Well I git this truck 2 years ago and gas mileage did not seem to bad---I did do Tune-up all normal things and had put new exhaust on so knocked out Cat. then, ---Installed ele, fans set-up too----
The truck runs Great!!--good power and idles perfect cold & hot and also added another cold air intake for air and all this gas mileage got worst??---When first got it drove on trip got 18.2 and around 14-15 city BUT now getting maybe 16 at most hwy and 13 or so in city??
ANY IDEAS??
I have a couple ideas for you. One question first. Did the mileage change slowly over the past few years or suddenly?
If slowly and if you have put a few miles on it, it could be engine or transmission wear. If you happened to do a compression check when you bought the truck you should do it again and see if there have been any drops. Even a good running engine not burning oil can sometimes have a drop in compression enough to cause a drop in power, which translates to fuel economy. If over a long enough period of time the power loss may not be noticed.
Is the torque converter in the transmission locking up? It should lock in 3rd and 4th, and is it shifting into over drive as it should?
The cold air intake is suspect. The ECM maybe enrichening the fuel slightly more if it's seeing much cooler air. Try disconnecting the cold air duct, drive for a few tanks and see if there is a change.
Knocking the guts out of the cat is suspect as well. Is improved exhaust flow good? Yes, but, if the cat was rattling or if you didn't get all the bits out when you knocked the guts out pieces can get lodged in the muffler causing back pressure. Back pressure can be checked with a gauge you may not have one but most shops will do a back pressure check for a 1/2 hour labor charge.
Outside of that this is an OBD1 truck so things can be a bit off with the computer system without the check engine light coming on. If it's been more than 3-5 years since the O2 sensor has been replace, then replace it. If it's not reading right the ECM may enrichen the fuel some, which may have contributed to the break down of the old cat if it did in fact breakdown.
Hook a scanner up to it and see that it is in fact reaching at least 190F when warmed up. Less than that and you'll see fuel economy drop. With that radiation and fan setup it would be easy to over cool especially if the thermostat isn't up to snuff.
With the scanner make sure it's getting into closed loop mode. I'm over simplifying here a bit but when you start it cold it'll be in open loop, where the ECM is in enrichment mode for cold run. Once it warms up it should go into closed loop, if it never or takes a long time to get into closed loop that'll hurt gas mileage. In moderate temps say over 45F you should be able to get into closed loop in 15 minutes or less. If it's colder then a little longer is ok. The times aren't really hard rules, but the quicker it gets into closed loop the better your fuel economy will be.
Hopefully this will give you some ideas.
