glfredrick
Explorer Addict
- Joined
- January 25, 2003
- Messages
- 2,427
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- City, State
- Louisville, Kentucky
- Year, Model & Trim Level
- 86-98 Ranger STX
There really is no fix for this problem. These engines are on the edge of a compression ratio that requires premium fuel. Just run it, or realize that all that "ticking" you hear under load is eating your bearings and pistons for lunch.
You might try replacing the knock sensor, which in turn adjusts the ignition timing, cleaning the intake system (as suggested above) or running premium fuel.
Here is a breif article detailing what is happening inside your engine:
Detonation - Causes and Effects
This article was taken from a series of postings on the Supra Club of New Zealand mailing list. The contributors were Stuart Woolford and Tony Bryant. The article was edited together by Cully Paterson.
Knock, detonation, pinging, pre-ignition, etc are basically two things:
1 - Compression ignition, usually from over-advance of ignition and/or excessive compression (e.g. having added a turbo to an NA engine without lowering the compression) - this only happens after ignition, and is where the last of the combustible mix ignites from over-pressure during the last of the burn, giving a sudden detonation. A little is OK, and keeps the cylinder clean
but causes larger stresses in an engine, this is what knock sensors monitor, as the detonation produces quite specific, and audible, noises.2 - Pre-ignition/thermal ignition, this is often caused by too 'hot' spark plugs or hot-spots in the carbon crud that builds up in cylinders. It causes the combustible mix to start burning from a point/cause other than the spark plug ignition spark. This can be VERY bad if it happens very early in the compression stroke, where it can destroy an engine in one cycle! This is, fortunately, less common, however it can be caused by runaway compression ignition. This causes additional heat to build up and the can cause pre-ignition to start, and to then happen progressively earlier in the cycle (until BOOM!).
The causes of knock can be:
* Excessive compression - increases pressure / temperature
* Not enough octane rating - fuel more likely to explode
* Wrong mixture - excessive heating / lowering effective octane
* Ignition timing - too much advance increases peak cylinder pressures a LOT
* Poor cooling - leading to high cylinder temps and pre-ignition
* Dirty cylinders - more candidate points for pre-ignition
* ECU/sensor fault - lots of options
and no doubt more.
The most 'efficient' place for an engine to run if often just on the edge of knock (from compression ignition) - so many modern ECUs use a knock sensor to hold the engine just on the edge of knock, but preventing damage due to excessive knock.
'Lean knock' is sometimes not actually a lean mixture (in fact, very lean mixtures will misfire, not knock), but rather an insufficiently rich mixture, as rich mixtures will delay the burn and burn cooler, thus reducing the chances of knock. This is very typical of the behaviour of turbo engines under boost, where they are deliberately run very rich to prevent the onset of knock. This explains why turbo engines in perfect condition can often be seen producing black smoke under power. On the other hand, get the mixture too rich, and the temperatures start coming back up rapidly...
- Is knocking the same as pinging and detonation?
We have a few possible heat sources.
1) Normal ignition. The normally controlled burning process obviously adds heat to the whole mixture.
2) The compression itself. In the same way a turbo heats its compressed air, the piston will heat the mixture at it compresses it.
3) Pre-ignition. Where a hot spot initiates ignition in addition to the spark plug, thus burning the mixture faster than usual.
When knock happens, its usually a combination of factors e.g. over-advanced timing combines the heating effects of 1 & 2, thus hitting the auto-ignition temperature, and wammo! the whole mixture goes up simultaneously, resulting in a "step" pressure rise, and general destruction eventually ensues.
