94-GMC 350 Made gas mileage Less By changes?

[Durfman77]

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??

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[84 M1008]

Is the check engine light on? You essentially removed the catalytic converter.

 

[rjohn]

Got a 94 burb do yearly plugs cap rotor an fuel filter and wires every 2 years make sure your injectors are clean and you have no intake leaks

 

[rjohn]

Also driving habits play huge role coast that bitch for lights and if ya added power u might feel it just ignore that lol don't drive her hard

 

[Grit dog]

Idk why mileage is worse, but that sure is a clean truck!
Have had a few of those trucks in the past and I don’t ever recall getting 18mpg with tbi or Vortec, unless maybe it was downhill of I was stuck behind someone’s grandma for 50miles at 55mph!

 

[80BrownK10]

16mpg seems right to me in that truck.

 

[SirRobyn0]

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.

 

[Big Chip]

If it's just recently it's probably the ****** winter blend gas they give us in Michigan. I drove my truck down to Florida in December and was getting 9.8ish mpg until I filled up towards southern Ohio. All of a sudden I was getting 11.6 mpg. Same experience on the way back, it dropped when I was somewhere in Ohio.

 

[84 M1008]

9.8 and 11.6 on the highway? That's ridiculous.

 

[80BrownK10]

9.8 and 11.6 on the highway? That's ridiculous.

This is the square body forum. That's not crazy??

 

[Big Chip]

I pulled a car down and a new trailer back. I thought it was pretty good mileage compared to driving the dually back from Oregon.

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[Ricko1966]

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??

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Well one thing cold air is denser so now you have a denser air fuel mixture. That means you're going to burn more fuel.Also if the truck has a positive backpressure EGR valve and you don't have enough backpressure to open it due to your hollowed cat no EGR means reduced MPG. I'm not going to argue the EGR point, it increases fuel mileage. Someone brought up blended fuel that maybe part of the equation also.

 

[Durfman77]

Is the check engine light on? You essentially removed the catalytic converter.

No engine lights ever--

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.

THANKs ALOT for all info--Millage did change quickly after I did all these things,,Have had truck be 2 years next month and I did put plugs--wires--new distributer in with new Icm --new fuel pump and exhaust [did not change co2 sensor?] After this it would start when cold run 15 seconds then ruff as heck?--put timing light on found timing all over the place so put old Icm back in and ran fine?--guess got bad new one??--I did check millage it twice on a 600 mile trip and got around 18 before I did all this--So your suggestion on co2 and cold air intake is worth trying next,, Converter is locking up ok did new filter and fluid in trans looked good,,,,and 195 thermostat is working like it should checked it,,Did not change fuel filter?-------I think I got all guts out of cat?--started it up when muffler off blew out good??--will post when I do these things and let you all know--THANKS!!---oh found this truck down south a 95 year old lady owned it [bought new] then past away,,truck was like in showroom--I got all service records also just normal things,,

 

[Bextreme04]

another thing to consider is since you unbolted the exhaust to knock out the cat you might have put an exhaust leak in there. If you have an exhaust leak close to the O2 sensor it can mess with the air/fuel mixture. I'd also look at the voltage range of your coolant temp sensor. If it goes bad it can give you some crazy fueling when cold and cause the rough running.

 

[Ricko1966]

Just noticed the electric fans. Thermostatically controlled? I hope ?or is it just running full time when the trucks running slowing down warm up, and delaying closed loop?If you do a new o2 sensor put in a 3 wire and wire up the heater circuit. That will get you into closed loop sooner