Bad Gas Mileage

Re: Changing OEM Theromstat

I just replaced my Thermo. @ 36,500 on my '06. Mileage was down to 35 mpg. After putting in a new stat. it went back up to 43 mpg. But, if you have a constant 3 bars....I'd look at the air filter. I also @ every 5K miles, run the appropiate amount of Techtron F.I. cleaner through the system. Hope you get it fixed. The air filter is pretty important for mileage figures.

Re: Changing OEM Theromstat

I just replaced the air filter about 2000 miles ago so I'm thinking it should not be the filter but I'll take a look just in case something got in there.

Re: Changing OEM Theromstat

freetime2247 said:

theinstrument cluster was showing a low of 28 and a high of 34 while cruising at70mph

Click to expand...

Before I did anything I'd run a full tank through the bike and get an accurate gauge of the MPG by actually calculating it rather than relying on the instrument panel.

Re: Changing OEM Theromstat

Yup done that too! I'm only getting about 200 miles before it goes to one bar on the fuel gage. Looks like I'll have the dealer run the diagnostic test on all the electronics and I'll change out the T-stat also just in case.

If it runs up to 3 bars it should be good as far as the Thermostat is concerned however, you really only notice that if the ambient temps are at least in the 40's.

How much fuel are you putting in when you fill up and you have 1 bar on the fuel gauge? The upper tank uses a float and the lower tank uses a thermistor, so it's possible you're having issues with that which appear to be bad mileage but are only a bad fuel indicator.

I believe the fuel gage is reading acurate as I'm calculating the mpg based on miles riden and the fuel quantity pumped during refueling. Still hard to believe my mileage is down by as much as 25%. Hopefully, the dealer will be able to properly diagnose the issue.

freetime2247 said:

I believe the fuel gage is reading acurate as I'm calculating the mpg based on miles riden and the fuel quantity pumped during refueling. Still hard to believe my mileage is down by as much as 25%. Hopefully, the dealer will be able to properly diagnose the issue.

Click to expand...

Could also be bad O2 sensors but they should throw an FI code.

I suppose it could be another bad t-stat, not the 1st time someone has had to change one back-to-back but it is somewhat rare... be sure and flush the system out real good if you do and you can put ice in the coolant res and shake to get the inside of that clean.

Thanks Joe! The orginial T-stat is still in the bike so this will be my 1st replacement. Plus the shop says they will be able to tell if the problem is an O2 sensor, spark plugs, fuel pump, pres regulator, filter or some other electronic problem.
I'm keeping my fingers crossed and hoping I'm not looking at several hunderd dollar repair bill.

freetime2247 said:

Thanks Joe! The orginial T-stat is still in the bike so this will be my 1st replacement. Plus the shop says they will be able to tell if the problem is an O2 sensor, spark plugs, fuel pump, pres regulator, filter or some other electronic problem.
I'm keeping my fingers crossed and hoping I'm not looking at several hunderd dollar repair bill.

Click to expand...

Ah, okay I read your orig post incorrectly, I was thinking you said another OEM tstat so I assumed you had changed it already.

My bet is on a sticking t-stat then. Unless the ambient temps are at least in the 40's you won't necessarily notice the 3-2-1 temp bar dance but you will exhibit bad gas mileage many times.

If your gonna spend $100 dollars on the dealer again, I'd recommend purchasing a Morgan Carb Pro, and doing the TB sync. instead.
Make sure you have fresh fuel, a clean air filter, new spark plugs, and replace the vaccume lines and clean the 5 way tee. Then do the TB sync.
I'm betting your gas mileage will improve 100%.
.02

I will be interested what your dealer comes up with as I am having the same issue.
My 04 with 82k is getting terrible gas mileage.This is an issue that I have been fighting for over 12k miles.Sigh!
What I've done so far..
Valve check and adjust @ 70k,new therm and coolant temp sender..temp gauge errattic after getting it back from shop.They found the wiring harness to temp sender deterioated from heat? and repaired wire harness.
Also was getting a FI light and poor cold start.Map sensor code.Found 5 way tee plugged up,replaced tee and vac hoses.Bike is running better now,no FI light.Map sensor replaced with a used one.
Notice bike misses when revving engine around 3k rpm and doesn't feel smooth.
Inspected plugs and boots,replaced plugs with irridium plugs,no problem found with boots.
On a ride in warmer weather bike starts cutting out and surging like out of gas.Let it cool down and made it home.
Ordered a fuel pump and filter,installed bike runs better.
Gas mileage still down 25-29 mpg avg.
Had starter valves synched,no improvement ...HELP!

Who did your TB sync? and was it done correctly?
A couple other members brought me their bikes with the same issues, and watched as I did the TB sync.
The lines were bouncing about 1/2 inch to 1 inch on a couple cylinders, when finished, they were dead even (on the little lines).
Both members reported an increase of a least 7-10mpg on the way home.
If a dealership told you it was done, I'd question if it was really done, or done correctly...
.02

o-2 sensors "wear " out and are a maintenance item.

Joe, are you sure that would throw a code ?

They would if the ecm thinks the readings are out of bounds.
O2 codes 21 and 22
O2 heater codes 23 and 24.

v8-7 said:

o-2 sensors "wear " out and are a maintenance item.

Joe, are you sure that would throw a code ?

Click to expand...

st1300r said:

They would if the ecm thinks the readings are out of bounds.
O2 codes 21 and 22
O2 heater codes 23 and 24.

Click to expand...

Yeah, but there may be a tolerance of some type before they threw a code.. I have no idea... it could mean bad mileage until the fault was considered a fault. I'd check the condition of the wires for anything obvious. But, I would think that would throw a code. The manual has a way to test them.

Very surprising that these bikes can go from 45mpg to 30mpg and no throw a fault code. I've never done a TB sync nor had it done on my bike. What is a Morgan Carb Pro and how difficult is the TB sync? How can you tell if your bike needs a TB sync? My bike is exhibiting no unusual performance issues other than the extremely bad fuel mileage.

freetime2247 said:

Very surprising that these bikes can go from 45mpg to 30mpg and no throw a fault code. I've never done a TB sync nor had it done on my bike. What is a Morgan Carb Pro and how difficult is the TB sync? How can you tell if your bike needs a TB sync? My bike is exhibiting no unusual performance issues other than the extremely bad fuel mileage.

Click to expand...

Great article here:
https://www.st-owners.com/forums/showthread.php?100936-ST1300-Starter-Valve-Synchronization

Mellow to the rescue once again! :bow1:
The sync is pretty easy and the only tools you'll need are a JIS screwdriver, 8mm & 10mm sockets, an extension, a ratchet, a 10mm wrench, and a 1/4 inch drive deep 7mm socket to turn the adjusters (makes it easier). If you've replaced your air filter, you almost were there! I'd go to an auto parts store and purchase about 4 feet of 4mm vaccume tubing, and replace the lines while your in there.

bad gasoline and ethanol will mess with mileage too, but I'd do what Mick said and do the math and not rely on guessing how much you've used.