I was doing a thermostat today on my XJR (the easiest-to-replace thermostat in the history of cars), and being under the hood reminded me of a not-so-easy job I was facing awhile back...

About 12,000 miles ago my XJR was kicking itself in and out of limp-home mode and the check engine light let me know a knock sensor was out. Any time I'd hit 3000+ RPM, the engine would slam into limp-home and run like crap.

Studying up on the issue, I realized that replacing this sensor was going to take a lot of work. It's located beneath the supercharger, which means a day of tearing the whole damn supercharger out, and bolting it all back together. A tiny simplistic job becomes a mess requiring multiple gasket replacements and a nightmarish back-bolt on the supercharger itself. It's a colossal pain in the ***.

I decided to try and bypass the whole job with a temporary relocation. After a bit of testing, I found a spot that seemed to work correctly. I left the existing broken knock sensor where it is and simply unplugged it, then plugged a new one in and high-temp glued it to the location shown in the picture below (pardon a bit of dust/dirt there, I live in the desert and the cat is due for a bath).

Here I am 12,000 miles later with no engine light and no limp-home issues. She's running like a top.

Obviously this is a bit of a shade-tree mechanic solution, and I'm not telling you this is the right way to do things (you should decide what level of risk you want to take on with your cat), but it worked for me and turned an all-day expensive job into a cheap 10 minute operation. Obviously I'll put it in the right spot next time the supercharger is out, but frankly, I'm pretty sure it could live the rest of its life right where it is without giving me any issues. If you're faced with that nightmare of a job and don't feel like tearing out the supercharger, this might be a quick-and-dirty way of getting past that problem for awhile.


Knock Sensor relocation on an X308 XJR

 

Probably okay, but be careful washing the engine. Not always a safe process.

 

I wouldn't endorse this. The 12,000 miles mean very little. This ranks almost as silly as the guy who hacked off his counter weights on his crankshaft to reduce rotational inertia on a Porsche 944- and then came onto the forums to brag about his skill the week before he had the crankshaft fail and his block crack. You've been lucky and a combination of the fuel used and load/duty cycle you drive hasn't warranted issues. If you were to take apart the engine- you may even find your pistons have minimal 'nibble' marks on the side (remember the AJ27 SC uses cast pistons)
A knock sensor mitigates knock under WOT and full load conditions. Its even more important on boosted engines. Jaguar engineering would have designed that for worst case sustained high end use. In engineering its a case of mitigating risk

The location is decided by signal to noise ratio considerations and making sure its mounted at a location where it can sense knock properly. It is then calibrated for that position.

 

I think it's a little safer than hacking counter weights off a crankshaft.

I'm not terribly worried about it. I use nothing but premium high-grade fuel and the car spends 99.9% of its time driving around at sea level at speeds 75mph and below. Scoping the two sensors shows no distinguishable difference between them with the engine running. The car also has two of these things, and one is still in the correct position and would presumably react if knock was detected even if the relocated sensor didn't, so again, I think the risk is generally low.

Obviously as soon as I have a reason to yank the supercharger (and spend all the associated time and money doing that), I'll relocate the sensor back to where it belongs. For now, it's just a quick and dirty solution to a problem that would have otherwise been a real pain in the rear to fix (and relatively expensive - the gaskets for the supercharger are surprisingly pricey).

At this point, the XJR is an older paid-off fun car that I enjoy owning and wrenching on. I'm willing to take the risk for now. If it turns out I made a mistake down the line, I'll own that. So far, I've gotten a few years of driving out the relocation with no discernible issues. Gas mileage is just as mediocre as always, and all of my horsepower are on tap. Not concerned even a little.

Your mileage may vary .