Knocking noise at idle

[Clifford Pope]

For some time the engine has been making quite a pronounced knock when idling slowly, and the car dithers up and down. It only does it when warmed up, and only at 800 rpm. If I touch the accelerator and increase speed to 900 rpm, it stops. It takes several seconds of idling before the noise cuts in. Some days it doesn't do it at all. Sometimes increasing the speed and then letting it drop stops the noise returning.
Otherwise running is perfect, and compression is good.

Counting the frequency of the knock, it corresponds to cam shaft speed, not crankshaft. (ie about 6 knocks per second, nothing like 12)
So I don't think it can be the common 2.3 piston slap.

What components could make this noise? Sticking valve, damaged tappet bucket or shim, cam lobe? It appears to be related to oil thickness.

Before I start investigating, any tips as to possible likely causes?

[Paul240480]

Sorry Clifford no tips, but can say that 'Tank' has, what I think are the same symptoms. Has done for a long time too. Like yours it had no effect on the cars running.
I shall keep an eye on this with interest
Tank is a 2.0ltr, so it would appear to be something that affects both versions.

[Clifford Pope]

I've just noticed that the noise is entirely absent when running on petrol, it only does it on LPG. But that perhaps merely reflects idling speed, as it idles slightly faster on petrol.

I know the cars are prone to uneven idling that stubbornly refuses to go away, and also to a bit of valve noise. It's interesting that another car shows the same symptoms as mine. It's almost as if there is an inherent tendency for something to vibrate or go into a dither at a certain speed, more pronounced in some than others.

The engine mounts are new, which made no difference.

It would be nice to be able to adjust the idling speed a bit to avoid this point, but the electronics don't allow that.

I'm going to try a thicker oil for the summer.

[honestjoe]

I"ve been wracking my brain on this funny problem..its true ..my bus does this at low speed..It still would be nice to localise the knock..no power loss..knock fades in and out on mine on tickover..(scratches head)..power steering pump?..oil pump?..ignition timing?..I know these engines have a bomb proof reputation but they still must wear over time..oh well..i think i"ll just wash and polish the motor..that should help!..hj

[Paul240480]

Mine is not LPG'd. The oil I use is Supermarket Semi-Synthetic & is changed between 5 & 6 k.
I must video mine sometime, to compare.........
Ah hj has the same. Mornin'!

[honestjoe]

Originally Posted by Paul240480

Mine is not LPG'd. The oil I use is Supermarket Semi-Synthetic & is changed between 5 & 6 k.
I must video mine sometime, to compare.........
Ah hj has the same. Mornin'!

Hi paul..i"ve got an hour to kill before rugger and beer..so i will video the sound as well...cheers hj.

[daveo2002]

What an interesting thread! I often hear a tapping noise at idle too on the old GLT - B230E, no LPG, in perfect tune etc. Goes away as the revs increase.

I'd always put it down to wear on the valve shims as she's done near 100k now and they've never been looked at. I presume the revs help pump the oil round and mask it...

[Paul240480]

Mines about 400 miles short of 200k. Will be getting it warm today, gotta pop out, so 'hopefully' will have it knocking & fully warm to video

[Paul240480]

Well here it is. Not sure if you will hear it on this....... The knocking I'm refering to is there, but it is subtle - very much drowned out by my noisy alternator bearing



Click the pic to start Tank up!

[Clifford Pope]

No, mine is slower than that. Like counting 1-2-3-4-5-6- in a fast but distinct way, with no slurring, if you see what I mean. That's what made me think it must be something at valve speed, not piston.