I'm having a hell of a time trying to figure out why my fuel mileage sucks so bad ! 99 z24/2.4 ld9/auto 4 speed. The engine runs excellent and has about 50k on rebuild, doesn't leak or burn oil. No engine lights. Fresh tune up with 2200 ignition conversion, all fresh fluids, etc... But it's still getting about 12-16 mpg. The only thing that I can come up with is that maybe the o2 sensors are bad, but not throwing a code ? Or maybe it's because it's been so cold outside, the car never reaches operating temp on most trips to work and such ? Anyone else have some insight as to what may be causing this car to pretend that it's a v8 and guzzle gas ?

not getting to full temp could be your problem.

The factory tune has settings for AFR target that is based on the engine temp. Also certain temp is required to allow the car to go into to closed loop, which is when it runs at Stoich.



FU Tuning

how's your transmission? is it shifting into 4th?

The transmission shifts perfect and always goes into 4th just fine. Sometimes it seems as though the torque converter doesn't lock up in 4th until the car has warmed up a bit, but I was told that this is normal. There's slight carbon build up on the exhaust pipe indicating that it's running rich. It's definately not burning oil though.

Could the 2200 SFI Ignition system conversion be related to the poor gas mileage ? It seems like the car got better gas mileage with the original IDI system, but maybe it's all in my head because I have read on here that people have experienced better gas mileage with the 2200 Ignition system.

My z ran at least 29 mpg average with 2.2 ignition... that shouldnt be an issue


Have a nice day.

I replaced a leaky fuel pressure regulator and the weather has warmed up a bit and now it sems that I'm getting about 19mpg city with the new fuel pressure regulator and the car actually reaching operating temp.

NEVERMIND ! It's still $hit @ about 12-15 mpg

Sounds like you need a new thermostat if you're not getting up to operating temp. If you're not warmed up enough you won't shift into OD, and it will run rich. If you know you have an issue with hitting full temp, fix that first.





i find it amusing that SHOoff has nothing better to do but follow me around & be an unhelpful dick in even cross-forum. - Jon Mick

I just had a brand new thermostat installed about 1000- 2000 miles ago when I had the timing chain and water pump replaced. It hasn't been reaching operating temp on most trips to and from work lately because my work is only about 3 miles from my place. I try to give the car a good 10 minutes to warm up before driving, but it still hardly gets to operating temp because it's been pretty cold here lately.

You could cover part of your radiator with cardboard that would help warm up a bit


Have a nice day.

Couple things here.

1 - letting your car sit and warm up, while probably good for the engine... does spend fuel. Especially when it's cold out as it spends more time in warmup mode.
2 - If you're not getting all the way warmed up, you're probably burning more fuel.
3 - In the winter months they add a bunch of Butane to the gasoline blends. This helps for cold starting. But reduces MPG at the same time.

One of those alone probably won't reduce your MPG to the level that you're getting. But if you're getting all 3 together you could get some serious damage in the MPG department.





i find it amusing that SHOoff has nothing better to do but follow me around & be an unhelpful dick in even cross-forum. - Jon Mick

Makes sense ! I bet it's because all of those things. The car runs excellent, just gets bad gas mileage and lately it hasn't been getting past the first 1/4 mark on the temp gauge because it's been so cold outside and I haven't driven long distances lately. I'll see what it does when the weather warms up if I haven't already sold it or traded it by then.

I always get worse mileage in the winter. I just contribute it to letting it warm up in the morning (idle) and the fuel blend they use in winter.

ld9z24 wrote:Makes sense ! I bet it's because all of those things. The car runs excellent, just gets bad gas mileage and lately it hasn't been getting past the first 1/4 mark on the temp gauge because it's been so cold outside and I haven't driven long distances lately. I'll see what it does when the weather warms up if I haven't already sold it or traded it by then.

That really sounds to me like a bad thermostat. I know you said it's new, but still it could be a bad one. I've been seeing as low as 6 degree temps in the mornings here, and I'm still getting up to 195 degrees. It will drop after a little WOT action when it's that cold out. But like i say, I still get to 195, just takes longer.

If you were a lil below that would be one thing, but when you're way below then cold or not, you have something wrong.





i find it amusing that SHOoff has nothing better to do but follow me around & be an unhelpful dick in even cross-forum. - Jon Mick

If the outside temps are above freezing and I drive more than 5 miles, it gets to the middle mark at 195. I live 3 miles from wehere I work, so most days it doesn't get enough time or miles to fully warm up.

What oil are you using in your car? synthetics will result in better fuel economy typically. I know for a proven fact, every time after an oil change, all my vehicles get much better mileage on a full tank.
bad air filters, O2 sensors can also cause it.
you said its running good, but faulty plugs can cause bad mileage.
check to make sure you are not getting a small fuel leak by the injectors. I had one for a while and didnt even notice. Cylinder 1 injector seal was bad (they're like $1.80 for 2 at any auto store).
the fuel pressure regulator diaphram can go bad too like mine, was leaking and never noticed until i saw ful down in cylinder #4 (thought it was the injector until i saw it coming out from behind the FPR).

Try as hard as possible on a full tank, to go to and from work at around 55-65MPH. Be very strict about it. Dont accelerate hard either. See if you get a consistency after 2 full tanks. Then drive normal and see if your fuel mileage suffers. It could be your driving habits.

Something is definitely wrong though...my B3000 dual sport (3.0L V6 auto) get better mileage than you're getting


04 Cav. 2dr. 5spd. My DD. 'Nuff said.

I replaced the spark plugs with NGK'S, as well as a new fuel pressure regulator. I inspected the injectors when I removed the fuel rail to replace the pressure regulator. All injectors looked good and o rings were in great condition. I only get up to about 40mph between home and work because that's the speed limit on my normal route. Once I get up to about 35mph it kicks into overdrive. With normal driving, it usually shifts between about 2600 and 3200 rpm just depending on the road, incline, etc... I just filled up again today and this time I'm trying to drive like a grandma and let it shift at about 2500 rpm. I filled up with 100% gas, no ethanol. I'll see what happens. The weather has warmed up a little and I actually reached operating temp on the way to work today. I'm going to have someone scan the car with a Tech 2 sometime this week and see if there's anything abnormal.

Oh yeah, I just changed the oil about 200 miles ago with Mobil 1 5w30 High Mileage and a purolator filter.

I also have a 99z24 auto, I dont really seem to get that great of fuel mileage in it anymore either. I keep all the maintenance stuff changed pretty regularly. I think the worse I've done in the past 6 months on mpg was 22ish on the highway but that was with a leaky fuel pressure regulator. Now im still only getting 26mpg on the high or so. Not really sure why mine does so crappy now it used to get 30-32 on highway.

have you had your car aligned recently?
if your front end is not aligned, the worse it is off the worse your mileage will be. Its like dragging them sideways over thousands of miles...uneven tire wear will show this though as well as impact fuel economy



Edited 1 time(s). Last edited Tuesday, January 29, 2013 8:35 AM

04 Cav. 2dr. 5spd. My DD. 'Nuff said.

I havent had mine aligned in a while, last time I had it done was when I got new tires. The tread wear looks fine but I swear when looking at the front tires they dont look lined up right. When I get my taxes back I'll get the alignment checked if I dont sell this thing before then.

My alignment is near perfect. The front wheel bearings are starting make a little noise though.

Took a recent trip that was mostly highway and got between 26 to 29 mpg. The engine was allowed to reach full operating temp. I guess my city mileage decreases drastically because most the time it never gets to operating temp and it's short distance, stop and go driving. I never knew that engine temp would make that big of a difference, but I guess it does.

I think its oil temp that matters more....which Im guessing is directly related to oil purity. Its a 100% fact that an oil change increase fuel mileage in any car Ive ever owned. And I reset the tripometer in every car every time after a fill up. I get 30 more miles a tank in my truck and car when I do a fresh oil change for a month or two..maybe longer. Depends on how much I drive it. Lower viscosity = lower friction and oil temps Im guessing..less friction is less wear on the components that are needed to produce power. Less power needed to move at certain speeds is less fuel consumed. Plus if you drive heavy footed like constant hard acceleration or you drive over 55/60, your mileage goes way down. i drive fast, so I dont expect to ever get great fuel mileage. Maybe 21mpg the way i drive which isnt terrible considering im running VERY rich and can see my gauge go down when we go cruising and mess around.

Im guessing most cavaliers should be getting at least 300 miles on a tank regular driving in the city. I get about 260-280 the way i drive, or if Im pushing it for an afternoon like half that lol. ive run through a half tank in less than 1 hour before cruising canyon roads flooring it constantly. But my tune is also set (for some reason right now) to dump fuel like its got a hole in the tank....


04 Cav. 2dr. 5spd. My DD. 'Nuff said.