I drive a 2005 Grand Caravan 3.3L with 55K miles and its getting 13.7 mpg in city (down from 18 mpg). Vehicle well maintained and in excellent shape overall. Had intermittent CEL (O2 Sensor bank 1 sensor 2) and replaced downstream O2 sensor. CEL stopped but mileage has not improved. Have not replaced upstream O2 sensor because no CEL codes indicated failing. There are no secondary indications of problems - engine idles, accelerates, cruises smooth. Have not checked for binding brakes but do not think that is cause. Does anyone have ideas on what to do next to find cause of very poor mileage? Can a weak upstream O2 sensor be the problem (even if no CEL/fault code)? Would it be good idea to replace it too? Thanks in advance.

 

There's the usual poor alignment, dragging brakes, topography or traffic changes to consider. Same driver driving it? After that number one on my list would be to get a scope or capable scanner on it and check fuel trims. Short of that, that upstream O2 sensor would be high on my list. They often wear to the point they degrade performance without throwing a code. Some articles in tech mags advocate changing O2 sensors at 100K miles as a scheduled item.

 

Thanks for the reply. All "usuals" have been eliminated. All that's left is (I believe) is fuel trim. So it could be a "weak" upstream O2 sensor not tripping a CEL/code but still causing excessive fuel consumption? If I go ahead and replace it and don't see a change what is left as culprit? Fuel injectors or something else with electronics/ECU? I am pretty much at the end of my knowledge level at that point. Is dealer only place to do that type diagnostics? Again appreciate the assist.

 

Bad thermostat? Running too cool would cause an overly rich condition which would lead to poor fuel mileage.,...

 

A partially plugged cat can cause bad mileage.
Our stratus had both cats almost totally plugged. No CEL, power seemed down slightly, but mileage was horrendous (14mpg).
Other sensors can be going bad, but still within range, causing poor mileage but no CEL as well.
Coolant temp sensor, throttle position sensor, for instance.

 

Thanks for the note offgas.
Thermostat seems to be ok... at "warmed up" op temp the temp gauge stays mid range. The car is used daily for mostly short (3-6 mile) city trips. I am north of Seattle ... ambient temp this time of year stays below 50 deg F so vehicle (thermostat) doesn't always reach 'warmed up' on the trips. Do you think running cool/not fully warmed much of the time could produce such bad mileage. Guess only way to verify is check highway mileage at nominal op temp.

 

Thanks Smokesxt.
Woa.... plugged cat never crossed my mind. How did you diagnose.... was there a test procedure? At this point my plan is to change out the upstream O2 sensor next and monitor any change in mileage. If nothing happens, on to thermostat and coolant temp sensor. Last will be throttle posit sensor. If it can be done I would prefer to use diag equipment to test these rather than blindly replace. Is it possible to test them in shop that has right equipment? The plugged cat is another matter...did it require replacing or could it be unplugged?

 

I'll start off by saying that I'm somewhat doubtful your van has any problems. I'd advise you to hold off on the part hanging for a moment, and reevaluate the background information on your mpg stats.

First of all, you need to consider that everyone's 'city' driving is different. So 13.7 mpg isn't automatically bad, or indicative of any problem. But for these vans, 18 mpg would always be at the top of 'city', so that's one of the things that's curious about what you've written. Not saying that 18 city is 'wrong', just a bit surprising. The answers to the following questions might be helpful.

1) Did this van have a long history of a consistent 18 mpg, prior to the drop? (i.e. how long had you been getting 18 prior to the drop?). And, has it ever dropped down from 18 prior to this recent event?

2) Did 18 drop to 13.7 very quickly, or was it gradual? Did the drop coincide with the approach of Winter? Depending on where you live, Winter additives in gasoline can cause 1-3 mpg reductions in some vehicles.

3) Has anything significant changed about your driving pattern? Modified route, new lights, stop signs, school buses (or any other changes)? Perhaps you've added a new side trip, which has a very slow driving pattern? Or, conversely, maybe you've eliminated a regular non-stop trip that had been previously 'improving' the mpg.

4) Do you calculate mpg using the exact number of miles traveled divided by exact gallons used? It may seem obvious if you do, but I've read too many accounts of people calculating mpg by the "tankfull' method. So, just checking to make sure we're on solid footing here.

You mentioned the van runs well outside of the mpg drop. This is the thing that leads me to believe your van may have no problems at all. With a true mpg drop like that, IMO there should be SOMETHING else going on with this van which you would pick up on. Perhaps you can find something about your city driving which has changed, and can explain the mpg drop you're seeing.

 

Good idea to verify highway mileage. If that's normal then I'd be scratchin the bean.

To t-shoot the cat you can connect a vacuum gauge at the intake manifold vacuum tree then get the rpms up to 2500 and hold it there. If the cat is clogged as back pressure builds the gauge will drop ever so slowly. A shop will do the same or test pressures fore and aft of the cat to check against specs.

What's after a sensor change? Really before you change the sensor you should get the fuel trims checked. That said the sensor is a logical "stab in the dark". After the sensor? Get the trims checked before you head down the road to an empty wallet changing part after part.

BTW - If you go for an O2 sensor change don't use a Bosch sensor. Many reports of problems with them on multiple forums. NGK or Denso work fine. I believe Denso is OEM.

 

Thanks Cougar41... definitely a test I will have a shop do. Yes I've seen the feedback on Bosch. Will go with NGK if I do replacement.