MPG...

[Blackstar]

A while back I had 2 ignition coils go bad, not at the same time. Replaced one and still had a misfire. Thought it was the replacement that was bad but it was a different one. '02 Envoy SLT with 146,000 miles. These were original coils. During this I changed the plugs. About 5 years ago I had installed NGK's and all worked well. Used them again this time. I also removed the throttle body and cleaned it thoroughly. Air filter looks fairly clean.

Now the curious part is that before I had the coil problems I was averaging about 17.5 l/100km (13.8 mpg(us). While I was working on the misfire problem the gas mileage got as bad as 25 l/100 km.

Now the truck is running very well, nice and smooth idle and acceleration but my gas mileage is currently averaging 10.2 mpg (US). I only drive this back and forth to work (about 2 miles) downhill one way and uphill the other. Couple trips every now and then that get speeds up to 40 or 50 mph.

Is there anything I'm missing that could be cleaned or adjusted that may have become fouled during the misfire episode?

 

[Mooseman]

After having misfires, maybe the cat got plugged? Try a backpressure test or remove the O2 sensor and see if things improve. That happened with my '02.

 

[TequilaWarrior]

@Mooseman What are the chances it's just his drive? If it's literally 2 miles, is the t'stat even opening all the way? TCC lockup probably isn't even possible in that short a distance, is it?

 

[TollKeeper]

He could be in cold mode (closed loop) for his SHORT journeys, and never getting the engine to peak performance. This will also cause problems over time with the Cat Converter, and the O2 sensors.

 

[Mooseman]

True, and that also could contribute to cat clogging as it never really gets up to operating temps.

 

[Blackstar]

It's a short drive but this is the quickest vehicle to warm up that I've owned. 5 minutes of idling from cold will have my temp gauge 1/2 ways to operating temp. It's fully warmed up after such short trips, according to the temperature gauge. What is TCC lockup?

This is how this truck has been driven since we bought a '12 Acadia 3 years ago. The Acadia gets most of the use. This MPG problem started when the coils failed and never returned to normal.

 

[budwich]

you can start at looking at fuel trims at initial startup and then after warm, along with running down the road at normal / constant speed. Go from there.

 

[TequilaWarrior]

TCC Lockup is Torque Converter Clutch. TCC Lockup reduces RPMs and only occurs when in a cruise mode and at temp. It only lowers RPMs by about 400 to 500 RPMS.
For some reason I perceived that your commute changed.
What does your temp gauge actually say? Do you have a OBD2 reader and can you get direct readings from the temp sensor? Operating temp should be around 204+ F (gauge essentially straight up). A marginal or failed thermostat will prevent your engine from reaching proper operating temperatures, therefor preventing TCC lockup and reducing your MPG. It would be happenstantial that the thermostat would fail at the same time as a coil - but stranger things happen.

 

[TollKeeper]

It's a short drive but this is the quickest vehicle to warm up that I've owned. 5 minutes of idling from cold will have my temp gauge 1/2 ways to operating temp. It's fully warmed up after such short trips, according to the temperature gauge. What is TCC lockup?

This is how this truck has been driven since we bought a '12 Acadia 3 years ago. The Acadia gets most of the use. This MPG problem started when the coils failed and never returned to normal.

It may be a short drive, but it can take a full 30 minutes to get all the system up to operating temp, especially the exhaust..

 

[Blackstar]

Temp gauge is straight up, 210F. Took her for a little longer drive (scenic route ) yesterday and MPG got down to 22.3L/100km. A little better. Maybe it just needs to be driven more. I have a borrowed basic code reader.

 

[TequilaWarrior]

Temp gauge is straight up, 210F. Took her for a little longer drive (scenic route ) yesterday and MPG got down to 22.3L/100km. A little better. Maybe it just needs to be driven more. I have a borrowed basic code reader.

Basic code readers can help solve a lot of problems - but their greatest achievement is the pulling of codes and reset of "check engine" lights. Of great value is an obd2 bluetooth reader (i have wifi one, good but bluetooth is better). You can get "realtime" readings from sensors - albeit a limited number of them - as well as basics like RPM, MPH, instant MPG, fuel trims, O2 sensor voltages, lots of stuff. There are free apps available that are good - "Torque (Lite)" is one and has a premium version. Some apps (not sure about Torque) can read and reset codes as well - although no reader can reset codes while the engine is running.

"Short" drives will brutalize your MPG average while longer drives should average out quick starts and breaking. 22.3L/100km is around 10.55 MPG - not stellar, not even "meh", definitely something not right somewhere.

Any changes to the vehicle since around the time of the coil failure? New tires? 4x4 stuck on?

 

[Blackstar]

Any changes to the vehicle since around the time of the coil failure? New tires? 4x4 stuck on?

Nope, except for a problem battery. I bought a good used (6 months old) battery from my neighbour last summer. It's a top post so I used adapter cables and I'm having problems with cold weather starting. I know I need a battery but this is the only other thing different.

Summer driving to work and back used to be about 17l/100km and winter would go to 18+ but never this bad. All my driving has been the same for the past 3 years. Coil failure drove the mileage all to heck and barely dropped back a little with the replaced coils and new plugs.

If she wasn't running and idling so smoothly I'd think I had left off a vacuum hose or something...

 

[Mooseman]

although no reader can reset codes while the engine is running.

Sorry but, yes they can. Done it lots of times with cheap wired readers, BT adapters as well as the Tech 2.

 

[TequilaWarrior]

Sorry but, yes they can. Done it lots of times with cheap wired readers, BT adapters as well as the Tech 2.

Hmm... I haven't had any that could. When I use my code reader to reset the SES light, I can hear it open&close the throttle, it won't let me do it while the engine is running. I'll have to check it out again.

 

[Mooseman]

Might not work if the issue is a "hard" error, like an unplugged sensor, but it will clear other codes. I once had an issue with one hard code that persisted but cleared all the other codes.

 

[HARDTRAILZ]

I do it all the time with my Scan-gauge II while driving and with the lil Bluetooth adapter and phone. Both work whether stopped or moving or engine on or engine off.