OT-Unexplained Mileage Increase

A couple months ago i posted about a 1997 GMC Sierra 305 that was getting terrible mileage. About 10 miles per cdn. gallon. Had to drive it about 15 miles to work every day so it was affordable but still an unneeded expense. The truck only ran at about 160F so i replaced the thermostat with a 195F and no difference in temp. I left it that way for a while but I decided about a week ago i would try blocking 2/3 the rad with some cardboard. Didn"t want to cover more because of the trans cooler. I ran it around the block. Total of about 6.5 miles. First 3 miles I ran it easy like normal but the temp didn"t go up at all so i ran it as hard as I could and finally got it up to 175f but it wouldn"t go up any higher. Now the truck is getting 16.5 to 17.5 mpg and would get better because quite a few of those miles were with 7-800 lbs in the bed.and a few 2 mile trips.Any idea what would cause the mileage increase? Would running it hard help somehow?

If you're going by that electronic readout, you are just fooling yourself. :>)

Allan

The increase in temperature could have cleaned some carbon out that could have been restricting flow. Like an intake valve, thus causing the engine to work harder to make the same power. I am a firm believer a truck and a tractor were built to work. That is there main purpose in life, every once in a while you have to work them to make them happy. You don't buy a truck for gas mileage (even though more mpg's are always good) and a tractor wasn't designed for parades and shows. Take them out and work them a bit and they will be happy. Thats just my theory, But then again I might just be crazy!

Allan is correct on the gauge reading idea.
An engine with electronic fuel control and ignition timing, needs to have an operating temperature in the 180 to 190 degree range (factory thermostat) if it does not run at full temp, it will sense that it is cold, and run richer, and be less efficient in thermodynamics as well.
Real mileage is determined by measuring fuel to the .1 gallon, and mileage on a corrected odometer to the .1mile. it is also reasonable to use test periods of 3 to 4 thousand miles under the same weather and road conditions. Jim

Running at proper temperature will increase mileage.
I had a '94 Explorer that ran cold all the time. I rarely ever drove it but my wife complained that the heater didn't put out much heat.
I put in a new thermostat and not only did it warm up my wife but it increased gas mileage significantly.

The best way to determine actual operating temp is to connect a scan tool and read the coolant temp that the computer is seeing. Then you can compare that to your gauge in the dash.
It's possible you got a defective stat that suddenly decided to start working. If the thermostat is working, you shouldn't ever need to cover the radiator. That's risky, especially in the summer.
Good Luck and God Bless

Probably a neighbor was siphoning gas.

Did you have a neighbor move away?

Or, perhaps his neighbor is playing "head games" and ADDING a little gas to the tank now and then!

That's what I was trying to do when I originally replaced the thermostat. I had read (and was told on here) that the computer would run things rich to try to heat the truck up. I'm not going to mess things up now that it's getting decent mileage. That rad must be way oversized for that engine because no matter how hard it is ran it won't go above 160 with the rad uncovered. I would run it harder but during normal use there's no need of it and when pulling a trailer the dang trans don't like being run hard. I actually had some cardboard in there with a 6000 lb trailer on behind and the temp still wouldn't go much over 160 running things as hard as the trans would allow. As soon as you run it too hard, every time you come out of lock up it slips real bad. Did I mention I hate automatics when pulling stuff? If i had it my way I would run it like a tractor (hard) but I can't. There is actually a good chance that the temp guage is out. I assume the truck is an american truck because of the gas tank and the fact that the odo. and trip are in miles. The speedo is way out. it's accurate at 0 and when is says 100km/h it is actually going about 87-88 km/h. 60 km/h on the dash is actually 50 km/h. When compared to an accurate gps. But if the temp did go up, then assuming the gauge is working(which it is) then it should show some sort of difference, even if not accurate. When i figured mileage i filled the tank right up and reset the trip. Drove 181.1 miles(trip is accurate) and put 11 gallons gas in. Works out to 16.5 m/cdn.g. Did it again and this time ran 88.6 miles and put in 5.1 gallons which is 17.4 m/cdn.g.

I do believe that as long as your temp is below 180 the computer tells the thing to hold the joke on.I know it don't have a joke but it does the same thing.My 1995 does the same thing,I just never thought of pluging the rad,scared to over heat it.But mine only gets 6-8 MPG

I have a 2003 Dodge Ram, Cummins diesel. I have checked EVERY tank for mileage and it is always the same as the electronic readout.

You are going to have to get the engine up to operating temperature(195) and get your speedo accurate to allow the computer to manage the fuel system properly.Make sure the fan clutch is not locked up. Right now the computer is trying to compensate for the low temperature and the inacurate rpm to road speed it was originally programmed for.

Never trust a gauge. Verify with a thermometer. Fuel mileage will drop off with a cool engine due to more fuel needed to achieve the combustion event. Kinda like trying to start a weedeater cold with the choke off. When your vehicle reaches closed loop (operating temp.) fuel control authority is given to the computer through the sensor input. After 1996 OBDII vehicles had codes for engine cold too long because it would increase emissions. Thermostats are responsible for achieving minimum operating temp. Hope that helps. Gerard

Did you hit the "Standard/Metric" switch by chance? That could explain the difference....

I think it has to do more with the dash panel being changed for a Canadian one than the signal sent being wrong. The computer should know how fast it is actually going if the sensors are working? The fan will turn with the truck off so I take it the clutch is working? That's why I think the temp might be off. The speedo has different units for Canada and the US as does the temp gauge, and oil pressure. Gas guage, volt meter and rpm gauge are the same and seem to be accurate, Compared to an actual canadian truck.

The computer reads the speed through the speedometer. If it is not getting the correct speed, the computer is adjusting the fuel rate for the speed the sppedometer is saying,not what the truck is actually doing. In your case the speedo is reading about 25% fast,so it is adjusting the fuel rate to maintain 80 KPH when it is actually running 60 KPH.
It is a closed loop system so everything has to be correct to obtain maximum mileage,while still retaining driveability. So the engine has to be at proper operating temperature, the truck has to be moving at the speed the speedometer is reading,etc.

With regard to your comment about the rad being too big for the engine, if the thermostat is working correctly the radiator does not even come into the equation until the engine reaches the proper operating temperature then it opens up and lets coolant into the radiator. Once the temp drops below it's operating temp, it closes and no more coolant goes through the rad until it heats up again.

I have a 1992 Chev 2500 Pick Up with a 305.Last year our son was using it for several weeks and complained about low milage. When he brought back, the Temp. Gage was low-I replaced it. It still had poor milage. On top of the engin next to the Gooseneck is a second Temp. sending unit that goes to the brain box. When I changed that one - the mpg went from 10 to 17 mpg.

(quoted from post at 13:28:14 08/28/10) The computer reads the speed through the speedometer. If it is not getting the correct speed, the computer is adjusting the fuel rate for the speed the sppedometer is saying,not what the truck is actually doing. In your case the speedo is reading about 25% fast,so it is adjusting the fuel rate to maintain 80 KPH when it is actually running 60 KPH.
It is a closed loop system so everything has to be correct to obtain maximum mileage,while still retaining driveability. So the engine has to be at proper operating temperature, the truck has to be moving at the speed the speedometer is reading,etc.

Click to expand...

Sort of but..............The readings from the O2 sensors will prevent odd sized tires from over richening or leaning the mixture.
As previously pointed out. The computer was running the too cold engine in an over rich condition. Perhaps even an "open Loop" mode that would be ignoring the O2 sensors.

Teacher in high School drove a Model A Ford to college daily.It was getting around 40 mpg.He later found his friends were adding gas at nite after he went to bed.