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MG MGF Technical - What could cause VVC mechanism to jam?

My VVC went to a garage after the cam belt tensioner failed. After head was removed, skimmed crack and pressure tested, valves replaced and reseated, it was reassembled with a shim and returned to me. It drove wonderfully for three days/50 miles and on the fourth day it suddenly ran very very badly. Rough running at idle, max rpm of 3000, max speed 25mph and very noisey.
Garage took it back and diagnosed a jammed VVC mechanism which they sent to a Rover dealer for calibrating.

Question:- what caused the VVC mechanism to jam?

I'm assuming the garage must have done something wrong when putting it back together. Blocked oilway? lubriction issue of VVC mechanics? Something not aligned?

I'd just like to understand it when I go and talk to the garage (and argue over the bill!)
Russell Parslow

The VVC mechanism can jam - I had to have one of the VVC solenoids replaced once (using the local MGR dealer), but was later told (by my regular specialist) that it can just be dirt and they can be cleaned. However, my symptoms weren't as drastic as yours. It just had a lumpy idle, but ran fine at higher revs, so I presume the mechanism had stuck at the profile for higher revs.
Dave Livingstone

Mike S makes an alteration to the oil-flow to the VVC mechanism as I understand it. Probably well worth to have done if the head is off for any other reason such as HGF or porting etc. / Carl
Carl

another board came up with this:-
"... they prob got the actuation rod channel junked up with the cam sealer and this causes vvc actuation plunger to stick,this is very common and i have seen it loads of times."

Sounds possible. I wonder if the garage will own up and if they'll try and charge me?
Russell Parslow

Mike Satur has also mentioned on this board about the need to take considerable care when reassembling the VVC mechanism: the mechanism is built to incredibly tight tolerances - and if assembled in a different order to the way it was originally put together will cause problems.

Makes me wonder whether this was the case here? Mind you, I would expect the problem to surface very quickly... perhaps within 50 miles?
Rob Bell

Would you have to disassemble the VVC mechanism to replace valves? I had the impression the VVC unit could be removed in one piece?
Russell Parslow

To remove the valves you need to remove the cams/ VVC mechanism Russell. Keeping the VVC together is difficult: the bottom half of the cam bearing is the cylinder head, and the top is the cam ladder - so removing the cam ladder would leave the VVC components unsupported. If you then removed the VVC components, it would be almost impossible to keep every thing together. What you have to do is keep the front and rear inlet cams in the cam ladder, complete with the VVC mechanism.

I've never worked with a VVC before (chat to the gurus such as Mike Satur and Dave Andrews) but IMO it would be tricky trying to keep everything together in one piece. If, for what ever reason, the VVC cams and mechanism were to become separated, then usually you are into complete unit replacement, unless you really know what you are doing.

As it happens, Valter has been doing this on his own car - will be interesting to hear what he thinks about this too.
Rob Bell

For completeness:-
The garage claims that the VVC jam was unrelated to their recent head work (removed,skimmed, new valves, pressure and crack test, shim added and replaced) as they didn't touch the VVC unit at all - it was left in place.
They seem sympathetic and have only charged me a small amount to cover costs.

It seems we will ever know why the VVC mech jammed. And thats what really bugs me.
Russell Parslow

Unfortunately Russell, there is a very good chance that you'll never find the answer here. I do have a sneaking suspicion that the mechanism could have come apart - you have to remove the VVC from the head in order to remove the valves. Plus you'd never skim a head with the valve train still in place!

Another potential cause of a jam, I guess, is metal swarf gumming up the mechanism - which is possible if the head wasn't cleaned thoroughly enough...

At the end of the day, finding out what caused the failure doesn't really help you - you need to replace the mechanism with either new or second hand components.

Would be worth getting the entire engine thoroughly flushed before reassembly though.

Would be worth while chatting to some K-series engine builders for some advice - Dave Andrews immediately springs to mind, along with Mike Satur...
Rob Bell

This thread was discussed between 10/06/2005 and 16/06/2005

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