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Thread: 280z fuel consumption problem...

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    The Z Milenko2121's Avatar
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    Default 280z fuel consumption problem...

    My 280z is only getting 14mpg...

    I've replaced the fuel filter, and the filter near the tank which raised me from 10mpg to the 14mpg.
    I've replaced the spark plugs and the wires with NGKs.

    My engine holds idle at 800rpms with no problems except when my engine is cold it gets stuck at 600-500 if it hits 500, it wants to die unless I rev it until it warms up.
    Sometimes when I start my engine when it gets stuck at 600-500rpm it will backfire at the intake when I rev it and then after roughly 2-3 backfires it will idle normally.
    It also backfires at the intake without the AFM attached so I don't see my AFM being an issue.

    I've already fixed my vacuum leaks and my fuel pressure is perfect (~35).

    What should I start diagnosing?
    February 1977 Datsun 280z 5 Speed

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    Registered User siteunseen's Avatar
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    Whow!

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    Check cts sensor and correct operation of throttle position switch, then check fpr.
    1978 280z 4sp

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    First check the TPS, as a quick test you could disconect it and see if fuel comsumption improves. Check the water temp sensor (and also the coolant level). check you're new plugs and see if they are black (probably). Also do a search for mpg and you may find a rather long thead involving the same problem I had, lots of info there. Also check the timing though it probably is not the cause.

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    Just read a guide on my TPS, and it was at fault. The second I hit throttle, it was telling my engine I was WOT. I followed the instructions and got it spaced exactly where it needs to be.
    Now I'm going to do a mpg test

    Also, my distributor timing was off by A LOT, I fixed that and set it to 7deg / 800RPM

    after my next mpg test if it's not acceptable, I'll test the sensors.

    Quote Originally Posted by cozye View Post
    Check cts sensor and correct operation of throttle position switch, then check fpr.
    What is the FPR ?
    February 1977 Datsun 280z 5 Speed

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    Fuel pressure regulator. but it seems you may be on the right track.

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    After a quick test run on the freeway and a couple red light stops; my car runs so much smoother, and it's accelerating much better. I actually said "Holy S**T" when I floored it from a dead stop to 65mph to get on the freeway.

    Filled up my tank to full, and I hope my mpg is better now. it was only 14mpg before.

    So just for future reference,
    I should check my Water Temp Sensor and my Coolant Temp Sensor?
    How would I check my Fuel Pressure Regulator as well as those other two?
    February 1977 Datsun 280z 5 Speed

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    It looks like you have found the culprit, but it is a really good idea to check all of you're efi system. To test the FPR you will need a fuel pressure gauge and a vacuum gauge. the water temp sensor can be tested at the ecu connector at the lower drivers side kick panel, you can test most of the EFI electrical systems there using a multi meter and the factory service manual:
    http://www.xenons30.com/reference.html
    check and clean all electrical conections and for sure inspect all vacuum hoses as a vacuum leak will cause the engine to not run optimally and could cause a miss diagnosis.
    Good job you are saving money in two ways: less gas and understanding you're engine so as to not throw new parts at it or paying a mechanic that does not understand older EFI. let us know youre progress.
    Last edited by grantf; 12-29-2011 at 08:47 PM.

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    Timing should be at 10 degrees minimum. A lot of people will advance it to 13 or so. Running it at 10 was lower than the previous years and beliefs done to lower emissions at idle.
    1978 280z 4sp

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    Quote Originally Posted by cozye View Post
    Timing should be at 10 degrees minimum. A lot of people will advance it to 13 or so. Running it at 10 was lower than the previous years and beliefs done to lower emissions at idle.
    I was told I needed to be at 7deg for california.

    I'm curious, where does this information come from that it needs to be at 10-13 ? And what would the benefits be

    Also, I've been reading a bit more, and I see people are able to go to 20deg and such.

    My timing won't let me go past ~12deg, but I can go the opposite direction way off the timing marker.
    Could anyone explain why this happens?

    Also, my engine ticks now at idle, but ever since I timed it to what I believed was 7deg(which I don't believe it truly is anymore) my engine has ran better than ever.

    One last thing, I had about 2 instances where I heard a weird metal grind from the engine bay when I revved up the engine around 4krpm that last maybe 1 second. Any ideas as to what this could be?
    Last edited by Milenko2121; 12-30-2011 at 06:33 AM.
    February 1977 Datsun 280z 5 Speed

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    Quote Originally Posted by Milenko2121 View Post
    I was told I needed to be at 7deg for california.

    I'm curious, where does this information come from that it needs to be at 10-13 ? And what would the benefits be

    Also, I've been reading a bit more, and I see people are able to go to 20deg and such.


    My timing won't let me go past ~12deg, but I can go the opposite direction way off the timing marker.
    Could anyone explain why this happens?

    Also, my engine ticks now at idle, but ever since I timed it to what I believed was 7deg(which I don't believe it truly is anymore) my engine has ran better than ever.

    One last thing, I had about 2 instances where I heard a weird metal grind from the engine bay when I revved up the engine around 4krpm that last maybe 1 second. Any ideas as to what this could be?
    Screw California.As far as timing goes, the figure is a base line,each motor will like what it's gonna like.Play with it untill it runs the best.
    Faster than anyone in Oceanside

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    Quote Originally Posted by Z train View Post
    Screw California.As far as timing goes, the figure is a base line,each motor will like what it's gonna like.Play with it untill it runs the best.
    how would you know what the best timing is?
    February 1977 Datsun 280z 5 Speed

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    Factory Timing is I think the 10 deg mark. I would start there and see if performance is improved or not. I set mine to 10 deg but latter moved it up to about 12. May be a stupid question but do you have it set 7 deg retarded instead of advanced? Anyhow another thing to test is the vacuum advance on you're dizzy to verify that it is working properly.

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    Quote Originally Posted by Milenko2121 View Post
    how would you know what the best timing is?
    Bump it in 2 degree increments.Sooner or later it's not going to like the increase.Then drop it down 2 degrees.Tuning is all trial & error.THere is no magical number that ANYONE can tell you will work best.
    Faster than anyone in Oceanside

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    Quote Originally Posted by Milenko2121 View Post
    I was told I needed to be at 7deg for california.

    I'm curious, where does this information come from that it needs to be at 10-13 ? And what would the benefits be

    Also, I've been reading a bit more, and I see people are able to go to 20deg and such.

    My timing won't let me go past ~12deg, but I can go the opposite direction way off the timing marker.
    Could anyone explain why this happens?

    Also, my engine ticks now at idle, but ever since I timed it to what I believed was 7deg(which I don't believe it truly is anymore) my engine has ran better than ever.

    One last thing, I had about 2 instances where I heard a weird metal grind from the engine bay when I revved up the engine around 4krpm that last maybe 1 second. Any ideas as to what this could be?
    10 degrees comes from the factory service manual, page ET-7. Its also a sticker on the hood if it hasn't been taken off. There is no separate value for ca cars.

    Running it low is just masking other problems, you will get lower vacuum, less power, poor fuel consumption.

    Historically, tuners have advanced timing as far as they can without causing detonation or spark knock. Typically some advance will give you slightly more power.

    That being said, your timing marks could be wrong. Check TDC of the motor and verify that the pulley mark lines up with 0. Sometimes the pully slips, which would make the mark wrong.

    Next thing I would do would be to take the valve cover off, do a hot valve adjustment, and while you are in there check the valve timing by getting motor at TDC at inspecting the cam pulley mark. Instructions are in th fsm

    No idea what your grinding noise was, but it shouldn't do that obviously
    Last edited by cozye; 12-30-2011 at 06:04 PM.
    1978 280z 4sp

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    Oh, and the reason why you are running out of adjustment it of the distributor is you either have excessive shaft play, the plates on the inside need adjusting, or the old ignition components are worn. Mine was like that until I put in an optical ignition trigger and phased it properly, now it's dead on with plenty of range.
    1978 280z 4sp

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    Thanks for the tips I'm sure my pulley is off then, but I like how my engine is running with this timing. It's smooth and clean, also my gas mileage has gotten much better. No true numbers yet, but I'm guessing I'm getting 18mpg maybe even 22. I'll find out in a week or so when I fill up.

    Also, found out that metal on metal noise was the skidplate on my catback. it's loose and rattles.
    And I apparently need a new exhaust. My cat has a hole that was welded, the exhaust pipe has multiple holes and is rusted as well as the muffler.

    I think that could explain my engine not running as well, and the shakiness.
    Last edited by Milenko2121; 12-30-2011 at 06:24 PM.
    February 1977 Datsun 280z 5 Speed

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    7 degrees is probably from the 1976 specs (my car and FSM). It's more likely that your pulley is fine and your timing is really at 7 degrees.

    Your distributor might be a little off. There are many common ways to install them incorrectly but also many ways to make it work, with no damage. 12 degrees will probably be just fine until you start tuning for performance. As you've found, 7 degrees runs well, as it should. You're just not getting the absolute most performance out of your engine.

    You could bump it to the 1977 spec. of 10 right away or even 12 if you're running premium, with no problems likely. It would probably be a little peppier and maybe get a little better mileage.

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    Find TDC and see if the line is at 0 or just a hair before.Really long odds on the balancer being the problem.
    Faster than anyone in Oceanside

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    You should be over 25 mpg on the highway at 65mph

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    Quote Originally Posted by Z train View Post
    Find TDC and see if the line is at 0 or just a hair before.Really long odds on the balancer being the problem.
    whats the best way to find TDC? ive been reading about it, but im still a bit confused on when to know how to do it perfectly
    February 1977 Datsun 280z 5 Speed

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    Use a wooden dowell rod in the spark plug hole, TDC is when that #1 piston is at its very top position. Get it close then mark the wooden dowel, then go a bit back and forth on the crank until its at the highest position. I usually take the fan off and use a wrench on the crank pulley nut to turn the motor
    1978 280z 4sp

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    Quote Originally Posted by cozye View Post
    Use a wooden dowell rod in the spark plug hole, TDC is when that #1 piston is at its very top position. Get it close then mark the wooden dowel, then go a bit back and forth on the crank until its at the highest position. I usually take the fan off and use a wrench on the crank pulley nut to turn the motor
    the #1 piston is the one closest to the firewall right? Just double checking >_<

    I'm still learning here, I've only owned this Z for a little over a month, and it's the first car I've ever worked on, and I've done a lot so far mostly thanks to this site.
    February 1977 Datsun 280z 5 Speed

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    Quote Originally Posted by Milenko2121 View Post
    the #1 piston is the one closest to the firewall right? Just double checking >_<

    I'm still learning here, I've only owned this Z for a little over a month, and it's the first car I've ever worked on, and I've done a lot so far mostly thanks to this site.
    #1 piston is closest to the radiator.

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    Quote Originally Posted by buysell View Post
    #1 piston is closest to the radiator.
    ...
    That could explain why the timing mark was off >_>

    I got a couple questions then.
    What happens when your timing is too advanced or retarded, and how would I know?

    Because when I set it to 7deg(using my 6th cylinder which I'm going to check tomorrow morning as to what it is with #1 now that I know)
    my engine has ran beautifully no hesitation at all up to redline.
    Last edited by Milenko2121; 12-30-2011 at 10:56 PM.
    February 1977 Datsun 280z 5 Speed

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    Quote Originally Posted by Milenko2121 View Post
    ...
    That could explain why the timing mark was off >_>

    I got a couple questions then.
    What happens when your timing is too advanced or retarded, and how would I know?

    Because when I set it to 7deg(using my 6th cylinder which I'm going to check tomorrow morning as to what it is with #1 now that I know)
    my engine has ran beautifully no hesitation at all up to redline.
    Put the timing light on#1. There is no telling where your timing is since you where timing it off #6. My guess is you are probably around 15
    1978 280z 4sp

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    There is some weirdness here. Shouldn't #6 be on the exhaust stroke, but in the same relative position as #1, when #1 is on the compression (spark) stroke? That wouldn't be 7 1/2 degrees off from where he is now. Seems like someone might have the plug wires in the wrong holes. That might be why he has limited adjustment at the distributor.

    You'll probably find that with the timing light on #1 you can't find the timing mark. The other possibility is that you've been looking at the wrong mark on the damper pulley.

    Might be best to start from scratch, put the pulley mark at 0 on the pulley, confirm that #1 is on the compression stroke, take the cap off of the distributor, find where the rotor is pointing to establish #1, and get all of the plug wires in the right holes. Then set timing off of #1 plug wire.

    p.s. Or, since it is running well, referring back to one of my original comments about making things wrong work right, just drive it and see where things are at before getting back in to it. Someone probably installed the oil pump a tooth off in the past and "made it work".
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    ^ true. I guessed 15 based on his statement that it's running well and there isn't much adjustment left on the distributor. If someone had the firing order wrong, there is no way it would be running well. My guess is that he was looking at a different mark on timing then, or its just wrong.

    #1 on the distributor is easy to find, most distributor caps have a line molded in on the side just under the plug wire. Should be towards the radiator side of it right by the latch
    1978 280z 4sp

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    didnt have much time to work on my car today, but i checked the timing, and the mark was exactly the same as it was using the 6th cylinder.

    also, what are the correct torque settings for the valve cover? and whats the correct way to bolt it on?
    February 1977 Datsun 280z 5 Speed

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    Have you downloaded the service manual?
    1978 280z 4sp

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    Quote Originally Posted by cozye View Post
    Have you downloaded the service manual?
    I have the EFI Bible and the Haynes manual, neither of them say what the correct torque is.
    February 1977 Datsun 280z 5 Speed

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    You should use the fsm, it's free and readily available.

    http://www.xenons30.com/reference.html
    1978 280z 4sp

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    Well, just got my MPG reading today.
    14.8mpg

    I did however find out that cylinder 1 injector wasn't firing. I apparently didn't get all the corrosion off, but it's fixed now.
    Haven't fixed the timing thing yet, been too busy, but I plan on adjusting my valves later this week, and doing the timing correctly.
    Also, my front tires were heavily offset which I got fixed today, and wow does it handle/steer better so that had to fix my MPG for sure.

    My rear tires apparently are off though, and I can't figure out how to adjust them. They'd toe'd in way too far.

    I also did a power test across all cylinders, and it wasn't equal with all of them. I'm not sure if it's my valves, but certain injectors dropped the RPM's more than others, and a few didn't even change a thing. I checked my spark plugs, and they are sparking.

    Just seems so odd considering it drives so smooth...
    February 1977 Datsun 280z 5 Speed

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    Have you read your spark plugs? That's the best way to look for differences between cylinders.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

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    You'll find that all of the little things add up. But the EFI 280Zs don't seem to get a whole lot better than low 20s mpg, mixed city and highway driving. Highway driving with a 5 speed would probably get you to mid-20s.

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    Quote Originally Posted by Zed Head View Post
    You'll find that all of the little things add up. But the EFI 280Zs don't seem to get a whole lot better than low 20s mpg, mixed city and highway driving. Highway driving with a 5 speed would probably get you to mid-20s.
    id love 20
    mpg vs
    my 14
    February 1977 Datsun 280z 5 Speed

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    Quote Originally Posted by FastWoman View Post
    Have you read your spark plugs? That's the best way to look for differences between cylinders.
    ive checked them and done a compression test they all look the same, and compression was8.6-8.7 across all cylinders
    February 1977 Datsun 280z 5 Speed

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