Hi all (again)

My car runs rough (almost like its not running on all 6-cyls) when it first starts but smooths out after ~30 seconds.

The car starts easily with only a few cranks, but then exhibits that behavior once running. This is without using the cold start injector (which I wired to a manual triggering switch). I read that it could be a fuel pressure issue or a failing fuel pressure regulator but it smoothing out after ~30 seconds tells me that wouldn't be it.

Any ideas?

I am attributing all my recent posts to the car essentially coming out of a 1.5 year hibernation.

What’s your fuel pressure reading beyond the filter? Have you installed a gauge yet? If not that would be the first thing on the list.

Just now, Yarb said:

What’s your fuel pressure reading beyond the filter? Have you installed a gauge yet? If not that would be the first thing on the list.

I haven't tested it since I didn't think the pump would change fuel pressure based on time. I'd have thought the fuel pressure would either be wrong for the duration of the engine running rather than improve over time, no?

Edited March 3Mar 3 by chaseincats

You seriously need that gauge to monitor the system. You will find yourself going down a rabbit hole. Fuel pressure and monitoring the gauge is key to diagnosing issues on the 280. I can’t make any determinations without seeing what the fuel system is doing.

Just now, Yarb said:

You seriously need that gauge to monitor the system. You will find yourself going down a rabbit hole. Fuel pressure and monitoring the gauge is key to diagnosing issues on the 280. I can’t make any determinations without seeing what the fuel system is doing.

Fair - I will grab a gauge next week after the storm passes and get back to you

An IR gun would be useful too. You could measure temp at each exhaust port and see if all six are firing and burning during that 30 seconds

This is where I located my gauge, see photo. Yarb is absolutely correct, knowing fuel pressure is key to diagnosis. I bought my gauge and clamps on Amazon. Also, be sure to use the fuel service rated clamps, not radiator hose clamps, otherwise you'll leak gas and a gas leak in the engine bay is bad news. As I recall, it should be somewhere around 35 psig if you have a healthy fuel pump. Might as well replace the fuel filter also if yours is old. Also, get a copy of the Fuel Injection manual - I think it's on the resource page of this forum. The manual has excellent troubleshooting scenarios and will provide a great understanding of the fuel injection system.

Edited March 4Mar 4 by Paulytunes

Thanks all and interesting idea on the IR gun. I do have one here, so I will put it in the car.

A couple of new/interesting observations in the interim:

Last time the car was running rough, I pulled/re-inserted the injector connectors one-by-one and found that cylinder 3's made a considerably smaller change in engine rpm when unplugged than the other injectors. Since that day, I've driven the car a few times and it hasn't exhibited that rough start behavior.

That said, I was driving it this evening and the car for a fraction of a second lost all power - almost as if the third injector missed its turn (if that even is the issue). The car did not exhibit the rough start prior to this drive. These injectors are only a year or 2 old, but did sit for a while with old gas in them, so I'm curious if one is possibly gummed up and is exhibiting these issues as it sifts through what's in it. I added a bottle of Lucas injector cleaner when I filled the tank up last week and I'm curious if that is helping keep the rough start at bay, or if it is just coincidence. I am curious about the new split-second power drop however.

I'm not sure if this helps, and am not sure if it was just the angle I put my ear, but when listening to the injector clicks using the 'put the screwdriver to your ear' trick it sounded like that injector's click was considerably weaker than the others.

Edited March 4Mar 4 by chaseincats

On 3/3/2026 at 8:53 AM, chaseincats said:

My car runs rough (almost like its not running on all 6-cyls) when it first starts but smooths out after ~30 seconds.

Is this on a cold start or after restarting with a warm engine? The 280Z's have a common hot-start problem.

40 minutes ago, Zed Head said:

Is this on a cold start or after restarting with a warm engine? The 280Z's have a common hot-start problem.

This is a cold start issue. The split-second power cutoffs were at operating temperature after driving for 10+ minutes.

Edited March 4Mar 4 by chaseincats

15 hours ago, chaseincats said:

the car for a fraction of a second lost all power

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.

4 minutes ago, 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.

Good to know - thanks. It happened twice during the same drive a few minutes apart after the car had been driving for 10+ minutes at around 3k rpm but no issues above or below that. I quickly glanced over at my air/fuel gauge and saw it go max lean (no combustion) then back to standard readings immediately after, but didn't think to look at the tach.

Do you mean the ignition box in the cabin, or the pickup coil inside the distributor? The 1978 car has a different ignition module since it is the only year that doesn't have a ballast resistor, right?

Edited March 4Mar 4 by chaseincats