ENGINE REBUILDERS PTY LTD
7 BARRETT STREET, FLEMINGTON VIC 3031,
TEL: (03) 93723111, FAX: (03)93723122 ACN: 006 798 492 ABN: 74 006798 492
TECHNICAL NEWSLETTER :- VALVE SIEZURE AT START-UP.
Two types of failure can occur:
1: Insufficient valve guide clearance:
Identified by seizures along the length of the valve stem. Guide material will be friction welded to the stem.
2: Abnormal temperatures :
This is mainly with the exhaust valve and related to very high exhaust gas temperatures. Typical evidence will be blue discoloration of the lower valve stem, seizure at the lower end but not at the upper end of the stem and, at extreme cases, cracked valve faces. Whilst less common, inlet valve seizure can occur where combustion is actually occurring in the intake port. Can be associated with a melted carby venturi and most often with LPG fuel rather than with petrol. In both cases, some anomaly is causing combustion to be going on in the respective ports rather than contained within the combustion chambers.
The following checks are suggested:
l. Confirm that the timing reference is correct.
Errors to find are: Slipped outer rim of the harmonic balancer giving the wrong TDC. Confirm with the piston at TDC and check the mark and the pointer. Has the correct timing covel been fitted? Some covers will interchange but with different pointers.
2. Check the operation of the distributor.
Is it advancing? Is the shaft badly worn? Is the vacuum unit working? If the engine is modified, has the dizzy been regraphed properly? Confirm that the total advance within the dizzy is correct. As a guide, the initial should be around 8 degrees before TDC and no more than another 32 degrees inside the distributor to give a total of 40 degrees. With a good performance engine, this will drop back to a TOTAL advance of around 33 degrees. That is less advance built in the distributor.
3. If an engine management system is in control, has the chip been done properly?
4. Look for vacuum leaks at the manifolds. If one valve has grabbed at the lower end, check the inlet and exhaust manifolds for a vacuum leak close to that cylinder. Additional air (from the vacuum leak) will assist to lift the exhaust temperature at the port.
In summary, what we need to find is a factor that is leading to buming outside of the chamber.
Consider:
1. In the ideal combustion process, valves are closed, the charge has filled the cylinder in an evenly distributed and gaseous condition, with only the one controlled ignition that has been actioned from a system that is in sycro with the position of the piston, and last, but not least, the coolant system is correct. This allows the flame front to start at the spark plug and slowly progress across the chamber as the piston goes over the TDC and successfully convert the combustion heat into energy that pushes the piston down.
2. When any part of this process goes wrong, unburnt gases leave the chamber still buming and exhaust temperatures go through the roof. As a guide, a normal exhaust discharge temperature will be below 700 degrees c. If an anomaly exists, expect 1000 degrees c and this acts as a blow torch on the valves and guides leading to rapid failure.
It follows that cam timing and ignition timing are the main areas to check. However, where the fuel is excessively rich, unburnt gases will still be in the process of burning when the exhaust valve opens, because the engine design allows for only enough time for a correct mixture to burn before opening the exhiust valve. A rich mixture takes longer to completely burn and it follows that exhaust port temperatures will exceed the normal and valve failure can quickly occur.
With inconect cam timing, the exhaust manifold will actually glow red whilst the mechanic has his head under the bonnet trying to establish why the engine isn,t running right.