just another bad mpg thread

[Str8sixfan]

So here's what I've done to the truck in the last almost 2 years.

Mileage started dropping when a check engine light came on in August of 2011. Right before that I was getting 23.5mpg fairly consistently highway miles. Pulled the PO171 code, ended up replacing the pre-cat O2 sensor. Took care of the CEL for about 2 months or so, but then came back on.

So mileage still sucked. Replaced the fuel filter later that fall
Last fall (2012) I replaced the plugs and wires
New air filter at the beginning of this year
Tried Seafoam a little later

Various stops along the way, I've re-set the battery and got the CEL to go off for a period. It's been off for a share of weeks right now too.

Anyway even with the new summer blend of fuel out, I can't even hit 20mpg (about a 19.5mpg avg now).

Where do I begin to look next? Compression check? Most vacuum lines look perfect. EGR? Plugged cat? Truck's never been speedy, so I can't really tell if it's lost power compared to what it was, doesn't seem really any different, though I could have suffered from the toad in a warm-boiling pot syndrome. I want to say it kicks down on interstate a little more than it used to, but that's really hard to quantify. I mean this truck is now 16yrs old, so the cat could be plugged for sure. I just want to know what types of things could I start checking on my own to make sure it's tuned up and running like it could.

 

[scotts90ranger]

Get some MAF cleaner and clean the MAF, also check for vacuum leaks (get a can of starting fluid and spray around intake gasket areas, if it increases in rpm then there's a leak), and check for exhaust leaks before the pre cat oxygen sensor.

If it thinks it's running lean, it means the fuel adapts have moved further out than they should be (bad mileage) for one reason or another, a dirty MAF will make odd things happen (if it's dirty and not seeing the right amount of air it will think there's less air then the oxygen sensor will be adding fuel to make up for it), an intake leak after the MAF will do the same thing, and I know it sounds weird, but an exhaust leak has a tendency to suck in air and trick the engine to think it's running lean and add fuel...

Just for the record, I got 14mpg on my last tank, but that was towing a trailer with a total weight with truck being 6800lb up and over a mountain pass... going the other way I got 12mpg... the turbo makes the engine thirsty but gets there faster

 

[bigmark303]

I would check your converters. You say you keep reseting the light and it comes back. When the o2s go bad it either runs lean or rich. If it ran rich the converters usually overheat from burning the excess unburnt fuel and vapors leaving the engine. When they overheat the msh inside breaks down and eventually collapses aka plugged cat.

Any type of vacume leak or steady issue usually trips the light again within 50-100 miles of driving because its a direct impact on the engine. The light wouldnt stay off for weeks. The cat on the other hand is sort of out of the way and in a failing state as long as the pre cat and after cat o2s are working wont show up for some time untill the engine has run out of adjustment from the o2s readings and then notices an issue.

Get under the truck and grab the exhaust pipe near the cats. Just try and wiggle it if it has play and see if you hear what sounds like sand in the cats. if the exhaust pipe is tight get a rag or thick cloth and put it between your hand and the cat then bang on it and see if you hear what sounds like sand moving around. When the inside mesh breaks down it turns into small metal fleks.

 

[Str8sixfan]

Thanks guys, both great pieces of advice. I know I looked at the MAF a while ago and it 'looked clean' but that's not to say spraying it with cleaner would be bade either.

I'll have to wiggle the cat. I did have an exhaust leak for awhile in front of the cat, but got it tightened up (the flange that is between the header and cat)....again fixing the CEL problem, but the mileage was still bad. This is sort of what makes me think that the cat is plugged. Seems like the back pressure is getting too extreme that it's causing problems further up stream.

 

[RonD]

What temp is your engine running at?

I believe the stock thermostat should be a 192deg, that should keep the engine temp up at about 200-220deg, giving you the best MPG.
Some people replace t-stats with 180deg or even 160deg, thinking cooler is better, but it is not if you want good MPG.

Also on Fuel injected engines there is an ECT sensor(engine coolant temperature), this is different than the dash board "sender" unit.
The ECT connects to the engine computer and tells it when the engine has warmed up.
Since there is no Choke on fuel injected engines the ECT serves that purpose, when it reports engine is cold the computer runs the engine rich with advanced timing, like a choke did.
This also places the computer in Open Loop, so it will ignore the O2 sensor until ECT reports that engine is warmed up.
If engine temp is not getting hot enough or ECT is not reporting that it is hot enough then you will be running richer than needed, so poor MPG.
ECT sensor can be tested with an OHM meter, test cold and then after driving and warmed up, about 40ohms when cold at 60deg and 2ohms at 210deg

The O2 sensor also doesn't work until it gets up to 650deg, so besides the ECT temp the computer has a timer that starts when you start the engine, it will look at the ECT value and then wait for the timer to expire before looking at the O2 sensor value, this is for warm starts, gives the engine enough time to warm up the O2 sensor again.
O2 sensors are not hard to test, once they are warmed up, .4-.7v is the normal range, below .4 is too lean, above .7 is too rich, .0v(after it's hot) or above 1.2v is a bad O2 sensor