Another gas mileage post

2000 GT w/160K

I've been dealing with the terrible gas mileage issue for some time now. I regularly take a 150 mile highway trip using cruise control (70-75mph) & have been burning a 1/2 tank of gas. This is ridiculous considering the Jeep & Avalanch I also drive use a 1/4 tank for the same 150 mile trip!

In the past year I've changed the fuel filter, fuel pump (Ac Delco), rear 02 sensor, k&n replacement filter, pcv, top intake manifold & water pump. Yesterday I decided to replace the spark plugs & wires & installed Ac Delco platinums. A few hours after dropping the new plugs in, I took the same 150 mile trip & realized my mileage was even worse! When I got to my destination I had between 1/2 & 1/4 tank which is even more ridiculous.

My car drives & handles fine otherwise & I'm running 30psi. Would iridiums make much difference?

While you are cruising at 70mpg with cruise control set what are your RPM's at?

Rpm's are at 2k if I remember correctly. My temp is a 1/4 of the way up when cruising & gets close to the 1/2 when sitting in city traffic.

2k RPM's at 70mph sounds normal. Anything higher than that and I'd suspect a trans issue either torque converter slippage or a stripped 4th clutch shaft.

Although 150 miles out of a half tank does seem rather on the high side, it doesn't seem terrible for. Next time try the trip at 60mph instead.

And after you drive your car next time, get out and feel the wheels, do any seem hotter than the rest? Maybe you have a dragging caliper.

I swapped the platinums for iridums now I'm back to using a 1/2 tank for 150 miles of highway driving. I guess I consider a 1/2 tank to be terrible cause I'm comparing the trucks I drive which only use a 1/4 tank. I'll check the wheels for any abnormal heat. Thanks for the feedback.

yeah, 150 isn't great by any means since your car is Naturally Aspirated. But there are many factors to take into consideration (wind, speed, traffic, stop lights, hills....)

When you took your first set of plugs out, what did the tips look like? Were they wet with gas, or covered in dark colored carbon buildup?

Next thing I would do is remove the IAC motor and clean it and its bore with carbon cleaner. Then take the MAF off and use MAF sensor safe cleaner on it.

Then I would remove the vacuum line off the fuel pressure regulator and check for fuel contamination. With the vacuum line on, turn the key to run so the fuel pump primes then check to make sure there is no gas in the vacuum port.

Other than that, you can check to make sure you have proper fuel pressure and that it holds. My GTP last summer was running rich (via scan tool), It turned out the be a leaky fuel injector.

you could also try putting a can of seafoam in the gas tank to give the fuel system a good cleaning.

I would also not rely on the gauge to tell you how much gas you are using. Fill the tank, go the distance, fill it again and see how much you used. Your gauge might be coming down too quick.

1/2 tank for 150 miles is not so bad. It means 18.8 mpg in highway. For a Grand prix with 3.8L engine and 160K miles it's reasonable. Better than my Windstar.

Don't compare with your Avalanche cause this one have a 31 gallon fuel tank so you have same fuel consumption. It's bigger, but better engine technology with direct fuel injection and 6 speeds transmission. And it's newer.

Normally, used vs new plugs don't make a big difference in fuel consumption. If the old one was working, new can improve to 0.5% only. Maybe the AC delco was very bad designed, it's strange.

A tips to improve gas mileave, draft semi. It reduces air drags a lot.

I can get close to 400 miles out of a tank of gas with my GP SE 3.8 L. That is an average of 60 Mph. With some stop and go traffic in the mix too.


A tips to improve gas mileave, draft semi. It reduces air drags a lot.

In the Chicago area the semi truck drivers are too crazy to follow. Plus with all the construction on the expressways, speeding in the construction zone the first offence will cost you $510. The next offence will be $1020, and the third offence is automatic supension of your DL for 1 year.

Possible bad lower intake manifold gaskets. None of your brakes are dragging and your tire pressures are correct?



***Also I see you have a K&N clean your MAF sensor with some MAF sensor cleaner and see if that helps. Oil from the K&N is bad for the MAF it distorts the readings when the wire get oil on them

Also I see you have a K&N clean your MAF sensor with some MAF sensor cleaner and see if that helps. Oil from the K&N is bad for the MAF it distorts the readings when the wire get oil on them

i was also going to suggest that, however noticed that the OP has a K&N drop in replacement filter therefore shouldnt really be oiling that (but then again, K&N does suggest oiling after cleaning) . IMO K&N is useless, and alot of people on this forum would agree that a good replacement Delco filter will actually do better than a K&N. again...just my :2cents:

When I took my old plugs out they all looked the same with a little white & no signs of oil/gas etc. I recently placed a can of seafoam in the gas tank & also cleaned the throttle body. I'm not sure if my lower intake manifold is leaking or not. I knew my top one was because it's easier to see! My tire pressure is 30 psi all around.

I'll give the IAC & MAF sensors a cleaning and go back to calculating my mileage (instead of using the gauge only.) I'll have to check my manual regarding checking fuel pressure & removing the vacuum line since I've never done this before.

I appreciate the tips

The white plug was probably caused by the intake leak or misappropriate plug heat range.

1/2 tank for 150 miles is not so bad. It means 18.8 mpg in highway. For a Grand prix with 3.8L engine and 160K miles it's reasonable. Better than my Windstar.

:confused:

:bs:

That is very bad gas mileage for any GM car with the Series II 3800 engine. My wife's '99 Regal (Naturally Aspirated) gets 26-28 highway and my '97 GTP (Supercharged) used to get up to 30 mpg highway before the engine mods. Cruising at 70 mph.

Bob, I love the BS flag guy! Great touch!

To the OP, have you checked the FPR vacuum line for fuel contamination?

No, I have not checked the FPR vacuum line for fuel contamination, not sure how to do this yet. The Hayes manual shows a little white on a plug as being normal after some use. I understand that the Avalanche has newer technology, but what about my '98 Grand Cherokee, this vehicle does 150 miles on a 1/4 tank & has close to 200k miles on it, & is two years older? This is what led me to believe my mileage was bad! Also, my relative drives the same distance & also uses a 1/4 tank on a '02 Bonneville.

I'll look into the vacuum line recommendation soon as I get a chance.

I get just over 150 miles out of half a tank of gas on my GTP with my mods, on winter grade gas, in mixed driving, mainly city. (just to put your milage into spec).

Check your vacuum line at the FPR for gas. It may be leaking.

You guys are all lucky, I get a big whoppin 200 miles out of my full tank! Once in a great while my car will be nice and give me 300 though but i've only seen it happen a couple times and i've owned the car since 2005! BLAH!

You guys are all lucky, I get a big whoppin 200 miles out of my full tank! Once in a great while my car will be nice and give me 300 though but i've only seen it happen a couple times and i've owned the car since 2005! BLAH!

On highway, city or mixt?

And what Year of car? GT, GTP, or SE?

It could be also an LE, STE, GXP.

Mixed driving I get about 23...all 70 mph I get about 30 mpg...but i definitely agree on not trusting the gauge, mine reads full until about the last quarter tank or so...

Mixed driving I get about 23...all 70 mph I get about 30 mpg...but i definitely agree on not trusting the gauge, mine reads full until about the last quarter tank or so...

That's is excellent! 30mpg at 70mph and 23mpg mixed is excellent for a grand prix.

So when you make 300 miles with your tank, I assume that you don't use all your fuel? Maybe 80% or something. Cause you've a 16 gal fuel tank.

But when you do 200 miles in a tank, it contradicts your gas mileage. Gas mileage should not fluctuate that much.

That's is excellent! 30mpg at 70mph and 23mpg mixed is excellent for a grand prix.

So when you make 300 miles with your tank, I assume that you don't use all your fuel? Maybe 80% or something. Cause you've a 16 gal fuel tank.

But when you do 200 miles in a tank, it contradicts your gas mileage. Gas mileage should not fluctuate that much.

Yeah at 300 miles it takes 11-12 gallons

So it's very good, 26 mpg. You doing 300 miles with 70% of tank. You can do 415 miles until it goes to empty.

So it's very good, 26 mpg. You doing 300 miles with 70% of tank. You can do 415 miles until it goes to empty.

actually it almost always comes out to 22.5-23 when I fill up, so maybe 13 gallons at 300 miles, or maybe I just usually fill up before 300 miles...

For mixed driving 23 is good. And if you're using A/C, it's normal that consumption increases a lot (20-30% more). So in summer, it'll increase.