Clarkie GT

noticed today as i was warming my car up for a oil change, i had my head under the bonnet manually reving my car with the throttle, and noticed a knock around 3k rpm, it's not a consatant knock with equall intervals. its a random knock happening every 2-3 seconds or so.
What's this likley to be?
it didnt go away with the oil change

Splitpin

Maybe you'd like us to predict Saturday's lottery numbers too? Trying to correctly diagnose something like that, over the internet, is like finding the proverbial needle in the haystack. It's made even more difficult by you not telling us what type and model year your car is, or whereabouts on the engine the noise appears to be coming from.

The best advice to give you in these circumstances is, if it concerns you, to get your car in front of someone who knows and let them have a listen. Is the repeat frequency of the knock totally regular, or does it vary, even though you're holding the engine speed at a constant level?

If it repeats at seemingly random intervals it could literally be almost anything from the onset of big end failure to a noisy injector to valve clearances out of tolerance to something as innocuous as a loose heatshield. If it repeats at regular intervals I'd look at the timing belt and tensioner - although you'd expect the frequency to be a little quicker than 2-3 seconds at 3000rpm with either of those.

Bottom line is that the internet is no substitute for a real engineer with his (or her) head under your bonnet.

dj219957

what he said^ take it to a garage, you will only end up going down the wrong track trying to fix it over the internet.

Clarkie GT

yes, i admit, it was a bit of a long shot, i just thought it might be a common/well known knock and i mite have been able to describe it to you.

anyway, ive realised what the 'knock' is now, i reset my ecu just before so was just pinking while the ecu learnt its paramters and octane rating in the fuel

thanx anyway

Splitpin

Whoa, steady there, what you're saying doesn't make sense. Det/knock/pinking isn't normally audible to the naked ear unless it is very bad, and besides which, if you really were getting it when the car was sat on the drive with you tweaking the throttle with your fingers, it'll be orders of magnitude worse when you're actually putting the engine under load.

Furthermore, immediately after a reset, the ECU will run on conservative settings (Ignition Advance Multiplier of 8 on a classic) and will then work up. It doesn't start with the wick all the way up and then turn things down, as you seem to be assuming. On a freshly reset ECU and a healthy engine, you shouldn't be seeing any noticeable knock at all, not even on 95RON fuel, especially when the car isn't even moving.

As things stand, I'd surmise that the noise you heard wasn't pinking at all, but, if you really think it was, it'd still be well worth you having your car looked at just to make sure it isn't detting itself steadily into oblivion.