Intake Manifold Pressure Dilemma

[Karabiner98k]

Hi everyone,

This is my first post in Tech Talk! I hope some of you could help me!

In this thread i want to know more about Manifold Absolute Pressure (MAP) Sensor.

I have read a lot about it and now i know what it is and what does it do but there are some aspects of it that i don't understand. I will mention them one by one:

1 - It is said that MAP sensor shows the difference between barometric pressure and the pressure inside the intake manifold. If this is true for example if my car's MAP sensor reads 323 millibars @ idle and the barometric pressure is 873 millibars then my REAL manifold pressure is 550 millibars (873 - 323 = 550). Is this true?!

2 - It is said that MAP sensor does not measure pressure! It measures manifold VACUUM. As you know when the engine is idling and the throttle is closed or when the car is going downhill with closed throttle, the VACUUM is at it's most because the cylinders are trying to suck air. By opening the throttle the air rushes into the manifold and the pressure goes higher. So, if this theory (MAP sensor shows VACUUM) is true when you open the throttle, MAP sensor should read a lower value than when it is closed (e.g if it reads 323 @ idle, it should read 290 @ 2000 RPM).

BUT! I have seen some cars that when you open the throttle the MAP value goes higher and also in some other cars it is the exact opposite and by opening the throttle it goes lower!! This is where the confusion starts!

3 - According to fluid dynamics, "faster moving air has lower pressure".
By opening the throttle the air velocity in the manifold increases but MAP sensor reads higher pressure!
This is contrary to the above-mentioned fact! By opening the throttle the pressure should drop!! because the air velocity has increased! This is where another BIG confusion starts!!

I have some other questions that will mention later.

 

[Giannis]

Nice topic. So, let's clear out some things.

1 - It is said that MAP sensor shows the difference between barometric pressure and the pressure inside the intake manifold. If this is true for example if my car's MAP sensor reads 323 millibars @ idle and the barometric pressure is 873 millibars then my REAL manifold pressure is 550 millibars (873 - 323 = 550). Is this true?!

1. The air as it comes in your engine's intake, it has atmospheric pressure. So, the absolute pressure is 1atm. The relative (to the atmospheric) pressure is 0. Given that, when a piston is going down, it creates a volume inside the combustion chamber, which has less pressure than the atmospheric. That way air is forced into the combustion chamber and fills it. The air moves towards the (initially empty) combustion chamber due to the difference in pressure (smaller in the combustion chamber). So, yes, your MAP sensor read a pressure less than the atmospheric. Therefore a negative relative pressure. Or positive vacuum pressure.

> Is the usage of the terms absolute, relative and vacuum clear, or do you need further examples?

2 - It is said that MAP sensor does not measure pressure! It measures manifold VACUUM. As you know when the engine is idling and the throttle is closed or when the car is going downhill with closed throttle, the VACUUM is at it's most because the cylinders are trying to suck air. By opening the throttle the air rushes into the manifold and the pressure goes higher. So, if this theory (MAP sensor shows VACUUM) is true when you open the throttle, MAP sensor should read a lower value than when it is closed (e.g if it reads 323 @ idle, it should read 290 @ 2000 RPM).

2. Vacuum pressure is just a negative relative pressure - see above.

3 - According to fluid dynamics, "faster moving air has lower pressure".
By opening the throttle the air velocity in the manifold increases but MAP sensor reads higher pressure!
This is contrary to the above-mentioned fact! By opening the throttle the pressure should drop!! because the air velocity has increased! This is where another BIG confusion starts!!

3. What you mentioned is just one side of the coin.

By opening the throttle more air comes in, as in mass. You have much more mass coming in, therefore higher pressure. The change in the speed of air is insignificant.

***

Most MAP sensors are actually MAF sensors, as in Mass Air Flow. They measure the mass of air that is going in. You can read an (air mass)/(time) reading in most cars with an OBDII port.

For example, in my S60, the maximum air mass the engine can suck is 140gr/s.

 

[Karabiner98k]

Hi Dear Giannis and thanks a million for your informative post! You have always been helpful to me!

You shed some more light into the dilemma but i still need more explanations.

If you could explain your facts by numbers and values it would be more clear.

You said that VACUUM is positive pressure. What does negative or positive pressure exactly mean?

Considering the fact that my car only has a MAP sensor you can forget about MAF sensor.

In the place where i live barometric pressure is about 873 millibars. My car's MAP sensor reads about 323 millibars @ idle (800 RPM). Does my car's MAP show negative pressure? or positive?

You can use my car and my conditions as your example.

According to what you said, MAP sensor readings are different in some cars. Some of them read negative pressure (VACUUM) while the others read positive pressure. Is this what you wanted to say? And is this why opening the throttle in some cars increases the MAP value but decreases the value in another car?

 

[Giannis]

So, relative pressure is absolute pressure minus atmospheric pressure:

Prel = Pabs - Patm

Your MAP sensor seems to measure absolute pressure:

Pabs = 323 mbar

So the relative pressure is:

Prel = 323 - 873 = -550 mbar

It's a negative relative pressure because it's less than the atmospheric (barometric) pressure.

You can also call this pressure "vacuum pressure" and then it would be + 550 mbar.

What exactly is vacuum pressure? Well, when relative pressure is negative, this means that it's less than the barometric pressure, there's therefore some sort of a vacuum where you measure the pressure. There's some sort of a vacuum, because if it weren't, the pressure would be greater or equal to the barometric. So, vacuum pressure is just a different name for negative pressure.

So, vacuum pressure of 550 mbar means relative pressure of -550 mbar (negative relative pressure).

Is it clearer now?

 

[Karabiner98k]

Yes, it is much clearer now!

Now i want to interpret the values of my car and my friend's car according to diagnostic data (MAP sensor).

We both have the same car.

My MAP pressure reads 323 millibars @ idle but his MAP reads 454 millibars @ idle! (both cars tested in equal conditions)

323 - 873 = -550
454 - 873 = -419

How do you interpret our cars? Does it mean that his car has lower pressure @ idle (419 < 550) because of some problems? (incorrect timing, manifold leak .. etc)

I have seen many cars of the same type and their MAP pressures @ idle were about 320 millibars (similar to me).

What does "higher reading @ idle" mean?

 

[Giannis]

How do you interpret our cars? Does it mean that his car has lower pressure @ idle (419 < 550) because of some problems? (incorrect timing, manifold leak .. etc)

Actually, your car has lower pressure @ idle, because the actual pressure is 323 and 323 < 454.

The 550 and 419 values are just the difference from the barometric pressure. Your friend's car's pressure @ idle is closer to the barometric (419 < 550), and therefore has higher pressure.

What does "higher reading @ idle" mean?

Hmm. I don't have a clear answer at the moment. I'll get back to you later on that.

 

[Giannis]

What does "higher reading @ idle" mean?

I thought of it and here it goes:

If every car like these two reads about 320 mbar @ idle, then this must be the normal and your friend's car has a problem. The problem could be one of the following by order of possibility (greater to lower):

1. If you have a higher relative pressure than normal then the most usual reason is a leak in your vacuum pressure circuit (the vacuum tubes that transfer vacuum pressure to the brakes' servo mechanism) or in the exhaust fumes circuit (that return back to the intake and are fed again to the engine). A leak in the vacuum pressure circuit is more common in force induction engines, while leaks within the exhaust fumes circuit is more frequent with N/A engines.

2. Your friend's car could have a leak in its exhaust but in the first few cm after the exhaust valves. Fresh air could leak INTO the exhaust, the first lambda sensor would therefore read too lean combustion. Thus more fuel would be sprayed in the combustion chamber, which in turn would require more air (all this @ idle), which would require a greater throttle angle. So, if more air goes in, your read a higher absolute pressure = lower relative pressure = lower vacuum pressure.

3. You could also have a hole or crack in the air intake. It happens!

4. The MAP sensor could also be faulty.

5. Possibly the throttle butterfly could need re-calibrating, if it is an electronic one only.

If the throttle butterfly isn't electronic, then it's 95% case (1).

So, why car/ engine are we talking about?

 

[Karabiner98k]

We are talking about Facelifted Peugeot 405 with 1.8 16V engine (XU7JP4):

The engine is a naturally aspirated 1.8 liters with 16 Valves.
Compression ratio = 10.4 : 1
110 PS @ 5500 RPM
155 nm @ 4250 RPM

Idles @ about 800 RPM ±50

One of my friends had it's timing belt changed at a dealer and immediately after that his manifold pressure went high @ idle! (higher than before) I read somewhere that valve timing can affect manifold pressure.

One thing I still don't understand is that "high manifold pressure @ idle", is it good or bad?

Why manufacturers don't announce the normal range of values for each sensor? For example Peugeot should mention in their manuals that MAP pressure of around 320 mbar is considered normal for this engine in X mbar of ambient pressure and Y degrees of temperature.

Is barometric pressure affect MAP pressure reading @ IDLE too? If my MAP reads 323 mbar @ 1300 meters above sea level, will it read 323 mbar @ sea level too?

I know that at sea level more air enters the manifold but that's the case ONLY when you open the throttle not when your car idles with closed throttle. Is it true? Is manifold pressure @ IDLE will always be the same regardless of elevation or outside pressure or temperature?

 

[Giannis]

We are talking about Facelifted Peugeot 405 with 1.8 16V engine (XU7JP4):

So, it has a good old cable operated throttle butterfly. Good!

One of my friends had it's timing belt changed at a dealer and immediately after that his manifold pressure went high @ idle! (higher than before) I read somewhere that valve timing can affect manifold pressure.

Hm, is he sure that they didn't damage anything else while changing the timing belt?

When changing the timing belt, you are supposed to use some specific locking pins and lock the crankshaft and camshafts to certain position, so that you won't lose the engine's timing. So, changing a timing belt won't affect the valves' timing.

Valve timing changes with variable valve timing systems, which are not affected by a change of a timing belt.

My bet is that they damaged something else while changing the belt.

One thing I still don't understand is that "high manifold pressure @ idle", is it good or bad?

Anything other than normal is not good.

Is barometric pressure affect MAP pressure reading @ IDLE too? If my MAP reads 323 mbar @ 1300 meters above sea level, will it read 323 mbar @ sea level too?

Of course the barometric pressure affects the pressure reading at idle. If your MAP reads 323 @ 1300 m above the sea, then at sea level, the air being more dense you would read a higher absolute pressure (and the barometric pressure would obviously be higher).

I know that at sea level more air enters the manifold but that's the case ONLY when you open the throttle not when your car idles with closed throttle. Is it true?

I think, no.

Is manifold pressure @ IDLE will always be the same regardless of elevation or outside pressure or temperature?

The manifold pressure is a function of both the barometric pressure (or elevation) and temperature.

 

[Giannis]

BTW, if I am not mistaken, @lafars works at a service garage. I wonder what his opinion is.

 

[Bucketslash11]

i'm only an apprentice but if i'd have to guess at something it would have to be the ECU, have you tried deleting the error, go for a test drive and reread the ECU?

it could also be a be a wire break and bad grounding

if it's not an electrical fault then it can be a small leak on the manifold somewhere (when was the intake gasket last changed?)

hope this helps

 

[Karabiner98k]

My friend's problem was about 3 months ago and by changing timebelt again his problem gone.
I want to know which factors make MAP pressure to drop?
Back Pressure (like clogged converter or muffler) is one of them.

I think worn-out piston rings make engine to create less vacuum and therefore if a car has worn-out rings it's manifold pressure should be higher than normal (I mean @ idle with CLOSED THROTTLE). Because with CLOSED THROTTLE a healthy engine should create enough vacuum. With worn-out rings, cylinder compression also drops and then MAP pressure goes high (again @ idle with CLOSED THROTTLE).

Correct?

 

[Giannis]

With worn-out rings, cylinder compression also drops

The best way to find out is to do a measure each cylinder's compression. It's very easy to do!

 

[Karabiner98k]

The best way to find out is to do a measure each cylinder's compression. It's very easy to do!

Yes, i know!

But in this topic i just want to discuss about manifold pressure and factors that contribute to it's increase or decrease @ idle.

 

[Giannis]

Yes, i know!

But in this topic i just want to discuss about manifold pressure and factors that contribute to it's increase or decrease @ idle.

Ok then, worn piston rings could lead to a higher relative pressure, but higher as in in the order of 2 to 3 mbars, which is within the error of the readings.

Very worn piston rings could lead to a noticeable difference in MAP readings, but then you'd have many more problems, like a white cloud coming out of your exhaust.

 

[Karabiner98k]

Hi guys,
I checked my own car's MAP reading 3 days ago.

@ idle it reads about 330-337 mbar and when i opened the throttle the reading dropped to about 260-270 and less than that in higher RPMs. (open throttle decreases vacuum).

So, i think my car's MAP is really showing the VACUUM because when decelerating with closed throttle the reading is the highest (the highest vacuum is created).

With engine OFF and ignition ON, MAP reads 870 mbar which is exactly the atmosphere pressure in our city.

 

[Giannis]

If with engine off and ignition on you read the exact barometric pressure, then your MAP sensor reads absolute pressure.

If it read vacuum pressure, then at those conditions you'd get a zero reading.

 

[Karabiner98k]

Last night i performed an interesting test with my father's car: Citroen Xantia 2.0 16V

I started the car and let the engine to warm up enough and go into Closed Loop.

I measured it's manifold intake pressure @ idle and it was around 365 millibars.

Then i blocked the exhaust by putting a rag into the muffler and hold it there for about 10 - 15 seconds to create back pressure in the system. The result?!

Manifold pressure went higher and reached 403 millibars and was keep going higher until i let the rag off and the pressure dropped again to 365 millibars.

So, MAP sensor responded to back pressure by showing a higher reading (403 vs 365).

How do you interpret this condition? Why back pressure made MAP sensor to show a higher reading? (instead of lower reading)

 

[Giannis]

Since your MAP reads absolute pressure, at idle and ideal conditions (free exhaust) you should read a certain pressure value which is smaller than the barometric pressure.

If you block the exhaust, then the cylinder can't suck air as much as it could, therefore you read a higher pressure, yet still lower than the barometric. There's still some vacuum generated, but less than it should.

 

[Karabiner98k]

Thanks Giannis.

According to what you said, blocking the exhaust LOWERS VACUUM and this LOWER VACUUM results in HIGHER READING.

But a counterintuitive fact occurs here!

When i was coasting downhill with throttle closed and around 3000 RPM in 5th gear the reading was very high (around 500 - 600 millibars). Yes, the reason is that with throttle closed and RPMs going higher there are lots of VACUUM to be produced and thus the reading goes high.

Now the thing i don't get is that why LOWERING VACUUM in first test resulted in HIGH reading?
And why INCREASING VACUUM in 2nd test again resulted in HIGH reading??

These 2 tests are exactly the opposite of eachother! One creates HIGH vacuum and the other LOWERS it! But MAP sensor responded the same to these totally different conditions! I think it's not logical!