Bad gas mileage all of a sudden..Possible reasons?

[davemsc]

Hey all,

Got a P5 with 212,000km on it.

Now keep in mind it is winter and I am doing a lot of short stop and go trips, but I just had the spark plugs changed recently (NGK, gapped to .04) and ever since then, it got bad. THis last tank was all stop and go short trips in the dead of winter and got 17mpg. Anyone else have that bad?

Also, I am starting to smell rotten eggs occasionally. So is my catalytic converter going as well? Any tests for that?

Thanks!
Dave

 

[The_big_dill]

- Tire pressure
- Dragging brakes
- worn O2 sensor
- losing compression in engine

Typically in the winter cars get poor fuel economy compared to summer.

Any engine codes?

Considering the mileage of your car, your catalytic converter may be starting to go bad.

 

[davemsc]

- Tire pressure
- Dragging brakes
- worn O2 sensor
- losing compression in engine

Typically in the winter cars get poor fuel economy compared to summer.

Any engine codes?

Considering the mileage of your car, your catalytic converter may be starting to go bad.

Hey,
thanks for the reply.

It was more of a sudden change...the last tank before the spark plug change I was getting about 24mpg in the winter..all of a sudden, 17mpg (granted a lot more cold starts, short trips etc..). Almost too coincidental after replacing the spark plugs.

No codes. In december (mostly highway) I got 29mpg.

How can I tell if an O2 sensor is going?

Also, like I mentioned, I am smelling rotten eggs periodically - not everytime outside the car after driving. Can a catalytic convertor PARTIALLY fail (i.e. only sometimes?)

Thanks

 

[The_big_dill]

Well I was going to say that spark plug brand has this effect sometimes, but you mentioned NGK so you should be fine.

Did you check your tire pressures? They should all be at 32PSI. What about brakes? After a drive, the longer the better, feel the air near all the brake disks, is one hotter than the other? (Don't touch the actual disk!)

O2 sensors usually deteriorate over long periods of time and become "lazy". Since you say its a rather spontaneous jump in fuel economy, and you don't have an engine light, I would rule this out too.

No, catalytic converters don't work intermittently, when they fail, you will smell it every time.

The only thing left that I can think of is that you mentioned that you only tried 1 tank of gas. I have seen gas affect the performance and fuel economy of a car even when fuelled at a regular gas station. Give your car another tank of gas from another gas station before you start diagnosing further.

Its also worth noting that short trips means your car is running more often below operating temperatures than it used to, this state is not very good on fuel economy. But I'm sure you knew that already.

 

[davemsc]

Well I was going to say that spark plug brand has this effect sometimes, but you mentioned NGK so you should be fine.

Did you check your tire pressures? They should all be at 32PSI. What about brakes? After a drive, the longer the better, feel the air near all the brake disks, is one hotter than the other? (Don't touch the actual disk!)

O2 sensors usually deteriorate over long periods of time and become "lazy". Since you say its a rather spontaneous jump in fuel economy, and you don't have an engine light, I would rule this out too.

No, catalytic converters don't work intermittently, when they fail, you will smell it every time.

The only thing left that I can think of is that you mentioned that you only tried 1 tank of gas. I have seen gas affect the performance and fuel economy of a car even when fuelled at a regular gas station. Give your car another tank of gas from another gas station before you start diagnosing further.

Its also worth noting that short trips means your car is running more often below operating temperatures than it used to, this state is not very good on fuel economy. But I'm sure you knew that already.

Yeah, Of course I filled up at the same gas station the other day...even after I told myself I was going to try a different one due to the reason you mentioned.

Brakes are good ...no dragging.

I mean, maybe it is just the winter and cold starts and very short trips. I mean, I've only had the car since October and this last month has been BRUTAL weather here (cold, tons of snow etc). So maybe it is all adding up. I just didn't think it would account for THAT much of a drop. I also owned a P5 6 years ago and it never dropped that low.

What is concerning me is the rotten egg smell that is certainly been happening lately. I don';t notice it all the time but maybe that is because of wind etc....I really noticed it after going up a hill at about 80kph and then stopping in a parking lot.

 

[pcb]

I had family members call me two separate times to say their cell went off. I told them to take it on the highway and run it hard for half an hour. Both times it cleared the code.

I say put half a can of seafoam in your gas then hit the highway for a spirited trip.

Change your air filter too,.. they're cheap and easy and made a 5 mpg difference on my car once. (and they are part of regular maintenance anyway)

There is a whole bunch of codes that your car will throw if either O2 sensor is shot or gets too lazy. The ECU will let you know when it's time to change it.

Here's the big list of things to consider if your gas mileage sucks.

PS... Repeated short trips in cold weather can foul any car up really fast... the engine never gets warm enough to burn off or vaporize all the condensed crap...

 

[kms1990]

i got 19mpg on mine with 2 days of trudging through ice and slush and it being 20-30 degrees and only 20-45 degrees the entire tank. but the one before i got 26 and this tank (according to were the needle is and mileage on my trip OD) i am getting about 27mpg these cars seem to suck on gas in the winter worse than others i have owned. 3 nissans never had a noticeable change in winter mpg from summer but this car gets from 2-5 less in winter than summer. my o2 sensors are factory and i still get 30-32mpg average if i drive freeways alot (most of my trips are down a highway with red lights to work) so i get about 28mpg average. i never replace o2 sensors unless they go bad as i have done that before and did not notice any change in mpg or how well that car ran.

seafoam through your vacuum lines (brake booster vacuum line) its easy 1. warm up the car to running temp. 2. turn car off and diconnect vacuum line from brake booster 3. start car up (you may need to open the throttle with thottle cable on throttle body if it starts dying) 4. slowly pour about 1/3-1/2 the can of seafoam through this line while car is idling (slowly so it does not choke and die) 5. shut car off, reconnect vacuum line let car sit for 20mins-3 hours (depending on how dirty you think it maybe) 6. go drive the car (it will smoke for awhile but its ok) and keep driving for awhile and shut the car off and your done. take the other 1/2 of the can and pour into gas tank (full to half tank of gas) and your engine will be a lot cleaner and it will help clean gunk off your o2 sensors as well which can all help mpg. i do this once every oil change.