I think you're on to something in post #19 Don, and I'm also wondering if vent solenoid error codes are causing the ECM to remain in "open loop" mode for an excessively long time. That would cause a mileage drop, and eventual damage to the converters if not corrected.

(Off topic, I stand corrected. No "Jet A" for the military outside the Navy, although JP8 is interchangeable with Jet A...JP8 must meet spec# MIL-DTL-83133. The Navy still uses the more volatile JP5 that was phased in during 1952. And yes, jet fuel smells close enough to 100LL to have caused accidents due to fueling errors on the ramp.)

 

It seems to have turned out in my case anyway that P0420 (catalyst below efficiency threshold), P0171 (system too lean) and P0496 (EVAP flow during during a non-purge condition) all came from the Purge Valve Solenoid. The failed open solenoid (vac leak) caused the #1 O2 sensor to think "too lean" so the computer dumped extra gas in making the #2 O2 sensor think it was "too rich", so the computer thought that the catalyst wasn't working. And all the time consuming more gas than it should especially at low and high speed, but seemingly normal MPG at 55-65 MPH.

 

As stated in the start of the thread, I run 91 or 93 depending on what's available. The 2.4 can run on anything from 87 to 93.

I've run 87,89,91 and 93. To me, there is a noticeable difference in performance.

Back to the point of the thread, my average MPG has gone up, but only slightly.
I'm still experiencing at least a 27% drop in efficiency from the 33-34 MPG average I've been getting in the two years I've had the car.

I'm due for an oil change soon, so I will see if that has any effect (doubtful) but I am also going to run a can of Seafoam in the next tank, and also Seafoam the intake valves. I did do 1200 miles in 4 days after all, and to be honest, I have no clue as to the formulation/quality of the fuel sold in Ohio. (That's where I was.)

We'll see what results these steps yield, then I'll start checking if there are any codes. (no idiot light on, but you never know.)

 

Here is an easy test for the Purge solenoid that may cause bad MPG: remove both hose and blow into either end of the solenoid, if you can feel air going through it it is bad and making the engine think it is running lean.

The purge solenoid is the one in the engine compartment. Don't force the hose clips, once you figure them out they release easily. A new one at Autozone is $15.

 

My recent drastic drop in MPG was fixed with a new air filter, I think. One day 360 miles mostly highway back near 30 MPG FROM 27 MPG. But I also dumped a pint of SeaFoam in the tank and reset the MAF connector (when replacing the AF).

 

My '08LS has not been getting the usual mileage for a number of months, and I've been scratching my head as to what to try. I also am noticing a slight bit of roughness when idling, and sometimes if I really have to get after the pedal up a large, long hill on the Interstate near my house, the car will intermittently hesitate. When it does this, it will do it in 3rd or 4th gear (auto), so I don't suspect a trans issue. There are no error codes of course.

I replaced the coils some months ago which seemed to fix a really bad hesitation the car had going up said hill. The car was almost cutting out, but after replacing the coils, I am only left with the occasional hesitation, and only when really getting after the pedal.

I am using a Specter oil type filter on a cold air intake, so air shouldn't be the problem. I may go and give the MAF sensor a quick cleaning. I've run 1 1/2 tanks with Redline so far through it. Otherwise, I'm stumped.

Where exactly is this purge selenoid?

 

Driver side of the engine bolted to the head, 2 plastic tubes and an electrical connection.

 

I removed the purge selenoid today and blew through both ends and nothing, so it should not be the source of my concerns.

I noticed a slight leak in the upper radiator hose clamp, and went ahead and replaced both the upper and lower hoses. Those stupid factory clamps are retarded. I ended up having to remove the thermostat housing completely to get that end of the lower hose off as there was no way to get the factory clamp off otherwise.