Mpg help, only getting 7.5mpg 280z

[Z Tyler Z]

So after finally making my 76 280z a daily driver I've discovered its getting horrible mpg. After my last fill up I only got 7.5 miles to the gallon. So I figure it's time to start trouble shooting this problem. My car is pretty stock with a 5 speed. Most of my driving is city.

After reading the EFI bible I'm still in need of some guidance.

Some ideas i have are

Leaky cold start injectors? I'm not really sure if this would led to 7.5 mpg though but I'm pretty new to EFI.

Bad AFM? Mine has corrosion on the circuit board on the AFM.

Sticking Air regulator?

Bad coil? Mine is the stock 34 year old one, this going bad could also explain my occasional lack of low rpm power too.

Bcdd sticking? Causes the rpms to stick occasionally, I thinking of just running a positive 12V connection to keep it closed.

A little more info about how my cars drives. It generally doesn't like to idle at the same rpm every time. Sometimes it idles at 1200 other times at 600. Generally more towards 1200, if I mess with the idle screw it will idle very low in the 500 range. When in the 500 range the car is gutless, I can go all the way down on the gas and it takes it's time moving up the rpm range.

Any advice would be much appreciated, I really can't afford to get 7.5 mpg.

[Z train]

Make sure your distributor is functioning as intended.OLug off the CSI and see if anything changes.

Do you see any smoke out the pipe?Another thing to check for is rips or tears in the manifold elbow(s)on both sides of theAFM).

Edited June 17, 2010 by Z train

[EScanlon]

How long since you made it a DD? If you only just got it working, it might need a shot of Seafoam run through it, an oil change, the gas-tank drained and cleaned, fuel filter changed and then a full tank run through it before you declare it as a gas-hog.

Other than that, as far as suggestions go:

Take your foot off the brake when driving down the road.

Don't sit in your driveway revving the engine for hours on end.

Take the Parking Brake off when you drive.

Get rid of the extra weight in the car that you don't need to haul around everywhere.

J/K on the second half, but serious on the first.

E

[Gary in NJ]

Fuel leak? Is the return not returning to the tank? Do you smell fuel at any time while driving? Assuming you are not driving at WOT, I really can't think of any conditions that would return 7-1/2 mpg.

[grantf]

I have a 78 that also gets crappy mpg. I will be waiting to see what you find. My car is slowly in the prosses of being dismantled for paint so it it will be some time before I trouble shoot my poor gas milage. I was thinking about a fuel leak leading to my symtems but I am not shure and at times I get black smoke from my exhaust.

[Zed Head]

It could be timing issues. It sounds like it might be retarded.

Check your vacuum advance. Read this thread (coincidentally recent and same year car), post #3 by Beandip, about checking your vacuum advance mechanism. http://www.classiczcars.com/forums/showthread.php?t=38890

Check your mechanical advance to make sure it is working. If it is stuck you'll have retarded timing at higher rpm, which will hurt gas mileage.

In general, the engine will be more responsive and get better mileage with properly advanced timing.

A bad water temperature sensor or connection will cause the engine to run rich all of the time.

A far-out possibility for the inconsistent idle - there is an electronic advance for 1976 non-CA models, that kicks in when the engine is cold then turns off when hot. It adds about 6 degrees of advance when cold. I don't know if this will cause a 700 rpm swing but it will cause some. It is controlled by a temperature switch on the thermostat housing.

Edited June 18, 2010 by Zed Head
Added "housing."

[Z Tyler Z]

How long since you made it a DD? If you only just got it working, it might need a shot of Seafoam run through it, an oil change, the gas-tank drained and cleaned, fuel filter changed and then a full tank run through it before you declare it as a gas-hog.

Other than that, as far as suggestions go:

Take your foot off the brake when driving down the road.

Don't sit in your driveway revving the engine for hours on end.

Take the Parking Brake off when you drive.

Get rid of the extra weight in the car that you don't need to haul around everywhere.

J/K on the second half, but serious on the first.

E

It's been my DD for about 3 months or so, I've been working on it for about a year now trying to get it to that status. I've yet to run Seafoam, but I've done a recent oil change and fuel filter was changed a while ago. Also if forgot to mention I already changed the water temp sensor recently.

I guess I can disconnect the full sized horse tailor I've been lugging around behind my Z. Didn't realize that would affect gas mileage. Also the parking brake is off when it's in the up position right?

It could be timing issues. It sounds like it might be retarded.

Check your vacuum advance. Read this thread (coincidentally recent and same year car), post #3 by Beandip, about checking your vacuum advance mechanism. http://www.classiczcars.com/forums/showthread.php?t=38890

Check your mechanical advance to make sure it is working. If it is stuck you'll have retarded timing at higher rpm, which will hurt gas mileage.

In general, the engine will be more responsive and get better mileage with properly advanced timing.

A bad water temperature sensor or connection will cause the engine to run rich all of the time.

A far-out possibility for the inconsistent idle - there is an electronic advance for 1976 non-CA models, that kicks in when the engine is cold then turns off when hot. It adds about 6 degrees of advance when cold. I don't know if this will cause a 700 rpm swing but it will cause some. It is controlled by a temperature switch on the thermostat housing.

That sounds very promising my car does the same thing at start up as his dose. It has a little hiccup right after being started for like 2 seconds then goes back to idle. I will check this out as soon as I can and get back to everyone.

Thanks for all the advice.

[Zed Head]

Z Tyler, I just remembered something about the 1976 Z that you should consider - the manual transmission model, as delivered from the factory, only uses vacuum advance in 4th gear. There is a switch on the transmission that closes in 4th, which activates a solenoid valve on the engine, allowing vacuum to get to the vacuum advance.

The engineers designed the engine to run without vacuum advance in all gears except 4th. So your problems are most likely not due just to vacuum advance problems, if your engine is still in stock form.

Don't forget it came with a ported vacuum source also, which can confuse things if you're checking it at idle.

[Z Tyler Z]

Z Tyler, I just remembered something about the 1976 Z that you should consider - the manual transmission model, as delivered from the factory, only uses vacuum advance in 4th gear. There is a switch on the transmission that closes in 4th, which activates a solenoid valve on the engine, allowing vacuum to get to the vacuum advance.

The engineers designed the engine to run without vacuum advance in all gears except 4th. So your problems are most likely not due just to vacuum advance problems, if your engine is still in stock form.

Don't forget it came with a ported vacuum source also, which can confuse things if you're checking it at idle.

How would not having the original transmission effect this, currently my car has a late 280z 5 speed in it. Would the vaccum advance just never open?

[Zed Head]

How would not having the original transmission effect this, currently my car has a late 280z 5 speed in it. Would the vaccum advance just never open?

According to page EC-9 of the FSM, if there is no power to the switch then vacuum is actuated. The valve closes when energized, opens when no power.

Odds are that whoever put the 5 speed in just disconnected the switch, so you probably have full-time vacuum advance if nothing else was messed with and things still work.

Really though, you should just put a timing light on your engine and check/set the initial value. Then while you're hooked up, rev the engine with the vacuum advance hose disconnected (plug it to keep things running smoothly) to check mechanical action. Then temporarily connect the vacuum advance to a manifold vacuum source at idle to see if it advances. The small port for the heater controls would work fine for that.

That would get you started to see if things are even working. If you wanted go further you could measure how much advance you get with an adjustable timing light (I think they call it a dial-back timing light) or adding some timing marks on the pulley.

Once you get in there with a timing light you'll figure out what's working right and what's not. Its just a big guessing game if you don't measure.

[Hardwyre]

Another thing to look at that I didn't see mentioned here is your Throttle Position Sensor on the side of the throttle body.

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

If your TPS isn't adjusted properly, you'll get fuel enrichment when you're not supposed to. Mine thought it was WOT just off idle. Fixing it gave me 2 mpg back, but I was already running 17. You have more issues then just a ganged TPS. Unless maybe it's contacting the idle and WOT contacts at the same time... that couldn't be a good thing.

[Z Tyler Z]

Today S30driver (James) and I went over to Zs-ondabrains (Dave) and worked on my car.

We adjusted all the valves (all way off) and fixed the timing. Both my mechanical advance and vacuum advance were working fine. We plugged my cold start valve off at the gas line. We figure it's summer now and I'll probably not need it as much.

We also did a compression check and got the following readings

Dry Wet

161 170

155 161

157 160

155 170

151 177

162 185

We also regressed all my linkage to ensure nothing is sticking. I'll give it a few days and get back to you guys on the next fill up and see if anything we did fixed my low mpg problem.