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Old 02-09-2011, 01:14 PM
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Question 360: how to tell a bad lifter from a worn cam from a burned valve?

I've got low compression in cylinders 6 & 8, leading to a misfire most noticeable at idle. It's gotten bad lately, but it seems to cruise just fine. Pretty sure it's only misfiring at idle because the plug looks pretty good.

Here are my compression readings:

#1 - 160
#2 - 150
#3 - 155
#4 - 160
#5 - 155
#6 - 135
#7 - 155
#8 - 130

When I remove the #8 plug wire from the distributor at idle, it doesn't change the idle behavior at all, so clearly something's wrong there. But compression isn't so bad as to indicate a burned valve, is it?

I'm leaning toward either a bum lifter or worn cam lobe as the culprit, but I'm really curious what you guys think. I thought about using a dial indicator on the ends of the pushrods to compare the travel...good idea, or is there a better way to gauge the condition of the cam?
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Old 02-09-2011, 01:23 PM
FSJ Guy FSJ Guy is offline
 
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I've never done it, but I think a dial indicator would definitely give you a good idea of the condition of the cam lobe.
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Old 02-09-2011, 02:46 PM
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may be as simple as a little carbon on the valve seat as well but an indicator should tell ya if its a cam lobe or lifter though
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Old 02-09-2011, 02:53 PM
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How many miles on the motor?

Mine was doing the same thing. Miss at idle. Once revved up, it ran fine, other than valvetrain noise.

It had both burnt valve and some well worn cam lobes.

If your oil pressure is good, consider having the heads done and doing a new cam, lifters and timing chain. The bottom ends on these AMC V8's will go a long ways. The valvetrain, not so much.

When you have the heads re-done, I recommend having hardened valve seats installed. That will solve the burnt valve issue for good.
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Old 02-09-2011, 03:15 PM
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Thanks for the replies.

There are less than 20K miles on this motor, but it's had this misfire issue for as long as I've owned it.

I did Seafoam through the brake booster recently, hoping to clear out any carbon buildup. It didn't really help. I'm going to try it in the crankcase tonight to see if if will free up any stuck lifters.

I found the cam lobe measurement procedure:



mweath1, you're saying the indicator measurement could also tell a bad lifter? I guess if the measurements are off, I'm looking at new cam/lifters since they're usually replaced together.
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Old 02-09-2011, 03:27 PM
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Lindel Lindel is offline
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Odds are, if it's the cam lobe/lifter, you won't need a dial indicator to tell which one is bad. BTDT. It'll be the one that doesn't move...
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Old 02-09-2011, 03:31 PM
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Quote:
Originally Posted by Lindel
It'll be the one that doesn't move...

Well, I know for sure they're moving. I pulled the valve cover last week just to have a look-see, and I turned the crank while watching the pushrods. They were moving, apparently plenty, but I didn't have a dial on them.

So signs are pointing toward burned valves. Any chance this is a head gasket problem? I don't have any issues with coolant mixing with oil, and it's never overheated.
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Old 02-09-2011, 03:39 PM
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It's possible, it doesn't happen often, but it happens. Either way you'll have to pull the heads to be sure, and to fix it from the sounds of it.

If it does end up being the cam/lifter/pushrod, it still wouldn't be a bad idea to pull the heads and have them freshened up. How many miles are on the engine?
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Old 02-09-2011, 03:44 PM
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Quote:
Originally Posted by elskeptico
When I remove the #8 plug wire from the distributor at idle, it doesn't change the idle behavior at all, so clearly something's wrong there.

Did you check for spark on this #8 wire? Might have a bad wire there...

IMO with 145 psi it should still fire. Not saying you don't still have a compression issue with those two cylinders, but just from my experiences, with those numbers it should still hit on those two.
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Old 02-09-2011, 04:03 PM
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Anyone mention a leak-down test? You can make a leak-down tester from junk and a gauge (maybe even a cheap compression test gauge from Pep Boys or HF). You'll need a compressor, but it does not need to be much of a compressor.

This will identify a leaky valve quickly.

A few links I found - lots more out there -

http://www.motorcycleproject.com/mot.../leakdown.html

https://www.mobiloil.com/USA-English...down_Test.aspx

http://www.carcraft.com/techarticles...ter/index.html
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  #11  
Old 02-09-2011, 04:21 PM
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elskeptico elskeptico is offline
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Only 20K or less on the engine, so I agree that a worn cam is unlikely.

I changed all the plugs, but didn't think to check the wires. Will do that tonight.

I hadn't thought of a leakdown test. I have everything necessary for that, I think. Will do that tonight too.

Thanks again for all the replies.
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Old 02-09-2011, 04:21 PM
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Quote:
Originally Posted by tgreese
Anyone mention a leak-down test? You can make a leak-down tester from junk and a gauge (maybe even a cheap compression test gauge from Pep Boys or HF). You'll need a compressor, but it does not need to be much of a compressor.

This will identify a leaky valve quickly.

A few links I found - lots more out there -

http://www.motorcycleproject.com/mot.../leakdown.html

https://www.mobiloil.com/USA-English...down_Test.aspx

http://www.carcraft.com/techarticles...ter/index.html

I was just getting ready to mention a leak-down test.

If you are convinced there is something catastrophic, you can skip the leak-down, and just hook air pressure to the cylinder and listen for where (or where all) the air is coming out.
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Old 02-09-2011, 04:23 PM
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Quote:
Originally Posted by biotex
Did you check for spark on this #8 wire? Might have a bad wire there....

oh...and I did check for spark on the #8 wire in a lazy way. I put my timing light sensor on it (more toward the distributor), and checked to see if the light would flicker. It did, but I realize now that was a bone-headed way of checking.
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Old 02-09-2011, 04:52 PM
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Do you have a misfire or backfire?

Spark plugs wires on the right plugs?

Since 6 and 8 are next to each other it is very possible you have a leak between them in the head gasket.

Maybe the gasket was put on wrong?

If you put the engine just past 8 on the firing order I believe the valves will be closed in both cyl. 8 should be on power stroke and 6 on compression stroke. Hook up an air line to either one and the compression tester on the other. See if you get air passed between them.

Someone correct me if I am wrong about where he wants the distributor rotor pointed when he tries this.

Second option, maybe better, is take off the valve cover and visually confirm that the 4 valves are closed.
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Old 02-09-2011, 06:03 PM
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It's a miss, not a backfire.

Double-checked wire placement last night, all good.

I'm going to try all kinds of tests with the compressor tonight, will add the 6-8 test to the list. I may end up pulling the valve cover and the rockers off to confirm the valves are closed.
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Old 02-09-2011, 06:24 PM
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That is the best way to do it. Air pressure is going to push the piston to BDC and may open a valve causing a false reading.
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Old 02-09-2011, 09:05 PM
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I am not as familiar with the AMC as I am with my 351C which has pedestal rockers. If you can, take the tension off the rockers and remove the push rods, then you will know they are closed.

We are on the same page here right? 6 and 8 are next to each other. They are the back two cyl's on the passenger side. I am confirming what you are saying as much as I am checking myself.
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Old 02-10-2011, 03:55 PM
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will e- you're correct. I ended up not doing the leakdown test for now because things changed last night.

The major game changer was adding Seafoam to the crankcase. This improved the idle significantly, and doing compression tests on #6 and #8 after the Seafoam revealed that it has come back up to 155 PSI on both of them. So now compression is good in all cylinders.

BUT, it still stumbles/shakes at idle. I'm beginning to think it's normal, but it doesn't feel normal to me. When I'm at a red light, I feel like I'm in some old hot rod with a heavily modified cam.

I'm going to upload a video of it and hopefully someone can set me straight. You can visibly see the engine shaking. Maybe it's not a misfire at all, but that's what it feels/looks like. Video to come...
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Old 02-10-2011, 04:39 PM
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A short clip of my rough idle is up: http://www.youtube.com/watch?v=qoP-0MBee_k

Any ideas? That isn't normal, is it?
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Old 02-10-2011, 04:48 PM
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sounds like a little miss maybe a good tune up and take it out on the highway and put some good hard miles on it( drive it like you stole it) that should clean all the old carbon deposits right up
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