Why does my car get poor gas mileage?

[aquajet]

I've been driven mad lately trying to figure out why my '99 VW Beetle with 170k miles seems to be chugging gasoline. I'm obsessed with trying to get this thing to be more efficient considering gas is above $3 again. It gets 25mpg on the highway these days. It used to get over 30. Unacceptable for a small car IMO.

Parts that have been changed within the last 1.5 years: catalytic converter, both O2 sensors, mass airflow sensor, coolant temp sensor, air filter (with a K&N), timing belt and related accessories. It also has new spark plugs.

My mechanic told me it has low vacuum under heavy acceleration and that it could be due to the foreign matter inside the muffler (probably from the old cat converter). But a muffler shop told me that there's no obstruction in the system. It doesn't burn oil so I assume it still has good compression.

So, before I bring it in again, I would like to try to figure out why the vacuum is low. I suppose this could be related to the decreased fuel efficiency?? What else should I be looking for?

 

[Abstract]

Are your driving habits different? Did you move to a new city?


Or maybe you're not a very efficient driver.

 

[MacNut]

Check all the seals and gaskets. I don't know if I would complain about 25 mpg tho.

An 8 year old car with over 170k miles is a lot of wear and tear, its not gonna run like new anymore.

 

[majiktt]

I can't comment on the mechanical aspects, but have you checked things such as the tire pressure, oil grade, and that you're not carrying around unneccessary load you didn't have before? Also, maybe your air filter is dirty already?

 

[aquajet]

Are your driving habits different? Did you move to a new city?


Or maybe you're not a very efficient driver.

Now that you mention it, it spent its entire life in Maryland until ~1.5 years ago when I bought it from my father. I'm in Texas. But it would still go longer on a tank even after it was moved. I guess it's been slowly degrading over the last year or so.

I drive it gingerly most of the time, but I run it up to redline every few weeks to blow the cobs out.

Check all the seals and gaskets.

Ugh. You try doing that under the hood of a Beetle!

I can't comment on the mechanical aspects, but have you checked things such as the tire pressure, oil grade, and that you're not carrying around unneccessary load you didn't have before? Also, maybe your air filter is dirty already?

Yeah, my tires are at 32 psi. I use synthetic 0W 40. Air filter is still clean.

It's usually just me in the car. You'd think I get better mileage since I've lost 50 pounds over the last two years.

 

[pseudobrit]

Parts that have been changed within the last 1.5 years: catalytic converter, both O2 sensors, mass airflow sensor, coolant temp sensor, air filter (with a K&N), timing belt and related accessories. It also has new spark plugs.

My mechanic told me it has low vacuum under heavy acceleration and that it could be due to the foreign matter inside the muffler (probably from the old cat converter). But a muffler shop told me that there's no obstruction in the system. It doesn't burn oil so I assume it still has good compression.

So, before I bring it in again, I would like to try to figure out why the vacuum is low. I suppose this could be related to the decreased fuel efficiency?? What else should I be looking for?

Check all your vacuum lines. Then think about getting a new cat back exhaust. Remember if you're doing lot of highway driving the Beetle is an aerodynamic nightmare with a Cd of .38 or something.

 

[aquajet]

Check all your vacuum lines. Then think about getting a new cat back exhaust. Remember if you're doing lot of highway driving the Beetle is an aerodynamic nightmare with a cD of .38 or something.

I miss my '97 Jetta with 5 speed and the same 2.0L engine and 36 mpg.

The only things I've done myself on this vehicle are changing the oil and air filter. What's the best way to go about detecting a leak in the vacuum lines?

 

[clownie]

Beetle Mileage

Check for fuel leaks. My last VW developed obvious (as in dripping) leaks in three of four fuel injectors... the rubber cracked and the cloth jackets just made some of the leaks harder to find...

 

[IJ Reilly]

The car should run very poorly if it's got an actual leak in the vacuum lines, so I doubt that one.

Your engine is pretty elderly. Even if you don't have any obvious compression issues, you're still talking about a lot of internal wear.

That being said, it's not clear how you are comparing mileage. A 15% difference is not that much, when you consider that even an increase in highway driving speed of 10 MPH can produce that sort of mileage hit, especially in a car which isn't exactly an aerodynamic wonder.

 

[Zwhaler]

Before I opened this thread, I thought, "I bet he has a VW". I don't know why, but those things just naturally get bad gas mileage. Even the small ones.

 

[aquajet]

The fact remains that I haven't changed my driving habits, so I'm not really convinced my highway speeds have much to do with it.

To be honest, I'm a bit suspect of the exhaust tech who examined the system by shoving a small rod down the length of the pipe. I'm thinking it might be time to disassemble the system and do a visual check instead.

Before I opened this thread, I thought, "I bet he has a VW". I don't know why, but those things just naturally get bad gas mileage. Even the small ones.

Like I said, my gas Jetta got 36 mpg cruising at 75 mph across the US. The turbo diesels can get 50+ if I'm not mistaken.

 

[7254278]

The fact remains that I haven't changed my driving habits, so I'm not really convinced my highway speeds have much to do with it.

To be honest, I'm a bit suspect of the exhaust tech who examined the system by shoving a small rod down the length of the pipe. I'm thinking it might be time to disassemble the system and do a visual check instead.

With a 170,000 miles and all the maintenece your doing wouldnt it just be better to put up with the bad gas mileage(which is still decent) and just save up for a new car?

 

[aquajet]

With a 170,000 miles and all the maintenece your doing wouldnt it just be better to put up with the bad gas mileage(which is still decent) and just save up for a new car?

A new car? I don't plan to ever buy a new car, used only and cash only. I don't plan to dump this car either until it breaks in half. Even if the engine exploded, I would just replace it.

 

[MACDRIVE]

My mechanic told me it has low vacuum under heavy acceleration

That's the way it's supposed to be; you have low vacuum under acceleration because of all the air rushing into the engine. You will have high vacuum at idle speeds because the air entering the engine is at a minimum.

How does the engine perform other than a decrease in fuel mileage?

Do you still have a smooth idle?

Do you still have plenty of power?

 

[kajitox]

I guess I shouldn't mention the mystery that is 18 MPG with a 1999 Nissan Maxima (80K mi).

Thus far I haven't found any problems with it, but still get really poor milage.

 

[Roger1]

Have you thought about getting a tune up? With my car, a tune up involves checking sensors, cleaning the air intake thingie, etc. It might be worth looking into.

 

[killr_b]

You may have a blown head gasket.

That'll eat right into your gas milage without adding new sounds or odd feelings to your car- it will still drive "normally".

 

[quagmire]

A new car? I don't plan to ever buy a new car, used only and cash only. I don't plan to dump this car either until it breaks in half. Even if the engine exploded, I would just replace it.

Why a used car? Why not new? I understand financially, but never planning to buy new? Frankly, I rather take the risk of a lemon then risk abused car.

Anyway..... it is probably due to wear and tear. At that mileage, fuel mileage would be down, power will be down, etc. It is all due to normal wear and tear. But, I would check for anything wrong with the car if this was a sudden drop if nothing changed on your side.

 

[adk]

I used to have a 2000 jetta with the same engine as your beetle, and never even once managed over 25 mpg.

 

[pseudobrit]

The best thing you could do is find someone with VAG-COM and log some data while driving. That way you keep yourself from throwing money at it.

Like I said, my gas Jetta got 36 mpg cruising at 75 mph across the US. The turbo diesels can get 50+ if I'm not mistaken.

Mine has gotten as high as 56 @ 75mph on a long distance trip.

 

[Porksword]

I'm surprised this wasn't mentioned already

I had a 97 Corolla with 115000 on her and changing the fuel filter on it helped my milage among other things. This would have been done during a 30k, 60, etc. dealer tune-up but if you haven't had one done in a while, it's worth trying. If you do it yourself its only a $35-ish dollar job.

 

[aquajet]

Why a used car? Why not new? I understand financially, but never planning to buy new? Frankly, I rather take the risk of a lemon then risk abused car.

Because I don't like the idea of absorbing the initial depreciation. I'd rather buy a well-maintained used vehicle with cash. No interest payments, minimized depreciation.

Anyway..... it is probably due to wear and tear. At that mileage, fuel mileage would be down, power will be down, etc. It is all due to normal wear and tear. But, I would check for anything wrong with the car if this was a sudden drop if nothing changed on your side.

VW's 2.0L motor is a solid one which will last a long time provided it's not abused and is well-maintained. My Jetta attained 36 mpg at nearly 200k miles. There's nothing to lead me to believe there's any significant wear inside the engine.

The best thing you could do is find someone with VAG-COM and log some data while driving. That way you keep yourself from throwing money at it.

Good thought. I'll look into that.

I decided to try and poke around the inside of the rear muffler because while shaking it back and forth, I can hear what sounds to be nuts and bolts and pebbles. I was also able to remove some material by using a coat hanger. I'm thinking it's time to cut it off and see how much crap is really hiding in there.

 

[daze]

A new car? I don't plan to ever buy a new car, used only and cash only. I don't plan to dump this car either until it breaks in half. Even if the engine exploded, I would just replace it.

You've got the correct attitude. I am never buying another new car ever again!

My 2002 Grand Am is running fine since I bought it 5 years ago... Oh, by fine, I mean that I've had to get the ignition module replaced twice, and the wheel hub broke, and the CD player's LCD died... Yes, and the last payment is due this month. It's got 73K on it, and guess what? I will probably never buy a GM car again. But I plan on keeping this one till it dies. I am hoping for another 3-5 years of usage out of this car.

The best feature of this car is the after market Pioneer CD player I put in it, which plays AAC files. Love it!

 

[quagmire]

VW's 2.0L motor is a solid one which will last a long time provided it's not abused and is well-maintained. My Jetta attained 36 mpg at nearly 200k miles. There's nothing to lead me to believe there's any significant wear inside the engine.

Was this a sudden drop in fuel economy or have you noticed a steady drop?

 

[climhazzard85]

You may have a blown head gasket.

That'll eat right into your gas milage without adding new sounds or odd feelings to your car- it will still drive "normally".

Um wow....


Anyway, has your area recently mandated a 10% ethanol mix in the gasoline? It did here, and it hurt my gas mileage.