Diagnosing nasty engine knock
Discussion
My 400 Chim has just developed a very audible knock. Sounds mechanical (rather than 'gaskety'), and a bit 'top-endy' (maybe on the near-side)... is intermittent (ie quiet then there with no discernible reason) and rises and falls with engine revs although it is difficult to say whether it is at engine speed or half-speed. It is louder in the car than under the bonnet. No obvious signs of engine distress in the form of loss of oil pressure, smoking, rough running, overheating etc. Oil seems clean on the dipstick and the water is not contaminated. Appeared after a 'decent' run (first noticed on gentle overrun) but no (recent) over-revving or 'hard' abuse. Has done just under 48k miles so I guess cam/valve train is favourite... pushrods/lifters? Are they prone to breaking pistons at all?
Whaddya reckon... sump off first or rocker covers?
Whaddya reckon... sump off first or rocker covers?
When the cam went on my Griff it was down to badly worn lobes. You could feel the knocking my touching the wing - the knocking was enough to resonate through to the outer shell.
It seemed very obvious at the time it was a cam failure - and it appeared in much the same way that yours has done too.
It seemed very obvious at the time it was a cam failure - and it appeared in much the same way that yours has done too.
Weird one this... been round the motor with a long screwdriver (as a stethoscope) and cannot pinpoint it... no untoward noises echoing out the oil filler either... now thinking it is not a main engine part but maybe an ancillary like the water pump or dizzy? One minute the thing sounds fine, the next it's clattering like a good one (seems to 'click' then clatter). Just to eliminate the cam thang, where's the best place to pick that up by 'listening' (tried all the 'heavy' surfaces and bolts ends on rocker covers, plenum, alternator etc)?
Just an update... and supplementary questions following further investigation. Have had the rocker covers off without any obvious signs of failure either to the visible valve gear or, by deduction (through measuring valve deflection), to the cam lobes. I've not pulled the cam or followers. I did find a loose fastener on one exhaust manifold and (optimistically) hoped that was the problem, but once back together and run-up the noise reappeared after the engine warmed. When cool, sounds cool. So, does temperature normally have a function on knocks?
You can pull the injection off and get at the cam lobes in about an hour. Taking the rocker covers off MIGHT tell you something IF you happen to have a stuck lifter AND it happens to be stuck when you whip off the covers.
Another couple of hours would have both of the heads off so you could check the bores for scoring which would help with broken rings (new head gaskets don't cost much but you'd have to allow for new pedestal shims too) - though from your description it sounds too 'heavy' to be rings.
On the Wedges at least, you can get underneath, drop the oil and pull the sump off for a look at the mains/ big ends. By that time you're about 80% into a full engine strip and may as well spend your wad....
Another couple of hours would have both of the heads off so you could check the bores for scoring which would help with broken rings (new head gaskets don't cost much but you'd have to allow for new pedestal shims too) - though from your description it sounds too 'heavy' to be rings.
On the Wedges at least, you can get underneath, drop the oil and pull the sump off for a look at the mains/ big ends. By that time you're about 80% into a full engine strip and may as well spend your wad....
Ok, again thats helpful thanks. If I did drop the sump for a squint at the crank, what would be the signs of big end failure... am I looking for shell material battered out the sides, or discernable 'play' (assuming there's nothing obviously 'broken')? Are shells available in a range of sizes that need to be determined using plastigauge, or are they all one size assuming a regrind isn't needed? Sorry if these are silly Qs
Normally you just need to drop one main and one big-end cap... the bearing shells should be an even mottled grey colour. Any sign of scoring (unless VERY light), copper backing visible or obviously melted white metal is bad news.
Of course if you caught it early enough you could miss the precise big-end that has failed, but what tends to happen is that metal particles from the damaged one rapidly get pumped round the others so they all end up scored anyway.
In extreme cases, yes, the white metal will have 'dripped' down between the conrod and the crank web... and you may or may not be able to discern play because the friction of the piston rings in the bores prevents you from getting much movement on the rods.
There are often a few undersizes available for crank shell bearings, depends on manufacturer and how much would need to be ground off the journals to get them useable again.
Of course if you caught it early enough you could miss the precise big-end that has failed, but what tends to happen is that metal particles from the damaged one rapidly get pumped round the others so they all end up scored anyway.
In extreme cases, yes, the white metal will have 'dripped' down between the conrod and the crank web... and you may or may not be able to discern play because the friction of the piston rings in the bores prevents you from getting much movement on the rods.
There are often a few undersizes available for crank shell bearings, depends on manufacturer and how much would need to be ground off the journals to get them useable again.
Something that you could try before stripping the engine is to purge the lifters. Hold the car at 3,000 rpm for two or three minutes and let it settle back to a natural idle. Also what grade of oil are you using? We recommend and use Mobil1 15/50 which is slightly thicker. You could go back to a semi or non-synthetic with a 'heavier' grade and this might stop any noises. If it's running ok turn up the stereo and enjoy it!! Bset regards, Tony
hexhamhc said:
Something that you could try before stripping the engine is to purge the lifters. Hold the car at 3,000 rpm for two or three minutes and let it settle back to a natural idle. Also what grade of oil are you using? We recommend and use Mobil1 15/50 which is slightly thicker. You could go back to a semi or non-synthetic with a 'heavier' grade and this might stop any noises. If it's running ok turn up the stereo and enjoy it!! Bset regards, Tony
Think it might take a braver man than me to do this, given the racket it's making. But say I did try, I assume I'd be trying to refill the hydraulic lifters with air-free oil? Why might this have become a problem? Would I not have seen less deflection on the affected valves if this was the problem?
I'm using 0/40 Mobil 1 (and have since acquiring the thing 4 or so years ago), as that's what it was apparently run on by the previous owner and I like the idea of the start protection given by such weight. Have to say things looked pretty clean and unworn or sludged/varnished under the covers so I'm not convinced the 'lightness' of the oil is the problem (although I know perceived wisom supports the 15/50 argument). Hell, shall I just go and turn the key... intrigued now?
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