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Help please


Simon Doherty

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I bought a 1982 280zx non turbo last month and since then I've encountered a problem. The car will start stumbling and misfiring around 2500-3000 rpms. But the really weird thing is that the car doesn't do this all the time. I will start the car up, and it accelerates as smooth as silk without missing a beat. However, when the car warms up, I'll turn it off, then back on, and then the car will start misfiring at that rpm range. I've checked the air flow meter, the tps, replaced the distributor cap and rotor, replaced the o2 sensor, and a couple other things all with no luck. The o2 sensor did seem to help it, but not for that long. Please help, it's getting quite frustrating LOL, and I'm slowly driving myself insane trying to find the problem. Any suggestions?

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I've got the fsm, and I've already checked many components. The chts has been brought to me before but I've not tested it. Didn't seem like a component that would cause my problem. The hard part about solving this is the fact that the car doesn't do it all the time. So whenever I get a new idea to test, I keep thinking, "well if it were that, then wouldn't it be doing it all the time?"

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When you say misfire is there popping back through the intake or just hesitation or something else? Sounds like the typical lean situation. Today's fuel and old electronics, a vacuum leak or two, bad fuel pump or FPR, CHTS reading hot...

When you turn the engine off, the CHTS probably picks up a little more heat from the cylinder heads since the coolant has stopped flowing. The little extra bit of leanness might be pushing the mixture in to the stumble range.

You could add a potentiometer to the CHTS circuit as a test. Cheap and reversible.

Could also be a worn spot on the AFM circuit. The narrow RPM range is probably the same spot on the AFM.

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I don't think it's the AFM. I took it off and popped the cover and, although a little old, it didn't seem worn out. Not to mention it's reading as it should on my ohmmeter on all terminals. And yes, there is a slight hesitation with the engine (all the time). And no, there is no popping through the intake, but there is some popping through the exhaust. However, there isn't usually a correlation between when it misfires and when it pops in the exhaust. I'm going to take the distributor apart this weekend and see if there is any play in the shaft. And I'm also going to do a pressure test on the fuel system while I'm at it. I tested the chts with an ohmmeter (from the ecu connector) and it doesn't seem to be reading what it should. (Though please keep in mind I'm not that experienced with an ohmmeter.) the car was warm when I tested it and it should have read below 2.9 ohms. However, my ohmmeter was showing me about 270 ohms.

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You might be on to something with wanting to check the dizzy. Those years are prone to issues regarding the vacuum advance. You might try disconnecting the vacuum portion of the advance and try driving it .

The plastic parts of those dizzys break and cause the vacuum advance to malfunction .

Check what scale you are on with your ohm meter. You were probably reading 2.7 ohms

Edited by madkaw
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There is no scale with my ohmmeter, it's a digital multimeter and the ohm setting automatically scales to the appropriate range....I think...

I'm not so sure that it's the dizzy. I mean, if there was a problem with it I feel like it would be having the problem constantly, not just when I turn the car off and on again.

My dad (a much better mechanic than me. He also worked for datsun 30 years ago) checked the vacuum advance and he seemed to think it was ok. I'll have to play around with it though. I kinda of doubt it's the fuel system, because he said the way it feels, it feels like an ignition problem. But who knows, by process of elimination it's becoming more and more likely. I'll keep you posted, let me know if you guys have any more ideas.

Edited by Simon Doherty
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Funny you say that. The tachometer does 'tweak' every now and then, usually when I let my foot of the gas. However, it doesn't really do this when the car misfires, so I would be kinda surprised if the coil was bad. It definitely is something to check though.

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That could be the ignition module on the side of the distributor, small black plastic box. Early ZXs have the E12-80 with only one plug, the later ones have E12-92 with 2 plugs. https://www.google.com/webhp?sourceid=chrome-instant&rlz=1C1BOFA_enUS494US494&ion=1&espv=2&ie=UTF-8#q=e12-92+280zx.

They have circuit boards inside so I'm sure they can fail at some point.

Edited by siteunseen
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That could be the ignition module on the side of the distributor, small black plastic box. Early ZXs have the E12-80 with only one plug, the later ones have E12-92 with 2 plugs. https://www.google.com/webhp?sourceid=chrome-instant&rlz=1C1BOFA_enUS494US494&ion=1&espv=2&ie=UTF-8#q=e12-92+280zx.

They have circuit boards inside so I'm sure they can fail at some point.

Alright well we've checked the distributor and the vacuum advance and it appears to be in perfect working order. I'm thinking it could be a gunked up engine or fuel system. I ran seafoam through the car, oil, and throttle body and it does seem to have changed the problem somewhat. It is a little smoother when when it misfires now, and it isn't quite as consistent also. This could be a coincidence, but running seafoam through the engine has definitely given me the biggest difference so far. I might try running seafoam through it a couple more times, or trying some kind of heavier product. And I'll probably check/replace the chts while I'm at it.

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Alright, well I have a question. Is there, to anyone's knowledge, a possibility that when a car gets above normal operating temperature it can cause a misfire? The reason I ask this is because the car started to do it a little more often without having to turn it off and back on again. And I noticed something else today. I stopped for food and parked while I ate it. It was hot out so I kept the car on. Since there was no air flowing, of course, the car got a little hotter than it normally did. And when I set off again, the car started stumbling much sooner than it normally does (about 1500-1800 rpm). And then after I got going for a few minutes, air cooled the car off to where it usually runs and it stumbled as it usually does. So, ya. Is there any possibility the thermostat is letting the car run too hot? And if so, could this, in any way, cause a stumbling problem?

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Ignition modules fail in different manners, but there's almost always heat involved. One trick people use to test them, for the ZX module, is to spray them with some sort of cooling spray or applyi ice to cool them down.

Stumbling could be ignition or fuel related but the EFI system will compensate for heat so simple overheating shouldn't really cause stumbling.

On the other hand though, if your air-fuel mixture is on the edge of lean, maybe the extra heat pushes it over. It might be worth your while to try the potentiometer in the coolant sensor modification. In your case it would in the CHTS circuit. I haven't seen ZX people with the problem but many 280Z people have added a little resistance to add a little more fuel to the intake air and found good results. The Z's may have been tuned to perfection and today's gas pushes them over, but maybe the ZX's have it too. It's a cheap easy test.

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  • 5 months later...

I bought a 1982 280zx non turbo last month and since then I've encountered a problem. The car will start stumbling and misfiring around 2500-3000 rpms. But the really weird thing is that the car doesn't do this all the time. I will start the car up, and it accelerates as smooth as silk without missing a beat. However, when the car warms up, I'll turn it off, then back on, and then the car will start misfiring at that rpm range. I've checked the air flow meter, the tps, replaced the distributor cap and rotor, replaced the o2 sensor, and a couple other things all with no luck. The o2 sensor did seem to help it, but not for that long. Please help, it's getting quite frustrating LOL, and I'm slowly driving myself insane trying to find the problem. Any suggestions?

have you checked the plugs/wires? and egr system? mine ran GREAT til one day my hubby parked it to change the brakes, then wouldn't start back. when she finally did start, she fell flat on her face. he checked everything from the afm to the plugs, turned out the plugs were fouled due to the egr being stuck. tapped it with a hammer and she cranked right up and ran perfectly again with no more fouled plugs due to running too rich.

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  • 2 years later...

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