R26 Cam shaft end plug blown out - cause?

Hi folks, been lurking on here for a few years but not a big poster. Hoping to pick the brains of some more experienced folks to see if they have any pointers on an issue that cropped up recently.

History is a stage 1 R26 Megane that hasn’t given me any trouble since I’ve owned it and mostly has had an easy life. It recently went to a specialist for a full cam belt service. It was commented on that some of the components looked like they’d been re-used the last time this service was done, including the cam shaft end caps which were a loose fit, but had been stuck in with a bead of silicone. Anyway, all was well after the service although I took it easy for the next few days. When I put my foot down for the first time since the service, again, all was well, but as I slowed for a roundabout, there was a distinct whiff of burning oil. I nursed the car a couple of miles home, and as I turned into our road, the smell got overwhelmingly strong. I swung onto the drive, switched off, and popped the bonnet to find the larger of the two end caps was nowhere to be seen, and there was oil everywhere.





Fortunately, the level is still above the minimum on the dipstick, but certainly over half a litre has left the engine. The person who carried out the service has been helpful and offered various options to rectify, but my real concern is what could have caused this to happen in the first place? I can replace the cap myself, but don’t want to lose another when I’m potentially further away from home. I’m at a bit of a loss to know what to check, the car doesn’t smoke, doesn’t use an excessive amount of oil, and doesn’t seem to run rough. I would like to think that the cap was fitted properly, so that would mean it has been pushed out - what could cause the head to pressurise realistically, and I have to assume that the silicone sealed old cap was able to withstand the pressure as I don’t think it is coincidence!

Any thoughts would be welcome, or places to look. I wondered if the breather system had any one-way valves or anything along these lines that could have failed and allowed it to pressurise.


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Sorry, I don’t think I was clear enough in my original post - the old caps were removed during the recent cam belt service and these were the ones found to be siliconed in place but not a tight fit. New caps were fitted (without silicone), and it is one of these that has been ejected.

 

I was told it was a full cam belt kit from Renault, and given that he is a well reputed specialist in the Renaultsport world, I have no reason to doubt. I’m not sure if they have any markings on them, but I could certainly check the one that remains in the block to see what it looks like.

I’ve just recalled something else - I was advised that one of the caps was weeping a small amount of oil, but only enough to drip infrequently onto the exhaust. If there had been pressure in the head, I’d expect to have seen more evidence of oil spray in this area over recent years, but I’ve never seen anything.

It is a little strange, and I don’t like not understanding it!


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The results are in, it's not pretty… 130, 160, 155, 150psi.

This is spark plug number 1 which is showing a bit of a crusty white residue:



Compared to all the others which are a much nicer brown:



Gutted to be honest, this car has led a pretty easy life, not been on track and most miles covered on a motorway. At least it saves me purchasing a leak down tester kit!

Time to consider options to rebuild or break…


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Thanks both for the subtle hint!!

I’ve been toying with the options, and I think I’m leaning towards keeping it at a similar power level to where it is now (RS Tuning generic stage 1 map), but would like to consider what I can do to future-proof the engine, or maybe increase the power slightly at a later date. I love the way these cars drive, and as much as I tell myself it’ll be fun to have 350bhp, my sensible side thinks I might fall out of love with a car that is trickier to manage on a damp road.

So assuming it is a ring landing problem, is this likely a cause of poor map / fuel starvation, or is it just that the standard pistons are weak and it is the luck of the draw when increasing the power? If it is the latter, would a set of forged pistons be a stronger alternative?

I am considering the option of fully forging, but understand that this is probably overkill if the likelihood is that I’ll never be over 300bhp. I’m sure I’ve seen discussion on here regarding using Megane 250 pistons and rods as these are stronger as standard - maybe these would be a cheaper, less overkill option to not be running on the limit of standard 230 internals.