Fuel Economy Just Got Worse

de3jr88

Original poster
Member
Dec 8, 2011
45
Airdrie, Alberta, Canada
My Envoy's fuel economy has recently gone down the drain, especially on the highway. I have the I6 4x4 with 4.10's. I was getting around 20 mpg (about 500 kilometres on 65 litres of gas) but my last 4 tanks have been around 16.5mpg (430 km's). The only driving difference is that the heater is on now with winter. I thought maybe it was the E15 fuel I had started using so I have run 2 full tanks of non-ethanol but it hasnt changed the economy. About 90 percent of the mileage on a tank is highway speed around 115km/hr. I have 135000 km's now, t-body and fuel filter 2 years (25000km's) ago.

I just had the thermostat changed 3 mths ago as my temp guage was not coming up to temp and I could feel the heater not getting as hot plus It was running a little rough at start-up. The guage was about a tick and a half below where it always is. It had a P0128 code for thermostat so I had it replaced plus the coolant flushed. Dont hate me but I had it done at the dealership that I trust because removing the alternator looked like it was beyond my skill level. Guage is normal now and comes up to temp fast plus it runs smooth again. I talked to the tech and he didn't think the coolant temp sensor needed to be replaced.

I was just reading MichEnvoy's thread about his MPG issues and i am now wondering if my sensor could be causing my issues too. I have not changed my plugs or coils yet although I should start thinking about as I am just past 135000 km's.

Anyone got any suggestions? Should I be looking at the coolant sensor or should I try plugs and or coils first.

Thanks
 

nbenjamin

Member
Dec 7, 2011
76
I had some mileage issues in the summer after changing plugs and exhaust manifold . I chased it and couldn't figure it out . I was throwing codes sometimes but not all the time . ROADIE suggested the 02 sensor . I changed it made a huge difference . been good since .
 

Mypetropig

Member
Dec 29, 2011
226
How many miles are you at? It might be time for a tune up. If not the try the o2 sensor as the previous poster mentioned, remember that its best to stick with ac-delco.
 

MichEnvoyGuy

Member
Dec 3, 2011
522
Ive inflated my tires to 36psi, changed my O2 sensor, AND replaced my t-stat and coolant sensor in an effort to make sure all the sensors that feed the fuel control map are in optimum condition. I replaced the plugs around 95,000 miles and Im at 150,000 now. With all of the stuff Ive done recently, I maybe bumped up my mileage to 17 from 16mpg average :crazy: Probably the .5mpg increase was from me babying the gas pedal..lol.

I want the 20mpg highway back that I used to get until this winter!! The crappy city mileage Ive always been used to, but this highway crap I cant stand.
 

gmcman

Member
Dec 12, 2011
4,656
Are you going by the DIC for the mileage or are you going by how many gallons of fuel you put back in against miles driven?

I go by the DIC mainly as well, but in your case just to be sure you could try that.

Are you letting your car warm up for a length of time before you drive away? Reason I ask is since it's cold now..I know I do it, and 3 or 4 times for about 10 min each can gobble up some fuel.
 

Chickenhawk

Member
Dec 6, 2011
779
Don't forget that all grades of gasoline switch to a winter blend for easier starting, and that tends to kill mileage a bit too.

If your gauge is straight up and down or a tick to the right, and it stays rock steady, especially on the highway, then I wouldn't bother with the coolant temp sensor.

The plugs are not due for change but you will see some incremental improvement with new plugs. Be sure to only use AC Delco 41-103 plugs.

Oxygen sensors tend to get a bit lazy after a few years and you can confirm this with any good scan gauge that has a live-readout function, but if it is working good, you won't see a dramatic improvement with a new one. However, if left too many years, they can seize in the manifold and become VERY hard to remove.

This is why I always urge folks to change out the front O2 sensor with every plug change every 160,000 km.

So I think the plugs and O2 sensor change is worth it because both are easy to do. You may see some small improvements but the biggest single improvement will be when summer comes back.

One thing to check though is your exhaust backpressure. Does your truck seem a bit more sluggish these days, especially when passing on the highway? The rich mixture caused from the bad thermostat may have partially plugged the cat. Cats are VERY sensitive to mixtures and if your thermostat was bad enough to throw a code, it may have been causing a rich mixture for quite a while before the code popped up. It would be worth the ten minutes at a good muffler shop or mechanic and get an exhaust backpressure test done.

They take off the O2 sensor and replace it with a pressure gauge and measure it at idle and at 2500 RPM. Pressure should be near 0 at idle and about 4 PSI at 2500 RPM.

Here's a tip: get a new AC Delco oxygen sensor but change it after the exhaust backpressure test. It will be WAY easier to take out the old sensor if they know they can just clip the wires and use a regular deep socket. Some of them can be in their very tight!
 

de3jr88

Original poster
Member
Dec 8, 2011
45
Airdrie, Alberta, Canada
Lots of great insight here, thanks to all. As of today, I am at 137000 km's so I am still thinking of doing plugs even though they are not due for another 20000 or so. I think I will also do the O2 sensor just because I know its the original. As for my temp guage, I am always half a tick to the left. It was there when we bought it 5 years ago at 69000 km's and stayed there until this past summer when it started reading lower. Since the thermostat replace in early October it has been back to normal at half a tick to the left. A co-worker has a scan tool and I am going to check it again to see if I have a code again, maybe I do need to change the coolant temp sensor.

We just moved out of the city in late August and now have a highway commute of about 80km's per day. When most of our driving was in the city we were getting about the same fuel economy as we are getting on the highway now, thats why this seems shocking to me. We always thought the mileage we got in the city was great, and I think overall it is. Obviously, the 4.10 gear ratio doesnt help much on the highway for cruising so we are trying to keep to around 115k/hr (70 mph).

If anyone has any other tips please offer them. I am going to try the O2 sensor and new plugs and see where we go from there.
 

Chickenhawk

Member
Dec 6, 2011
779
Too bad you are not two provinces east; I would lend you my scan tool.

Everyone should have a good OBDII tool with a live readout function. I actually have two or three. (On my Christmas list this year was an OTC 9450 that also scans ABS and airbag codes AND resets them too. It is a nice scan tool, and now my Innova 3130 is just sitting in my toolbox.)

If your temp gauge is still a tick to the left, then I would get your temp checked with a scan tool for sure. It sounds like you may still have a problem. Let us know what the temps are. You should be hitting close to 205 when warm. If you are closer to 185, then I think your coolant temp sensor will need changing.

Plugs, O2 sensor and ECT sensor should fix you up. But don't forget that exhaust backpressure test. It sounds like you were running rich for quite a while, and it will be worth getting it checked out. It should be cheap or free at any good muffler shop.

Let us know when you get your backpressure readings.
 

MichEnvoyGuy

Member
Dec 3, 2011
522
Chickenhawk said:
Too bad you are not two provinces east; I would lend you my scan tool.

Everyone should have a good OBDII tool with a live readout function. I actually have two or three. (On my Christmas list this year was an OTC 9450 that also scans ABS and airbag codes AND resets them too. It is a nice scan tool, and now my Innova 3130 is just sitting in my toolbox.)

If your temp gauge is still a tick to the left, then I would get your temp checked with a scan tool for sure. It sounds like you may still have a problem. Let us know what the temps are. You should be hitting close to 205 when warm. If you are closer to 185, then I think your coolant temp sensor will need changing.

Plugs, O2 sensor and ECT sensor should fix you up. But don't forget that exhaust backpressure test. It sounds like you were running rich for quite a while, and it will be worth getting it checked out. It should be cheap or free at any good muffler shop.

You know, I would change the coolant sensor anyway. Its relatively cheap. Replacing mine this past week instantly stabilized the swinging temperature that I observed with my scan tool. I have yet to go on a highway run long enough to be worthy of seeing an mpg increase, but I gained about .5mpg in my overall average (calculated by gassing up, not the DIC). Now the temps stay right at 190 driving and creep up to 195 when Im stopped. No wild variations anymore.

Chicken- I have the Innova 3130 too - bought it for myself as a Christmas gift 3-4 years ago, and now Im thinking its time to upgrade. I want a scan tool that can at least get me some codes from the BCM/ABS/SRS modules. I got an email from Innova a few years ago saying I can download the ABS firmware but I'd rather not put any money into the tool seeing that it still wont ever be able to read BCM/SRS, etc.. codes. But in the meantime, the 3130 does a great job of giving me streaming live data. Side note, I noticed on ebay the price of TECH II's have really come down.
 

Jkust

Member
Dec 4, 2011
946
Are you telling us that you can actually choose whether you want ethanol or not in your gas in Canada? For us it is 10% ethanol or 85% ethanol with very few no ethanol pumps. It is 100% politics here.
 

Chickenhawk

Member
Dec 6, 2011
779
Mich, check out the OTC 9450. I got mine from ntxtools and it can read and reset OBDII, SRS, ABS and stability control codes.

otc-9450.gif


One of the advantages is that it is upgradeable using a simple USB cable, although there are no upgrades available yet.

OTC 9450 OTC9450 OTC TOOLS 9450 - Bilingual Obd II Scan Tool, Abs & Airbag (Srs) Code Reader

One problem with scan tools is that, up to a few years ago, consumer-level tools were not allowed by law to read ABS or airbag codes because regulators didn't want backyard mechanics working on these systems. This means that most older tools will not read these. I found that my Actron ABS code scanner would also read the stability control system codes on my 07, even though it only goes up to 06 in the database, and Actron had an update available that would also allow me to reset my ABS codes.

I haven't used my OTC 9450 much yet but it appears it will be able to replace my Innova and my Actron all in one tool.

The scary part is reading the code histories and picturing how previous owners must have taken it in for some expensive warranty work in the first four years of its life! Thankfully, it all seems to be solved and we have gotten one brief airbag code that went away right away, and the occasional service stability control, relating to the steering wheel position sensor.

(That steering wheel position sensor code is quite common and if one thinks it through logically, it makes sense. I have never seen inside one of those sensors, but I am assuming it is a type of variable resistor that uses contacts to determine the angle of the steering wheel. Obviously, one is driving with the wheel straight ahead the vast majority of the time, and so the contacts outside of the straight ahead position are not used as much and can be prone to a bit of corrosion. So this is why, if you park your car with the wheel pointed to the side a bit and leave it for a day or two, the light comes on occasionally. Once you drive a bit and restart, the light goes away. It is amazing how much these scanners tell you if you think logically and understand a bit about basic mechanics. Everyone says that working on cars these days is just plugging in a computer and replacing the part that it tells you to ... and this is FAR from the truth.)
 

Forum Statistics

Threads
23,271
Posts
637,475
Members
18,472
Latest member
MissCrutcher

Members Online

No members online now.