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[NiallyV6CDX]

Hi all
I have recently done a top end overhaul - 3.0 cams in, newer tappets from a low milage engine (mine were rattling so I though I would change them), all the usuals, cambelt, spark plugs, HT leads, oil and coolant change ect ect...
The engine ran but had a missfire on one cylinder throughout the rev range, so i've just taken the plenum off, checked the injectors, spark plugs, and done a compression test.
All looked ok, but the compression test showed I was getting virtually no compression on cylinders #3 and #6.    I noticed that when turning the engine over with the spark plugs in, there is an odd "chuffff" sound coming from the manifold pipes to cylinders 3 and 6. I put my hand over it and there seems to be a bit of blowback into the manifold  It must be that the valves arent closing fully and thats what causing the low compression and resultant missfire.
However I am sumped as to why they wouldn't be closing or how to fix it
Anyone got any ideas?

[Bent valve]

I take it you have double checked the cam timing?

[Andy H]

Did you squeeze the oil out of the followers before you put them into your engine?

[NiallyV6CDX]

Yes I have looked at the timing, it looks fine the notches still line up with the ones on the cam wheels.

No I didn't squeeze the tappets before I put them in. I didn't think you had to as they are (supposedly) self adjusting right? :-?

[Andy H]

They are self adjusting but.....

When you stop your engine then (depending where it stops) some of the valves will be held open by the camshaft. If the self adjusters got pushed back in by the pressure of the valve springs each time you parked you would have 2 minutes of rattling every time you started the car.

I think that in normal operation they will self adjust when there is light pressure on them but they lock when a greater pressure is applied.

[NiallyV6CDX]

So is it safe to drive and will it adjust its self with some longer drives or do I need to rip it all up and start again?

[Andy H]

It depends how lucky you feel.

Worst cases are

1. backfire through inlet system. If you don't have LPG it probably wont do any permanent damage.
2. Burnt exhaust valve. If you drive any distance with an exhaust valve that isn't sealing properly the hot gases that should be driving the piston down squeeze past the valve and cut a groove in the edge of the valve.

If you do decide to let it cure itself then leaving it idling would be safest. I wouldn't drive it anywhere with one or more valves not closing properly.

[NiallyV6CDX]

Hmm, well I have driven it a couple of miles and it sounds and performs fine, except it sounds really rough on idle. Oddly there doesn't seem to be a problem with the exhaust valves....also it passed it emissions test! (god knows how )

I thought the spring inside the tappets were slightly less stiff than the valve spring and the oil pressure was supposed to fill them out to the optimum size. Am I not correct in thinking this?

[Andy H]

I don't think the spring is very strong at all, I think the oil pressure is more significant.

The problem is that if a cam follower is too long it will always have the full pressure of a valve spring acting on it and will stay locked.

[Marks DTM Calib]

Are the plug leads connected up correctly on the DIS?

Idle will be very slightly rougher with the 3.0 cams?

[NiallyV6CDX]

Yes I have double checked all the electrical bits, only the compression test results and funny noise when cranking seem to be amiss.

So basically, the tappets are designed to adjust outwards, but not back inwards is that what you're saying?

[Andy H]

So basically, the tappets are designed to adjust outwards, but not back inwards is that what you're saying?

Yes

In normal operation the valve clearances might close up fractionally over thousands of miles so there must be some way of coping with this (maybe natural seepage?) but they would never normally be subjected to such a big change in that direction.

[NiallyV6CDX]

I see. Looks like I've gone from having rattly ones to ones that are stuck open 
More trouble then they're worth if you ask me!

Thanks for the advice anyway I'll just run it for a while and hope it improves. If not, well...Alfa 166 here I come.

[Andy H]

This maintenance howto explains how it should be done http://www.omegaowners.com/forum/YaBB.pl?num=1163080380

[NiallyV6CDX]

Does anyone have any experience with lifter treatment fluid? The stuff that you add to the oil.
If so what does it do and is it any good. :-/
If it helps unstick them I might try it.