It sounds like you're on the right track. Another thing to try is to run the tank down and fill with 89 octane and see if that helps at all.

If you plan to keep the truck, you may want to invest in an SCT tuner from Hemifever. I have one for mine and it's the best money spent so far. It'll give you 3 different tunes for different octane fuels, remove the factory death-flash, remove torque management, and move the torque converter lock up further up the mph range helping the engine to run better.

I keep mine pretty well tuned. With 4.10's. 315 tires and the 89 octane tune, I'm getting 8.5 mpg's around town and 13.5 highway. I don't think these V10's will do much better than that.

 

Was it the left bank rear plug? That one is a pain to get at, and I have heard of mechanics 'forgetting' to change it out.

The RC12LC4 is what my '95 called for.

Are only getting 5.7MPG? (or did I read that wrong?)

I get 9 to 10 with mine, unless I'm doing a lot of plowing (lot of backing up)

Worst I have gotten was 6.3mpg, that was towing my 12K deckover with 2 Jeeps on it, running 75 on the freeway.

If I keep it 65 or under , I usually pull 9 even when towing.

 

That's entirely too common, and on every vehicle there is in which there's a difficult to reach spark plug.

My mother had a first year Chevy Astro, bought new, and it had a problem of randomly just dying and then taking anywhere from a few seconds to several hours to start up again. It went back to the dealer about a dozen times for that failure, and every darn time the monkeys there who didn't know the first thing about troubleshooting electronic ignition systems would say that a tune up would fix it. They never replaced the plug in the #1 hole. I eventually got manipulated into fixing the thing, and swapped out that plug while I was at it.

 

Hi,
omg I had to reregister having much trouble with password and then I spent 30 min. writing long reply only to find out I wasn't actually signed in and lost it so this is short.
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Hood latch linkage was actually the problem . cinched with zip ties. (Update ) 1/5/15- NO! still tinny vibe! So, I went under again and bent the edges at the top and bottom of the heat shields behind the engine to maybe stiffen them and make them stop vibrating and IT WORKED! What a relief!
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Putting in back corner plugs was actually quick and easy using 6 in. extension. Was able to insert new plug quickly with fingers with my eyes closed leaning over engine on a short ladder. Seeing with my minds eye. ha
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Checked on SCT Tuner- I wonder how more power means more mpg. SCT says you may get 1-2 mpg more.
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Yes, 5.7 mpg. Checked manually. Pulled trailer 40mi. out of 155.( Update ) 1/5/15- Without pulling trailer I do get 9.3mpg and the digital display showed 9.5. So, I confused the issue but am sad that it drops so much- but then, 4.10 diff. Dam! What will I get pulling at 6k ft. when I go to Utah!
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I replaced the plugs and the 2 front o2 sensors but the display still maxes out at 10mpg at 45-55mph today. I am at 0 elevation too near El Centro, Ca. Checked vacuum and it's rock steady at 21 and according to it- no drop off at 2,500rpm so exhaust is not restricted nor cats- I guess.
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Notice Dupli-color copper plate spray paint on grill and hubs. 1.5 kits.
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Update 7/15- late, I know- Well, after all that I found the tinny vibration was my dogs food dish container placed by the tc case shifter, which is made of wrapped wire and the wire handle was rattling. My friend riding with me detected it. Alone, I swear it was in the engine compartment.