# Acceleration stumbles after warmed up



## Billbob2 (Sep 19, 2007)

I have a 98 Altima with 114,000mi and under normal acceleration it stumbles. It idles smooth and if I punch it hard, it pulls smooth and hard like normal. It's just under normal acceleration. Initially when engine is cold after start up, acceleration is smooth, but after about 5-10 minutes of driving it consistently stumbles when accelerating. Any suggestions where to start?


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## Ryan's Altima (Aug 29, 2006)

I have the same problem starting. It doesn't get up to speed as fast as it should and it seems like it tries, then tries a little harder then harder even when I have the pedal in the same position. I'd just assume to let it warm up. It'd be nice to figure this out.


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## Tozia600 (Oct 8, 2007)

I also Im having the same stumbling problem with my 2000 Altima. It has 112,000 miles and Im starting to find out that a lot of other people are having the same problem with the 2.4L. It feels like between 2500 to 3500 RPM it loses a lot of power then at about 4000 it picks back up again. It almost feels like a cylinder is not firing. I have heard a lot of different reasons for this and have replaced a lot of things hoping to get the problem but it still stumbles. Would really like to figure out what it is.


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## Ryan's Altima (Aug 29, 2006)

I take back the air filter solution, mines still sluggish from 2k-3k rpms then picks back up after the warm up.


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## jo31b (Oct 24, 2007)

Tozia600 said:


> I also Im having the same stumbling problem with my 2000 Altima. It has 112,000 miles and Im starting to find out that a lot of other people are having the same problem with the 2.4L. It feels like between 2500 to 3500 RPM it loses a lot of power then at about 4000 it picks back up again. It almost feels like a cylinder is not firing. I have heard a lot of different reasons for this and have replaced a lot of things hoping to get the problem but it still stumbles. Would really like to figure out what it is.



What things have you replaced so we don't do the same thing?

Could you post a list of all of the solutions people offered you that didn't work?


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## jo31b (Oct 24, 2007)

Billbob2 said:


> I have a 98 Altima with 114,000mi and under normal acceleration it stumbles. It idles smooth and if I punch it hard, it pulls smooth and hard like normal. It's just under normal acceleration. Initially when engine is cold after start up, acceleration is smooth, but after about 5-10 minutes of driving it consistently stumbles when accelerating. Any suggestions where to start?


Does it still stumble once at operating temperature? Or does it only happen during the warm up phase?

If you know all of this stuff bare with me. I am posting this theory to work through my thinking process for you so you know exactly where I am coming from.

When your engine is cold it needs much more fuel to accelerate then when your engine is warm; this need would allow the computer to ignore a rich signal from the O2 sensors or even see it as necessary to warm up the car or get it moving the way we have come to expect a car to move. 

Once your car starts to warm up, the O2 sensors' signals become very important to control the air fuel mixture to control emissions and to prevent damage to your Catalytic converter. If your O2 sensor is always sending a signal saying the mixture is too rich, your ECU will cut fuel to cut fuel to stay within allowed emission levels.

The O2 sensors generate a voltage signal from 0-1v. A high voltage signal (>0.5v) indicates a rich mixture and a low signal (<0.5v) indicates lean. The voltage is created by the difference between outside oxygen content and exhaust oxygen content. If oxygen from the exhaust is not able to make contact with the electrodes in the O2 sensor (by the O2 sensor being clogged or the air fuel mixture being too rich) you will have a high voltage signal produced and sent to your ECU telling it to add less fuel.  When you open the throttle to Wide Open Throttle (WOT) Your ECU ignores your O2 sensors completely and gives you an air fuel mixture for maximum power.

The ECU varies the air fuel mixture between a little rich, for power, and a little lean, to supply oxygen for your Catalytic converter to store for completing the combustion during the rich time. So your voltage will constantly vary from around 0.35v-0.6v. If your O2 sensor is partially gummed up with carbon, it will produce a voltage signal from say 0.4v-0.65v because there is not enough surface on the O2 sensor to keep the voltage as low as it needs to be. So your ECU will add less fuel until there is enough Oxygen in the exhaust to lower the signal to what it wants to see.

If your O2 sensor(s) are completely gummed up, they will produce solid high signal or a very high varying signal (0.6v-0.8v). A very high varying signal or a solid high signal will set a code in your ECU. It will only turn the check engine light on if the system thinks it will eventually damage the Catalytic converter. I'm guessing if you are in California it would come on sooner to prevent high emissions.

I think that one or both of your O2 sensors are partially covered in carbon from the symptoms you described.

I was thinking a vacuum leak at first, but one big enough to hinder acceleration would also mess with your idle. So for now this is the most simple problem I can think to check, any other cause seems more complicated and less likely. Other than base ignition timing i guess.

Good luck.


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## Tozia600 (Oct 8, 2007)

*O2 Sensor*

That is exactly what is happening. I just recently was talking to a friend that owns a shop and he said it would problebly be my O2 sensor as well. So now I need to find out if its the sensor near the cat. or the one on the headers. Or bolth


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## jserrano (Oct 27, 2004)

Before you go on with an O2 witch hunt, try this to rule out the EGR system,

1. Disconnect the vacuum hose the feeds the top of the EGR valve and plug it with a big enough screw. 

2. Take the baby for a ride, and if the problem remains then it is NOT EGR system related, ie. lean misfire.

3. Continue on with the witch hunt.


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## jo31b (Oct 24, 2007)

Good call for the EGR check. I should have also said to look for exhaust leaks like a crack in the manifold and at all the joints. Any bit of soot at any point of the exhaust shows a leak, you can also put your hand around each sealing point to feel if any gas is escaping. (not trying to One Up Mr. jserrano He just jogged my memory) I guess my idea would be a witch hunt since I didn't include the testing procedure for it.

The first step in ANY engine management problem should be to have your codes read to look for any codes that exist that are not turning on the check engine light. ie. multiple cylinder misfire.

Note: The only way you should have a stumble with no check engine light AND no code involving the O2 system is if both sensors have a similar issue. If sensor one is gummed up a bit and the computer leans your air fuel mixture to get the signal it wants from it, and O2 sensor #2 is fine, then the computer will see that there is too much oxygen in the exhaust and should trigger an O2 sensor code in the computer. It may not light up the Check engine light either.

Once you have checked your EGR they way Mr. jserrano said you can also try these two tests:

A caution about test one. I haven't read the way these cars deal with having no signal from the O2 sensors, the car's reaction may be to use a lean air fuel mix and retard the ignition timing to keep emissions low without O2 sensors. If this is true you may have very low power once the car is warmed up. 

Test 1-- If you unplug both O2 sensors your computer will be forced to operate in "open loop" mode (open loop just means to run the car without feedback form the O2 sensors). Because it will not be getting a signal from your sensors the computer should revert to basic air fuel mixtures (without going lean, rich, lean, rich) to avoid driveability issues. This WILL also set at least two codes in your ECU and will turn the check engine light on after a maximum of two trips, or possibly even cause it to flash during the first trip (flashing indicates your catalytic converter will be damaged if you just continue to ignore the flashing light but for this test it is OK). I don't know if the codes will be "hard" so once the sensors are plugged in again and you drive a few times with them working the light may shut itself off, or if it was flashing it may just come on solid. Just remember to have your codes cleared as soon as possible after doing this to avoid confusion down the road (no pun intended).

Once they are unplugged, and their wires are secured so they won't burn or get caught, get the car up to operating temperature and go for a drive to see if you have your problem still.

Test 2-- You could do this before test one, but you may damage a good O2 sensor trying to take it out so I put it as test 2. Check the O2 sensors by take them out to see if they are noticeably gummed up. Let everything cool off for at least 3 hours to avoid burning yourself and to let all the metal shrink so the sensor will actually turn. You will need a special O2 socket to remove it without damaging the wires.











Don't forget the caution before step one, if you don't have a service manual yet I'd get one for yourself. I listed the Nissan Publication number in my signature.

And please if anyone sees I messed up some info let us know.

Thanks Mr. jserrano


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## jserrano (Oct 27, 2004)

The Altima ECU has an O2 sensor diagnostic mode test you can use to quickly ascertain if the front O2 is working. The service manual should mention how to set it up. You then just run the engine to about 2000 rpm and watch the CEL. It should flash ON/OFF with 5 iterations of this within 10 seconds. Of course, the engine has to be at operating temperature in order to be conclusive. 

If the rear O2 is bad then you should get a code for it or a code for "inefficient catalytic converter. 

These are just general tips since it doesn't isolate a lazy O2 sensor which can also cause problems. Good luck.


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