Square lobe on K4M cam

[elandman]

Sup guys,

Could someone please explain what the square lobe on the exhaust cam of my K4M engine is for? Seems to also have a tappet that follows cam profile. The tappet sits in the tappet cover housing yet it doesn't drive anything. Very strange.

I cant seem to find out what its for or any mechanic locally knows what its for.
Reason I need to know is that I suspect that the tappet is faulty or damaged, since its making a horrible rattle between 2000 and 3000rpm while holding rpm.

Been speaking to a Renault tech and he seems to think that's the fault as he received a mandate recently from Renault France about the same issue occurring on a few Meganes/Scenics. Similar rattle to a faulty dephasor, except noise isn't coming from dephasor. Its from between coil pack 4 and 4, same area as VVT solenoid.

 

[elandman]

BUMP

 

[Sean6545]

Seen them before. Never found out what it is though.

 

[MicKPM]

I believe its the lobe that works the VVC camshaft position sensor pickup... a very common failing part on the later K4M which causes a nasty knocking and "injection fault" to appear on the display when faulty (Also DF137 iirc).

Mick

 

[Sean6545]

But this is on the exhaust camshaft though mick? I might have a dig on dialogys and get some more info tomorrow.

 

[MicKPM]

Good point... Scrap that then... Must read properly.

 

[singlespeed]

I'm wondering the same thing about the square lobe and tappet as I have a K4M-854 head in bits from an RS133, ready to be ported.

The tappet should be retained in the tappet bore by a screw, comming through the top of the cam cover and locating in a groove in the tappet.
The tappet is lubricated by an oilway comming up from the head and feeding the anular groove around the side of the tappet. The spring side of the tappet is drained back into the head through two drillings which become fully covered as the tappet follows over the tips of the square cam lobes and are fully open when on the flat part.

Being as the lobes of the square cam are between the lobes for the exhaust valves, is it possible that this square cam is there to maintain a more constant loading on the cambelt, filling in the tension as the exhaust valves open and close?

 

[elandman]

I'm wondering the same thing about the square lobe and tappet as I have a K4M-854 head in bits from an RS133, ready to be ported.

The tappet should be retained in the tappet bore by a screw, comming through the top of the cam cover and locating in a groove in the tappet.
The tappet is lubricated by an oilway comming up from the head and feeding the anular groove around the side of the tappet. The spring side of the tappet is drained back into the head through two drillings which become fully covered as the tappet follows over the tips of the square cam lobes and are fully open when on the flat part.

Being as the lobes of the square cam are between the lobes for the exhaust valves, is it possible that this square cam is there to maintain a more constant loading on the cambelt, filling in the tension as the exhaust valves open and close?

Thanks for the reply!

I can only guess hey. It must be for something like that. I've been told it might be called a "backlash compensator". But I have yet to find a mechanic who doesn't give me a skwiff look when I ask them about it. At Renault or otherwise :|

Or even a part number since I think mine is cooked or faulty. Hence my tappety noise between 2500 and 3000rpm at a hold speed throttle level. Perhaps at gentle throttle there inst much pressure on the cams from the valves, so thats where this tappet takes over and that's where it gets noisy.... who knows.

I'm opening up the head in the next week or two to take a look, as well as do my timing belt and check whether dephasor is funky too.