MAP Sensor Diagnosis and Understanding Function- Pt2

let me hook up the scan tool and we’re just going to sort of reproduce the voltage measurements but I just wanted to show the voltage measurements for people that don’t have the scan tool we’re going to do some things that involve removing the map sensor and experimenting a little bit and I don’t want to do the voltage measurements with that because there’s too much chance of crossing my leads up so for safety we’re just going to do everything from the scan tool but really it’s the same thing it’s just kind of going to read from the voltmeter measurements and give an interpreted value for the map sensor so let’s look at what we get ok so what I’m using is auto ingenuity as my scan tool here and what we are looking at in this left corner up here is going to be the intake manifold pressure much to my surprise there actually is not a voltage measurement that is an option on my scan tool generally there will be a voltage measurement but either way it’s the same thing this manifold pressure measurement is going to be low right now at idle but of course we’re going to see the same phenomenon if I step on the gas pedal let more air in we’re going to see this number increase I don’t really know how much but it is definitely going to increase and then when I let off on the pedal the D cell condition it’s going to go below a measurement of 9 millimeters of mercury so let’s go ahead and just validate that real quick okay so either way that we do it either with the scan tool or with the voltmeter we can see that we have if nothing else if nothing else we do know this while we may not know what the values should be hey maybe these values are way off and if you had a map sensor check engine light it’s because these values are out of they’re out of order but one thing we do know just from the voltmeter alone because we have variance in the signal wire we know that our electrical connection to the map is good we know we have a five volt reference signal getting to the map we know we’ve got a ground and we know we’ve got variants in the signal so we at least know our electrical is is solid to the computer that we do know for sure but there could of course be some calibration issues with the sensor itself so we might be able to do some things to look at that one way that we could look at a calibration is using this pit over here we could look at the barometric pressure and seeing that the barometric pressure is reasonable we know that we would know that the map sensor is calibrated properly so that would be another way to check the map sensor again pretty rare that you’re going to have a map sensor issue to begin with as you can see the map sensor reacts greatly to engine conditions so you have that thing where is your map sensor recording an engine problem for example if I were to make this engine really stumble by maybe removing a spark plug wire and causing a misfire that’s going to have a dramatic effect on my map sensor reading but the map sensor would not be the problem the map sensor would just be recording an issue and you might get some type of out of range map code or something possibly more likely you’d of course easily identify that from the misfire code though but let’s do another experiment here real quick what I’m going to do is show sort of some manual manipulation of the map sensor and see how our scam tool reacts so what I’m going to do is come over here and what I’m going to do is just pull off this vacuum hose right here to the intake manifold now when I remove that vacuum hose what would we expect to see from the map sensor I’ll give you a second while I go ahead and grab this here okay and if you guessed that we are going to see this map sensor increase then you are correct we will see it increase because we are going to draw more air into the intake I almost kill the engine there and there’s our D cell condition by the way but you can see that we there’s a vacuum leak we do indeed increase so that’s how the map responds to a vacuum leak so if you have a vacuum leak

and you continually have a high map sensor reading that maybe even sends a code is that the map sensor that’s faulty absolutely not the map sensor is just reacting and reporting another condition and it is not at fault for the problem it is just reporting the problem so you can see how you might get a map sensor code from a vacuum leak but not have a map sensor problem now of course another thing that we can do is this now what I’m going to do here is for you guys that are really advanced and you saw my fuel trims video we’re going to do something a little special for you let’s pull up the short term fuel trim here and this car is actually relatively new so it’s actually at a perfect short term fuel trim and here’s what I am going to do I am going to actually remove the map sensor and then plug the hole with a with a stopper and just lift the map sensor sit outside of the engine but the there will be no vacuum leak because I’m going to seal the hole up what do you think that is going to do to my fuel trim when the map sensor is actually outside of the engine at idle and there are no vacuum leaks what would that do to my fuel trim think about that while I go ahead and set this up okay and if you guessed that the fuel term is going to go negative in other words we are going to create a rich condition you are absolutely right let me go ahead and cause this to happen first and then I will explain to you why that is the result we would get so let me go ahead and get this off of here and I’ll imagine that we’re probably going to kill the engine with the massive vacuum leak that we’re going to get try to do this real quick oh that was pretty slick okay now let’s go ahead and look at our fuel trim okay so as you saw it did indeed go rich not not hugely rich maybe as you would expect but you have to remember this engine is also a math engine so it’s getting more of its airflow data from the mass airflow sensor so the map probably not really affected the fuel trim as much but you can imagine if I had a map only engine or if I disconnected the mass air flow sensor well actually I imagine probably the engine would die at that point I think but either way we saw that as predicted we got the rich fuel trim so let’s talk about why that happened in case you don’t understand all right and this concept is going to be extremely important actually because variances in the map sensor are often predicted by the computer and also they are expected by the computer so there’s a little difference there and if things that are expected by the computer are not showing then the computer is going to start looking for someone to blame maybe maybe it’ll be the map sensor maybe it’ll be something else but let’s talk about that rich condition that we had for those of you guys that are more advanced and familiar with fuel trims so what happens is of course we’ve got the engine idling everything’s normal we’ve got some air coming in our map sensor is about what was at 1.5 volts I think is is what it is normally at idle on this vehicle so and actually that’s voltage there let me let me put it to eight because that is the inches of mercury that we saw on the scan tool I want to keep consistent with the scan tool so we’re in our normal idle condition here and then what did I do well what I did was I took the map sensor and I plugged that vacuum leak up so my map sensor is now open to just atmospheric pressure okay now what does this do well as far as the air coming into the engine that has not changed we still have the same amount of air coming into the engine but the problem is the amount of air that’s increasing the pressure in the map sensor has dramatically increased so at that signal wire it’s going to increase the voltage because the map sensor is reading a lot more air coming in and increasing the pressure but the engine itself does not actually have that happening so when this higher voltage gets to the PCM it’s going to start adding fuel because the computer thinks this is because more air is

coming in and therefore it’s going to increase the injector pulse length to increase fuel now when that increased fuel gets in there remember we were at fourteen point seven to one just fine but now with the added fuel coming in but no extra air coming in this ratio is going to be way off maybe fourteen point seven to three who knows but whatever the case is it’s a rich condition that is going to be detected by the o2 sensor so the o2 sensor is going to feed back to the PCM if you are familiar with my fuel trim video it explains all this and the PCM is going to then show a long term and short term fuel trim as rich and it’s going to want to scale back the fuel delivery now it doesn’t know why there is this rich condition do you see where in this event you might get a throttle position error code because the engine has to be thinking there’s a lot more air coming in but the throttle is still showing is closed so depending on the programming that the engineers put into the PCM s you may get any number of different check engine light codes maybe not even for the map sensor on that condition but that could have well been a map sensor code do you see how this could happen if that hose became disconnected you would of course still have a vacuum leak so it would be a little different condition than this but you can see where you can have codes for things that are caused by the map sensor or not those things fault and you can have things at fault that give a code to the map sensor when the map sensor is not as fault so it’s very important to understand fully how the map sensor integrates with all of this complex system before you start replacing a map sensor in the car because even though as we saw it’s very easy it is bad practice and it can be expensive and it certainly is not the philosophy of this channel alright so pretty much all of that said you can see where it is very difficult sometimes to interpret a map sensor problem because the map not only can cause various engine effects as we saw but more importantly the map sensor responds to engine conditions so there’s always that difficulty of are you reading a fault code for a map sensor is that a response to the engine condition that the map sensor is accurately reporting or is there an engine condition that is resulting because you have a map sensor problem so it’s it can be very difficult but as you can see I’ve shown you many ways to validate the electricity to the map sensor validate the map sensor output itself verify that the map sensor is responding as anticipated to conditions and at least on this car on some anticipated values that you would expect so you know hopefully that will help you in your diagnosis obviously you know I’m not a huge fan of you know like what do they call flow charts where you can see where a flow chart would be sort of problematic on a map sensor because there’s so many variables that you can eliminate right off the bat and sort of bypass 90% of your flow chart but you know honestly flowcharts are for people that do not understand the parts that they are troubleshooting or even the concepts that they’re trying to troubleshoot not that there’s anything wrong with a flow chart like flow charts they can be helpful but the idea is if you’ve spent this time watching this video and understanding then you can make your own flow chart that is custom to the particular diagnosis that you’re trying to make and that’s what I really hope that I’m helping you guys to do so that pretty much covers it for the map sensor hopefully you’re an expert in that now and you can use it not only to you know this knowledge to diagnose a map sensor problem but also you can see where you could possibly use the map sensor to help diagnose other things in the engine if you know that the map sensor is working so hopefully it’s taken some that mystery out of this little talked about components in the engine controls so thanks for watching I hope you found this helpful