Knocking '78 SC engine -- compression OK?

[hilandscott]

After the engine oil temp reached midpoint in the summer heat, the car started to stall out, then smoke, with a loud knocking noise on the right side of the engine.

The compression results:
Cyl 1 -- 125psi
Cyl 2 -- 125psi
Cyl 3 -- 130psi
Cyl 4 -- 130psi
Cyl 5 -- 120psi
Cyl 6 -- 135psi

In addition to being the lowest, Cyl 5 was slow in getting to the maximum, stalling on the gage pressure with turns of the engine (valves?)

After removing the valve covers I can find no loose parts or obvious signs of failure. Aside from being on the low side of compression, the cylinders seem okay. I am thinking that the problem has to do with the valves in Cyl 5.

Any suggestions?

Thanks!

[Early_S_Man]

Did you check all 12 lower Allen (barrel) nuts for tightness with a 10 mm Allen drive bit?

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Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[Superman]

Yeah, and cross your fingers when you check those twelve (12) 'barrel' nuts. But I'm not sure a loose or broken head stud would cause the stalling and smoking and knocking. Perhaps.

A leakdown test would isolate a bent or burned valve if you think that's the problem, and of course I favor making sure the ignition system is working properly as well as the fuel injection before you start blaming things on internal engine parts.



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'83 SC

[hilandscott]

Thanks for the replies-- here is the latest:

With the new 10mm driver and the torque wrench, I checked the allen head "barrel bolts" from the exhaust valve area as suggested. I found no loose studs (six bolts in all on the right bank), and all were torqued to at least 33Nm (24ftlb) (on the side that showed the lowest compression and was the source of the noise). I can still find nothing obviously wrong. Any suggestions?

By the way, I did notice the handywork of some genius, who got the bright idea to repaint the valve covers inside and out. The outside paint layers are flaking off, while the inside of the covers are loosing paint over time with exposure to the oil. Needless to say that will all be cleaned up before the reinstall.

Still crossing my fingers on this one,

John