So, I had a multiple cylinder misfire a month ago in my 03 STR and had oil in the plug wells. I cleaned everything up temporarily and replaced a bad coil. After this happened, I realized that it was 2.5 quarts low on oil. I have since had an oil service and am getting a tapping noise under load that is most noticeable when slightly depressing the accelerator causing the car to drop down one gear. The car goes in next week to have the intake manifold gasket replaced as well as the rest of the coils and new plugs (no labor cost for these as they will already have all the coils out for the intake gasket). The mechanic took it out for a test drive and noted the noise, but said it doesn't sound like a typically knocking noise due to a rod from the low oil, but is not sure what is causing the noise. It cannot be replicated on idle, only under load. I have scoured the forums for the past two weeks and am at a loss as to what might be causing this noise. Any suggestions are very much appreciated.

 

The noise may be "knock-ping" as it is often called. It only appears under load, not idle.
Because you can't get high-octane petrol anymore (the forbidden tetra-ethyl-lead additive), a high compression engine such as a Jag. with 10.5:1 compression ratio has to have it's spark-timing retarded slightly to prevent the knock-ping during heavy load such as acceleration. This is achieved by a "knock-sensor", an acoustic device tuned to the characteristic sound, the output of which is fed to the micro controlling the spark timing. Under light load, the spark timing is advanced to normal to get better fuel efficiency and less emissions. Any experienced mechanic will recognize knock-ping at once. Of course your noise may be something else, this is just a suggestion. But check the knock sensor, it's the most likely cause if the noise IS knock-ping.
Explanation of knock-ping:
You can't keep increasing the compression ratio for more power of a petrol engine beyond a certain point for available petrols and their octane rating because of this "knock-ping" or what is sometimes called "detonation" effect. It also stops you advancing the ignition towards the best power and fuel consumption.
The higher the octane number of a petrol, the slower it burns; completely opposite to cetane rating of diesel. The higher the cetane number the faster diesel burns.
In a high compression petrol engine, if the petrol is of lower octane, the fast-burn will create an acoustic shock-wave ahead of the flame front, erratically and spuriously igniting the end gas in noisy spats. This is a post-ignition problem fixed by temporarily retarding the ignition point at high power demand using the knock-sensor and micro. It is NOT a pre-ignition phenomenon with spark-ignition point as datum; if it were, the retardation of the ignition would have no effect on knock-ping.
Huge effort has gone on for years to make a very high compression petrol engine that won't suffer from knock-ping. The newish stratified charge petrol engines are a step forward, but have their own problems.
Leedsman.

 

I have been using 93 octane gas regularly since I bought the car and it had just started making the noise. If it is knock-ping, will changing the knock sensors solve this issue? I can't hear it with the windows up, only down.

 

JagSTR, you need an experienced 'mechanics ear' to initially determine the nature of that sound. Not doing that can lead you up time wasting and expensive blind alleys. The good old boy mechs., or almost any involved in racing will know.
Leedsman.

 

I have now had the spark plugs changed, new coils, intake gasket changed due to a leak, and the transmission software updated to the latest version. Since this software update, the car seems to hesitate if moderately accelerating and changing gears (drops from 4-3, 3-2, etc. - not full throttle, just one down changes). This seems to also coincide with the tapping noise mentioned earlier. We have a Jaguar tech in town that has said that he is not sure what the noise is - almost positive it is not a rod as it only happens under load during light acceleration. Noise is absent on full throttle acceleration and light acceleration - only happens in the band between there... The mechanic said the only way for him to find the source of the problem would be to tear the engine down and that it would most likely be a waste of time and money. I can't tell if this noise is related to the engine, supercharger, or possibly the transmission. Has anyone experienced these issues before? It is mildly embarrassing to have a knocking Jaguar, but more important than that, it is concerning that it could be a failing component that could cause more damage if not taken care of immediately. My tech said to keep driving it and see if it gets worse... Sorry for the long rant; I am just trying to explain it the best I can think of.

 

Well it's easy to tell if it's supercharger related. Just remove the belt and go for a drive.
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jaguarSTR, clubairth came up with a fine suggestion: Remove the belt (s). Not only will you remove the s/c from the equation but also a failed a/c compressor, which by the way will make a tapping noise when turned on (compressor packs) or off (bearing).

 

No, the octane rating is the fuels resistance to ignition, not how fast it burns. The higher the number, the higher the ignition point. Ping, knock, Detonation, whatever you like to call it can result from several things but you are right, most common is the fuel is igniting due to hot spots, or compression before the spark.

I do however agree with him on checking out the knock sensor.

 

How easy/difficult is it to remove the belts to take the S/C out of the equation? Any good write ups on the boards?