Hi all, I've read the forum, but this is my first post...

I have a 2001 Suburban with the 5.3 engine. Today, while pulling a small trailer uphill on the interstate, I heard some pinging / ticking, and then my service engine light came on. P0304 cylinder 4 misfire. I cleared this code many times, and it came back every time while driving. However, when idling, code p0300 comes on (random multiple misfire), and NOT p0304.

Symptoms are: rough, noisy idle, rythmic noise consistent with RPM. Not a clanging noise like metal on metal, but more of a "sucking" noise at idle, changing to a faster "ticking" noise at higher RPMs. Seems to be coming from the top center of engine, intake manifold. Also, under heavy load / strain at higher speeds, I get some louder "pinging" or clattering. But power at higher rpms doesn't seem too bad.

Exhaust seems mostly normal with maybe an unburnt fuel smell.

Already checked spark, spark plug, coil, and injector, swapped them with ones from other cylinders.

But I just checked compression. Cylinder 4 is dead. 0.
Now I think I remember cyl. 4's spark plug being wet before. And it looked bad / carbon fouled today too. But the problem today seemed to start all at once.

Is it blown piston ring? Or head gasket? Stuck valves, lifters?
Where should I go next?

Thanks in advance for any help!

 

Welcome to the forum.

Squirt some oil in the cylinder and rerun the compression check. If it's better then probably rings. If it's not, then it could be head gasket or valves.

 

Update, I disabled fuel and spark, pulled all plugs, and did a compression chekck on all cylinders. They are all reading between 125 and 130. Except cylinder 4. Zero, dead.

I squirted some heavy 20w50 oil in and checked it wet. Same thing, zero.

So I guess I'm gonna get some valve cover gaskets and take my valve cover off and take a look.
I've never been in this deep, what should I look for under there?

Do I have to pull the head to know if the valves are seating properly?
How do I know if I have the "collapsed lifter" thing?
What else do I check for?

Thanks...

 

If it's valves or head gasket, you'll need to pull the head.

You could pull the valve cover and check for a stuck valve or broken spring.

 

Well, I found it. I took the valve cover off. Nothing out of the ordinary. I rotated cylinder to top dead center on the compression stroke, and blew compressed air into the spark plug hole. It was blowing out like crazy. Put a small piece of paper in front of the little hole in the intake manifold, and it was blowing in the wind.

So I figured intake valve is sticking open, has gunk on it or whatever.

Took the intake manifold off, loosened the exhaust manifold and got the cylinder head out.

Behold! My piston and rings were in perfect condition, still holding the oil I squirted in hours before at the compression check!

But the intake valve! It was warped?? The valve stem wasn't bent, but the actual part that seats was warped, leaving at least a 16th inch of gap on one side! No wonder no compression. I see no other damage whatsoever. Weird. Maybe I'll post a picture tomorrow.

Went ahead and lapped all the other valves / seats.
Intake valve is on it's way and will be here tomorrow morning. 8 dollars.

8 dollars (plus gaskets, grommets, and bolts). And you don't want to know how much time...
Actually, doing it didn't take that long, it was studying about it and running to the parts store 3 times.

But I figure any shop would have a few hours of diagnostics in it, and then probably want to rebuild the whole engine. Probably saved myself a couple grand?

Hope this helps the next guy.... good night.

 

Glad to hear you got it figured out. Be interesting to know what caused the intake valve to get warped/bent. Was there any evidence of something hitting it?

 

Here it is, next to a new one... Any ideas what could have caused it? I don't see any dings like the piston hit it, it's actually warped TOWARD the piston.... Weird. I replaced it and the gaskets / bolts, put it back together and everything runs great like it's supposed to.

By the way, how much money do you think I saved by not taking it to a shop?

 

Best gas is a foreign object got in there and hit the valve when the piston pushed up against it.

 

The valve looks thicker on the one side, or is that a camera illusion?