It has gotten to where it starts fine in the morning or when it has cooled off. When it is hot and it starts it almost sounds like it is dieseling for a bit and then it evens out. It is running a little rough and has 145,000 miles on it but it does not smoke, the tailpipe is clean as a whistle and is not really down on power. Mileage on the interstate is about 18 but around town it is THIRSTY, estimated at 8. any idea where to start?

 

Suggest you buy or borrow a scan tool. Even if you have no "check engine" light on, you may have a code you can look at. Go through the basics. Spark, air, vacuum lines, pvc, filters. Dirty sensors. I own the same year and power and have found dam near everything can throw a code. How old are the plugs, cap, rotor and leads? Maybe time to change them anyways? Not enough info to be of any more help than this.

 

Low MPG is a common symptom of a blocked fuel filter. How many miles on it? Before attempting to remove it, read this thoroughly:



After that, check fuel pressure with a gauge:

 

Do the dealerships scanners tell you anything that a portable scanner doesn't? It is getting a full tune up tomorrow. How many miles do plugs realistically last and is there a specific plug that I need to run? It has a 4.9 in it.

 

Yes they do. There are some really good hand held scanners out there now, but they just don't compare to a shop diagnostic system. Not all shops will give you a copy of your scan though, so I would qualify that before you agree to pay for one. Most car dealers are required to have and maintain a certain equipment standard so you should be OK with where your going. But not all dealer service centers are created equal. Some are just extentions of their parts department. I bet half the members on here have stories they could share about someone being sucked into a long list of parts installed and no success finding the actual cause. Take the reason why I joined. I had my parts store trying to convince me my PCM was bad and a mechanic saying my MAF was bad. KUTO'S to steve83. Until he told me that the "R" wire supplied all the sensors from the PCM, I was looking hard at replacing the PCM. Think the mechanic that convinced me to buy a new MAF knew to tell me to test the wire 1 foot over to the right to know the PCM was good, after I told him I lost all power to the new MAF? Guess we'll never know.

Plugs can seem to last forever in some motors while others eat them like candy. If your finding a plug(s) failing next to others that aren't, then you possibly have broader issues. Most repair manuals have pics of bad plugs in the tune-up section of the manual. So take a look and see. Ask your service dept. to keep and number the old plugs if they show any signs of trouble.

Did you replace your fuel filter as suggested? If you're not sure how old it is, I would follow his advise.

 

Nothing in your original post suggests any real problem. It clatters - that's normal for a 4.9L. It MIGHT get bad city mileage - that's normal for a fullsize truck. But your highway mileage says the truck is in exceptionally good condition, and you shouldn't mess with it. Before you start throwing cash at it, why not find out what the city mileage actually is? If it's above 12, forget it & just drive.

Plugs can last over 150Kmi if you want them to, but they're so cheap, it's hardly worth the effort to keep them more than 40Kmi. The ones that perform the best in these old smallblocks are also among the cheapest - plain old resistor AutoLites.

The WDS & later dealership scantools CAN show a lot, but on such an old truck, there's not much worth seeing. It's like hiring a NASA engineer to make a paper airplane fly straighter.