Misfire out of nowhere

[NC280z]

Good evening everyone,

 

It seems that I can't get a break with my 280z. 2 weeks ago I had it running great, and today I went to prep it for paint. The car starts and idles fine, but when I blip the throttle or rev up past 2,000 rpms I get a good bit of stumbling and misfiring, and the air fuel ratio goes lean (into the 17~18 range). I'm perplexed at what the cause could be, since it ran perfect a few weekends ago and it idles nicely. The only conclusion I can come up with is low fuel pressure or bad fuel from sitting.

 

Anyone run into issues from letting their Z sit, and what was the cure? Thanks for your time!

[MicroMachinery]

Change out your fuel filter. Start with the easiest/cheapest solution; it's often the problem.

[datzenmike]

The 'flapper' on the AFM may be sticking. Here is what to look at. Not saying to take it totally apart, maybe just reach in and push the door open and see if that frees it. .... http://www.atlanticz.ca/zclub/techtips/afm/

[Middle man]

Hey Mike this is a great link. If I understand this info. does this mean I can more accurately adjust mixture on my carb with a thermo gun. At idle at least? waiting between adjustments for temp.variations? If so where would you point the gun?

[datzenmike]

You would want to use a wide band O₂ sensor and adjust for the correct or best air/fuel ratio.

[NC280z]

Ran some checks today. AFM flap isn't stuck or sticking (it's a new AFM), the fuel pressure is 28~30 at idle and jumps to 36 on WOT. Timing is 7' BTDC, and plugs all are correct brownish color for normal running condition. Thinking that this is a bad fuel issue, I ran some Iso-HEET in the fuel to see if it'll help any.

 

One thing I have noticed is that it idles just fine, has excellent vacuum (18~20 inHg) and holds steady at higher RPMs just fine. But when I blip the throttle quickly I tend to get a bit of stumble, more so when the engine is warmed up. The stumble is also very intermittent, and not a constantly occurring thing.

[NC280z]

So I drained out the old fuel, replaced it with fresh premium and a new fuel filter and the problem still persists, and is actually a bit worse :confused:. I'm at my wit's end with this damn car!

[NC280z]

It turns out that the inlet tube on the fuel tank that leads to the fuel pump was cracked, and letting the pump suck in air and dilute the fuel being fed to the injectors. On top of that, the inlet tube's other end (the end that's immersed in fuel in the tank) was partially plugged, causing further issues. D'oh! I'll need a better approach to things next time I troubleshoot, so I can save myself some time.

[Draker]

It turns out that the inlet tube on the fuel tank that leads to the fuel pump was cracked, and letting the pump suck in air and dilute the fuel being fed to the injectors. On top of that, the inlet tube's other end (the end that's immersed in fuel in the tank) was partially plugged, causing further issues. D'oh! I'll need a better approach to things next time I troubleshoot, so I can save myself some time.

 

It's interesting you didn't see this when you had the fuel pressure gauge hooked up. Maybe you didn't stay on the gas long enough for it to drop in pressure?