911SC: Broken intake rocker = no compression

Hello All,

True or False?

Will a broken intake rocker cause the compression test to show "no compression."
In other words, the intake valve on a cylinder is not opening. Will this prevent the cylinder from creating pressure on the upstroke?

Think about it? If not, it looks like I am pulling the head off of my 911SC to find out what is going on.

Thanks!

There are several things that can cause zero compression in a cylinder, including: holed piston, broken rocker, burned valve, bent or stuck open valve, carbon chunk stuck to valve seat, broken valve head, broken or pulled stud causing blowby which has flame cut cylinder and cylinder head severely ... see Bruce Anderson's 'Handbook' p. 113-114 in 2nd ed. or p. 104-105 in 1st ed. If you suspect a broken rocker, you can confirm or eliminate that particular malady just by pulling an intake valve cover and inspecting with eyeballs and fingers, and maybe an inspection mirror and drop lamp!

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Warren Hall
1973 911S Targa

Thanks S_Man!

The rocker arm is broken. This arm controls the intake valve. I cannot see any other problems. Will this cause no compression?

Also, why would a rocker arm break? The only scenario I can imagine is that the cam timing was severely off and the piston kissed the valve and inturn broke the rocker arm. I can't see the cam breaking the rocker arm. What do you think? In that case, I guess the piston would have a big dent in it or possible a hole ($$$$$). I hope that is not the case. I cannot see in the cylinder to check the piston for damage.

There could be a couple of causes for a broken rocker, and cam timing being off could be one of them, but unless one of your tensioners has failed, I have my doubts about that! Have/had you done any 'enthusiastic' or agressive driving prior to finding this problem, and could you have over-reved the engine, even 'a bit' ... say 300-500 rpm?

If not over-reved, there is a possibility that particular intake valve spring has broken, and hence even 'normal' range revs could have caused contact between piston and valve! Fortunately, the cast rockers first introduced with the 'T' engine in 1968 have proven to be a very good 'safety valve' for the valve train, and engines with five, or even six broken rockers have been found with no other damage! Go ahead and pull that rocker shaft out, and find as many of the broken rocker pieces as possible! So, my recommendation would be to get one of the tools that allows pulling the valve spring retainer with the engine in place ... using compressed air with the cylinder at TDC, put the trans in 4th or 5th gear and lock the handbrake securely!!! Then, pull the retainer, carefully, so as not to lose either valve lock! Remove both valve springs, and any extra broken pieces, and inspect carefully! I suspect a broken outer spring, and you will be able to tell, once removing.

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Warren Hall
1973 911S Targa