Automechanicfilter: The Case of the Thirsty Tank

November 22, 2009 8:47 PM

What could be some reasons why my gas mileage is very suddenly and dramatically craptastic?

The decrease in fuel efficiency happened directly after I got stuck driving my car on a flooded road through deeper water than it should've been asked to handle. The car didn't stall, but it felt very close to stalling, and the steering was "stiff" for a few minutes. However, the functioning of the car returned quickly to totally normal. Except that my gas mileage is reduced by nearly half.

It's not running particularly rough or loud, using more oil than usual, running hotter than usual, or anything else that would signal an obvious "car is working too hard" issue.

The gas tank has been run down nearly to empty and filled, fuel injector cleaner has been used once. It's been a couple of months, so it's not resolving itself, that's for sure. And no, nothing's leaking when it sits.

It's a 1996 meticulously-maintained Neon. So yes, it's getting up in years, and I know that some cars lose fuel efficiency as they age, but this was a VERY dramatic change.

posted by desuetude to travel & transportation (19 comments total)

Catalytic converters can get gummed up and take mileage down bigtime. Not sure how the flooding would have caused that though.
posted by Hardcore Poser at 8:58 PM on November 22, 2009

The steering was stiff because power steering is hooked to the engine. So, if your engine is close to dying, it doesn't have enough power to run the power steering as well.

Try replacing the spark plugs and the spark plug wires. If you're not getting a hot spark, it can fail to burn all the gas and thus your fuel efficiency tanks. Spark plugs don't play well with water, but wouldn't necessarily cause the engine to run roughly or otherwise indicate problems.
posted by bookdragoness at 9:01 PM on November 22, 2009

A vacuum leak can ruin your gas mileage. You might be able to hear a hissing under the hood.
posted by neuron at 9:09 PM on November 22, 2009

I'm wondering if the water damaged the MAF sensor. The MAF sensor tells the engine how much air it's getting, so it can inject the appropriate amount of fuel. With a damaged or broken MAF sensor it may be running very rich, burning too much fuel. Is there a check engine light on? In any case it's probably time for a trip to a mechanic.
posted by 6550 at 9:19 PM on November 22, 2009

Was the water at the level of the air intake? Was the distributor submerged? Are your brakes dragging? Is the car shifting normally?
posted by zippy at 9:19 PM on November 22, 2009

It's definitely going to the mechanic, but I wanted some good questions to ask and possibilities to consider. Forewarned is forearmed and all that.

zippy, can you tell me how high the water would've had to have been to be at the air intake or distributor?

My brakes seem fine and the car shifts normally. No check engine light.
posted by desuetude at 9:30 PM on November 22, 2009

As an easy-to-do thing yourself, you could check your air filter and clean it - even if the problem turns out to be something else, it is still worth doing.
posted by MuffinMan at 12:32 AM on November 23, 2009

The steering was sluggish because the belt that runs the power steering pump got wet and was slipping.

I'm not sure where the alternator is located on those, but I'd wager that it is down low. It may have gotten wet, failed in some non-spectacular way, and is adding drag to the engine.

The water would have had to be (roughly) to the top of the tires for that kind of issue.

I'd suggest changing the oil and trans fluids just for good measure.
posted by gjc at 4:49 AM on November 23, 2009

One of my cars' air intake was piped to down low, near the air dam, presumably to find a source of cooler air than would be found up behind the radiator. Driving through one deep puddle allowed it a big enough drink to hydro-lock (not bending anything in the process, fortunately). The fuel injection / emission controls could never quite hold a decent idle speed, after that.
Where's your intake?
posted by TruncatedTiller at 5:58 AM on November 23, 2009

The water was definitely to the top of the tires, but probably not much higher than that, except for some ripple-wave action from other cars.

From some googling, it looks like my air intake sits pretty high -- I'll pop the hood to confirm this tonight.

(Oil and filter will be changed this week.)
posted by desuetude at 6:56 AM on November 23, 2009

The steering was sluggish because the belt that runs the power steering pump got wet and was slipping.

This.

I'm not sure where the alternator is located on those, but I'd wager that it is down low. It may have gotten wet, failed in some non-spectacular way, and is adding drag to the engine.

Not this - the battery hasn't gone flat and it is almost impossible for an alternator to 'fail' in a way that adds drag to the engine. Quite the opposite in fact unless it is a spectacular failure that stops it from turning - although in this case it would stop charging and the battery would go flat.

Try replacing the spark plugs and the spark plug wires. If you're not getting a hot spark, it can fail to burn all the gas and thus your fuel efficiency tanks.

No. This would not cause such a significant difference on it's own. 10% difference? Possibly but not in any way through the submersion issue, just through usual service life.

Water did not get into the intake to any degree to cause an issue - a hydro lock is usually terminal and would certainly have stalled the car. This didn't happen at all, and I suspect the drop in rpm was just as much part of the force of pushing all that water around as an indication of the lack of good spark from water dissipation.

There is no need at all to worry about changing oil (especially transmission oil) because of driving through a flood - this come up a lot in these types of questions and it is garbage. If oil isn't leaking out your engine or transmission (and in most cases even if it is) there is no way that water can get into the oil system through submersion. It would have to actually pour in the top to get into the oil system, and you'd have to have left something (like the oil cap) off for this to happen. Water, even to the depth of the tyres, doesn't have enough pressure head to force it's way into an engine.

The car needs to go to the mechanic, as there really isn't any way to make an accurate guess for suitable issues. The diagnostic computer that every reputable garage uses will be able to find out which sensor has gone south (possibly through shorting out the loom by being submerged) and it may be that drying out the plug will fix it, or it could be that a new item (Maf sensor, temp sensor even etc) needs to be replaced. Without any other symptoms other than high consumption, you'll only get wild guesses until someone actually puts hands on the car and starts fiddling, basically.

Best bet is to make sure you have all the details and all the symptoms clear in your mind and be totally open with your mechanic - something you may think is important could be completely irrelevant (like the steering issue) or something you think is unimportant may be useful to them.

Good luck, and let us know what happens.
posted by Brockles at 8:25 AM on November 23, 2009

Re-examining symptoms in my head for things I may have rationalized:

It seems like the gas consumption goes from craptastic to super-craptacular on the last 1/3 or so of the tank.

If it gets below a 1/4 tank, I feel like it maybe it runs a little rougher sometimes. Nothing really obvious, like a specific noise, just that it sometimes feels a little "off."

(It's a little hard to tell, because my little car isn't so young, and isn't exactly the smoothest-riding car anyway.)

Clarification: It's going in for an oil and filter change because it's due, not specifically for this issue.
posted by desuetude at 8:45 AM on November 23, 2009

If it gets below a 1/4 tank, I feel like it maybe it runs a little rougher sometimes.

Interesting. I wonder if this is a fuel tank issue rather than an engine issue, then. Perhaps the breather is clogged and the water immersion was coincidental, or perhaps there is sludge in the bottom of your tank that gets pulled against the fuel pick up when you fill up (Occasional resulting drops in fuel pressure will cause rough running and 'hesitating' from the engine). It is unlikely to be water in there, as that will sink to the bottom and cause the engine to stall (the fuel pickup is usually the lowest point in the tank, obviously) but something that was flushed back through the breather by the water immersion maybe?

If the consumption depends on the fuel tank level in a repeatable fashion, it is unlikely to be anything to do with the engine, but more to do with the fuel tank and the delivery system. The tank, crap in the tank, the fuel pump, the filter and the breather system are all suspect if that is the case.

When you take your car in, I'd highlight the consistency of the fuel tank level versus the fuel consumption and add "in case it matters, I drove through a flood a while back" as your description, rather than the other way around. The flood may actually be either coincidental or just a trigger for the true issue.
posted by Brockles at 10:04 AM on November 23, 2009

The flood may actually be either coincidental or just a trigger for the true issue.

I am so positive that the flood was not coincidental. It was THAT dramatic and immediate of a difference.

But I'll make sure to not gloss over the possibility that there may be a pattern depending on the the fuel tank level. That's really helpful advice, because this pattern is a really subjective assessment, and I was inclined to think that perhaps I was imagining it out of paranoia that I was going to run out of gas.
posted by desuetude at 11:56 AM on November 23, 2009

You said in one place you are going to the mechanic. In another place you said you are getting the oil and filter changed. Do not go to a "quick oil change" operation for this service. If you have a mechanic that you use, he/she will be familiar with the car and will be thinking about solutions, not upsells. My own guess is that you have a partially clogged fuel filter. I don't know where it is on a Neon, but they are generally visible to a trained mechanic.

Another thought is that gasoline is highly volatile. If you park out in the open and drive at moderately high speed, you may never notice a gasoline leak. It could be anywhere from the tank to the engine. Have your mechanic check for this as well.

Good luck.
posted by Old Geezer at 2:59 PM on November 23, 2009

The failure I was thinking of was in the nature of cracked case or bad bearing. Still making juice, just adding drag.
posted by gjc at 4:08 PM on November 23, 2009

You said in one place you are going to the mechanic. In another place you said you are getting the oil and filter changed.

Sorry if this was unclear -- oil change/maintenance/car trouble goes to the same auto repair shop with my almost unbelievably honest and humble dude. Not a quick-change place. Those places are evil incarnate.

I park on my city street in the open. I'm not always driving at high speed, though I'm either 70 straight down the highway or 10-20 mph in the city. But I'll add a leak to my list of things to ask about.
posted by desuetude at 6:22 PM on November 23, 2009

Update: Diagnostic computer didn't indicate anything wrong. So, that rules out the sensors pretty well, huh?

It could still be a fuel tank/delivery system problem, though, I guess? Perhaps my mechanic is too cautious and should've poked around more aggressively.

I've got a recommendation for another mechanic in my neighborhood -- I'll see what he says.
posted by desuetude at 11:38 AM on December 3, 2009

Diagnostic computer didn't indicate anything wrong. So, that rules out the sensors pretty well, huh?
Not necessarily. It's not unusual for a sensor to produce a signal that's in the range expected by the ECM while not producing a signal that's precisely accurate to current conditions.
Just the other day, I was working on a car that had a really vague fault stored that had something to do with a possible lean condition. It turned out that the Mass Airflow Sensor was underreporting the amount of air by just a few grams per second. At a glance the reading was approximately correct (as far as rules of thumb go) and it would have been really easy to dismiss, especially since there wasn't any fault highlighting that sensor in particular.
But, this condition didn't result in a 50% mileage decrease. Just a virtually symptomless Check Engine Light.

Another possibility is that water has infiltrated the gas tank. There's a ventilation system that prevents the tank from collapsing due to pressure fluctuations. I don't know the specifics of the Neon tank, offhand, however.
It's easy enough to sample the fuel in your tank, put it in a clear container, let it rest, and see if any water separates out from the fuel.

And, PS. Your profile says you're in South Philly. I am also in South Philly and I just happen to fix cars for a living. MeFi mail me, if you'd like more information.
posted by Jon-o at 6:29 PM on December 3, 2009