Bad mpg on 280z

[Joey J]

I do mixed driving. About 50/50 city/hwy. driving with a light foot. Shifting at 2-3k rpms. It's a manual 5 speed.

 

The car is new to me and the last couple of tanks I've been keeping track of fill ups and I'm getting 13-15 mpg.

 

I did a quick google search show some ppl getting 20+ mpg.

 

What should I get done to increase my mpg?

[Jeff G 78]

Start with the obvious.  Are you sure your odometer is accurate?  Get on a highway and watch the mile markers.  Go as far as possible and verify that your odometer agrees with the mile markers.  A GPS obviously will work too.  What size tires do you run?  Have you made any gearing or transmission changes?

 

If that checks out, check the basics of the engine.  Get a compression tester and PROPERLY check the compression.  It should be even across all 6 cylinders and somewhere north of 150 psi.  When you have the spark plugs out, take a look at them.  Are they light tan, of black and sooty?

 

A vacuum gauge will also give you an idea how it's running.  Pull one of the vacuum hoses off the intake and hook up the gauge.  If it doesn't read 18 - 20 inches of Hg at idle, the engine might be getting tired.  

 

My 10:1 CR '78 280 gets 23/25, so yes, mid-20's are very possible.

[datzenmike]

A lot can be accounted for by how you drive. (not saying you do this but if you)....

 

Follow too close behind someone.

Foot is either on the gas or on the brake.

 

You will find that you use the brake more than you need to. Remember that any speed that you are going was paid for in gas used... so any time you hit the brake you are throwing away the gas used to get you to speed. Try to anticipate stops or slow downs and let up of the gas and let the car slow down gradually. This can often result it actually not having to stop because the light turns green. When following someone stay back several lengths and you won't have to constantly brake. Speaking of brakes... are the tires free to rotate? Could a brake be dragging? E brake part on?

 

 

Are the tires correctly inflated? Lower than recommended is harder to push and flex generates heat which over time wears them out. Harder rolls easier. Better over inflated than under.

 

Are the tires aligned properly?

 

Carrying junk in the trunk? Takes gas to accelerate junk too.

 

 

 

Last, is engine in tune. Is it running right?

 

Could the AFM be dirty? Sometimes the variable resistance track is worn and gives a faulty reading for the fuel/air mixture. http://www.atlanticz.ca/zclub/techtips/afm/index.html

 

 

Throttle position sensor?

http://forums.hybridz.org/index.php/topic/86041-tps-throttle-position-sensor-adjustment-preguide-76-280z/

[Ryoskatekov]

or don't drive a z if you want to get good milage period...  

 

But then again, extremely terrible mpg usually does tell you something is wrong somewhere. 
 

[racerx]

yeh that's too low mpg..have you done a compression check?  how many miles on the engine? Like Mike said could be getting tired. If compression okay then I would do a complete tune up, adjust the valves, change the gas filter, change oil and see if this makes a difference.  Good luck.

[Joey J]

Thanks to everyone who replied, I appreciate all the help! :thumbup:

 

 

I do drive trying to use the brakes as little as possible. I'm confident my driving style is not the culprit here. I also checked out the odometer and it is correct. So that's not it either.

 

I haven't got the car checked out yet so that's at the top of my list. Maintenance wise I've only done an oil change and air filter change. I'm going to give it a try and do the compression test on my own and I'll also buy the vacuum gauge to check that out.

 

The only thing I've noticed is that sometimes the car wont idle correctly when it starts. It's only happened twice. The idle bounces from 500-1000 and I can hear it and it sounds like its about to turn off each time. I give it some gas and drive a bit and it starts to idle normally again.

[racerx]

Thanks to everyone who replied, I appreciate all the help! :thumbup:

 

 

I do drive trying to use the brakes as little as possible. I'm confident my driving style is not the culprit here. I also checked out the odometer and it is correct. So that's not it either.

 

I haven't got the car checked out yet so that's at the top of my list. Maintenance wise I've only done an oil change and air filter change. I'm going to give it a try and do the compression test on my own and I'll also buy the vacuum gauge to check that out.

 

The only thing I've noticed is that sometimes the car wont idle correctly when it starts. It's only happened twice. The idle bounces from 500-1000 and I can hear it and it sounds like its about to turn off each time. I give it some gas and drive a bit and it starts to idle normally again.

What about changing your spark plugs, wires, dist. cap and rotor? Fuel filter..yeh do a comp. check. before changing parts.

[datzenmike]

Inspect the thick pleated rubber hose between the throttle valve and the MAF. It can crack between the folds and leak air in.

 

The temp sender may be bad reading cold all the time so the engine stays in warm up mode and runs rich. Not the gauge sender, the one that goes to the ECU.