My Experience Is Blinding Me...Need Some Ideas

[MrCritical]

Been in automotive in some capacity since 1977, now in service in several capacities.

Wife's '06 G6 GT approaching 100k, so decided to do the scheduled maint where I work. Senior driveability tech doing the work (I shoveled him gravy for years when I was an advisor). I trust him with my family's safety.

Car has averaged 33-34 mpg on highway both on in car computer and manual calculation since it was broken in (we're the original owners. The combined mileage has always been 25-26. (3.5 V6, 4 speed auto).

Since new AC/Delco plugs, wires, coolant flush, and injector service with throttle body clean we have lost a smooth 4 mpg on both city and hwy calculated on car computer or manually.
Everything looks in spec, car runs like a top, no codes, From
my training and experience I know customers have come up with outlandish claims and I guess now it's my turn. Any ideas?

 

[The Critic]

Does the throttle body on this car needing any adaptation or reset after cleaning?

 

[daves87rs]

Fuel filter. Not clogged, but dirty maybe?

 

[spasm3]

Is this one the first tank since the service? Was the Battery disconnected? If not then i would make sure the engine is coming up to operating temp, if after the flush, the thermostat was not closing, it might be running cold and rich. Also did he change the tire pressure from what you were running? One last thing, if the o2 sensors are starting to go, you lose mileage before they throw codes, i lost 4 mpg on my escape , changed upstream sensors and got right back the the same mileage, never had a code.

 

[Steve S]

Re do all you have changed and check the throttle body. Does any body at work use an oscilliscope[sp]?Nobody seems to know anything outside of duh there is no codes. When I went through continuing auto education classes we learned how to trouble shoot devices with an oscilliscope. In the old days that is all that was used and was a life saver . The only other thing is the temp sensor but I think it has to be what was messed with. After that the o2 sensors etc the old change the parts till it works type of repair.I have been out of the busisness for 11 years and all is forgotten bit by bit.

 

[The_Eric]

Originally Posted By: spasm3
Is this one the first tank since the service? Was the Battery disconnected? If not then i would make sure the engine is coming up to operating temp, if after the flush, the thermostat was not closing, it might be running cold and rich. Also did he change the tire pressure from what you were running? One last thing, if the o2 sensors are starting to go, you lose mileage before they throw codes, i lost 4 mpg on my escape , changed upstream sensors and got right back the the same mileage, never had a code.


Yes, maybe the O2 sensors were damaged by the throttle body and injector cleaning chemicals?

 

[JoeWGauss]

Originally Posted By: The_Eric

Yes, maybe the O2 sensors were damaged by the throttle body and injector cleaning chemicals?



Very possible!

Also could be that the chemical washed a lot of carbon onto the sensor and damaged it.

 

[AdRock]

I'd bet that the battery was disconnected and for tis first tank the ECU is relearning. I bet it will go back to normal after a few tanks.

 

[MrCritical]

We've run over 4 tanks of fuel since the service, both ethanol and non. It's a steady 4 mpg loss on both hwy and combined.

I would think a degredation of the o2's would have triggered the CEL, but I'll sure check it. I honestly didn't think of that.

Thanks a lot.

 

[hattaresguy]

I'd say 02 sensors, they won't always throw a code when they get a bit weak.

 

[mechtech2]

Clean your MAF sensor.
Check your front O2 sensor
Check for exhaust obstructions. [bad cat, bent or collapsed pipe, etc.]
A 4MPG change is a LOT.

With no codes, this is odd.

 

[punisher]

Originally Posted By: mechtech2

With no codes, this is odd.


You got that right. 4 mpg difference and no codes.

Verify your L and R fuel trims against each other. Doubtful a TB cleaning could harm both O2s. See if fuel pressure is in spec (regulator not stuck closed). DI or distributor? Ck timing (bit my [censored] once on a lack of power/low mileage complaint.....once).

No real solid ideas. Anything burning that much fuel above normal would set a code. Screwed up knock sensor/harness retarding timing?

Shots in the dark man.

 

[MrCritical]

Will start scanning for sensor performance/ect tomorrow and not just failures.

The odd thing is that the performance is fine. (Not DI). It wouldn't be so odd except the car has been so on the money it's entire life on fuel mileage. Even changes in fuel formulations for seasons have never made 1 mpg difference. Ethanol is more readily available here, but regular unleaded mpg losses are almost identical. Different brands haven't made any difference whatsoever.

 

[CBR.worm]

Could be a MAF, totally unrelated to the service. They are one sensor that frequently doesn't give a code, and they don't last forever.

 

[MrCritical]

Originally Posted By: CBR.worm
Could be a MAF, totally unrelated to the service. They are one sensor that frequently doesn't give a code, and they don't last forever.



And it would still run like a top?

 

[mattd]

Have you checked fuel trims?

 

[CBR.worm]

Originally Posted By: MrCritical
And it would still run like a top?


No, my experience is that performance would suffer, but it is possible for it to run reasonably well and get poor fuel mileage.

Did you find anything out?

Can you see if it is going into closed loop mode? Dragging brakes? Low tire pressure?

 

[disturban]

I am a GM Tech and I have to agree on the O2 sensors, The cleaning agents used to clean fuel injectors are bad for o2 sensors and they should have been removed for this procedure. You could have possibly damaged a fuel injector during the cleaning procedure as well.

As long as the the ECM can see the the sensor it will not trigger a CEL, however O2 sensors can become lazy and your cross count will be slow. Sensor output ranges from 0.2 volts lean to 0.8 volts rich, an A/F ratio of 14.7 gives an average reading of around 0.45 volts, when the sensor reads above 0.45 volts and then below 0.45 volts that is known as a cross count, this should happen many times per second with a healthy O2 sensor, you will be able to see the cross counts on a live scan tool such as a TechII or Snap On solus.

If the average voltage from the O2 sensor is running high more than 0.50 volts it indicates a rich condition possibly due to a bad MAP, MAF or Air Flow sensor or leaky injector.

If the average voltage reading is running low less than 0.45 volts the mixture is running lean possibly due to a vacuum leak.

You really need to take it in and have it scanned with a Tech II to figure out what is going on, it will be cheaper to pay a GM Tech or other qualified mechanic $50 to scan it and figure out exactly what the problem is and fix it rather that throw a bunch of parts at it and still not solve the problem and continue to get bad gas mileage.

If the O2 sensors are lazy/damaged replace them with AC Delco or NGK sensors, stay away from Bosch, GM vehicles tend to be real finicky with their O2 sensors!

 

[morris]

the spark plugs have the wrong gap.