I have a 1996 Maxima. I just replaced the head gasket and had a valve job done. Now the car is acting funny and the cel is on. It seems to run fine once it gets moving. When I stop and it is at an idle it loads up and sputters. The rpms fluctuate and the car feels like it is going to die. It has plenty of power and runs fine once I take off. I took the car to Autozone so they could run a check on the cel. They told me it was the MAP sensor. When I did the self-diagnostic test, I got a code for the absolute pressure sensor according the link from the forum. I assume these are two names for the same part. I called the Nissan dealership to check the price and they had no idea of a MAP or an absolute pressure sensor for my car. They said I would probably need a MAF sensor for upwards of $400. I called my local NAPA store to get their price and they too said my car uses a MAF sensor instead of a MAP sensor. I am totally confused. I searched the forums and read where some cars use a MAP and a MAF sensor, but it's Greek to me. Can anyone explain this in English? Does my Maxima have both sensors or only one? I don't understand why one store has a MAP sensor for my car and the dealership doesn't even know what I am talking about. Are these symptoms indicative of a problem with them? Where can I go to get one cheap?

Andrew

Oh boy! I will try to explain this as best I can.

Ok yes your car has both components, some dealer parts personnel are not always the sharpest knifes in the draw. The MAP/Absolute pressure sensor are one in the same. But they will have no effect on how your car runs. The sensor is simply used for OBDII monitering purposes. It moniters the 2 readings when the car is conducting self tests on the emissions evap system. That is its only purpose. the sensor has one vaccum line to it coming from a solonid that has 3 ports. 1 port is open to the atmosphere to measure absolute pressure. 1 port is plummed into the manifold to meausre manifold vaccum [map portion] the other port goes to the MAP/Absolute pressure sensor.

The other sensor MAF [Mass Air Flow] is for measureing the amount of air entering the engine and is used for fuel control. This is the system nissan uses for measuring engine load to control fuel.

Now heres where the confusion is coming from. Different manufacturers use different methods of measuring engine loads. For example GM used to use the map sensor to mesure engine load for fuel control. So If you encountered someone who has more GM experiance then Nissan experiance they may interpit the code you got as the source of your drivability problem, But when you went to a different source that was familier with nissan knew the MAF was the primary control for fuel they would have taken a different stance as to what the problem was. The reason they sent you to the MAF.

Most manfactures use the airflow method now and the days of the MAP are pretty much gone. The only use for the MAP any more is for OBDII self tests. It will set the check engine light if there is a problem since its an emission related componenet.That is really all the check engine light is for is to tell you something is wrong that could adversely effect emissions being emitted from your car.

What you should do is get the code number and look it up here to be sure you got the right code.

Hope that explination cleared most of it up for you.

Scott

Thanks so much for the reply. That does clear things up. Are you saying that replacing the MAP sensor probably wouldn't help my problem, rather maybe check the vacuum lines running to the sensor? What do you think should be the next step to fixing the way the car is running?

the code likely is a result of low manifold vaccum. you probably have a vaccum leak,pretty good one judging by how your car is acting. start checking all your vaccum lines, be sure to take a look at the PCV hose and the brake booster hose closely. also check the accordian tube for splits going from the air cleaner to the throttle body.

I just pulled some more codes from the ecu. Now it is not giving me the map sensor code. It spit out three codes. 1)0201-fuel inj. system left bank: lean 2)0115-fuel inj. system right bank: lean 3)0503-front right heated o2 sensor.

This is really getting annoying. Would replacing the o2 sensor fix things? Is it expensive? I have already spent a ton of money on this thing. It feels like it will never get fixed.

I think I will try resetting the ecu and seeing what codes it gives me when it comes back on.

now these codes more reflect what your experiancing, and indicate it is runing lean. As I mentioned earlier check for vaccum leaks. The map code says the ECM thinks the sensor is screwed up by looking at other sensors and determining it should read more then it is, But the ecm doesnt realize there is a condition causing it to read incorrectly. Now the Lean code is generated by the O2's primarily they are seeing a condition indicating more oxygen is coming through then it logically should have. Now in all accuality the ECM is a moron as it doesnt have the ability to do the math from the different codes it got and determine the car has a vaccum leak. Also your drivability issue you are experiancing is also pointing to a vaccum leak.

NISTECH wrote: check for vaccum leaks.

I agree. From your descriptions, sounds like vacuum leak.

Thanks NISTECH. You helped me out tremendously. I think we have the problem solved. I had the guy that has been working on my car check the vacuum hoses like you suggested. They were fine. His final diagnosis was that the place that did the valve job set the valves incorrectly. He called me tonight and had reset all of the valves. I haven't driven the car yet, but he says it is fixed and I am crossing my fingers. I am not going to get my hopes up until I drive it myself though. I really appreciate your help.

Andrew