Help please

[Simon Doherty]

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 , and I'm slowly driving myself insane trying to find the problem. Any suggestions?

[madkaw]

Find the download for the FSM for the car and get your ohm meter out and start checking everything . CHT comes to mind for your issues.

[Simon Doherty]

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?"

[Zed Head]

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.

[240260280z]

try cleaning the throttle valve switch contacts and its connections to the wire harness.

[Simon Doherty]

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.

[madkaw]

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 July 22, 2014 by madkaw
Because

[Simon Doherty]

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 July 22, 2014 by Simon Doherty

[madkaw]

Seems okay and working right are two different things. If he worked on Datsuns he knows that these dizzys screw up --a lot!!

Simple test to accomplish - pull vacuum hose, plug, drive:)

[Simon Doherty]

Well the distributor was his first theory. Also, he stopped working for datsun in 83, so he never saw the problems that these cars developed as they aged.

[siteunseen]

Does the tachometer stay constant with the motor's RPM or does it bounce around? I ask thinking of a failing coil.

[Simon Doherty]

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.