Ever wonder why engines fail? Engine builder Marvin McAfee of MCE Engines/Los Angeles has answers.
Marvin McAfee of MCE Engines/Los Angeles teaches us something about engine failure. It isn't always for the reasons you think.
Engines don't always fail due to a thrown rod, busted piston, broken rocker arm, valve spring/keeper/retainer failure, broken timing set, or snapped oil pump shaft. When engines auger in as they sometimes do, it's typically a chain reaction. The ultimate and obvious failure isn't what started the failure. Never underestimate your engine's potential for failure even when things looks good at the instrument panel and spark plug readout.
When you knock a blown engine down, examine everything. Then, ask yourself what started the failure.
1: Engine failure due to low oil pressure? Why?
2: Metal trash in the oil pan? Why?
3: Broken piston rings and lands. Why?
4: Snapped oil pump shaft? Why?
5: Broken valve spring and valve into a cylinder? Why?
6: Overheat and engine failure? Why?
7: Blown head gasket? Why?
8: Snapped timing set and bent valves? Again why?
An engine's ultimate failure isn't always the primary reason why it failed. For example - a baffle screw comes out of a valve cover and meets a spinning crankshaft en route to the oil pan. Crank tears it up and fragments fall to the pan. Fragments go through the pick-up and cease the oil pump. Oil pump shaft snaps. At the same time, the cam ceases and breaks the timing set. While all of this is happening, oil pressure to main, rod, and cam bearings goes to zero. To add insult to injury, we're at 6500 rpm under hard acceleration. Rod bearing journals superheat, oil breaks down, and we have rod failure. Although it looks like we threw the rod first, that was the last thing in a chain of catastrophic events.
Oil wedge breakdown happens when journal temperatures increase and oil flow across the journal slows down. Remember - oil isn't just a lubricant, it is also a coolant. Regular engine oil begins to break down at 260 degrees F. Synthetic engine oil begins to break down at 300 degrees F. Average rod/main/cam bearing journal temperature is around 400 degrees F. When oil moves across the journal quickly while maintaining a wedge, it carries heat away from that journal. It is when we drive journal temperatures skyward with tight clearances or extreme operating conditions that we cause oil break down and ultimately journal/bearing overheat and failure.
If you are lulled into thinking a brief overheat doesn't do engine damage, do not kid yourself. It does do engine damage by causing oil breakdown. When you rebuild, double and triple check bearing clearances, side clearances, and endplay. And remember, as engines get hotter, clearances get tighter.
Ignition timing and fuel mixture directly affect your potential for engine success or failure. A lean fuel mixture and hard acceleration will cause detonation (spark knock) in a nanosecond. So will excessive ignition timing. Both mixture and timing drive combustion temperatures up. Do a spark plug reading and check ignition timing with a timing light just to be sure.
Engine failure must be evaulated carefully to understand why failure happened - and how to prevent it from happing again.
For more information contact Marvin McAfee at MCE Engines/Los Angeles 323/731-0462.