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Thread: Rough idle and bogs and backfires a little when accelerating

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    Default Rough idle and bogs and backfires a little when accelerating

    I picked up a 1976 280Z 2+2 over the weekend. According to the seller, "starts right up, really purrrs, positive acceleration". However, that is far from what's happening (buyer beware). When I try to start it, it doesn't start very well, really idles rough, and when I give it some gas (not even under load), it stumbles, boggs, and backfires a little before it picks up rpm. Once I get it up around 1500 to 2000 rpm, it smooths outs and runs OK and doesn't make excessive noises.

    The good; The oil pressure seems good; there isn't any blue or black smoke from the tail pipe up[on start up or reving; the exhaust coming out the tail pipe at say 2000 rpm is steady and smooth. The antifreeze in the radiator looks good (no oil signs) and the oil looks good (no antifreeze signs).

    The bad; As noted above, especially concern over the backfires. I kind of hate to start it for fear of it jumping timing or doing some damage before I figure things out.

    The ugly; There is none. These are really cool looking cars! I love the looks of the car and really want to get this thing fixed up.

    I haven't had a chance to check the compression yet or do other mechanical test of the engine. Nor have I had a change to check the timing and the distributor/rotor for cracks, etc, or how it advances, etc. It looks like the seller just put new plugs in and so I haven't even pulled them to see how they are burning.

    I guess I was looking for some advice on where to start looking/testing. The engine is the original with about 80k miles and it doesn't look like the fuel injectors have every been changed, or the fuel pump, etc. So before I start, I was wondering if anyone else has had similar "symptoms" and has advice.

    Thanks
    Gary

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    Could be a vacuum leak, letting air bypass the AFM, causing a lean condition. It sounds like the fairly common lean back fire. Higher RPM might overwhelm a small leak, letting the engine run correctly. At low RPM the vacuum leak dominates.

    Check the big rubber hose after the AFM, the PCV hose, the breather hose to the rocker cover and the various vacuum ports on the intake manifold. Spraying carb cleaner or starting fluid on the intake/head interface and listening for RPM change will find a leaking manifold gasket.

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    Thank you Zed Head. Before I jump into anything major, I'll first check all the hoses as you suggest.

    I once heard that instead of the carb cleaner or starting fluid approach, using a small propane torch (not lit of course) will also alter the RPM as the propane gets drawn in by the leak (safer and less messy also). So I think I'll start with your suggestion and check every hose and fitting as best I can.

    Thanks
    Gary

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    I finally got a chance to get back to looking over the car. I checked the vacuum hoses as Zed Head suggested and found the line that goes from the distributor and AFM had a hole burned in it from the exhaust manifold. I fixed that hoping it would run better. However, I could just barely get it to start and it ran very rough for a less than a minute before it died and now it won't even fire.

    I'm kind of an old carb guy, This FI stuff looks pretty complicated. I do have an official service manual for it but I'm not even sure where to start. Does anyone know of a link to a technical paper on these engines that would outline steps for trouble shooting (eg, do I first check for spark and how, then check fuel pump flow and pressure, then fuel injectors, etc, etc.) Or is it kind of beyond the typical shade tree mechanic to trouble shoot this stuff and I'll have to find someone who's familar with these cars and pay the $$$$$.

    Any advice or words of wisdom or warning are apreciated.

    As mentioned above, I really like this car, the cool looks and practicality, and really want to get it fixed up.

    Also, I live way up in Northern Michigan and Winter is closing in so unfortunately I may have to let it sit until Spring.

    Gary

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    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


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    Running lean is quite common for the 280. There are plethora of causes. Download the fsm and start a thorough check of the efi system. You can do most of it with a simple ohm meter. Not hard.

    Once that's done do a search for a thread "purrs like a kitten" there are several of us here including fast woman who have gone to hell and back with the fuel injection on this car. You will get there, it just takes patience and thorough troubleshooting.
    1978 280z 4sp

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    Thanks guys, lots of great info which I'll be throwing on my favorites and collecting for when I get a chance to really dive into it. For now however, because I live in Northern Michigan and Winter is fastly appproaching, it may have to stay parked until the weather get better this Spring.

    Blue, this knd of off topic, but your car looks awesome. I need to do something with my chrrome bumpers and rotted rubber end caps. Is the bumper/air dam you have the one they refer to as fiberglass type 2 on the Zcar source site? And your photo doesn't show the back bumper, is it the matching one (eg 50-4073)?

    Thanks
    Gary

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    Hi Gary,

    Download the FSM for your car at www.xenons30.com. It's free, put out there for the collective karma.

    In all likelihood, there are many things wrong with your engine, so trying to diagnose it can be an exercise in frustration. My advice would be to just go through it from end to end, check every component and every connection, fix whatever you find that needs fixing, and you'll end up with a running engine.

    There will be one final component which might pass Nissan's diagnostics, but which might still be out of whack. That's the ECU (the "computer"), which Cozye and I have found can drift in its calibrations over time. (It's analog, not digital). There are remedies for that too, as you'll find in the "purs like a kitten" thread.

    Eventually you'll get there, and once you're done, you'll understand every part of the EFI and realize it's not so complicated after all. (To me, CARBURETORS are complicated, as they depend somehow on magic!)
    Last edited by FastWoman; 11-15-2011 at 09:22 AM.
    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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    Quote Originally Posted by Healeyalt View Post
    Blue, this knd of off topic, but your car looks awesome. I need to do something with my chrrome bumpers and rotted rubber end caps. Is the bumper/air dam you have the one they refer to as fiberglass type 2 on the Zcar source site? And your photo doesn't show the back bumper, is it the matching one (eg 50-4073)?

    Thanks
    Gary

    It came with the car. I believe it was an msa aero 2 kit w/o the side parts (here). I sold the parts and now have a gnose and open back.

    http://www.ridejudge.com/ride/38427/1977-nissan-datsun
    Last edited by Blue; 11-15-2011 at 09:54 AM.
    There is scarcely anything in the world that some man cannot make a little worse, and sell a little more cheaply.
    The person who buys on price alone is this man's lawful prey.


    John Ruskin (1819 - 1900)



    ZCars in Eastern Canada seaport ready for shipment to Europe

    http://ZSportCanada.com


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