I recently had a shop repair a head gasket on my car. When they were done with the repair they called to tell me the head gasket repair went well & fixed my car. But, when they test drove the car after the repair it developed a miss. They tore the car back down & now say it has a “sheared cam lobe” also. The car had no miss before taking it to the shop, it was just getting hot. What would cause, as the shop put it, this “mysterious” cam lobe shear? Thank you for any help.
Usually ‘cam lobe’ problems are where the nose of the lobe becomes worn off. You may not have noticed the miss because of the problem with the head gasket. The surface of the cam lobes and the lifters are specially treated to resist wear. Once this heat treatment is worn through, the cam lobe and lifter wear fairly quickly. One cause of this can be lack of adequate quantity of oil, poor oil quality, or extended oil change intervals. Another cause I just considered is that the head gasket might have been leaking coolant into the oil. If that was the case, the lowered lubricity of the oil might have caused the accellerated wear.
Unless the cam lobe actually ‘sheared’ off, it is unlikely the head gasket repair caused the damage. If the ‘shear’ is due to a cracked cam shaft, head warpage might have caused this. But head warpage would probably have been the result of your original problem.
The only shear plane on a cam lobe is through the camshaft so I’m guessing the sheared cam lobe translates to snapped camshaft.
So what can cause it ?
Lack of lubrication, incorrectly timed engine (this would cause piston collision) and could snap the shaft, manufacturing defect - possible though doubtful.
Which engine is this ?
Another possibility is that the metal in the cam lobe could have become crystallized due to excessive heat from the overheating and just gave up; especially if the overheating was severe and/or chronic.
Saw a Ford some years back that suffered some chronic overheating and the entire camshaft snapped in half. A fluke type of thing but in the auto world anything weird can happen.
Assuming this is the 4 cylinder in your case.
I had an Olds Acheiva come in once where the owner had thought the transmission had failed. When I went to remove the flex plate to torque converter hardware by turning on the crankshaft bolt, the engine turned but the flex plate didn’t. It turned out that the crankshaft had snapped in two just at the rear main bearing. The engine was never overheated nor run low on oil. When I got the engine out, the flex plate would rotate but the engine would not, and you could rotate the engine but the flex plate wouldn’t rotate. I attributed it to material failure.