Knocking Sound on Startup


Well, when I went to start Harlem ('00 XJ8 - 180K miles) up after her long hibernation I noticed what sounded like a quick knocking sound from the engine that went away as soon as the engine started. To be sure, I re-started her a couple more times after fully-warmed up and it was still there, and even a bit louder. WTH? :open_mouth: I think my car had the same issue before hibernation, but more “softly” in volume/pitch.

I was thinking that it might be a bad spark knock sensor, but if that were the case wouldn’t it continue after the engine started running (and there’s no trouble code being triggered for it)? Same for a rod bearing. :thinking:

Perhaps it’s low oil pressure … that would explain why it goes away once the engine is running, but don’t know what would cause such low pressure at start-up only to go to needed pressure while running. :confused: Maybe bad gas from sitting for many months w/o running (and only a few gallons in the tank)?

Hopefully, this is not the dreaded sound of a loose timing chain b/c of a tensioner about to bite the dust (I don’t know whether this engine had the relevant upgrade or not). :grimacing:

Other than for that starting sound, the engine runs very smoothly and quiet, as it always has. I may try to record the sound on my cellphone and post it as a wav file w. link here, later on.


(gary breyer, 09 X358 LWB Sovereign, Liquid Silver/Warm Charcoal. Ex Ministerial car.) #2

Is it a loud rattling sound or a quieter rhythmic ticking sound? If the former, it could be tensioner/chain related, if the latter it could be (quite common and not really an issue) the VVT solenoids.



k. Hmmmmm … not sure it is a “rattle”, but more like a few quick knocks (puck, puck puck) … I think I’ll have someone stand outside it at the front with the hood open while I start it to see how it sounds to them. Thanks, Gary.


(Michael Garcia) #4

take the fuel filter out
turn it where the lead from the tank faces down
see what comes out…


(Graham Knight) #5

If it’s a “deep” knock, it might be indicative of wear in the big end bearings aka rod knock, it goes away when the oil pressure starts to rise. You could try to isolate which cylinder by pulling off spark plug wires one at a time with rubber handled pliers till you find the one where it quiets down.

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Hi. I had a similar mysterious knocking noise when starting up. It turned out to be a faulty headlight height adjuster motor. It made a hell of a racket, as it is effectively in an acoustic chamber, in the inner wing panel. Disconnect the electrical connection. If the noise stops, then you have found the faulty motor (one each side). Hope this helps.



Hmmmm … I didn’t know we even had those … on a '00 XJ8? :confused:


(Robin O'Connor) #8

Some do some don’t, if you have a small rotary knob on the dash with 1,2,3,4 then that is the manual adjustment for the different number of passengers.



I don’t know about 00 xj8 (mines a 96 x300). When it happened on mine, it was noisy enough and regular enough to suspect a major engine fault, although it would (mostly) disappear after a few minutes. Sometimes it wouldn’t, so was very embarrassing. Anyway, the motor is like a stepper motor or solenoid with a linear motion applied to a spigot which pushes and pulls on a lug on the headlight. The noise is created when the motor repeatedly attempts to move the spigot and fails. Replacement of the motor is the cure. I found one on ebay.

Hope this helps.



Wow … very interesting, and quite “Jagish” … :open_mouth: As for the # adjustment, I take it that it re-aims the height of the headlight beams based on the # of passengers - and hence the increased/decreased weight … Never knew that one more (or less) passenger (e.g. 180 or so lbs.) made that much of a difference in aim … :thinking: