Poor gas mileage

**jackdup** · 10-07-2010

I bought a 2005 GTP about a month ago and the gas mileage around town is terrible. The car runs great and has the power it should but gas mileage is about 11.5 MPG around town. The first fillup I calculated the MPG and thought perhaps the gas tank wasn't completely filled on the previous fillup and thus the poor mileage, but just filled it again today and the exact same thing. Between the last fillup and this one I took special care in how I drove to see what it would do for mileage but it made no difference. I haven't had it on the highway for any appreciable distance so don't know what highway mileage is like.

Any suggestions aside from taking it to a GM dealership and spending $300 to have them tell me everything is okay.

Thanks

**blueguy** · 10-07-2010

Check the TB...for one the '04+ cars get gunked up really bad...might be an issue there...also, I'd check the o2 sensor (Top one), air filter, plugs/wires...don't have to worry about the fueling system since its all returnless though.

**machinegunsquid** · 10-07-2010

It needs a good tune up! Get a new O2 sensor, new plugs, air filter, and fuel filter. Get new plug wires if you want, or if any look questionable.

**jackdup** · 10-07-2010

Thanks for the replies.

Is there any easy way to test the O2 sensor or do you need a scan tool?

Also as far as the TB can it be cleaned with a spray type carb cleaner or does it have to be removed and cleaned?

Plug wires "appear" good. Was planning on changing the plugs anyway but with about 60,000 miles on the car didn't think they would be bad enough to reduce the mileage that much.

Do you know what one can expect for mileage in the city? I know what the EPA ratings are but they are often not what the average driver gets for mileage?

Thanks

**Fivefingerdeathpunch** · 10-07-2010

remove and clean, not sure on what to use tho.

change them, i say every 40-50k.

i drive 85% city and get around 16-19 depending on stoplights and stoplight road rage and normal puttering around. 30 ish freeway.

**machinegunsquid** · 10-07-2010

Well my 2002, i get around 25-26city driving, and around 34-35 on the highway.
As far as testing the O2sensor, not a real good way to do so, other than changing it. Dont use carb cleaner, use an intake cleaner meant for use with cars that have MAF sensors.

Go ahead and change the plugs, stick with copper plugs, dont go fancy, its just a waste of money.

**jackdup** · 10-07-2010

Thanks for the suggestions, will give it a try and hopefully that will be the answer.

**cerick08GP** · 10-07-2010

Originally Posted by jackdup

Thanks for the replies.

Is there any easy way to test the O2 sensor or do you need a scan tool?

Also as far as the TB can it be cleaned with a spray type carb cleaner or does it have to be removed and cleaned?

Plug wires "appear" good. Was planning on changing the plugs anyway but with about 60,000 miles on the car didn't think they would be bad enough to reduce the mileage that much.

Do you know what one can expect for mileage in the city? I know what the EPA ratings are but they are often not what the average driver gets for mileage?

Thanks

Q#1 Well, you can test the output voltage on the O2 sensor. But that isn’t 100% accurate, also consider the fact it could “work” but be slow responding. I’d get the car scanned before worrying about the O2 sensor ATM. The sensor “should” last a least a 100k, but then again just had one go bad on a friend. It was on a Toyota with only 20k on it, I thought Toyotas never broke.

Q#2 You can clean it with regular carb cleaner. You don’t need to remove the TB. In fact, I recommend that you don’t. Remove the intake tube, and unplug the TB cable. Use your finger to open up throttle plate. Clean the back side of the plate and the TB walls. Then put it back together. Easy cheesy.

I clean mine once a year, so far zero problems. Let me just add that this is good maintenance to do, however a gummed up TB shouldn’t effect your mpg. The normal symptoms are starting and idling issues.

Q#3 Your plugs and wires should be just fine. However, I would pull a couple plugs just to check them out. Even though the plugs “should” last for 100k, you may want to consider replacing them. The difference in time it takes to check them vs. replacing them is nearly the same.

Q#4 IMHO most ppl with un-modded cars get slightly higher than EPA mileage. I personally get 2 mpg higher than EPA on average. I track my mpg like a hawk, and the most significant factor in my mileage is my average speed. If your one of those ppl who waits in drive through lines every day at banks and fast food places your mpg will suffer. If you city drive all the time in a wise fashion you should be able to get at least 16 mpg. If you give me your DIC avg speed, I’ll tell you what your mpg should be.

These are the big tune-ups for mpg:
1) new O2 sensor
2) new spark plugs
3) MAF cleaning
4) proper tire pressure

If I were you I’d first clean my MAF sensor, only because your car is so new. Then I would have the car scanned.

Get back to us.

**jackdup** · 10-08-2010

How do you clean the MAF sensor? I don't want to damage it and that is something I haven't done before.

As far as avg speed, I assume that is reset when you reset the Trip mileage, which I did after the last fuel-up so will have to wait a few days so it is a little more representative. If it is not reset after reseting the Trip mileage then I can check it and post it here. I don't do a lot of idling aside from sitting at red lights. I would probably only go through a drive thru once a week or so.

Thanks for your reply and suggestions.

**SlowNA06** · 10-08-2010

Buy a can of CRC Mass Air Flow Sensor Cleaner. Pull the MAF tubing out of the intake, spray the **** out of the little wires (the 04 I did this for had BLACK wires before being cleaned... it kind of flaked off after half a can) and let it dry for 5 minutes.

While it's off, you can take a can of throttle body cleaner, spray it on a clean rag and wipe down the brass valve and the backside of it, and the surrounding throttlebody walls to get the gross out. A toothbrush might also be helpful. Just don't spray directly into the open throttle body - that's a bad thing when you have a supercharger. When it's clean, use a rag or something to hold the throttle body valve open for 5 minutes to dry thoroughly before re-assembling everything.

The car might take a couple extra seconds to start, but will be fine afterward.

The other (and more thorough) option to clean the throttlebody is to remove it completely and do a full cleaning and even wipe down some of the intake... but that's an awful lot of work and might even require a gasket replacement.

**cerick08GP** · 10-09-2010

Originally Posted by jackdup

As far as avg speed, I assume that is reset when you reset the Trip mileage, which I did after the last fuel-up so will have to wait a few days so it is a little more representative.

Ummm, nope. As far as I know the AVG speed doesn’t reset with any other function. You need to reset it independently of the trip counter. I reset my avg speed every fill up, most people never mention this function in mpg discussions, but that one number can deliver quite a bit of insight on mpg.

**jackdup** · 10-10-2010

I'll check it tomorrow and post and then I will check it again next fill when I reset the trip odometer and see if it does reset. You could very well be right about it not reseting, I have never checked it and assumed it reset based on reseting the trip odomter.

Last time I checked it it was 20.8 km/h but again will check it tomorrow

**cerick08GP** · 10-10-2010

Originally Posted by jackdup

Last time I checked it it was 20.8 km/h but again will check it tomorrow

I’m nearly speechless, 20.8 km/hr? Really?

That is a crazy low average speed, just for a comparison my average speed is about 61 km/hr. I didn’t make my number up it is based off 10k+ miles of driving data. Assuming you gave a good number I have a mixed bag of news for you. The good news is your car doesn’t have a problem; it is actually getting the gas mileage it should. I personally had to extrapolate my data out significantly to compare with such a low average to be honest. I rarely average less than about 54 km/hr. I’ll I can say is you must travel in constant bump to bump traffic, or you let your engine idle way too much. As you mentioned get back with some more data, we’ll figure this out.

**jackdup** · 10-10-2010

I just checked and my average speed is 21.8 km/h. If that is the average since new then the previous owner may have done a lot of idling. I did reset a few thing when I bought the car and that may have been one of them but can't remember specifically if it was reset. The only time I am idling is at red traffic lights, and the odd visit to a drive-thru which is no frequent. Our speed limit here is 50 km/h so average speed would have to be considerably lower than that. Photo radar gets you at 61 km/h so am never over 60 just to be on the safe side. Many of our main drags have numerous traffic lights. We don't have bumper to bumper traffic but there is a lot of starts and stops due to traffic lights so getting the average speed up isn't likely.

My previous car was a 94 Olds Cutlass with a 3.4 DOHC and I got 16 with it and was disappointed, but to drop to below 14 was a real surprise and disappointment.

The Olds was rated at 14L/100km and the GP 12L/100km so certainly expected better mileage from the GP as I drive the same way and for the most part the same routes.

**rah575554eg** · 10-10-2010

Try a new fuel filter and a can of Sea Foam.

**Biohazed** · 10-10-2010

had MPG issue with my '00 ended up being the Fuel pressure Regulator ...

**cerick08GP** · 10-10-2010

Originally Posted by jackdup

I just checked and my average speed is 21.8 km/h.
My previous car was a 94 Olds Cutlass with a 3.4 DOHC and I got 16 with it and was disappointed, but to drop to below 14 was a real surprise and disappointment.

The Olds was rated at 14L/100km and the GP 12L/100km so certainly expected better mileage from the GP as I drive the same way and for the most part the same routes.

Well, the truth is you bought a car that slightly heavier and has about 100 hp more than that Cutlass. GPs get great mpg for what they are, however ALL cars (especially bigger ones) take a hit in mpg in stop and go traffic. Let me ask you this; did your Cutlass get 33 mpg on the highway? I’m thinking not, but that is what I get.

Look at it another way, for how slow you are actually traveling you could get to work MUCH faster on a bicycle.

**jackdup** · 10-11-2010

The Olds did 30 on the highway. I haven't had the GP on the highway yet but hope to in the next couple of weeks and will see what it does for mileage.

In a city with a 50km/h speed limit I'm not sure how i would get my average speed up much from where it is. It takes a few seconds from a stop to get to 50 and a few seconds slowing down which brings down the average speed significantly. Add to that being stopped at traffic lights would reduce average speed even more. I'm not exactly sure how it calculates average speed. If it is total time the engine is running which would include idling at red lights etc then I can't imagine getting the average speed up much from where it is. We really don't have many long stretches of road where a person would be able to keep the speed up at 55-60 in order to raise the average speed.

I have talked to others here that have similar sized cars and drive in the same conditions I do and they are getting 20-24 MPG. I think there is something definitely wrong aside from the low average speed so will try a few of the suggestions given here but would be surprised if the spark plugs were bad enough at 109,000 kms to cause that poor gas mileage, and one would think the O2 sensor would set a code if it was bad.

Thanks to all that responded for the suggestions.

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