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Preamble (Rant)

I was reviewing this question in which the OP presents a repair quotation that includes replacing the "intake camshaft" to fix an engine that had overheated.

To me this recommendation is incredulous. I can understand the need to replace a warped cylinder head, but struggle to see how an overheating problem could lead to the need for a new camshaft.

As an aside, some aftermarket tuners will create new camshaft profiles by grinding down existing OEM ones; if there was a sensible reason to use a new one I'm pretty sure they would be among the first ones to do so.

Questions

What things could happen to a camshaft that warrant its complete replacement?

For the different ways that a camshaft can fail/break, are there any techniques out there to salvage a damaged camshaft?

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up vote 12 down vote accepted

Obviously any physical damage; cracked, broken, bearing surface damage, sheared gear, etc... Any catastrophic failure warrants cam replacement without possibility of repair.

A warped cylinder head could theoretically also warp the cam shaft but i would not just replace it, i would check it first. Place the cam in a set of V blocks. Put a dial indicator on the bearing surface and spin the camshaft. Watch how much the dial indicator deflects. The appropriate engine manual will have the warpage specs.

The more subtle problems arise from lobe wear. A valve that does not open all the way could cause strange problems. At idle everything is fine but a misfire develops under power at high RPM. If the lift of the valve is reduced it won't allow as much air in which is most visible at high RPM. This is difficult to detect. A compression test may not reveal a problem. If every other possible issue is eliminated, pull the valve cover and compare lift of several valves. At worst the lobe is completely round and there is just no air entering or leaving the cylinder. This is easily detectable with a compression test.

Theoretically the worn cam lobe can be reground to regain the lift. The problem with this technique is that not every valve train can compensate for the reduced base circle. Any car with adjustable valves can usually tolerate this. Any car with only hydrolic lifters normally can't.

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These are great points. Typically, I'd expect to see excessive lobe wear on high mileage vehicles. Any other possible reasons for lobe wear? Lack of lubrication, perhaps? – Zaid Jan 3 at 17:02
    
@Zaid anything that can cause a lifter to bind can cause lobe damage. There is nothing specific that is an "aha" moment. Manufacturer defect is a possible cause. A broken retainer on a roller lifter is a surefire way to trash a cam but that falls under catastrophic failure. I had a 4.0L Jeep that trashed a cam at 25k miles but was 10 years old. The owner only used it as a summer driver. The only plausible explanation that i could think of is lack of lubrication form extensive siting. – vini_i Jan 3 at 17:18

What things could happen to a camshaft that warrant its complete replacement?

There are couple of things which I can think of off the top of my head:

  • Performance Gains: The user wants some gains in performance so will replace the camshaft so the engine can breath better (more air in = more power out).
  • Cam failure: A cam can fail for many different reasons
    • Cam lobe failure due to lubrication issues or failed lifter
    • Cam bearing journal failure due to lubrication issues
    • Cam breakage (I've seen this happen due to a warped head)

For the different ways that a camshaft can fail/break, are there any techniques out there to salvage a damaged camshaft?

If a cam is wiped out (cam lobe failure) or bearing journal failure, you can have the area rebuilt (welded) and ground down to spec again. If a cam breaks, you don't have much of a choice but replacement.

For almost any of the more common engines these days which accept performance improvements, there is most likely a camshaft design for you. There are many different aftermarket cam manufacturers out there who will build a camshaft to your specifications. They will also help you figure out exactly what is going on with a different cam shaft which will be right for your application. They can even build to suit. In most cases it's going to be easier/cheaper to buy a new cam than it would be to have your cam shaft ground down with a new profile ... that's most cases.

There are always the odd-ball engines which don't have an aftermarket which you'll need to do some extraordinary means to accomplish your goals. One of the ways which used to be utilized to see performance gains was to regrind a cam. That process would be to utilize a stock cam and regrind the base circle of the cam. The base circle is the where the lifter rides when there is no lift applied to the valve. If you grind this area down, the lift from this portion to the top of the cam lobe becomes a larger distance, and thus you have more lift and more potential air flow going into the cylinder. With so many good aftermarket cam manufacturers out there and the ability to have them grind pretty much whatever you want, this procedure has pretty much fallen from favor. Machining costs are greater than what something from a cam manufacturer would cost, so why go through the hassle.

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Excellent answer as usual, not that I'd expect anything less ;) – Zaid Jan 3 at 16:49
    
Haha ... thank you kind sir ... I think ... :D – Pᴀᴜʟsᴛᴇʀ2 Jan 3 at 16:49

Other answers have well covered how cams are inspected. This case history makes me wonder if the tech who took this engine apart damaged that cam in some way. The quick discount is suspicious. Dealer techs are mostly paid per job and they move fast. This is a low pay job that would not have been wanted in the shop. Explaining why the service dept has to buy an expensive part to a tough, cost conscious boss, is no fun; better to sell it to the customer as failed. This does not happen often, but the temptation is high and the risk low. It does happen. If this is my engine I want that cam so me or my expert can inspect it before proceeding.

I have repaired hundreds of overheated engines. Warped heads are very rare, say 1 in 2000, I have never seen a heat damaged cam. The first damage seen would most likely be oil boiling out of the bearing surfaces causing metal to metal contact damage. Heat damage that was enough to warp the head would almost certainly have destroyed the piston rings also.

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Possibly a seriously over heated cam, maybe from a lack of oil. Serious overheating could do this, but could also allow an oil leak in the feed to the head (possibly leaking internally).

Many years ago I had a motorcycle engine where the loss of an O ring resulted in low oil pressure (but not low enough to trigger the oil pressure warning light). While it killed the crank, the cams were also heavily blued from the lack of lubrication.

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