Symptoms of a burnt exhuast valve?

[s10mk]

Hello everyone,
What are the driving symptoms of a burnt exhuast valve? My truck idles and drives very well, and it still has good mid-range power. But recently it developed a high-load/ high rpm miss. I did a compression test, and cylinder 5 has 75 psi of cranking pressure, cylinder 2 has 95 psi of cranking pressure. The rest of the cylinders all have 120 psi while cranking. I then did a leak down test, and cylinder 5 is clearly leaking out of the exhuast valve. While cylinder 2 appears to be leaking past the rings, I also squirted some oil in number 2 and compression jumped from 95 pounds to 115.

I guess my question is, what would these issues "feel like" while driving?
Thank you!

[NoNeck]

The symptoms you’re likely to notice the most with a burnt exhaust valve is a loss of power from the engine and an increase of fuel consumption as the engine is run harder to do the work expected of it. Other symptoms will be dependent upon how badly burned the valve is.

When the edges of an exhaust valve burn, the combustion chamber no longer seals and not only do you lose compression in that cylinder, unburnt hydrocarbons will be exiting before they can be ignited.

Running an engine low on oil will often burn one or more valves before the main bearings start to pound. Otherwise, a lean fuel mixture can cause this, too.

On vehicles with a full exhaust system, the sound can be hard to detect. A cylinder leakage test will pinpoint it fairly well. Your pressure loss will be fairly high and you’ll hear the pressurized air escaping through the exhaust system, rather than through the intake or the crankcase. If you hook up a vacuum gauge, you may notice a steady loss of vacuum at idle if a valve is burnt or sticking.

You’ll be looking at a valve job to correct it, or else swap out the cylinder head altogether.

POP

[Wrenchbender Ret]

A burnt valve will be most notable at idle or low RPM. It will have a miss that seems to go away at higher rpm. If it is an auto trans, it will be very noticeable idling in drive. It will have a bad shake.

[s10mk]

Originally Posted by Wrenchbender Ret

A burnt valve will be most notable at idle or low RPM. It will have a miss that seems to go away at higher rpm. If it is an auto trans, it will be very noticeable idling in drive. It will have a bad shake.

Huh, maybe my driveability issue is coming from somewhere else, becuase my truck sure does seem to idle and run good at low rpm. I don't have a problem until higher load/rpm

[BCOWANWHEELS]

More likely a sunken valve seat

[special-K]

I think this could be unrelated. Your valve is probably not burned that bad yet, but you discovered it early by your test. Could be something like a lifter or rocker thing. Ignition, like plug wire?

[rsavage]

Compression is fairly low on all cylinders. Maybe time for a rebuild of both block and heads.

[s10mk]

Originally Posted by special-K

I think this could be unrelated. Your valve is probably not burned that bad yet, but you discovered it early by your test. Could be something like a lifter or rocker thing. Ignition, like plug wire?

That's what I thought too, it actually feels like an ignition problem. But everything seems to check out ok. The only thing that I found suspect, was that the + side of my coil has 10.5 volts while the engine is running. But I don't think that would cause my problem.

I have a wideband o2 sensor, in going to put it in my exhuast just to be sure I'm not running out of fuel.

[s10mk]

Originally Posted by rsavage

Compression is fairly low on all cylinders. Maybe time for a rebuild of both block and heads.

That's actually my plan for this winter, I'm gonna pull the engine and give it a good going over. Rings, bearings, valve job, etc

[s10mk]

Originally Posted by special-K

I think this could be unrelated. Your valve is probably not burned that bad yet, but you discovered it early by your test. Could be something like a lifter or rocker thing. Ignition, like plug wire?

Hello gents, I do believe I figured out why my truck has been running funny as described above. The other day, it started running really poorly, and it had fouled a set of plugs. From weak spark. I looked over my entire ignition system, and everything looked really good. But with my multimeter, I found that the yellow positive wire that powers my coil(resistance wire) has a short too ground. I haven't found the short yet, but with the wire removed from the coil, it has continuity to ground. So it looks like that circuit wasn't able to supply enough current to my coil on account of the short.

[BCOWANWHEELS]

that points resistor wire is supposed to cut voltage

[s10mk]

Originally Posted by BCOWANWHEELS

that points resistor wire is supposed to cut voltage

I understand that part, the resistance wire is supposed to lower voltage to around 9 volts. But my resistance wire is shorted to ground, I don't think that's right

[BCOWANWHEELS]

does it drain the battery

[s10mk]

Originally Posted by BCOWANWHEELS

does it drain the battery

Nope, not yet. But I'm looking for a short or something stuck on

[Zone47]

yeah, a slight miss at idle is the first sign.

Disconnect the coil wire and crank it. You might hear a slight rise in cranking speed when it gets to that cylinder with the lower compression. That is another test.

*edit*

Ok never mind!

[s10mk]

I wanted to update my thread, as it turns out, the coil wire wasn't the problem. I did some checking and I have two cam lobes that are flat, one intake lobe on #4 and one exhuast lobe on #2 cylinder. So a new cam will be on the way shortly

[RustyBucket]

A new cam won't fix 75lb. cranking compression on #5 if the lobes on #'s 2 & 4 are wiped. Have you disassembled the engine & seen the cam or determine this from the rocker arm travel? If the cam is flat the metal went someplace.

[s10mk]

Originally Posted by RustyBucket

A new cam won't fix 75lb. cranking compression on #5 if the lobes on #'s 2 & 4 are wiped. Have you disassembled the engine & seen the cam or determine this from the rocker arm travel? If the cam is flat the metal went someplace.

Cylinder #5 has a separate problem, it failed a leakdown test which appears to be from a burnt exhuast valve. In #5s case, the exhuast valve won't seal, and air whistles right past the valve during a leakdown test.

I diagnosed the flat lobes on 2 and 4 by watching rocker arm travel, I actually used a dial indicator on them but it was really unnecessary. They only move about .050 of an inch at the pushrod. So I think after all, the miss I felt was from number #2 cylinders exhuast valve not opening and backfiring through the intake.

[Tquillen]

It was stated earlier that a burned exhaust valve can be caused by an overly lean condition, and this is absolutely correct. Also, it can be caused by retarded timing; if there's not enough ignition lead, the mixture in the combustion chamber continues to burn as the exhaust valve is opening, and the valve overheats. If the lifter preload is set too tight, the same thing can happen...the exhaust valve hangs open, the valve edge overheats, and the valve fails. This is what it will eventually look like.

[rsavage]

If you wiped lobes, there was a lot of metal that went through the engine. Pull your oil filter and cut it in half so you can look at the element. If there are metal particles, there is a good chance that they have damaged your crank and bearing surfaces. Probably worth your while to pull the engine to at least look at the bottom end and the cylinder bores (you are going to have to remove a head to have it repaired and would normally do both heads at the same time). Sounds to me like this could be a more extensive than a cam and lifter replacement. Good luck with whatever you do.

[s10mk]

Originally Posted by rsavage

If you wiped lobes, there was a lot of metal that went through the engine. Pull your oil filter and cut it in half so you can look at the element. If there are metal particles, there is a good chance that they have damaged your crank and bearing surfaces. Probably worth your while to pull the engine to at least look at the bottom end and the cylinder bores (you are going to have to remove a head to have it repaired and would normally do both heads at the same time). Sounds to me like this could be a more extensive than a cam and lifter replacement. Good luck with whatever you do.

Thank you, you are absolutely right, its looking like this old engine has seen better days. And considering that I really don't want to put much money into my old heads, since they have the small 1.72/ 1.50 valves. I'm quickly moving towards just buying a new gm crate engine.

[s10mk]

Originally Posted by rsavage

If you wiped lobes, there was a lot of metal that went through the engine. Pull your oil filter and cut it in half so you can look at the element. If there are metal particles, there is a good chance that they have damaged your crank and bearing surfaces. Probably worth your while to pull the engine to at least look at the bottom end and the cylinder bores (you are going to have to remove a head to have it repaired and would normally do both heads at the same time). Sounds to me like this could be a more extensive than a cam and lifter replacement. Good luck with whatever you do.

Oh, and ill go ahead and check the filter, but I changed the oil and filter about 500 miles ago, right when this problem first crept up. So it might not tell me much, or I might get a false indication from the filter, but I'll go ahead and give it a look

[mike16]

two things;

learn how to use a vacuum guage. nobody knows how any more but it was an important tool back when our trucks were new...ish. Older chiltons manuals for 40's and 50's vehicles usually have a good instructional explaination on how they are used, read and understood.


when you go shopping for a vacuum guage get the one with the biggest dial you can find.

[jabborabbo]

Originally Posted by Tquillen

It was stated earlier that a burned exhaust valve can be caused by an overly lean condition, and this is absolutely correct. Also, it can be caused by retarded timing; if there's not enough ignition lead, the mixture in the combustion chamber continues to burn as the exhaust valve is opening, and the valve overheats. If the lifter preload is set too tight, the same thing can happen...the exhaust valve hangs open, the valve edge overheats, and the valve fails. This is what it will eventually look like.

Did that valve come out of a “running” (obviously not running well!) engine?????

[Tquillen]

Originally Posted by jabborabbo

Did that valve come out of a “running” (obviously not running well!) engine?????

It did. It idled low, and sounded terrible, but we were sorta committed to making the run. Race car on methanol; the valve got hung open slightly from a pushrod problem on a previous run (long story). Interestingly, I have pulled valves out of daily driven Chevrolet small blocks that looked absolutely identical to this. The exhaust note out of the tailpipe will have a very distinctive sound; once you hear it, you won't forget it! LOL