Chasing a few lost MPG's

gmcman

Original poster
Member
Dec 12, 2011
4,656
I believe over the last few months, maybe more since I didn't pay too much attention when the winter blend of fuel came into play, I have lost a couple MPG's and I want to try to hunt down what may be the culprit. I want to say it's possibly fuel related but just a hunch.

I have changed the plugs (AC), fan clutch, compression is very good, vacuum good and steady needle, TB cleaned, upstream O2 replaced (AC from dealer) all new bearings at each corner. Tires are good and are LRR, MPG's were fine when I purchased the tires, brakes are not dragging.

I did reset the codes after the O2 and fan clutch but I'm letting it sit overnight with the battery disconnected.

Driving around with my OBD scanner I have been watching the fuel trims, mainly the long-term and while I'm not well-versed on fuel trims, I have a basic understanding of them. What has me thinking fuel is when I'm on a flat road, steady 60 MPH I can almost hold a steady 20 MPG on the DIC, but spends more time at 19..and mine is about 1 MPG off, reading high. I would generally hover around 22-23 between 55 & 60.

At a steady cruise at 50-65 ish my LTFT stays around -2 to -3.5 and sometimes raises to -1.7 but not for long...this is with a steady foot at 60 MPH, flat road.

I recorded some live data but don't have serial port on this PC so I need an adapter before I can post the results, maybe someone that knows the trims well enough can help. I want to look at dirty injectors, MAP sensor, fuel pressure or maybe even a vacuum leak but something tells me it's fuel related.

Another possibility is a restricted exhaust, the cat's temp last night after a drive was about 375-400 deg in and just over 600 deg out, downstream sensor stays around .475-.575V and is fairly steady. I guess this doesn't mean it's not restricted but I will test backpressure....I plan on building my own gauge from an old O2 sensor so we'll see how that goes.

Given the fact most members here are creeping up on miles here as well, hopefully this will address some MPG issues some may be having as well. I will get started on it this weekend or just before and post results as I progress.

I'm barely on the gas at 60 but just seems like I need to push on it harder than normal to pick up some speed....almost like I have 500 lbs in the cargo area. Owning this since day 1 you just get used to things and something is amiss.

All input welcome.
 

CaptainXL

Member
Dec 4, 2011
2,445
gmcman said:
I believe over the last few months, maybe more since I didn't pay too much attention when the winter blend of fuel came into play, I have lost a couple MPG's and I want to try to hunt down what may be the culprit. I want to say it's possibly fuel related but just a hunch.
.......
............

I'm barely on the gas at 60 but just seems like I need to push on it harder than normal to pick up some speed....almost like I have 500 lbs in the cargo area. Owning this since day 1 you just get used to things and something is amiss.

All input welcome.

Exact symptoms here. I have strong evidence to believe that my case is gas related. Recently I filled up with 87 octane with no ethanol and my throttle response increased notably. I normally get E10 at Costco but wanted to see what the ethanol free stuff is all about. My advice is to experiment with different gas stations that are known to have ethanol free gas. See what happens.
 

mcc

Member
Apr 8, 2012
49
CaptainXL said:
Exact symptoms here. I have strong evidence to believe that my case is gas related. Recently I filled up with 87 octane with no ethanol and my throttle response increased notably. I normally get E15 at Costco but wanted to see what the ethanol free stuff is all about. My advice is to experiment with different gas stations that are known to have ethanol free gas. See what happens.

I have the same suspicions- when I travel out-of-state and am forced to buy 87 with ethanol, my throttle response seems to degrade. I always fill up with top tier non-ethanol 87 when I'm in Iowa.

I get better economy without ethanol as well- enough to offset the additional expense. 87 w/o ethanol tends to run about 13¢ more than 89 w/ ethanol.

I hate ethanol. It's a terrible idea. It drives up worldwide food prices and, last I researched, is either carbon neutral or positive compared to using straight gas. The production/transport/etc. of ethanol releases CO[SUB]2[/SUB] of its own.
 

gmcman

Original poster
Member
Dec 12, 2011
4,656
I went to Costco last fillup as well and went with 93 just to see how the fuel trims behaved...as well as driveability. The motor runs strong and doesn't hesitate but couldn't really get a good basis since I pulled the trailer a little bit.

I just filled up again with 87 and added my spring dose of 44K, I will also pick up some Redline SL-1 for the injectors before I have them profesionally cleaned. I'm going to pick up a used fuel rail with injectors to keep it on the road. Fuel atomization is a huge variable, cleaners don't really get them like new once they are gunked up. Once clean you can keep them clean but when dirty you can get them almost clean unless they are taken apart, $25 per is far cheaper than a new set.

I'll verify fuel pressure first then go from there.
 

gmcman

Original poster
Member
Dec 12, 2011
4,656
mcc said:
I get better economy without ethanol as well- enough to offset the additional expense. 87 w/o ethanol tends to run about 13¢ more than 89 w/ ethanol.

There is a non-ethanol station about 25 miles away, likely won't offset the drive but I want to try it once I get this straightened out. Fact is I was still getting decent mileage with the ethanol, about 265-275 per tank (small tank) combined cty/hwy and that was last spring, with a bad fan clutch... now it's about 235-240 if I take it easy.
 

mcc

Member
Apr 8, 2012
49
gmcman said:
There is a non-ethanol station about 25 miles away...

Wow! I'm always surprised to see and hear about the difficulty of finding non-ethanol fuel outside of Iowa. As far as I know, all stations here have ethanol-free 87. The ethanol content of premium varies per station.

A major ethanol producing state, of all places. :confused:
 

C-ya

Member
Aug 24, 2012
1,098
I just put a bottle of Star Tron in my truck. Initial dose is 1 oz per 3 gals, 8 oz bottle. After that, it is 1 oz per 6 gals. I also found another type of additive that also prevents phase separation and assists the burn. I want to say it was a Gunk product, but I can't think of it right now. I'll check when I go home for lunch. It says to use 1 oz per 5 gals. It comes in a bigger bottle and is cheaper than the Star Tron, so I put that in the wife's '04 Impala to compare mileage and "pep". With either one, an extra $2 or $3 at each fill up may be worth it if they work as advertised.

On a related note, my range showed over 300 miles for the first time in a few months. I think the winter blend gas has something to do with it, but I can't figure out how. When I bought the truck in August of last year, it would show 320 miles of range when I filled up. Sometime in October or so, it started showing 280 miles after a fill up. I filled up Monday (before the gas prices jumped $.23/gal) and it showed 310 miles. Odd...
 

Jkust

Member
Dec 4, 2011
946
I am 100% certain my substantial drop in mpg's is winter fuel related. I could tell which fill up the station switched over to non-winter blend. I'm back at 17.9mpg's per tank and I had been as low as 12 this winter for the same route to work driving. I may try some injector cleaner as well. I used to brush cleaners off as gimicky until I tried some in my van and it actually worked smoothing out the idle among other things.
 

gmcman

Original poster
Member
Dec 12, 2011
4,656
Good call on the temp gun, I will check that this weekend when I look for an exhaust leak :mad:. I'm not going to base all this on my LTFT being -3, that is supposed to correlate with the STFT so it likely is fine, good thing is they tend to stay within the 4% limit.

When I changed the front O2 sensor I disconnected the battery for 30 min, may not have been enough, who knows. I disconnected the battery last night for 10 hours and when I fired it up, the HVAC actuators were jumping around, alarm went off, maybe this time it reset.

Winter gas plays a part but for me I was getting much better mileage with it than I am now.
 

gmcman

Original poster
Member
Dec 12, 2011
4,656
The MAP sensor is original so I will check the voltage and see how close it may be to the acceptable limits. I want to have one just in case so I may just pick one up and compare.
 

rjpoog1989

Member
Dec 4, 2011
116
Jkust said:
I am 100% certain my substantial drop in mpg's is winter fuel related. I could tell which fill up the station switched over to non-winter blend. I'm back at 17.9mpg's per tank and I had been as low as 12 this winter for the same route to work driving. I may try some injector cleaner as well. I used to brush cleaners off as gimicky until I tried some in my van and it actually worked smoothing out the idle among other things.

My gas mileage has also been very poor through the winter. I'm averaging around 17 where in the sumer I was averaging 20. I think the outside temperature has a lot to do with this as well. Does anyone know when they go back to the summer blends?
 

DDonnie

Member
Mar 26, 2012
2,631
Captain, where the heck did you get 87 octane ethanol free?
 

gmcman

Original poster
Member
Dec 12, 2011
4,656
Well, slight progress as I probably didn't reset my ECM properly back when I changed the O2 sensor. I pulled what I believe is the ECM fuse and left it out for 30 min. The other night I disconnected the battery overnight, so yesterday and today there is a noticeable difference in driveability. From starting off it's much smoother and doesn't feel like I have a 500 lb trailer behind me. The last O2 sensor was very lazy and I'm surprised it didn't throw a code.....1.5 sec average to change values.

Fuel mileage appears to be slightly improved but we'll see how the rest of the tank does.

Does anyone have the MAP sensor values, mainly the corresponding in/Hg per volts? I have just over 18 in at idle and I want to say it's slightly low, either case I had 1.14V at 18.25 in.
 

CaptainXL

Member
Dec 4, 2011
2,445
Removing power from the PCM will just reset learned values and shouldn't affect mileage at all. It's going to relearn everything again and you will be right back where you started.

Don't worry about your MAP sensor. If it was out of range the PCM would throw a code. Same with O2 sensor.

P0133 is the code for "Oxygen Sensor Slow Response Bank 1 Sensor 1" which means the sensor is not switching rapidly enough. If you aren't getting this then the PCM is content and is delivering fuel in a non-wasteful manner. You can bet the EPA made sure the factory has this monitor tight.

What you want to look for is stuff that is outside the PCM's control such as vacuum leaks or stuck injectors.
 

gmcman

Original poster
Member
Dec 12, 2011
4,656
CaptainXL said:
Removing power from the PCM will just reset learned values and shouldn't affect mileage at all. It's going to relearn everything again and you will be right back where you started.

That's what I'm worried about

Don't worry about your MAP sensor. If it was out of range the PCM would throw a code. Same with O2 sensor.

I agree, and the O2 sensor will compensate if the MAP is off slightly. One thing the MAP also does is influence the spark timing for the load, this plays a large role in economy.



What you want to look for is stuff that is outside the PCM's control such as vacuum leaks or stuck injectors.

That's today's project :cool: Building an exhaust pressure tester and I am checking for vacuum leaks.
 

Jkust

Member
Dec 4, 2011
946
rjpoog1989 said:
My gas mileage has also been very poor through the winter. I'm averaging around 17 where in the sumer I was averaging 20. I think the outside temperature has a lot to do with this as well. Does anyone know when they go back to the summer blends?

Well, here in MN they have made the switch. The cold does take some mpg's away that's for sure. I've never owned vehicles so sensitive to these factors as my three v8 gmt's.
 

gmcman

Original poster
Member
Dec 12, 2011
4,656
I made an exhaust pressure gauge and I will write that up later, but there is no restriction. Since the hard reset it drives much better, noticeable difference and it seems like it shed about 500 lbs. I still want to check the MAP to known values but at idle I believe it's within spec. Just need the breakdown for the vacuum to volts.
 

gmcman

Original poster
Member
Dec 12, 2011
4,656
Haven't seen this on the DIC in quite sometime...:cool: even on summer gas. Took this right when the light tripped and was a mix of approximately 60%hwy/40 cty. Not sure if it was a summer blend but when I asked the station attendant he wasn't sure if they switched yet...we'll see how the next few tanks go, this tank averaged 17.1 and I have been struggling to stay above 15 even before winter.

Only major change was the upstream O2 sensor and there wasn't a code...just a tired sensor.
 

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mcc

Member
Apr 8, 2012
49
CaptainXL said:
Got a progress report?

Not yet- I just finished finals week as a senior in Computer Engineering, moved to a new apartment, spent lots of hours shopping for furniture, and am now helping two others move. And I'm starting an internship soon. Hopefully I can take a closer look at the issue this coming week.
 

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