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L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

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Old 11-26-2010, 11:37 PM   #1
cumbercr
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Default What are the symptoms of a worn cam lobe?

Like the title states, how can I determine if a cam lobe is worn without removing the cam?
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Old 11-26-2010, 11:40 PM   #2
Marv02
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The car will run ruff.

Pull the vavle covers and wacth the rockers arms either turn the motor by hand or you can bump the motor with the starter if 1 or a couple arms dont have full movment as the rest most likly you have a cam that went flat on you.
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Old 11-27-2010, 01:38 AM   #3
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measure how far the valve stem depresses into the head as the valve opens. compare with other valves.

pull suspect lifter. if the lifter face or roller is worn, concave, or shows damage or pitting, its pretty certain that the matching cam lobe is a goner. the mrtal surface of the cam lobe is only hardened for a few thousands of an inch. once the softer metal underneath is exposed, failure follows.
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Old 11-27-2010, 08:28 AM   #4
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Quote:
Originally Posted by Marv02 View Post
The car will run ruff.

Pull the vavle covers and wacth the rockers arms either turn the motor by hand or you can bump the motor with the starter if 1 or a couple arms dont have full movment as the rest most likly you have a cam that went flat on you.
... ... the engine will miss , roughly ... will make clickity-clack noises ... and observation will determine which rocker(s) have diminished motion ... its rare on roller cams but not unheard of ... good luck
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Old 11-27-2010, 09:37 AM   #5
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Thanks.

Checking rocker motion is easy enough to do. The engine has been running rough. I have gone through the fuel system, checked ignition and checked compression. I'm running out of ideas. This is a flat tappet cam with approx 50,000 miles on it. Seemed like the next logical thing to check.
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Old 11-27-2010, 10:19 AM   #6
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........ Flat tappet cams are very much at risk to wear due to a dramatic decrease in ZDDP additives in motor oils ... recently , Valvoline Racing Oil has added increased amounts of ZDDP ( zinc-di-dithio-phosphate ) and , of course , there's always STP ... for a while there a lot of enthusiasts were using Shell Rotella Oils that were primarily intended for diesels , but they also have had to reduce ZDDP for emissions reasons ... use the search function for past threads addressing this ...............
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Old 11-27-2010, 10:34 AM   #7
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The rocker adjustment will get loose. It will start clicking, you adjust it. It gets loose again. After a while you will get a miss on that cylinder.
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Old 11-27-2010, 05:29 PM   #8
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With a dial indicator and the valves covers off, you'll be able to tell exactly if there is any wear. You would only be able to see very severe wear by eyeballing the rockers.
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Old 11-27-2010, 09:51 PM   #9
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With a dial indicator and the valves covers off, you'll be able to tell exactly if there is any wear. You would only be able to see very severe wear by eyeballing the rockers.
That's what I was thinking. This cam only has .444 lift. And I don't think it's that far gone. I think Pete had it right. If a lobe is worn enough to effect performance, it should show up with lower compression. I think I'll redo a compression test. The last time I checked compression, I had one cylinder that was out of family but was still in spec according to the FSM. That could have been the beginning of the problem. Thanks guys.
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Old 11-28-2010, 01:37 AM   #10
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Originally Posted by cumbercr View Post
That's what I was thinking. This cam only has .444 lift. And I don't think it's that far gone. I think Pete had it right. If a lobe is worn enough to effect performance, it should show up with lower compression. I think I'll redo a compression test. The last time I checked compression, I had one cylinder that was out of family but was still in spec according to the FSM. That could have been the beginning of the problem. Thanks guys.
If an exhaust cam lobe is going away, you might not notice a drop in compression during a test. In fact, the compression might even rise in that instance.
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Old 11-28-2010, 02:44 AM   #11
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If you have a flat cam on the exhaust lobe, it will pop or backfire thru thru the carb or TB at full throttle.At light throttle it will still run marginal. If the intake lobe is flat, then you would have compression problems, because it can't pull any air in the cylinder making it act like a dead cylinder. But flat cams are alot more common with flat tappet cams than roller cams.
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Old 11-28-2010, 02:55 AM   #12
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agree with above post ^^^ took a few posts to mention it, but a flattened exh lobe backfires like crazy out of the intake manifold.

Also, don't forget to go old school when trying to figure out core mechanical failures..try a vacuum gauge.
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Old 11-28-2010, 02:55 AM
 
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