How does a mechanic treat non interference engine? (vehicles, valve, exhaust)
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I am looking for a guidance and like to pick up your insight on non interference engines.
Non interference engine is designed with enough clearance between pistons and valves to allow the crankshaft to rotate (pistons still moving) while the camshaft stays in one position (Several valves fully open). If this condition happen normally, no internal engine damage will result.
I know that you have worked on cars with Non interference engine with snapped/broken timing belt.
Did you find bent valves (piston hitting valve conditions) on such vehicles?
How did you make sure, that valves were not bent on such vehicles?
Did ASE certification help to work/diagnose on Non interference engines?
List of interference engines will help you to tell if your particular one is or not. No certification required.
From that on, valves are not bent on non-interference one and MAY be bent on interference one.
You need to pull head off, release valve springs and check for normal movement/obstructions.
Don't over think this. Not a spaceship.
a non-interference engine by design will not have the pistons hit the valves should the camshaft not rotate in time with the crankshaft. So there will be no bent valves from this cause in the engine.
After restoring the camshaft drive, a compression test will reveal if there are any bent valves. As well, a visual inspection to observe the normal movement of valve train opening/closing would be a means of checking the engine mechanical condition, usually seen by removal of a valve train cover. If a tech had a concern about any given valve, another means of checking normal valve travel would be to use a dial indicator to measure that travel as the camshaft was turned; a bent valve will not have the normal range of travel found on most of the other valves.
If the concern was to verify no valve damage before repairing the drive system, there's two ways that a tech could do so:
1) a leak-down test on each cylinder by rotating the camshaft/valve train to a closed position for each cylinder. Ideally, you'd rotate the crankshaft to TDC for the cylinder being tested, but for the purposes of verifying a bent valve, you could do so with the piston at BDC. A bent valve would be revealed by a low numerical leak down reading and generally you can hear the airflow in the intake manifold or out the exhaust.
2) some engines might permit a visual inspection of the valves inside the combustion chamber by use of a borescope through a sparkplug hole. Again, you'd rotate the camshaft to a position where the valves should be closed on the cylinder being inspected.
Of course, if the engine is test run after a repair to the valve train and is performing normally, that's further indication of no mechanical damage to the valves.
I have never seen an ASE certificate perform any diagnostic or repair procedure on a vehicle. There's good and bad techs who have these certificates, and good and bad techs who do not. The repair question you've posed is well within the grasp of techs with a basic mechanical knowledge of how an engine works.
What I have always wondered is who thought interference engine design was a good idea?
Better performance? A smaller diameter valve or 4 valves, in a smaller diameter cylinder has to open further along with a smaller squash area when the piston is at TDC. Engineers aren't looking further down the road that if you don't replace the timing chain you'll destroy the engine when it breaks, they're just looking at that it achieves their performance goals. Then they recommend replacing timing components later. They don't worry about repairing cars, just assembling them.
As for the OP back in the late 60's early 70's Pontiac used a nylon camshaft gear, (nice and quite) after 30K miles or so the chain would stretch more than the nylon gear could take, so it would strip. I worked in a gas station across from a supermarket, people would come over that the car wouldn't start. They actually acted like a flooded engine. So in those days with carburetors you'd put a screwdriver in the carb to hold the choke open and try to start it. Well they'd backfire, flames would come out of the carb. A compression test would confirm, wouldn't even have to check all the cylinders, just 3 or 4.
We'd replace the timing chain and never had a problem with a bent valve.
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