Help needed to identify engine noise on an SL500

[DaveG38]

For years now, my 2005 SL500, which has just 58k miles on it has had a curious tapping from the front end, probably under the bonnet, rather than at the wheels. Performance is normal, the noise hasn't get any worse, but the noise is mildly irritating, and so I am wondering what the cause may be. The sound is a light slightly hollow sounding tap, and manifests as follows:

1. When first started with the engine cold, there is no noise over the rev range.
2. When in neutral, there is no noise over the rev range, whether the engine is hot or cold.
3. The noise is only present after about 1/2 mile of driving i.e. as things warm up.
4. The noise isn't present on light acceleration but starts to appear at around 1400 rpm.
5. The noise disappears at revs above 2000 rpm.
6. The noise is not present at any revs on the overrun.
7. The noise seems to be at about half engine speed and the tapping follows the revs of the engine not the speed of the car, as gear changes occur.

Anybody got any ideas? All suggestions gratefully received.

 

[Uncle Benz]

I’d say it’s likely to be a little end knock. You occasionally hear them on M113 V8’s. I have a theory, but it’s only a theory as to why it occurs. The rods are gun-drilled to lubricate the little end, and the upper big end bearing shell has a small hole to allow oil to pass into the drilling. The connecting rods are made in one piece and broken so they fit together perfectly. Sometimes the postion of the break and the machined slots that locate the shell bearing heel tags conspire to allow the bearing shells to move slightly and the hole and drilling no longer align. I’ve seen a few where the drilling witness on the back of the shell is right next to the hole in the shell, but would have been effectively blocked. I have a picture somewhere

 

[Uncle Benz]

I’ve seen plenty with a little end knock, never seen it result in failure. It’s just annoying.

 

[Uncle Benz]

 

[Uncle Benz]

The second picture is the back side of the upper shell. If you look closely you can see the little hole. You can also just to the left of the hole see a larger circular witness mark. It’s on the surface of the material and is the impression left by the gun drilling hole in the connecting rod. The drilling is bigger than the hole in the shell that is supposed to feed oil through. You can see that the two didn’t marry up.

 

[DaveG38]

Thank you for this advice. One thing I forgot to mention is that the noise is very much reduced when the engine is fully hot after a long run. Would this be compatible with your thoughts on the tapping?

 

[Uncle Benz]

Possibly. It’s very hard to be certain without seeing it in the flesh, but what you describe sounds to me like what I’ve heard from a few over the years. Like I said, I’ve never seen it lead to a failure, or get appreciably worse over time. Maybe get an experienced independent specialist to cast an ear over it, they will be aware of what’s normal and what isn’t

 

[Uncle Benz]

In fact, looking again at that picture I think you can actually see the circle of the drilling was originally over the hole, and it has moved over time. There are almost half moons where it moved and settled a bit, and then moved again, finally ending up completely off the oil hole. If you zoom in you can just make it out. I’ve fascinated myself with that!

 

[ajlsl600]

If that gets out of line or blocks you have no worthwhile feed to piston pin. That's eventually a walking job! Cracked rods a reduction in production cost issue , I not a fan. I have seen locals in developing world challenged by that. (Square peg, round hole story)
Also if u end up with partial sieze, little end. skirts will suffer excess wear and in some cases the skirts will crack, break off
I wud expect locating tang to eventually wear off, leading to bearing spin and a large bill.
Op says half speed. Cud this be a timing tensioner guide occasionally taping away against its retaining pins.?

 

[Uncle Benz]

If that gets out of line or blocks you have no worthwhile feed to piston pin. That's eventually a walking job! Cracked rods a reduction in production cost issue , I not a fan. I have seen locals in developing world challenged by that. (Square peg, round hole story)
Also if u end up with partial sieze, little end. skirts will suffer excess wear and in some cases the skirts will crack, break off
I wud expect locating tang to eventually wear off, leading to bearing spin and a large bill.
Op says half speed. Cud this be a timing tensioner guide occasionally taping away against its retaining pins.?

Loads and loads of cars have no lubricant to the little ends other than oil splash from below. They don’t fail. Gun drilling of the connecting rods is pretty rare really. I’m currently building a FA20 Subaru engine for a pal. We need connecting rods to handle 500bhp, and Manley appear to be the best solution. Little ends are splash lubrication only.

I’ve seen a lot of M113 with this problem and little end knock. Never seen one fail. Ask anyone how many blown M112 or M113 they’ve seen… it won’t be many.

 

[ajlsl600]

OK get you.. why does maker send press supply there then.?Its either lube, cooling or both. And or skirt jets spraying pressure oil up inside skirts , so wot happens when one falls off.(jet that is) that pin, rod, piston gets scrapped.
Splash lube was I believe considered OK for lower rpm motors. I remember migdit with spoons as part of lower rod. Specifically to pick up sump oil and "splash" it around into underside of skirt, some of it lubed the small end. Spoons cud break off.

 

[ajlsl600]

Not saying can't work. You say it does , I know ford's didn't lube piston pin directly either . But with today's higher state of tune and requirements to dissipate heat I think more do and will be doing.

 

[Uncle Benz]

Not saying can't work. You say it does , I know ford's didn't lube piston pin directly either . But with today's higher state of tune and requirements to dissipate heat I think more do and will be doing.

if my theory is correct it does end up with a little end noise, so they do do it for a reason. I’ve just never seen it result in an engine failure. They aren’t a particularly highly stressed engine, no vvt, 3 valve heads, not high revving, relatively low bhp per litre output. It’s that under stressed design that makes them so reliable and long lasting. Still one of my favourite engines

 

[ajlsl600]

That's why I loved my Vdoz, never going to strain it! I have seen small ends sieze, had top end clatter. Spoke to owner with 50,50 solution. He OK, d it I thrashed the motor a few min at standstill, eng at op temp did 2 times clatter disappeared. Not ideal but alt was eng strip. He was happy camper.

 

[star]

Just a thought, pulsation damper on abc system??