blending* or some safe additive to slow burning down.
Methane IS used as a vehicle fuel, known as CNG (compressed natural gas) but needs very strong tanks due to the high storage pressure needed. In UK I have not encountered anyone using CNG, but lots using LPG (usually propane, but sometimes mixed with butane esp. in France). I limited my piece to fuels commonly available at petrol stations. In UK, about one in eight stations have LPG on sale. The connector is 'captive' you have to turn a lever to lock it onto your filler. When finished and you disconnect, there is a huge "whoosh" of escaping gas which can be scary if you're not used to it.
I learned about combustion chamber phenomena and fuel characteristics a long time ago from the the American Society of Automotive Engineers back in the 1960s. They had a big interest at that time due to vehicle pollution esp. in California. NOX emissions from tailpipes were creating a kind of brown gas in the air due to the effect of sunlight. Athens in Greece was having the same trouble.
Leedsman.
*Benzine was used as a blend, but due to carcinogenic effects, is also banned. Benzine can be produced from the destructive distillation of coal, and was the basis of South African motor fuel and in wartime Germany. In UK it was called "Coalene".

 

Leedsman,

You are still advancing the idea that different octane levels of gasoline burn at different speeds. This is false.

http://www.2strokeheads.com/tech-octane-detonation.htm

 

We must then beg to differ.
Leedsman.

 

Interesting debate on the technical details with various fuels, but it seems y'all have totally forgotten about the OP's original question (not to mention the first wrong answer in post #2).

The fact is that using lower octane fuel CAN cause the CEL to illuminate, although this is not common. Usually the engine control system will simply retard the ignition to prevent the detonation, but if there are other conditions that predispose the engine to detonate, the lower octane fuel can push it beyond the ability of the control systems to maintain the proper conditions. Each cylinder bank has a separate knock sensor, and the computer code will clearly indicate if the CEL is due to knocking (and if it is, which bank).

If you are trying to diagnose such a problem yourself, you cannot simply rely on having some auto parts store clear the CEL for you and wait to see if it comes back on. Depending on the severity of the problem, the computer will set a preliminary code but not trigger the CEL until the problem continues longer or gets more severe. It is easier to purchase a tool to read the codes yourself and check them frequently until the actual problem is found and fixed.

 

Wrong - on several points - You've embellished your answer with extraneous conditions which were not part of the OPs question. His original question was answered correctly. Even in the special scenario you've created, "the other conditions that would predispose..." would be the diagnosable condition, not the 87 octane. Also wrong on the knock sensors in any Jaguar engine, and even more wrong if this is an X-Type. And wrong, in general, on the CEL protocol you've described.