Update for you: I noticed the spark plug gap was small (I believe .9mm but I don't remember how small specifically) so I gapped them all to ~1.1 which is the maximum gap in the fsm.

The car seems to be considerably down on power and I need to press the pedal further to get the same power as before I messed with the gap. Oddly, the car idles noticeably smoother than before, it just is considerably down on power which doesn't make sense. Any idea why that would be and if I should reduce the gap to below 1mm like it was before?

Edited April 1Apr 1 by chaseincats

How does that relate to 45 thousandths of an inch.

51 minutes ago, Yarb said:

How does that relate to 45 thousandths of an inch.

I’m not sure what you mean, can you elaborate a bit?

IIRC the plug gap should be set at 45 thousandths on your plug setting gauge.

Can't remember if your ignition system is stock or not. 1978 is high-energy, big gap.

Why did you decide to mess with the spark plugs if everything was working well? Your post of March 16. Is it running oddly? If you have created a misfire with a big gap it should show up on your AFR gauge. But a misfire will also cause non-smooth running.

I'd guess that the engine is running differently for some other reason than plug gap. Maybe you bumped a hose or something.

Maybe a mouse has moved in to your air fillter box. Worth a check. It happens.

Edited March 30Mar 30 by Zed Head

2 hours ago, Zed Head said:

Can't remember if your ignition system is stock or not. 1978 is high-energy, big gap.

Why did you decide to mess with the spark plugs if everything was working well? Your post of March 16. Is it running oddly? If you have created a misfire with a big gap it should show up on your AFR gauge. But a misfire will also cause non-smooth running.

I'd guess that the engine is running differently for some other reason than plug gap. Maybe you bumped a hose or something.

Maybe a mouse has moved in to your air fillter box. Worth a check. It happens.

I have a cone air filter so it shouldn't be a mouse nest issue especially since it's kept in a sealed garage.

It really should be the plug gap modification since it was an immediate change in power loss.

I gapped all of the plugs because I dropped one (like a dummy) when I took it out and when I checked the others - they were all considerably smaller than the 1.0-1.1mm gap in the picture you sent from the FSM. If I had to guess, I might have done the Canadian gap by mistake back in the day. Is there a reason Canada uses the smaller gap?

17 hours ago, chaseincats said:

It really should be the plug gap modification since it was an immediate change in power loss.

Is there a reason Canada uses the smaller gap?

Reset them to the narrower gap and the question should be answered.

I can't recall ever seeing a Canada spec 280Z. Hard to say. Maybe they used the older ignition modules with the ballast resistor.

On 3/30/2026 at 4:25 PM, Zed Head said:

Reset them to the narrower gap and the question should be answered.

I can't recall ever seeing a Canada spec 280Z. Hard to say. Maybe they used the older ignition modules with the ballast resistor.

I pulled the plugs and they were gapped to 1.0 not 1.1 even though I was sure I gapped them to 1.1. Just to cover all of my bases before I really went overboard with below-spec spark gaps again, I changed the gap to 1.1 and the car runs great.

I don't understand why the car ran well at less than 1.0, poorly 1.0, and great at 1.1 considering factory spec is 1.0-1.1.

Either way, we're sitting pretty now. Thanks all and hopefully this is the end of the drama!

Edited April 2Apr 2 by chaseincats

I apologize for not understanding the plug gap measurement you are speaking of. Forgive my ignorance so to speak. This is the first I’ve heard of this measurement. Please educate me on this.

9 hours ago, Yarb said:

I apologize for not understanding the plug gap measurement you are speaking of. Forgive my ignorance so to speak. This is the first I’ve heard of this measurement. Please educate me on this.

The factory manual says the spark gap can be set anywhere between 1.0 millimeter and 1.1 millimeter. I just wrote that instead of the thousands of inch measurement because it's shorter/easier to visualize for me.

As a side note - when I removed one of the plugs last night, its ceramic surround cracked/shattered. It makes me curious if there were small cracks in it causing electromagnetic interference...or if really was just the gap.

Edited April 2Apr 2 by chaseincats
added info about the cracked plug ceramic

On 3/4/2026 at 2:14 PM, Zed Head said:

If it happens again, take a look at your tachometer. The ECU uses the tachometer signal to determine when to fire the injectors. Failing ignition modules are a common problem with the 280Z's.

You're describing two distinct problems now. Best to try to diagnose them seaparately.

@Zed Head I haven't had this issue again, but tonight I was driving, stopped at a stoplight after being on the freeway for about 20 minutes, and the car shut off like a switch was pulled (didn't stumble and die so it would be a spark issue). Could this be related? This is the first time I've had this happen ever.

If it helps, I turned the key and it immediately fired right up like nothing happened.

Edited May 2May 2 by chaseincats

Intermittent electrical problems are really hard to diagnose. I was testing a spare 1978 ECU and it did what you describe. Died, started right up, died again a half mile after, wouldn't start again.

Had an ignition module that worked fine until it over about 3,000 RPM. Then the tachometer got weird, reading high and needle jumping around, and the engine ran poorly. Turned off the engine, restarted, and it was back to normal.

All you can do is collect the clues and make your best guess.