Best way to test for bad cam lobe?

[Big6ft6]

What is the bet way for me to determine if I have a bad cam lobe? I have low compression on cylinder #6 in my 95 TBI 350 (80 psi ). I'm narrowing down causes, adding oil didn't seem to change compression reading, so this is supposed to point me towards the valve train. My biggest valve train fear is of course worn-down cam lobe. I had to stop for a time and I'm currently planning my next diagnostic.

Can I tell by running the engine without the valve cover and just looking? Is it that noticeable that one rocker would not move as much as others? Should I turn the engine by hand and use a dial indicator on the push-rod end of the valve? What is the best practice?

Thanks!

 

[sweetk30]

best pull intake and look in valley .

but as you said roll engine over and look at rocker movement to be much less than known good other cylinder .

not perfect test but dam good cheepy way .

 

[Big6ft6]

Thanks sweetk30.

Can anyone chime in on the likelihood of a worn cam lobe on a motor with 130k on it? If I recall the cams would wear prematurely if people didn't use assembly lube and/or break the cam in properly resulting in a bad cam pretty early in the engine's life. Does the fact that the engine has made it this far make it less likely to a bad cam lobe or not necessarily? Just curious on people's experiences.

I'm really hoping it is just a bad/burnt valve so I only need to pull one head off to get it fixed. I'm going to do the compressed-air-in-the-cylinder test to see if I hear leaking one place or another...but waiting for a chance to do more diagnostic is driving me nuts...I just want to know.

 

[pblaze725]

You make a good point that if was worn on break-in it probably wouldn't have made it to 130k miles.

I'm thinking carbon buildup on the exhaust valves would keep it slightly open reducing compression.

If you have the equipment you should do that leak down test and see where the air goes.
My money is on the exhaust valves.
You could pull the exhaust and take a peak in there for excessive carbon buildup.

 

[sweetk30]

I purchased a 79 c-10 burban with factory 454 in it 1 time under 70k miles . had 4-5 flat lobes on the cam.

fired up good / ran good / reved up good / but under load on highway pop-pop up threw the intake over 2k rpm .

pulled the intake and sure thing few flat exhaust lobes.

darn thing is still on a stand in my shop . haven't done anything with it in years .

 

[pblaze725]

I purchased a 79 c-10 burban with factory 454 in it 1 time under 70k miles . had 4-5 flat lobes on the cam.

fired up good / ran good / reved up good / but under load on highway pop-pop up threw the intake over 2k rpm .

pulled the intake and sure thing few flat exhaust lobes.

darn thing is still on a stand in my shop . haven't done anything with it in years .

Want me to come haul it off for ya?

 

[sweetk30]

Want me to come haul it off for ya?

right amount of cash . . . .

had great oil psi and no real smoke.

781 heads on it also .

 

[Big6ft6]

Thanks guys, I hope it is a valve. I don't have a true leak down tester that gives a % of leakage, but I can hook my compressor up to my compression tester fitting and pressurize the cylinder, I'm hoping I can either feel or hear air either going out the intake, exhaust, crank breather or see bubbles in the radiator (Bad head gasket).

Here is my plan in order
1) Seafoam the engine through brake booster, hopefully knock of some carbon
2) re-test compression
3) if still bad compression, put cylinder #6 just before TDC (visually watch rockers while I turn engine) and fill with compressed air for listening
4) if I can't hear anything that helps, I'll use a dial indicator to actually measure rocker movement to try and prove bad cam.
5) if I can confirm valve....I'm going to keep truck and fix
6) if I can't tell what is going on....I might just flip the truck.

 

[Stomis]

Just pull the lifters out of the low cylinder. You cant have a flattened came lobe without apparent issues on the lifter.

 

[diesel4me]

I've always done 2 things on an old engine to find a bad cam lobe..

One--rev it up with the air filter off,to the redline--if you hear it start gargling about 3500+ rpms,maybe some backfiring too,then its a bad cam lobe most of the time..

Two--I yank off the valve cover on the bank suspected of having an issue,and 9 times out of ten you can run the engine long enough with oil deflector clips or duct tape over the rocker arm oil holes,after cleaning them off with carb cleaner,and watch the rockers as its idling--it'll be readily apparent which valve isn't getting the same amount of lift as its neighbors are..

You can still have good compression with a bad lobe,but without enough valve lift that cylinder will either starve for fuel/air mix if its an intake valve not opening--or it will pop back thru the intake at high rpms if its the exhaust valve not opening enough to let exhaust out..it builds up in the cylinder and the next time the intake opens,it tries escaping thru the intake..on most of the chevy's I've had with flat cam lobes,its been an exhaust valve lobe that went flat..

A bent push rod can also reduce valve lift--to determine that you may have to remove any suspected of being kinked and roll them on a flat surface or piece of glass and see if they are bananas or straight...sometimes just watching them with it idling you can spot a bent one too..

 

[Big6ft6]

diesel4me, Thanks that is really helpful information. I've only had the truck a week, but I of course put the throttle down while test driving it and I did not notice any high rpm stumbles, burps, or backfires. There is only a slight vibration at idle, feels like a miss would feel, but as rpm increases the engine is smooth.

I also often, but not always, get a little chirping out the tailpipe when it is idling. It sounds like a squeaky belt would sound, but the noise is definitely coming from inside my exhaust tailpipe. Like there is a bird our a mouse squeaking up in my exhaust system.

I'm getting more and more confident it is a burnt, or obstructed exhaust valve that is causing my low compression on cylinder #6.

 

[diesel4me]

A burnt exhaust valve can make some strange sounds out the tail pipe,like "jiffy pop" erratic misfiring sounds...doing a "wet" compression test usually wont show any increase if the valves are leaking--if it were rings it would...could be a piston has some damage around the top ring from pre-ignition causing the leakage too..