1997 Nissan Altima Check engine light on

Hi ,

I have a 1997 Nissan altima. The check engine light came on all of a sudden one day and I too it to the mechanic. They told me it is due to cylinder 4 misfire( I think code is P304) . So I did a engine tune up, changed spark plugs, wires, distributor cap. It cleared the check engine light, but it again came on the next day.

So my automechanic suggested replacing the fuel injector. replaced cylinder 4 fuel injector. But still the check engine light is on and it is giving misfire on cylider 4.

Current condition of car: The car runs fine if I am pushing the pedal and accelerating. Once I remove from gas pedal, once it reaches 40 mph, there is hesitation and I can feel the vibration all the way to my feet. It runs fine when accelerating.

I have already spend good money for engine tune up and changing fuel injector.

I would be really thankful if you could share your expert opinion and advice on this.

Before replacing anything else, have a compression/leak-down test performed on the engine. This will indicate if there’s a mechanical problem with cylinder #4.


I would want to know if the compression in cyl 4 is ok.

(tester beat me to it, I didnt’ click on submit button fast enough.)

forgot to mention.
After replacing the fuel injector , they did a comperssion test . the compression came out to be 138 psi, but my mechanic told me for older cars it is good enough to run decently.

Then the problem might caused from excessive carbon build-up on the #4 cylinder intake valve. This can act like a sponge where it aborbs the fuel from the injector intended for the cylinder and causes that cylinder to lean out causing a misfire.

Has your mechanic tried a decarbonization of the engine?


No, my mechanic mentioned he might have to open the engine and do a visual inspection and could cost lot money.

How do you do decarbonization of th engine ?

Is it bad for tne engine if I use the car with misfire? Can I put off the repair for a month and still keep using the car. How critical is it ?

How much does it cost to do decarbonization ?

If you replaced all the seconday ignition components yourself, you can do this.

Purchase a can of Seafoam Motor Treatment. Get the engine up to operating temperature and shut the engine off. Remove the brake booster vacuum hose from the brake booster, and adapt a hose that fits inside the brake booster hose and into the can of Seafoam. Take a pair of pliers and pinch off this hose. Have someone start the engine, and bring the RPM’s to 2,000. Slowly open the pliers so the Seafoam starts to be drawn into the engine. It’s here where the RPM’s have to be kept up, and where the hose is pinched off to prevent the engine from stalling. Did I say the engine will smoke like hell? It will.

Once all the Seafoam has been drawn into the engine, shut the engine down. Reattach the brake booster vacuum hose. After a half hour has past, restart the engine and bring the RPM’s back up to 2,000 until the smoke clears from the exhaust. Take it for a cruise to see if the Check Engine light comes back on.


have the same car with the same problems did everything you did including injector with no results even replaced engine brain box one day I unpluged the egr valve and pluged the vacume line and dam car runs fine did you use the sea foam to clean the carbon deposits because thats what I was told is wrong but they gave me a estimate of $1200 to pull intake manifold and clean it