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I bought a 95 Altima last month that had a P0301 (cylinder 1 misfire) code. The dealer told me the timing chain had been changed recently and he had just replaced the exhaust system. If I realized how serious the problem was I wouldn't have bought the car, but the dealer made it sound like it was no big deal. Since then, I have replaced the spark plugs, wires, and fuel injectors, and the check engine light went off and the car was running great. It has had trouble starting ever since I did the fuel injectors. I have thought maybe the fuel pressure is low, but I'm not sure. I did purchase a new fuel filter, but have not replaced it yet.

In the last few days it's started running rough (jerking/lurching around 2000 RPM) and the check engine light came on for about a day and then went away. I scanned the OBD and it showed cylinder 2 misfire (P0302) and engine knock (P0325), and recommended new fuel injectors (which didn't make sense to me because I just replaced them). I did have a mechanic look at it a while back and they said the compression on cylinder 2 was about 20 lbs below the others, and cylinder 1 was 5-10 lbs below the others, but they didn't do a wet compression test or a leak down, so I don't know much about what's causing that. I did check the oil just the other day and it's still full, so I am doubtful that it's the rings, because I would expect that to burn oil.

I have wondered if maybe there's something wrong with the head gasket or perhaps the intake manifold gasket, but I don't know enough to really identify that for sure (I don't know much about cars except just what I've read online and learned on my own). I did check the ignition coil with a multi-meter, and the resistance seems to be right about what it should be, and the distributor cap looks to be practically brand new, so I don't think it's that either. When I drive it with the windows open I do smell a strange burning smell, not like gasoline, I don't quite know if it's like a burning coolant smell or perhaps like burning rubber. Can anyone help me figure out how to pin down the exact issue?
 

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Bad compression on one or more cylinders can cause hard starting and rough running. Run a compression test on each cylinder and post the actual readings here on the forum; Standard 178 psi, Minimum 149 psi, maximum allowable difference between cylinders 14 psi. If you find a bad cyl reading, you can follow it up with a leakdown test to determine the cause.

Several things that could cause misfires:
* Is the misfire occurring on any particular cylinder? 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 at the output side of the fuel filter. The readings at idle should be as follows:
- with vacuum hose connected to the fuel pressure regulator: 34 psi
- with vacuum hose disconnected from the fuel pressure regulator: 43 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 or defective fuel injector(s). If you replaced the fuel injectors, were they new or re-manufactured or rebuilt. With rebuilt parts, rebuilders will use a vehicle's old part and replace just the worn components. If the vehicle's old part cannot be rebuilt because it is too worn, they will use a part from another vehicle (referred to as a 'core'). If a core is used, than they will replace only what is needed in the core. With re-manufactured parts usually (not always) refers to a part that, for all practicable purposes, has been completely re-manufactured to the standard of a new part. So to summarize, new or re-manufactured are the best to buy. A rebuilt part may still be defective or DOA when you're ready to use it.
 
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