1998 GMC Bent Valve

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kstanf150

Joined
Aug 4, 2011
Messages
2,572
Hey Guys, just want to share a story with ya.
The company I work for owns a ‘98 GMC
3/4 ton van, bought new and now has 180K miles
The van has served us well with very few issues.
Three days ago while driving it at highway speed the engine suddenly developed a bad miss and when stopped wouldn’t idle. So, fast forward
Apparently due to high oil usage over the last 40K miles or so the engine has accumulated a lot of carbon build up and a piece broke loose and stuck the #5 cylinder exhaust valve open and the piston hit the valve and bent the valve preventing it from closing . So if anyone has an engine using more oil than it should
May want to do some checking and de-carbonizing before your like our van and having two heads serviced or maybe even worse.
Thanks
 
This is a good reality check. The scenario you just described is how many engines end up burning valves, or have piston to valve contact.
 
Sure it's not a Valve Guide too tight? Gen 1 Vortec's were notorious for this & usually caused misfires under heavy load. But bent valve's also happened. And it was usually #5 but could also effect the other center cylinders 3, 4, or 6.

If you're having the heads rebuilt.....make sure to have the machine shop hone the guides for 0.002" clearance regardless whether this was the cause or not!!
 
clinebarger said:
Sure it's not a Valve Guide too tight? Gen 1 Vortec's were notorious for this & usually caused misfires under heavy load. But bent valve's also happened. And it was usually #5 but could also effect the other center cylinders 3, 4, or 6.

If you're having the heads rebuilt.....make sure to have the machine shop hone the guides for 0.002" clearance regardless whether this was the cause or not!!
Click to expand...
Hello sir
Yeah it’s a bent valve. Got the head off today and disassembled. The piston has a small scare from hitting the valve and carbon build up is a lot.
Thanks for the info, I’ll pass that on to the machine shop guys to make sure they allow for .002 clearance.
I interested in seeing if the head job with new valve seals will help with the oil consumption.
we’ll see soon
 
I'm not disputing the bent valve....A bent valve is a bent valve. But failure analysis is a lot harder. It would take a huge chunk of carbon to hang to valve open enough to cause interference with the piston. L31's (Assuming this is a L31 5.7L) have generous valve reliefs. So the piece of carbon would have to hang the valve at it's max opening.
That's a lot of stuff lining up just right.
 
I totally understand what your saying.
The valve spring is not broken the keepers were still in place as was the retainer. Just a bent valve and I will say the mechanic that’s repairing the engine is somewhat speculating that it’s carbon build up. So your correct that it may turn out to be something else that caused the valve to get bent
 
clinebarger said:
Sure it's not a Valve Guide too tight? Gen 1 Vortec's were notorious for this & usually caused misfires under heavy load. But bent valve's also happened. And it was usually #5 but could also effect the other center cylinders 3, 4, or 6.

If you're having the heads rebuilt.....make sure to have the machine shop hone the guides for 0.002" clearance regardless whether this was the cause or not!!
Click to expand...
Can we have a drum roll please for Mr. Cline 👍👍👍
Bent was due to more than one issue
1. Valve guide clearance
2. Leaking intake gasket at #5 cylinder
3. A very nasty clogged radiator ( low flow causing heat issues)
4. Fuel pump and dirty injectors

But the main thing looks like heat played a roll in the valve to stick open. Causing a lean burn issue which lead to excessive heat. Both heads had cracked in the center bolt hole that went all the way into the water jacket causing coolant lose. Good news is the block cylinder bores looked and checked out to be in good shape
So two heads a radiator a water pump and a fuel pump and injectors cleaned were back on the road
Thanks Cline you were spot on again 👍👍
 
clinebarger said:
Sure it's not a Valve Guide too tight? Gen 1 Vortec's were notorious for this & usually caused misfires under heavy load. But bent valve's also happened. And it was usually #5 but could also effect the other center cylinders 3, 4, or 6.

If you're having the heads rebuilt.....make sure to have the machine shop hone the guides for 0.002" clearance regardless whether this was the cause or not!!
Click to expand...
Good information.

I was unaware of this issue and was surprised when a shop told my friend that his 98 K1500 needed heads a month or so ago to fix a misfire that progressed to not even running. My friend had exhausted throwing parts at it including upgrading the fuel injection though. The truck only has 150k and is in stunning condition, but is seldom driven. It's back in business with two replacement reman heads and running well.
 
kstanf150 said:
Can we have a drum roll please for Mr. Cline 👍👍👍
Bent was due to more than one issue
1. Valve guide clearance
2. Leaking intake gasket at #5 cylinder
3. A very nasty clogged radiator ( low flow causing heat issues)
4. Fuel pump and dirty injectors

But the main thing looks like heat played a roll in the valve to stick open. Causing a lean burn issue which lead to excessive heat. Both heads had cracked in the center bolt hole that went all the way into the water jacket causing coolant lose. Good news is the block cylinder bores looked and checked out to be in good shape
So two heads a radiator a water pump and a fuel pump and injectors cleaned were back on the road
Thanks Cline you were spot on again 👍👍
Click to expand...
Those "Vortec" castings don't like to be hot, The center cylinders naturally run hotter because the siamesed exhaust ports. Glad it got sorted!!
 
01rangerxl said:
Good information.

I was unaware of this issue and was surprised when a shop told my friend that his 98 K1500 needed heads a month or so ago to fix a misfire that progressed to not even running. My friend had exhausted throwing parts at it including upgrading the fuel injection though. The truck only has 150k and is in stunning condition, but is seldom driven. It's back in business with two replacement reman heads and running well.
Click to expand...
I've not been aware of this issue either. I've owned my 98 K1500 for almost 23 years and have never had any mechanical issues with it, just several intake manifold gasket replacements. Mine only has about 140,000 miles on it and has never gotten hot. Hopefully I don't run into this.

A friend of mine from work had the same truck as mine with the same engine and he had over 400,000 miles on his with no mechanical issues.
 
My father in laws 2005 4.3 has this problem since it was new and got progressively worse until I fixed it at about 70,000 miles. It had very tight valve guides. Drove it it for another 80-90000 miles then traded it in with never an issue again.
 
todays build quality is prolly better than in 72 when my NEW 307 nova was fowling the plugs at a few thou. dont know what the warranty fix was but as time went on it was an oil burner using a qt on 500 miles, a real POS GM engine!!!
 
The Vortec's are well known for their "tight valve stem" issues; in fact, GM sent out a TSB describing the issue and repair. Their solution was a complete rebuild of the head.
An alternate solution for those who are experiencing the misfire under load but don't want to pull the heads is to try this:
Run a can of BG's EPR through the crankcase before the next oil change. This should help loosen some of the carbon deposits on the valve stems.
The Vortec engines are also known for their weak valve springs. Replace the springs with GM beehive LS2 springs. These are cheap and sometimes take-offs can be had for free if you're friends with any GM speed shop guys. The only other thing you'd need to buy is aftermarket retainers made by Comp Cams. Stock valve locks can also be used with them.
All in all it's an easy job, and pretty much guaranteed to fix it for a fraction of the cost and time of pulling the heads. Make sure to use a good synthetic oil on refill.
 
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