So I heard these cars are rated to 27mpg highway?

[yhlz]

Gas milage will depend on your engine's condition. Two words that will solve your question: OLD CAR. 

[osirus9]

Gas milage will depend on your engine's condition. Two words that will solve your question: OLD CAR. 

 

That is an utterly unhelpful answer...

[Tony D]

260Z, Round Top SU's---late five speed, 3.70 gear out back, stock tyres.

17mpg in town driven aggressively

27 mpg highway never exceeding 65mph averaging 60mph over a tankful

Highway mileage was a linear slope from 27mpg@60mph average to 19mpg at 110mph average over a complete tank.

 

I80, Oglalla NE to Iowa, across Iowa & Illinois to the mess of Chitown, that's where....before you even ask.

 

You gotta work to crack 20 in the car if it's running right.

 

76 Fairlady 2/2 with swapped L28 and EFI from a 1976 stateside model that had 176,000 miles on it before being removed, early 5speed, 3.90 gearset stock tyres....just about 22 whereverthehell I went, towing an 800# trailer over 3,200 rpms most of the time. 24 mpg in LA commuting at 120kph+...

Edited November 6, 2013 by Tony D

[osirus9]

So essentially, there is something wrong with my car causing it to run rich. I am planning on buying some supra 440cc injectors for my turbo build, and I've got an extra set of stock injectors laying around. I think I'll buy some cheap supra injectors, and send them along with my spare stockers to witchhunter (I've used them before, they're great) and get the lot cleaned and tested. Thanks for the info once again guys.

 

At the very least this will rule out my injectors as the culprit. But if they're not the problem then I'll just deal until I get my Haltech installed.

Edited November 6, 2013 by osirus9

[osirus9]

OK, so bringing this thread back from hibernation. I finally got my car back and am trying to troubleshoot my bad mileage and low power issues. First thing I did was check the coolant temp sensor, that had a shitty connector so I replaced it and I got good readings at the ECU connector. Next was the TPS. That was totally out of alignment and it thought I was cruising at idle and ~5% throttle was WOT. Got that adjusted but I haven't driven enough to see if mpgs improved much. The car still smells very strongly of gas and doesn't have as much power as I feel like it should, (my gf's miata feels WAY faster, I feel like they should be pretty comparable).

 

Anyway, next to check was the AFM. That failed all the resistance checks, so I swapped it with another that passed them spot on. Tested with these numbers: http://www.atlanticz.ca/zclub/techtips/afm/ car ran like total garbage! I swapped back to the "bad" AFM I was using before and the car ran like normal again.

I also found that the ECU had been replaced with an ECU supposedly from a '77 automatic (yellow junkyard marker written on the side), does that matter? My car is a '78 Manual.

 

Here is a video of my fuel pressure and vacuum: 

Does that seem normal to anybody? Fuel pressure looks if anything maybe on the low side to me...

I also monitored fuel pressure over time after shutdown to check for a leaking injector. Do these numbers look normal to people?

 

31psi @ Shutdown

31psi @ +2 minutes

29psi @ +7 minutes

24psi @ +15 minutes

23psi @ +17 minutes

22psi @ +20 minutes

21psi @ +23 minutes

20psi @ +26 minutes

19psi @ +29 minutes

18psi @ +32 minutes

12psi @ +52 minutes

 

I know fuel pressure bleeds off over time, but this seems like it is bleeding off faster than it should. Possible leaking Injector?

 

Also, I have to crank the car a few times to get it to start. It will start then die usually 3 times if it has been sitting for a while, then start fine on the 4th try. If I try to rev it it will make angry noises and not rev until about a minute has passed then it will rev freely and normally. If it has just been running maybe an hour or two ago I can start it on the first try no problem.

Edited June 21, 2014 by osirus9

[Phantom]

The fuel system is NOT supposed to bleed down over time. That is why it is taking a bit of time to crank the engine as the pump has to build pressure. If your pressure is bleeding off then you either have:

a. leaky injectors

b. a bad check valve in the fuel pump

c. the pressure regulator is bleeding by - it should maintain about 35-39 PSI if I remember right.

d. you have a leaky hose or fitting somewhere.

Reading what you've said I would start with C and replace it.

Pull your plugs and look at them. If they look wet then you still have a problem with you still have a problem with your coolant temp system and you're running too rich.

Pull your dipstick about 30 minutes after you shut down and see if you smell fuel on it. If so, you probably have one or more leaking injectors.

When I had my original L28 and 4-sod in my '77 280Z I never did better than 23-24 MPG. The engine had 202K on it when I pulled it so it was a high mileage L28. It also had original injectors,etc. The only thing I had done ws track down the bullet connectors in my wiring harness about 18" from the thermostat that were oxidized and causing a huge problem with my coolant temp sensor.

[osirus9]

Thanks Phantom. I just pulled the dipstick and it smelled like gas about 3 hours after I was running the car. Would you suggest just replacing all the injectors? I have 2 sets so it it pretty likely I could make 1 good set out of the 12 I have, but how can I test them to see which one is leaking?

 

I'll order a new FPR now and see if that helps first though.

 

EDIT

 

Just tried starting the car with the starter unplugged. Fuel pressure goes straight to 33psi. Shouldn't it be at 36psi?

Edited June 21, 2014 by osirus9

[Six_Shooter]

I've never owned a car that DIDN'T bleed down fuel pressure. Yes, I've installed fuel pressure gauges on many of my cars for monitoring and/or diagnostic purposes.

 

Most cars have bled off in less than an hour. I had one that would take a couple hours. Never a problem starting.

 

I find it absolutely hilarious that people cite this as a reason for long crank times.

 

Next time, watch how fast the fuel pressure comes up when the key is turned on and fuel primed (most cars work this way), it's quicker than you can get the key all the way to start, usually.

 

Some EFI systems have it programmed into to wait for a certain number of ignition pulses before enabling fuel, in order to assure there is oil pressure. I don't know if the Nissan system is designed this way, I doubt, since it's pretty archaic, but it is something that is a part of some systems.

[PMG]

My fp used to drop similarly to yours. It turned out that there were a few clamps on the fuel rail that weren't as tight as they should be. Tightened them up and solved the issue. Can you smell gas after shutting off the car while in the garage?

[Tony D]

Haven't seen a "leaky injector" YET that wasn't an EXTERNAL leak. If you don't see it, they're not leaking.

 

FPR seat is far more likely BUT...

 

THE FUEL EXPANDS due to heat soak after shutdown. As it does the FPR dumps pressure. (This can happen from 5-15 minutes after hot shutdown)---after that point, the fuel cools and contracts. Maybe a 10% volume change of a hydraulic fluid in a closed container can make for a big difference in the pressure that remains.

 

Do this: tomorrow morning with the engine STONE COLD, crank up the car with your fuel gauge in place. Once it's started and the fuel pressure is stabilised shut it off. We're talking maybe 30 seconds of run time max.

 

Then repeat your checks and please post the results. Use the same gauge and same tming intervals for best comparison.

 

THEN...

 

Without starting the car AT ALL...rig up your Fuel Pump to run "key on"... You may have to move the AFM Flap, jumper the Oil Pressure Switch, etc... Run the pump ONLY until pressure stabilises, the repeat your test as above at the same testing intervals.

 

Remember this: in a RUNNING CAR with the shown 17" HG idle vacuum, fuel pressure will be FPR set point minus 8.5psi. AS SOON AS YOU SHUT OFF THE ENGINE the FPR changes to a 3X pis regulator. So the car WAS running with 24-27 psi, then rises to FPR set point and relieves, and then cools and contracts and as a result the pressure will drop dramatically. As soon as you crank the car the pressure should be 24, and the initial pulse of the cold start injector may be killing your pressure if it's slightly low. That's why they ran the pump to prime on the ZX's, to prevent extended cranking and flooding of the engine!

 

You will see (should) marked differences in the two tests above between where the car starts running vacuum, and funning straight upon the FPR. From your starter test, your fuel pressure is FINE!

 

Smelling gas on the dipstick is VERY subjective. I don't know if I would jump off that bridge just yet!

[NewZed]

The 3.7 is going to make your odometer readings high unless you changed the speedo gear in the transmission.  Same with low profile tires, it's diameter that matters, not width.  so your calculations may be off a little bit.

 

My 76 essentially stock 280Z gets about 20 mpg, mostly city with some highway and it runs great, with plenty of power,and very little gassy smell.  I've seen a variety of "surveys" on forums and it seems like most people are around 20 to low 20's on mileage, on the pre-O2 sensor cars.  I also have a potentiometer on my cooling temperature circuit and can "trim" the circuit while driving, from lean to rich (I have an adjustable FPR so start a little bit lean).  The engine is much more responsive on the rich side.  Probably because there is no acceleration enrichment until WOT on the early EFI system.  So I run on the rich side, for fun.  The higher gas mileage people may be leaving some engine response on the table to get their high numbers.  It's not obvious either, unless you can play with enrichment  while driving.  Mileage is also worse in the winter than in the summer, possibly because the air temperature compensation is not quite right in the stock Nissan ECU algorithm.  Just a guess on why, but I get 1-2 mpg higher every summer when it's nice and hot out.  The 20 is a summertime number.

 

The starting then dying with the angry noises isn't normal.  You haven't mentioned your timing specs. either.  "Fine" at idle may not be fine while driving.   Ignition timing can have a big impact on gas mileage, and performance.  The distributor advance mechanisms have to work properly, they're often corroded on the old distributors and don't.do what they're supposed to do.

 

Have you given the engine a good tune-up, including valve lash adjustment?  It sounds a little noisy in the video and your vacuum readings are on the low side.  By the way, those $15 JEGS gauges aren't very good, I had one that hid a fuel pressure problem from me for quite a while, a few years ago.

Edited June 22, 2014 by NewZed

[osirus9]

Haven't seen a "leaky injector" YET that wasn't an EXTERNAL leak. If you don't see it, they're not leaking.

 

FPR seat is far more likely BUT...

 

THE FUEL EXPANDS due to heat soak after shutdown. As it does the FPR dumps pressure. (This can happen from 5-15 minutes after hot shutdown)---after that point, the fuel cools and contracts. Maybe a 10% volume change of a hydraulic fluid in a closed container can make for a big difference in the pressure that remains.

 

Do this: tomorrow morning with the engine STONE COLD, crank up the car with your fuel gauge in place. Once it's started and the fuel pressure is stabilised shut it off. We're talking maybe 30 seconds of run time max.

 

Then repeat your checks and please post the results. Use the same gauge and same tming intervals for best comparison.

 

THEN...

 

Without starting the car AT ALL...rig up your Fuel Pump to run "key on"... You may have to move the AFM Flap, jumper the Oil Pressure Switch, etc... Run the pump ONLY until pressure stabilises, the repeat your test as above at the same testing intervals.

 

Remember this: in a RUNNING CAR with the shown 17" HG idle vacuum, fuel pressure will be FPR set point minus 8.5psi. AS SOON AS YOU SHUT OFF THE ENGINE the FPR changes to a 3X pis regulator. So the car WAS running with 24-27 psi, then rises to FPR set point and relieves, and then cools and contracts and as a result the pressure will drop dramatically. As soon as you crank the car the pressure should be 24, and the initial pulse of the cold start injector may be killing your pressure if it's slightly low. That's why they ran the pump to prime on the ZX's, to prevent extended cranking and flooding of the engine!

 

You will see (should) marked differences in the two tests above between where the car starts running vacuum, and funning straight upon the FPR. From your starter test, your fuel pressure is FINE!

 

Smelling gas on the dipstick is VERY subjective. I don't know if I would jump off that bridge just yet!

 

Actually the two tests you are describing are exactly what I did in my last post. Maybe I didn't describe it as well as you though. The car was cold and I just ran it for a few seconds to get the fuel pressure numbers I posted. I then disconnected the starter signal wire and cranked the car to get the 33psi number. I didn't monitor it as thoroughly the second time but the pressure dropped off at about the same rate. The pressure also never stops bleeding off and if I let the car sit for a few hours the pressure is 0psi.

 

There is definitely no external leak on the car, I see no drips and if I pressurize the system without starting the engine I don't smell or see any gas at all. So this leads me to believe it is either A. Leaking Injector, or B. Bad fuel pump check valve. 

Then again if it was just the fuel pump check valve then I wouldn't be getting bad mileage...

 

EDIT

I am going out to do a valve adjustment now and see if that helps anything. car has 95k on it and I have no idea if anybody ever did a valve adjustment. I have verified the timing is set correctly with a timing light as well.

Edited June 22, 2014 by osirus9

[Phantom]

I installed a fuel pressure gauge on my Z back when it had the L28 in it. Frankly don't remember if it consistently read 36 or 39 psi. What I do know, however, is that it really didn't bleed down over time. I could pop the hood a day later and it might be down 1 psi. I do understand what Six-Shooter is saying. My LS1 280Z bleeds down pretty fast but that's because I'm running an aftermarket fuel pump with no check valve. I turn my ignition to the accessory position and listen to the system pressurize before hitting the ignition. It usually takes about 4 seconds. If I wait it starts immediately. If I don't it will crank a couple about 3 seconds before catching.

Not sure what to tell you how to easily bench test your injector for leaking. It would be a wild shot but you might pull all the plugs and see if one indicates the presence of fuel or an overly rich mixture. That could give you an indication but is definitely not the best way to do it.

If you have a strong raw gasoline smell on your dipstick you could also have your oil analyzed and get a report on the condition of the oil. I know every mechanic I've worked with gets really nervous if they notice that. It can be caused, however, by an overly rich mixture as well as leaky injectors. My sons LS1 240Z has been sitting in the garage for 3 years now waiting for him to come up with the cash to get it properly dyne tuned to eliminate the rich mixture condition it has.

[osirus9]

Just finished doing a hot valve adjustment and 4 of the valves were really off. After that I started the car and it idles better than it ever has, so that definitely helped. As for the fuel pressure going down I think the next thing I'm going to do is pull the rail off and put a paper towel under the injectors and just lay them on the manifold but still hooked up to the fuel lines, then unplug the starter signal wire and pressurize the system. If an injector is leaking I'll be able to see it on the towel. If I get no leaks then I'll know I need a new fuel pump.

 

Phantom, I used to have a fuel system like that on my rx7 running a haltech, but watching the fuel pressure gauge on the Z, pressure goes up a little on accessory (like 20psi), but it goes up more (33psi) when I disconnect the starter and "start" the car. 

Edited June 22, 2014 by osirus9

[Milenko2121]

Jeeze, I'm jealous of these people saying they get 20-30mpg.

 

I'm lucky to see 17mpg. I'm getting 15mpg most of the time and it doesn't matter if it's city or highway.

Two different motors, same Z31 ecu swap. Both got same gas mileage.

 

I wonder why my car is just eating my fuel. I need to get a wideband on this thing...

[Tony D]

Actually the two tests you are describing are exactly what I did in my last post. Maybe I didn't describe it as well as you though. The car was cold and I just ran it for a few seconds to get the fuel pressure numbers I posted. I then disconnected the starter signal wire and cranked the car to get the 33psi number. I didn't monitor it as thoroughly the second time but the pressure dropped off at about the same rate. The pressure also never stops bleeding off and if I let the car sit for a few hours the pressure is 0psi.

 

There is definitely no external leak on the car, I see no drips and if I pressurize the system without starting the engine I don't smell or see any gas at all. So this leads me to believe it is either A. Leaking Injector, or B. Bad fuel pump check valve. 

Then again if it was just the fuel pump check valve then I wouldn't be getting bad mileage...

 

EDIT

I am going out to do a valve adjustment now and see if that helps anything. car has 95k on it and I have no idea if anybody ever did a valve adjustment. I have verified the timing is set correctly with a timing light as well.

 

You most likely have the bad pump check valve as stated.... 

To further isolate it, you can clamp the feed line immediately after shutoff, and if it leaks down then it's on the FPR return side.

To go the next step, clamp both the feed line and return line Immediately after shutoff. if it leaks down THEN, it's in the injectors.

 

The troubleshooting steps I've posted before. No need to take anything apart a gauge and a couple of vice grips or fuel line clamps are all you need to zero in on the section of the fuel system responsible for your bleed down.

 

Aftermarket check valves are available, and have been posted previously.

[Tony D]

Have you seafoamed the injectors and cleaned the fuel system? dirty and damaged pintle caps along with backside valve deposits wreak hell on mileage as well.

 

but you needed to have all items in the basic tune up proper first. You are WAAAAY down the line skipping the basics. Valves out of adjustment cause low vacuum which causes higher fuel pressure which causes poor mileage.

Ignition timing (centrifugal or vacuum -- PARTICULARLY VACUUM -- advance malfunctioning can affect mileage similarly with a double whammy of low vacuum and more throttle angle required due to running retarded from optimum.

 

Do a basic tune up according to shop specs FIRST, then move on to the bizarre and obscure stuff. Don't start there!

[Tony D]

Keep in mind the Bosch System defaults RICH. That's safe. And the cars run GREAT when rich. So many times people think they have a 'great running car' but have multiple failures in components resulting in rich conditions which give poor mileage.

 

additionally, the way you drive DRAMATICALLY affects the numbers. The 27mpg should be possible  if you steady-state run the thing on flat land at between 35-55 mph with gradual throttle applications, and never, ever exceed 60-65. Which was the test conditions used to get 27mpg!

[osirus9]

Oh wow for some reason it never occurred to me to just use a clamp on the line. Great idea!

 

Also I'm not saying I think I should get 27mpg the way I drive, but I think 20 is achievable. 14 is way too low. I'll run some seafoam through the injectors as well and see if that helps. I only run premium gas so hopefully that is helping to clean any gunk out of there too.

 

I'll look at the FSM for how to check the vacuum advance too. That is definitely one thing I forgot existed.

 

Thanks!

[Tony D]

I would agree that 14 is borderline too low. Likely from the aforementioned rich running, it makes a bunch of low down torque....

 

Now, 14 in the city I say borderline as I got 15-17 in city driving...but I CAN get as low as 10-12 if I know the cops aren't around and I drive like I'm Auto-Xing it!

 

So really the driving style will dictate quite a spread. Also during northern winters  and cold weather---that would be FINE! The cold start loop can take FOREVER to get the car off the enrichment loop and short trips KILL your mileage as you are always running on the enrichment loop. I would block off at least half my radiator, run a 190 thermostat and idle for a while to warm up before even THINKING of moving during the winter months up north. The colder it is, the worse it is!

 

I'm good for simple troubleshooting methods. Most of the stuff on this car can be troubleshot in 45 minutes or less on the electrical end of the engine, and even less on fuel! It's either working or it ain't!