[BrandonRay]

I used the search tool and checked my service manual but couldn't find anything. I threw a P0171 code... running lean on bank 1, so I wanted to clean the MAF sensor. Anyway I couldn't find it in the service manual index. All I found was a MAP (Maximum Absolute Pressure) sensor.

So I was wondering, is this just a new name for the Mass Air Flow sensor??? I have a 2005 LJ 4.0 I6.

Thanks

 

[tknarr]

No, Mass Airflow Sensor is usually found in the air intake system and measures airflow to the engine.
Manifold Absolute Pressure sensor is usually found in the intake manifold and measures vacuum? (anyone fell free to jump in here and correct me).

 

[BrandonRay]

Yeah Manifold Absolute Pressure not "Maximum"... sorry for the typo... anyway I can't find anything on the MAF in my service manual... I'd think that would be something worth listing in the index.. thank you for the input!

 

[Sundowner]

The MAP sensor works alongside the TPS to measure the amount of air coming into the engine, if I am not mistaken. It's a speed-density system, of sorts. No MAF sensor is needed.

 

[Sweeney]

Well, your Jeep doesn't have a MAF sensor.

 

[joshr1987]

Yeah, while they are not the same, the do serve the same purpose. You wouldnt have both...

 

[Sundowner]

Yeah, while they are not the same, the do serve the same purpose. You wouldnt have both...

Well, kind of...so, just for the sake of clarity:

MAF directly measures the mass of the air that is entering the engine, and the computer adds fuel accordingly.

MAP measures the pressure of the air that's in the manifold...i.e. air density...and correlates that value with the computer and the throttle position at the time, which allows the computer to know how much air is actually entering the manifold; the computer then adds fuel accordingly. It's sort of an extra step in the process, so to speak.

So yes, in that they both serve the purpose of measuring how much air goes through the manifold, they are very similar. However, it's not quite accurate to say that they serve the same purpose, as they are designed for totally different applications.

 

[BrandonRay]

Yeah it doesn't seem like I'd need both... so I guess I'll clean the MAP and hope the MIL stays off and the MPG's improve a little.

 

[BrandonRay]

This is a little of what I found..

The MAP sensor is used as an input to the Powertrain
Control Module (PCM). It contains a silicon
based sensing unit to provide data on the manifold
vacuum that draws the air/fuel mixture into the combustion
chamber. The PCM requires this information
to determine injector pulse width and spark advance.
When manifold absolute pressure (MAP) equals
Barometric pressure, the pulse width will be at maximum.

 

[Sundowner]

P0171 can also be caused by low fuel pressure, and at times by a vacuum leak. The Bank 1 lean condition is detected by the upstream Bank 1 O2 sensor, and all it's telling you is that for one reason or another, it thinks you have too much air and/or not enough fuel in the front half of the engine. The MAF sensor - in this case, not present - may or may not have anything to do with it.

 

[BrandonRay]

P0171 can also be caused by low fuel pressure, and at times by a vacuum leak. The Bank 1 lean condition is detected by the upstream Bank 1 O2 sensor, and all it's telling you is that for one reason or another, it thinks you have too much air and/or not enough fuel in the front half of the engine. The MAF sensor - in this case, not present - may or may not have anything to do with it.

Yeah my basic plan is to clean the MAP sensor, then replace if that doesn't work, if I still have the issue move on to checking the vacuum and PCV hoses, then the fuel filter, and the fuel pressure. Let me know if that sounds like the right path to take... I'm just learning here.

 

[Sundowner]

No offense, but why are you going to spend money on a new sensor before you check things that will cost you nothing? Looking over the vacuum lines and checking your fuel filter and fuel pressure costs you exactly $0. I promise a new MAP sensor will cost more than that.

 

[BrandonRay]

Hopefully that pic goes through... if it does is that not a MAF on the air intake tube between the air filter housing and the throttle body? You can see the MAP in the bottom of the pic next to the throttle body.

 

[BrandonRay]

No offense, but why are you going to spend money on a new sensor before you check things that will cost you nothing? Looking over the vacuum lines and checking your fuel filter and fuel pressure costs you exactly $0. I promise a new MAP sensor will cost more than that.

None taken... the MAP's just under $20, and I wasn't sure what a PIA it would be to find a leak somewhere else... I haven't done it before. I was just thinking systematically.

 

[Sundowner]

I don't see any picture, but I don't need to. You don't have one. Trust us.

 

[Sundowner]

None taken... the MAP's just under $20, and I wasn't sure what a PIA it would be to find a leak somewhere else... I haven't done it before. I was just thinking systematically.

Systematically, you troubleshoot simple problems first. Logically, you are not likely to fix a problem without knowing what's broken. Throwing money at a problem rarely solves anything, save for lightening your pockets. Besides, if you troubleshoot thoroughly, you learn a lot more about how the engine works.

If you replace a sensor that's already working, you haven't fixed anything. You've just wasted money, and your problem is still present. That's why "I replaced everything!!! How can it still not work???" happens.

 

[BrandonRay]

Systematically, you troubleshoot simple problems first. Logically, you are not likely to fix a problem without knowing what's broken. Throwing money at a problem rarely solves anything, save for lightening your pockets. Besides, if you troubleshoot thoroughly, you learn a lot more about how the engine works.

If you replace a sensor that's already working, you haven't fixed anything. You've just wasted money, and your problem is still present. That's why "I replaced everything!!! How can it still not work???" happens.

By systematically I meant that if I rule out the MAP, I don't spend my time chasing a leak that doesn't exist, but I see your point and thanks for the advice.

 

[Sundowner]

Yes, I know what you meant. You can go that route if you want...but I never spend money that I don't have to spend just to "rule out" something unless it's the last option. Been there, done that...and I'm poorer for the efforts. If you spend three hours chasing a vacuum leak that doesn't exist and it does turn out to be your MAP sensor, then I look at that as first-rate education on how the vacuum system works.

Perhaps you can learn about testing MAP sensors to see if they're functioning or not...then you can rule it out without spending that $20, and educate yourself in the process.

 

[BrandonRay]

I don't see any picture, but I don't need to. You don't have one. Trust us.

You're right... what I saw in the pic was the Intake Air Temperature sensor

 

[5-90]

No, Mass Airflow Sensor is usually found in the air intake system and measures airflow to the engine.
Manifold Absolute Pressure sensor is usually found in the intake manifold and measures vacuum? (anyone fell free to jump in here and correct me).

Half marks.

As I recall, the Mass AirFlow sensor uses a "speed density" calculation to measure the amount of air flowing over a heated wire, by determining the amount of current needed to maintain the wire at a given temperature (or measuring the drop from a given temperature, which is derived from a steady current applied to the wire. I don't recall which offhand - MAF is a Ford thing, and I avoid Fords...)

The MAP is part of a "temperature/density" measurement setup. It accomplishes the same results by providing a "density" measurement (absolute pressure,) which is then correlated with an intake temperature reading (from the IAT,) to derive airflow into the engine. It doesn't directly measure the airflow proper, so it's a shade less accurate - but that's what the feedback/trim signal from the HEGO is for (in either case.) GM and ChryCo use the temperature/density setup.

Given a choice, I'd prefer to use the temperature/density system anyhow. You've got a feedback loop for fuel metering trim, so that's not a problem. The volumetric efficiency (how much air is actually moved vice theoretical displacement) should be known at all RPM ranges by the factory, so that's a workable correction. Between the temperature/density signal, the HEGO feedback loop, and the variation of practise with theory, you should be able to chart a workable map of fuel values and only require slight trim to correct.

Besides, the IAT and MAP together tend to cost less than the MAF unit; and are more reliable, and you can replace one or the other when it fails (for $30-50, vice dropping $200-300 on a MAF sensor...)