Hello all!

 

I have been slowly getting my '77 280Z 2+2 5 speed manual up and running. It seems to be going well for the most part. However, the last two tanks have been giving me some pretty poor MPG!

 

When I first got the car running, I filled the tank, set the tripometer to zeros, and drove until I used what I thought to be about a quarter tank of gas. I refilled, did the math, and I was getting about 15 MPG. This was with a combination of in city and highway driving. I would have liked it to have been better, but could have lived with it if I had to.

 

However, over the last two tanks of gas, when calculating MPG I am getting between 11 and 12 MPG! Again, this takes into consideration both in city and highway. However, the majority of the driving has been highway. According to what I've read, the 280Z should be getting 15 MPG in city and up to 27 MPG highway, so an average of 11 or 12 is pretty darned low.

 

What sorts of things should I be looking for to improve this and where would you recommend I start?

 

Thanks in advance for any help.

the driver is first. do you have a lead foot?

 

have you checked the condition of my spark plugs?  running rich?

Edited July 13, 20159 yr by hr369

A pic of the spark plugs would tell us quite a lot.

Have you compared your speedometer readings with GPS?  My speedo was off by 15mph before I swapped to the correct speedo gear for the rear end.

Spark plugs then air filter. Mine averages about twenty mpg, dogging the s**t out of it.

You should also check the bullet connectors from the coolant temp senor. Shine them up.

I have not checked the speedometer for accuracy, but it appears to be accurate when compared to other drivers on the highway and when I am following a friend in their car, etc. It also "feels" like its going the right speed compared to what the speedometer is saying, too.

 

The 5 speed tranny and rear end are all stock, so the gears should be as they are supposed to be for this car.

 

I can drive a little fast from time to time, but not enough to knock it down to 11 MPG!

 

The spark plugs are all new, as are the fuel injectors and pigtails.

 

The air filter is new, too.

Edited July 14, 20159 yr by SilverSurfer

Take pics of the plugs and post them. Let us see what they look like.

"Speedometer" was probably meant to be odometer.  You can check your odometer using mile markers on the freeway.

 

They wanted you to check spark plugs because they can indicate how rich the fuel-air mixture is by color and carbon buildup.  The smell of the exhaust is also a clue.  Does it smell gassy?  

 

My 76 suddenly started getting terrible mileage when the fuel pressure regulator (FPR) went bad.  Check fuel pressure if you can.  It's very important.

 

And you can get a good idea of the quality of the coolant temperature sensor circuit by measuring resistance at the ECU and comparing it to the temperature chart in the FSM.

 

You have a lot of "should"s and "should be"s in your post.  You have to get numbers and take a close look to really know.  All of your shoulds say you should be getting better mileage.  So they must not be what they should be.

New injectors? What are they? Where did you source them? They could be larger or leaking which would contribute. Check the plugs first, if it's running really rich they will be black.

You have a lot of "should"s and "should be"s in your post.  You have to get numbers and take a close look to really know.  All of your shoulds say you should be getting better mileage.  So they must not be what they should be.

That was the first laugh of the day, thank you!

It's been a while, but I wanted to add an update:

 

I drove the car for about 4 hours the other day (took 'er on a road trip, I did). My MPG ended up a little over 17 MPG on the highway. Not great, but better.

 

The speedometer is off by about 5-7 MPH depending on how fast I am going ... in this case, up in the 65-75 MPH range.

 

The tripometer appears to be working properly as I checked it against mile markers on the highway.

 

I will try to pull a spark plug and photo it tomorrow and post photos here.

sorry in advance to the kind folks who helped me troubleshoot my 78 through many a dark time, but i gotta chime in here...

i know others may have different experience, but i never could get the efi on my 78 to do better than 16 mpg on freeway driving. the car ran well (not great) and i replaced every single component, hose, fitting, connector and wire in the system - sensors, injectors, csv, fpr, filters, aar, everything (the ecu was less than a year old, as was the afm) as well as new pcv, plugs, wires, cap, rotor, etc. i'm not saying it's impossible to get good mileage out of it (others have) i just know i couldn't, so i finally bailed on it.

 

yanked the whole thing, harness and all and swapped in a set of su's with sm needles and i'm consistently hitting 22-25 mpg and i'm driving it like a lunatic most of the time.

old school just plain works.