Sudden horrible mileage, no CEL

Thanks in advance for the help.

I have an 05 GC with about 175,000 miles. Last week it suddenly started getting horrible mileage. I was getting about 300 miles per tank, now im lucky to get 200. I drive the same route every day, with cruise control so that variable is constant.

My scanner shows no codes related to fuel (there is a b2331 code, vehicle needs degaussed but again, no CEL)

02 sensors are all at .9-.9.4 volts,
Thermostat is new, plugs, coils have maybe 20,000 miles on them. I first noticed it after an oil change, but the shop had lowered the tire pressure from 38 to 32psi so i thought that may be it, but that didnt help either. My scanner does show system A is a closed loop, system B is open. But i dont know what to do with that info.

Does anyone have any suggestions? Again, thanks in advance

 

Welcome @bradley300 ...just some thoughts: check air filter? brakes dragging? decreased cat efficiency? bad tank of gas? higher tire pressure usually lowers rolling resistance.

 

I dont think i can check cat efficiency on my scanner but i can see temps and its normal. Bad gas is a small possibilty, but im at least 2 full tanks into this issue and i generally get gas at the same spot. Air filter looks good.
What really has me stumped is how quick it happened. Literally like someone flipped a switch, but no CEL but it does still light up on startups so i know it works

 

For starters, and O2 sensor should vary from .1 to .9 volts, at least mine does. When it gives a steady reading, it is bad. Interesting about your system B, as shouldn't it be closed loop? You can check a cat condition with a pressure gauge. Unscrew the sensor and screw in a pressure gauge. The less pressure there is the better, say no more than 3 p.s.i. The procedure should be on a YouTube video.

 

From everything ive read system b should be a closed loop so im thinking that may at least be part of the issue. I should clarify what i said was the o2 voltage max, it goes down and then peaks about every two seconds so im pretty sure it is good. Heres some info from a 2500rpm run

Intake man absolute pressure 8.6inHg
Short term fuel trim bank 1 (-4.7%)
Short term fuel trim bank 2 (-.8%)
02 bank 1 sensor 1 fuel trim (-4.7%)
02 bank 1 sensor 2 fuel trim N/A
02 bank 2 sensor 1 fuel trim (-.8%)
Long term fuel trim bank 1 (10.2%)
Long term fuel trim bank 2 (5.5%)
02 bank 1 sensor 1 voltage (.665)
02 bank 2 sensor 1 voltage (.665)
02 bank 1 sensor 2 voltage .685
02 bank 2 sensor 2 voltage .625
They both have a slow climb to that number while all the others look like a heart beat monitor constantly up and down. this makes me wonder, so does bank 1 sensor 2 fuel trim being n/a but again, i have the info but dont know what to do with it

i also have these same stats at idle if someone thinks they will be helpful

 

Finally threw a code! It developed a shimmy under load tonight and a scan showed Cylinder 1 misfire. Took out the plug and its coated pretty heavy in oil. Cleaned it up, re-installed it and swapped coils with cylinder 3. New scan still showed the misfire in cyl 1 but the shimmy is gone for now, my guess is it will return if i give it time to accumulate some oil on the plug. So i guess my next question is what can cause the oil on the plug?

 

My $0.02 says you’ve found it....now for what’s causing it... see if there is a full signal to the #1 coil. I’m guessing the coil itself is now working fine on cyl #3.

As for the oil on the spark plug, a severe misfire (no spark at all) may be responsible for that cylinder drawing a tiny amount of oil mist into the combustion chamber with every revolution, coating everything (think: blowby caused by cylinder vacuum).

A bad misfire will certainly kill mpg.

 

If coil is fine..Bad ring(s) in cylinder 3??? It would explain oil covered plug.

 

Its not running fine, but it is definately running better, but i think that was probably from cleaning the plug. The plug itself was a little loose as well, it was in all the way but didnt take much at all to turn it. Does this engine have spark plug seals? Im going to pull the rest of the plugs on that bank this week and see how they look.

 

Yes definitely clean the plugs, or at least the bad one. I was hung up on an ignition problem, but, if that plug oil fouls, yes, that will still cause a misfire. The only other way oil can get in there is through the valve seals, but an instantaneous failure like you mentioned is pretty much unheard of with valve seals...they go bad slowly.

What’s weird is how quickly it failed, by that I’m thinking about the quick mileage drop. A compression check will tell a lot about the health of each cylinder, and is relatively simple to do. If it is a broken ring, it certainly could be like flipping a switch like you mentioned.

Bob is certainly on to a possibility with a bad/broken ring. It’s rare, but not unheard of. Has that engine ever overheated? Is there gunk in the cooling system per chance that might cause it to overheat? I’m trying to think of what would cause some damage. Also, if oil is going through the cylinder into the exhaust, keep your eye out for an engine code involving that catalytic converter and/or O2 sensor(s).

The seal that the spark plug creates is a result of either the bevel or flat gasket at the top of the threads, whichever it has. They simply need to be snug and preferably torqued in place.

 

Thanks for the replies.

so is there no replaceable seal around the sparkplug inside the valve cover? That was an issue on my wifes old Acadia, seal was bad and oil was escaping into the cylinder through the sparkplug area.

Valve seals... i have seen some blue smoke on start up in the past, always in the morning after it sits all night. But its only happened 2 or 3 times and not happening now, so idk

its been hot, never stalling out and boiling over hot, but close (thermostat stuck closed) That was over a year ago though and this is the first time this problem has cropped up