I'm planning on getting the Front Knock Sensor replaced next week. This is on a 98 LS1 A4. I'm not 100% clear on how to check for false knock and true knock. But, would like to figure this out before I go and spend money on getting this sensor replaced. I do have loss in power, loss in gas mileage, KR during acceleration, and a slight stumble during the 3-4 shift. Anyone who knows, please give me some of your advice.

 

True Knock: Caused by detonation in your engine. Sounds like marbles in your engine. This is caused by improper detonation of the fuel (pre-ignition, uneven burning of the fuel, yadda yadda). The one tried and true method of eliminating True Knock is to use higher octane fuel (100+ unleaded racing gas). This fuel burns at a slower, more predictable rate, and is much less likely to pre-ignite or burn at variable rates.

False Knock: This is something rumbling around in your engine. Whether it be your exhaust rattling, a noisy lifter, a noisy bearing. It is something that is setting off the knock sensor. The knock sensors are very sensitive to concussion signals, so they are easy to set off with a FALSE signal, even if you can't hear it.

How to Test if you have False Knock or True Knock:
As stated above, True Knock is caused by improper detonation of the fuel, and one tried and true method of eliminating this is by running race gas (just for a little bit). So, go put some 100+ unleaded race gas into your car. If the knock goes away, then you know that it is true knock. If the knock does not go away, then you know that it is False knock.

Now, if it true knock, you have a problem. There are many things that will cause true knock to take place in an otherwise healthy engine. Here are some more common ones.
1) Carbon Build-up in your combustion chamber: Does your car burn oil. If so, then this may be something you will want to look into. The carbon buildup in the cylinder stays hot even after the combustion has subsided. So, when more fuel is sprayed in for the next combustion, the carbon has a tendency to ignite at the piston as well as at the spark plug. This causes two colliding flame fronts, and thus detonation.
2) Running Lean: Too much air, too little gas. The lean a/f ratio will cause the air/fuel ratio to heat up too much during the compression. The more "low" octane gas is heated, the more volitile it becomes. The gas may actually heat to a point where it pre-ignites prior to the spark (like a diesel) or the excessivly heated charge will burn too quickly, cause colliding flame fronts, or uneven flame fronts.
3) Bad gas quality. Bad gas will exert the same qualities as described in the running lean condition, however, it will happen at lower compressions.
4) To much compression. This happens to those that like 11:1 compression ratios, but don't like to run around with 94+ octane gas.

There are many other items that will cause true knock retard, but at least this will get you going.

Good Luck
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