I finally got my ’93 Nissan 300ZX out of storage! Unfortunately, it didn’t pass the emissions inspection on the first try. So I got a chance to reconnect with my Fairlady Z the best way I know how – giving her a tune-up.
Failed emissions due to high HC (hydrocarbons)
The 300ZX is a fickle lady. If you have one, then you probably know that passing emissions can be a challenge. Especially now that they’re 20+ years old, keeping them running perfectly clean is hard work. If you happen to live in a state that doesn’t do emissions testing on pre-OBDII cars, I hate you and I’m extremely jealous of your awesomeness.
My test showed that CO and NO levels were fantastically low. The car actually runs smoother than it ever has in my possession. But the hydrocarbon level was over double what is allowable. So I needed to do some work.
If you have high HC but no CO issues, you can rule out a bad catalytic converter. If your HC is only a little high, check for a vacuum leak. But if you’re in my shoes and your HC level is off the chart, that means a whole heap of fuel is making it to the tailpipe unburned.
Emptying a 300ZX gas tank
Since my car has been sitting for over a year untouched, and barely driven the year before that, the culprit is probably (hopefully) just bad gas and carbon buildup in my intake. I needed to empty a full tank of year-old gas and here’s how I did just that.
- Release pressure in your fuel system. (Easiest way to do this is just let the car sit overnight and don’t put the key in until after the next step.
- Loosen the clamp and remove the return fuel line after the hard line atop the plenum. This is the rubber hose that runs alongside your fuel filter.
- Take a piece of fuel line and attach it to the hard line atop the plenum and use the clamp you took off in the last step to secure it. Place the other end of the line in your gas can.
- If everything is secure and all spills are cleaned up, start the car and let it idle.
- When you have filled the can, turn the car off. Wait a minute for the extra gas to trickle out of the hose before removing it to swap cans.
If the car shuts off by itself, that means you’ve drained all the fuel out that your fuel pump can reach. Do not keep cranking it over trying to get it to start. You will very easily kill the fuel pump. Instead, add new gas and off you go!
Why is it better?
I’ve seen on numerous forum posts that “all you have to do is disconnect the fuel filter“. Well, that’s just not true. You see, the 300ZX automatically shuts the fuel pump off after two seconds when the car isn’t running. That’s a great safety feature, but doesn’t help us empty the tank.
Some people recommend finding the drain plug on the fuel tank (a feature that almost all cars are lacking). But if you don’t have a Twin-Turbo, you may not have a plug, or may not be able to get to it. Even on the Twin-Turbo tank, it’s not an easy job and it sounds like a big mess if you have a full tank.
My assumption? None of these people have ever tried to empty their tank.
To be honest, there shouldn’t be a reason to warrant emptying your tank. You shouldn’t – and most people don’t – let your fuel sit so long that it’s too far gone to use. I put additives in my tank before letting it sit for a year so it wouldn’t go bad. But when it comes to emissions testing, you’ll need fresh gas anyway.
So taking the return line off and draining from there while the car is running is the best option. Mostly because it’s the only practical option. I emptied a 15.8 gallon tank in less than 30 minutes.
Have a go at it, be safe, and enjoy your Z.