Several things that could cause misfires:
- What brand of spark plugs are you using? You should be using OEM NGK plugs; other brands such as Champion or Bosch many times cause driveability problems in Nissan engines.
- The cam position sensor may be marginal.
- Incorrect fuel pressure. Tee-in a temporary fuel pressure gauge between the fuel feed hose and the fuel rail. The reading at idle should be 51 psi.
- There may be a major intake system vacuum leak. To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be 21 InHg. If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.
- Dirty fuel injector(s). Run some good injection cleaner, like BG products 44K, through the system; give the cleaner about a week or two to do it's job.
First thing to check is your engine ground. With the car running, put a voltmeter on the lowest scale and measure from the block to the negative battery post. It should read no more than 50 millivolts (0.05V). If the ground is good, inspect the MAF for lint, butterfly wings, ETC. Nissans have no MAP sensor to double-check the MAF, so the ECM basically has to trust it. If it lies, bad things can happen. Once it's clean, consider doing an IAVL (Idle Air Volume Learn). It's just a matter of the silicon aging and it's harmless to the MAF, but if it's over- or under-reading because of old age, the ECM needs to know about it. That's what IAVL does, it re-matches the MAF to the ECM. Lastly, depending on your driving habits, you could have a bad case of carbon buildup. If you drive conservatively and don't "stomp it" very often, carbon buildup on the piston crowns and intake valves can occur. Easy enough to fix, just warm it up fully, have an assistant hold the RPM's around 3500, and put a teaspoon of ATF (any kind works) down a vacuum port. The brake booster port is usually easiest. Any carbon in the engine will literally "crack loose" and go out the exhaust from the momentary temperature differential caused by the film of ATF.