The vehicle Idles rough and the check engine light came on. I'm experiencing heavier fuel consumption, Also the Short term fuel trim is at 24% warm or cold. The Long term fuel trim is at 9%. The code first showed was P0701. I sprayed carb cleaner around the number 4 fuel injector the there is a distinct change in the RPM. The intake manifold is also in that area.

I replaced the fuel injector seals, both 02 sensors, the spark plugs and cleared the P0701 code. This did not solve the problem. Within a short period of time the check engine light came on again. The code this time shows P0304. I went and purchased the intake manifold gaskets from Nissan which I planned to install asap. Will provide an update after installation.

Update from Dec 21, 2011: Problem Solved!!!. Replaced the intake manifold gasket with the help of my mechanic buddy.

Cost $180.00

My 1998 Nissan Altima which has been reliable for years is now suffering from an extremely erratic idle. The rpms would start up around 1200 then suddenly drop to 200 up again then die. After changing and replacing everything everyone else here seem to had done also I came to the conclusion that it is my intake manifold gasket. Took it to mechanic and they could not fix the problem. Seem like he didnt know to much about japanesse cars. I cant afford to have someone fix it so Im going to bite the bullet and do it myself with all the manuals I can get my hands on. Nissan built this car so complicated that you have to remove about 10 different things just to get to a bolt. I guess that is their way of getting the customer to take their car to the dealer and spend extra money. This is the last repair Im going to do on this car. Wish me luck as I heard this is a pain in the azz job to do.. Here I go.