Possible Drivability problem with 98 Chevy Venture

My 98 venture has 100,000 miles on it and I am just getting ready to replace the plugs - they have never been replaced and according to the maintenance schedule in my owners manual they are not due to be
replaced until 100,000 miles - anyway generally speaking it has been running fine - the idle is getting a bit rough but I figure the plugs will correct that
Anyway one thing I am noticing is that the end of the tail pipe is black with carbon - this wouldnt bother me that much except that I have noticed on other vans of this model and vintage that this is not something that I see on them.
My knowledge of diagnosing and fixing driveability problems ended with the last car I owned that had a carberator and no computer - ( a 79 Firebird) - I am just wondering what the possible causes of this situation would be
Thanks in advance for any help
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Your black tailpipe suggests carbon from unburned fuel. Your planned tune-up may well take care of it. H
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Hairy wrote:

Those engines have a chronic intake manifold gasket failure problem, which often lets antifreeze get into the oil and/or the combustion chambers.
As a basic precaution, pull and oil sample and have it analyzed for traces of coolant. You can do so for $20 using www.blackstone-labs.com or any of a number of other oil analysis companies.
As for replacing the plugs. Good idea, and the back ones are a PITA to get to. Have a look at a service manual first as it isn't as easy as on most vehicles.
John
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I have the same year van also noticed the tailpipe carbon. I had the check engine light come on several times, then go off again a few trips later. Changing the plugs is a good idea, I changed mine already. A bit of advice, if you're going to change them yourself, be prepared for a bit of a battle. it's not that hard to do, just takes some time. I would also recommend changing the wires while your at it. As for me, the carbon is still there,next step is to replace the oxygen sensor. As soon as I can figure out which one should be changed.

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The replacement of spark plugs it is relative can't go by owners manual. Firts I will go for will be the Air filter, Fuel Filter and Pcv valve. and a good induction service(Injector cleaner service) then a good road test, and take it from there if spark plug will be the cause your vehicle will be miss firing and a check engine light will pup-up by now, save the money on the spark plugs. GM Certified Technician.

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