Knock Sensor Issues


Jun 21, 2009
Stock long block - never touched the knock sensor - trying to track down a snapping sound coming from engine under boost (>8 psi). Started a month ago and have tested several things: coil and ign module, plugs replaced, fuel pressure is good (under boost), methanol spray is working, no leaks in tubing-intake-throttle, MAP voltage agrees with boost pressure readings. I haven't checked the injectors - 42# from TT, 6 years old.

The snapping sounds like detonation but i don't see KR on scan master. I tried testing knock sensor but get no KR unless I directly tap on the knock sensor with large screw driver (in park, RPM's 1500-2000, TT Alky chip). Hard, continuous tapping on the engine block or intake manifold near knock sensor will not produce KR with same conditions. Searching several posts about knock sensor tests, and people tend to think that tapping engine block in the back drivers side should show KR on scan tool if idle RPM are high enough for chip to respond.

Is my knock sensor worn out or not torqued properly? I'm still trying to find out the source of the snapping sound (detonation?) - haven't found a way to be under the hood during boost.
Knock sounds like a when you shake a spray paint can. Does the engine skip when the snap sounds? Maybe a sparkplug wire is cracked and you're hearing the spark jump?
Knock sensor is listening for a specific frequency so it will not pick up just any noise. It gets pretty loud under your hood but that doesn't mean you've got detonation. Are you running a power logger? If you are then I would record while driving around and see if you register any knock on the shifts or at any time. I never see knock on the scan master but often find it in the logs. There is also the ESC module that translates the knock sensor signal.

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Don't look for knock retard, look for knock counts. It's normal to accumulate 1-10 counts when you start the engine. That's a good way to know that the knock circuit is doing something
New idea for what is causing the sound. I changed out hydraulic lifters and flat-tappet cam a while ago - possibly 10k miles. Lifters are TA V1405 (can't recall their brand). I got push-rods with length such that carefully measured lifter pre-load was ~0.035". Lately the valve train is noticeably more noisy after engine gets hot and oil pressure is 15 psi at idle. If the engine is still high oil pressure after start-up, I don't hear the detonation-like sound at any boost level up to 20 psi (running ~5min), but the sound starts after 15-20 min of driving as oil pressure drops due to higher temp & lower viscosity. I speculate that with the hot oil pressure, the lifters no longer pump up enough to keep the valves open when cylinder is under higher psi. If this is true, changing back to stock length push-rods should help - until lifters don't pump-up withadditional, extra pre-load.

This thread was very informative about stock engine pre-load measurements: