I have a 2000 Dakota Club cab 2WD 4.7L V8 I have a bed cover on it. Until recently I was averaging 20/21 HWY. This truck has 213,000+ miles on it. Every year I go to AL. to visit family and I average about 21MPG on the trip. Its aroun 650 mile I usually fill up when I leave and about half way I fill up again. I usually get there with about a 1/8 tank left. This year it took about 2 and a half tanks to get there. Acording to my little computer in headliner I averaged about 16. The loss of feul mileage has been going on for a little while. I had the plugs replaced at 100,000 miles at the dealer. 2 weeks ago just before the trip I replaced the plugs again myself. I used the exact same plugs as what was in it. I was also hearing a wierd knock. I thought it was spark knock but it always goes away after about 5 min of driving. You can drive the truck for 30 min or 5 hours, shut it off and restart it immediately and it will knock for about 3-5 min. no matter what. It doesn't sound like a lifter or a rocker arm knock. Its sounds higher pitched. All I know is when the knocking sound started is when I started losing my fuel mileage.

I should also say that when I took my trip for thanksgiving I stopped and got fuel for the third fill up and I was driving for about 30 miles and I noticed my Check Guages light was on and My Voltage guage was reading 8 volts. I was about 50 miles north of Nashville TN. when I noticed it. I drove to the south side of Nashville and I found a parts store. I pulled off the alternator they tested it and it tested fine. Which blew my mind so I put it back on the truck. Doing this all in thier parking lot. I started the truck and everything was working just fine. good voltage and everything. The onlything I can figure is maybee corrision, since I live in Northern IL.
If anybody has any suggestions to the fuel mileage problem. aND THE kNOCKING issue. I am open to all sugestions.

Thanks
Jason

 

Something I forgot I may have a exhaust manifold leak. I can smell and hear the exhaust when I first start it up. But then it goes away.