Rough Running Engine

I have a 1997 GMC pickup with the 5.7L V-8 Vortek engine. The problem I have is that it has a bad miss/shudder down the road and rough idle. I recently pulled all the plugs to check the condition and see if
there was anything unusual. The spark plug on the #6 cylinder is covered with carbon. I also noticed that I have been lossing a little bit of anti-freez, about a half gallon every few months. I think I may have a burned valve on the #6 cylinder?? Also maybe a leaky intake gasket possibly both on the front and rear. Any ifon that anybody may have would be great or any ideas as to weather my diagnosis is correct on the burned valve. Thank you very much
Nick
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NickHanson wrote:

The first thing I would do is get the intake gasket replaced. Not only do they leak antifreeze on these engines, but I've seen a number of them get distorted and they will go out of shape and develop a vacuum leak at a cylinder or two. The vacuum leak will be inside the engine, so spraying stuff around the exterior of the intake will not find the leak. Anyway, if you are using antifreeze, the gasket needs to be replaced. I'd do that first and then see how she runs, she may smarten right up.
Ian
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shiden_kai wrote:

While you have the intake manifold off, do a thorough cleaning of the EGR valve & passages - mine had 100% blocked passages which, when cleaned, smoothed the motor immensely.
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"Stephen Young1" wrote: > shiden_kai wrote: > > NickHanson wrote: > > > > > &nbsp;>>I have a 1997 GMC pickup with the 5.7L V-8 Vortek > engine. The problem > &nbsp;>>I have is that it has a bad miss/shudder down the road > and rough idle. > &nbsp;>>I recently pulled all the plugs to check the condition > and see if > &nbsp;>>there was anything unusual. The spark plug on the #6 > cylinder is > &nbsp;>>covered with carbon. I also noticed that I have been > lossing a little > &nbsp;>>bit of anti-freez, about a half gallon every few > months. I think I > &nbsp;>>may have a burned valve on the #6 cylinder?? Also > maybe a leaky > &nbsp;>>intake gasket possibly both on the front and rear. > Any ifon that > &nbsp;>>anybody may have would be great or any ideas as to > weather my > &nbsp;>>diagnosis is correct on the burned valve. Thank you > very much > > > > > > The first thing I would do is get the intake gasket > replaced. Not > > only do they leak antifreeze on these engines, but I've seen > a number > > of them get distorted and they will go out of shape and > develop > > a vacuum leak at a cylinder or two. The vacuum leak will be > inside > > the engine, so spraying stuff around the exterior of the > intake will > > not find the leak. Anyway, if you are using antifreeze, the > gasket > > needs to be replaced. I'd do that first and then see how > she runs, > > she may smarten right up. > > > > Ian > > > > > While you have the intake manifold off, do a thorough cleaning > of the EGR valve & passages - > mine had 100% blocked passages which, when cleaned, smoothed > the motor immensely.
Thanks for the suggestions. I will do the manifold gasket first and do a good cleaning inside when I have apart. But I can’t see that causeing #6 spark plug to be covered with Carbon?? Maybe I am wrong but I think there are further problems. Last night I unplugged my mass airflow sensor and it did not take the roughness away but did give me all my power back and improved my gas milage quite a bit too. Anything else you guys can think of would be great!!
Thank you
Nick Hanson
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NickHanson wrote:

It's not always this way....but often a vacuum leak internally on one cylinder will cause that cylinder to develop carbon on the plug. Why? Because that cylinder can end up sucking a lot of oil vapour from inside the engine. This used to be very common on the older small blocks when the EGR valve would burn the intake gasket in the #6 and #8 cylinder location. Those two cylinders would exhibit carboned up plugs. Your engine is a different design, but does have intake manifold gasket problems too.
Ian
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"shiden_kai1" wrote: > NickHanson wrote: > > > Thanks for the suggestions. I will do the manifold gasket > first and > > do a good cleaning inside when I have apart. But I can't > see that > > causing #6 spark plug to be covered with Carbon?? Maybe I > am wrong > > but I think there are further problems. Last night I > unplugged my > > mass airflow sensor and it did not take the roughness away > but did > > give me all my power back and improved my gas mileage quite > a bit too. > > Anything else you guys can think of would be great!! > > It's not always this way....but often a vacuum leak internally > on one > cylinder will cause that cylinder to develop carbon on the > plug. Why? > Because that cylinder can end up sucking a lot of oil vapour > from > inside the engine. This used to be very common on the older > small > blocks when the EGR valve would burn the intake gasket in the > #6 and #8 cylinder location. Those two cylinders would > exhibit > carboned up plugs. Your engine is a different design, but > does > have intake manifold gasket problems too. > > Ian
Great, thanky you very much for the great info. It was my suspision all along that this was the problem but was nice to have people here to confirm the issue. Originally I was told Fuel PRessure Regulator and I knew that was wrong and was a very expensive part on the truck to replace and not have it fix the problem. That diagnosis came from a mechanic too so I guess I will not ask him again in the future.
nick
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NickHanson wrote:

Hang on....not so fast. You haven't pulled the intake manifold yet, and figured out what is wrong. The other mechanic's diagnosis could be correct too, there are problems with the pressure regulators leaking inside the upper plenum. I'm merely giving a "suggestion" which is about all that we can do, since I'm not present to actually diagnose the problem. Don't write that other guy off too fast.
Ian
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Get a new mechanic as soon as possible!
He'll be glad you did.
Bob
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