# 99 Altima runs better with IAT sensor unplugged



## RabidMatt9000 (Mar 12, 2020)

I have a 99 Nissan Altima with a 5spd and I have had an issue where if I barely press on the gas pedal it will sputter for a second and then level out once I give it more gas. So I unplugged the IAT sensor, (I don't remember why exactly) forgot to plug it back in and my car ran great, just like when I started it for the first time after the engine rebuild. So I replaced the sensor, plugged it in and......the sputtering returned. So I unplugged it again and it runs great like before. I know this isn't exactly good so would anyone have any ideas on what could be causing this? My only idea at this point is to check the wiring but the more ideas the better.


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## xX0sledge0Xx (Jul 29, 2020)

Having the exact same issue, following!


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## rogoman (Dec 16, 2004)

RabidMatt9000 said:


> I have a 99 Nissan Altima with a 5spd and I have had an issue where if I barely press on the gas pedal it will sputter for a second and then level out once I give it more gas. So I unplugged the IAT sensor, (I don't remember why exactly) forgot to plug it back in and my car ran great, just like when I started it for the first time after the engine rebuild. So I replaced the sensor, plugged it in and......the sputtering returned. So I unplugged it again and it runs great like before. I know this isn't exactly good so would anyone have any ideas on what could be causing this? My only idea at this point is to check the wiring but the more ideas the better.


There could be another problem. With the IAT unplugged, the ECU might have switched to "fail safe" mode giving the appearance of smooth running.

Is the MIL lighting up? You might want to perform an ECU code readout with a portable scan tool to see if any fault codes are set. The tool can be purchased at most auto parts stores or online at Amazon.com. Post the actual codes here on the forum so that we may be able to help you further. If there is one or more fault codes set, they can help point to the malfunction.


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## xX0sledge0Xx (Jul 29, 2020)

I'm going to assume the original poster has the same issue as I, code p0111, and same as him I also replaced the iat with a brand new one, plugging in new iat causes rough idle, poor acceleration, vehicle luchring, and makes vehicle almost undrivable, the prongs on my iat were touching and sensor was shorted when I pulled it so idk if that can kill the pcm, I've also hered a clogged cat can cause a backfire into the intake causing the iat to read false higher temps but I don't think that's the problem because it runs the same cold as it does warm, I assume the original poster is having the same problem, I have not checked pcm yet nor know how to


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## RabidMatt9000 (Mar 12, 2020)

The only code I'm getting is p1336 which is for the crank sensor that I already replaced (twice). After doing some digging I believe I may have a broken tooth or two on the flywheel. If I remember correctly the crank sensor works with the Iat and Map sensor when the motor is at operating temperature ( closed loop) and if there is something wrong with the flywheel teeth it can mess with the idle and driving and such. I literally replaced everything but that and I don't have a vacuum leak either. Also it starts pretty rough and I've heard it grind here and there. I will be trying that next and will post back whether it fixes the problem or not. Might be a bit because it's going to be hot as shit these next two weeks.


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## rogoman (Dec 16, 2004)

RabidMatt9000 said:


> The only code I'm getting is p1336 which is for the crank sensor that I already replaced (twice). After doing some digging I believe I may have a broken tooth or two on the flywheel. If I remember correctly the crank sensor works with the Iat and Map sensor when the motor is at operating temperature ( closed loop) and if there is something wrong with the flywheel teeth it can mess with the idle and driving and such. I literally replaced everything but that and I don't have a vacuum leak either. Also it starts pretty rough and I've heard it grind here and there. I will be trying that next and will post back whether it fixes the problem or not. Might be a bit because it's going to be hot as shit these next two weeks.


The crankshaft sensor on the Altima KA24DE engines prior to 2002 is used only during diagnostic testing; it has no effect during normal operation. So the P1336 fault is a non-issue to your problem.


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## RabidMatt9000 (Mar 12, 2020)

Well then I guess I'm running it into the ground because I'm out of ideas at this point hahaha. thanks though


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## rogoman (Dec 16, 2004)

With the IAT unplugged, the ECU switches to "fail safe" mode giving the appearance of smooth running. The ECU will just assume the intake air temperature is 68°F. This is called a fail-safe mode. The ECU will still use all the other sensor data. Even if the actual intake air temperature is much different than 68°F the ECU will still adjust air-fuel ratio using the O2 sensor feedback.

The O2 sensor may be causing the problem. The easiest way to check the O2 sensor is to view the live data with a scan tool and check that the voltage output is correct.


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## xX0sledge0Xx (Jul 29, 2020)

After checking for vaccume leaks, I am down to tps or ECU/pcm one of the two is causing the problem but if tps is bad the vehicle shouldn't run with iat unplugged, double check yours to be safe, my car wouldn't run good with new iat plugged in yesterday, now it does and runs great, so I'm leaning towards a pcm issue, hope this helps, will update when new pcm arrives


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## RabidMatt9000 (Mar 12, 2020)

The O2 sensors are only 2 years old but after checking the live data on my scanner it's telling me that the downstream voltage is reading 0 volts, I hope the upstream one is anywhere from .08 to .5 on the reading. What are the appropriate voltages exactly?


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## RabidMatt9000 (Mar 12, 2020)

RabidMatt9000 said:


> The O2 sensors are only 2 years old but after checking the live data on my scanner it's telling me that the downstream voltage is reading 0 volts, I hope the upstream one is anywhere from .08 to .5 on the reading. What are the appropriate voltages exactly?


Also I want to point out that the only things I haven't replaced on this car is the power steering pump, transmission, and the alternator ( was brand new when I bought this thing). With how much money I've put into this I am still having this problem for almost 2 years now, and I haven't even come close to fixing it


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## rogoman (Dec 16, 2004)

RabidMatt9000 said:


> The O2 sensors are only 2 years old but after checking the live data on my scanner it's telling me that the downstream voltage is reading 0 volts, I hope the upstream one is anywhere from .08 to .5 on the reading. What are the appropriate voltages exactly?


According to the FSM the upstream O2 sensor voltage reading should switch somewhere between 0 - 1.0 V. The downstream O2 sensor reading should switch somewhere between 0.5 - 1.0 V. The rear O2 sensor has a much longer switching time between rich and lean then the front O2 sensor.


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## RabidMatt9000 (Mar 12, 2020)

rogoman said:


> According to the FSM the upstream O2 sensor voltage reading should switch somewhere between 0 - 1.0 V. The downstream O2 sensor reading should switch somewhere between 0.5 - 1.0 V. The rear O2 sensor has a much longer switching time between rich and lean then the front O2 sensor.


Yeah the only time it gets up to .2 V for the rear sensor is at 2000 Rpms other than that it's 0 v at idle


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## rogoman (Dec 16, 2004)

When performing O2 sensor checks, it is important to take measurements at idle and 2000 rpm. Be aware that O2 sensor pre-conditioning is important, even on the newer styled heated O2 sensors (not afr sensors). Pre-condition the O2 sensor by raising the engine speed to 2000 rpm for about 15 seconds or so. The O2 sensor has to be above 600° f. to be able to operate properly. Long periods of idle time can render a non-heated or older O2 sensor too cold for it to function at all. At the same time, do not try to force a heated O2 sensor into operation. An O2 sensor with a faulty heater will go into closed-loop after a good warm-up session.


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## RabidMatt9000 (Mar 12, 2020)

So my O2 sensors are good. Now I'm getting the codes p1336 and p1105


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## rogoman (Dec 16, 2004)

RabidMatt9000 said:


> So my O2 sensors are good. Now I'm getting the codes p1336 and p1105


The P1336 code is for a problem with the crankshaft position sensor. However according to the FSM, the sensor is only used during diagnostic testing; it is not used during normal engine operation, so it should have no effect on your driving.

The P1105 code is for a problem with the map/baro switch solenoid valve. Could be a bad harness connection or the solenoid itself. It's located just behind the air filter box.


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## RabidMatt9000 (Mar 12, 2020)

P1336 also can be the crank and the cam sensor are out of sync right? I only ask because my distributor is aftermarket, wondering if I should of got a refurbished Nissan one


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## RabidMatt9000 (Mar 12, 2020)

So I rescanned the downstreamo2 sensor voltage and it doesn't go higher than .2, and stays at 0 when at idle, also discovered I have a pretty bad exhaust leak in the flexible exhaust pipe right beforebefore the downstream o2 sensor, so I'm wondering if I were to get the pipe replaced if this would fix my problem? Does the downstream o2 sensor affect performance or is it just letting you know the cat is bad?


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## RabidMatt9000 (Mar 12, 2020)

RabidMatt9000 said:


> So I rescanned the downstreamo2 sensor voltage and it doesn't go higher than .2, and stays at 0 when at idle, also discovered I have a pretty bad exhaust leak in the flexible exhaust pipe right beforebefore the downstream o2 sensor, so I'm wondering if I were to get the pipe replaced if this would fix my problem? Does the downstream o2 sensor affect performance or is it just letting you know the cat is bad?


I will see how much it would cost to weld this in. Hopefully not much


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## RabidMatt9000 (Mar 12, 2020)

UPDATE: had the flex pipe replaced and now the o2 sensors are working properly, however there is still hesitation/bucking when maintaining the same mph. P1105 no longer appears once I replaced the connector with one from a junkyard, so p1336 is the only code that pops up (wishing some other code would). I checked vacuum and it holds at 22 and all the fuel components are brand new, so on a hunch a bought a new rotor for the distributor. Ran without hesitation or bucking for a few miles but unfortunately that returned. Will be checking plugs next since the wires are also brand new to see if I can track where the misfire may be coming from, even though I would think it would give me a misfire code letting me know what cylinder is the problem. May just have a mechanic look at it to find the problem mainly because I would like this motor to last me a few years, especially after tearing it down and rebuilding it myself (would do it again no questions asked, expect maybe on a frontier or xterra)


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## rogoman (Dec 16, 2004)

RabidMatt9000 said:


> P1336 also can be the crank and the cam sensor are out of sync right? I only ask because my distributor is aftermarket, wondering if I should of got a refurbished Nissan one


Just for the hell of it, disconnect the harness connector at the crankshaft sensor; now run the car on the road to see if the hesitation/bucking goes away before messing with anything else.
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You've gone quite a ways to getting everything fixed up, except the hesitation/bucking which could possibly be caused by that *aftermarket distributor; r*eplacing the distributor with a *rebuilt OEM* unit may be the fix. However spending the money and finding out that it doesn't fix the problem would be very frustrating. You might consider using an oscilloscope to look at the pulse waves at the camshaft sensor output from the distributor and at the coil/power transistor to insure you're seeing a steady pulse stream. If the pulse stream is skipping considerably, then you've found the problem. Here's two pictures:


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## RabidMatt9000 (Mar 12, 2020)

rogoman said:


> Just for the hell of it, disconnect the harness connector at the crankshaft sensor; now run the car on the road to see if the hesitation/bucking goes away before messing with anything else.
> ----------------------------------------------------------------------------------------------------------------------------
> You've gone quite a ways to getting everything fixed up, except the hesitation/bucking which could possibly be caused by that *aftermarket distributor; r*eplacing the distributor with a *rebuilt OEM* unit may be the fix. However spending the money and finding out that it doesn't fix the problem would be very frustrating. You might consider using an oscilloscope to look at the pulse waves at the camshaft sensor output from the distributor and at the coil/power transistor to insure you're seeing a steady pulse stream. If the pulse stream is skipping considerably, then you've found the problem. Here's two pictures:
> 
> ...


Thanks for the pics. I unplugged the crankshaft sensor and it ran perfectly for a few miles and then the problem came back just like when I replaced the cap and rotor, so I'm definitely thinking it's the aftermarket distributor, but I'll try to test it before I replace it. Found a remanufactured oem one for a good price, it should be here Friday or Saturday.


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