Engine/Transmission (1998.5 - 2002) MAP sensor values ???

[KGrotte]

Truck seems to be having fuel timing issues at about 2000 rpm. I was just checking a few things and hooked up my code scannner and happened to look at the MAP reading it said 58in with the engine off. so i unplugged it and it went down to 29in so i plugged in a spare with out installing it and it went back up to 58. I did not check it when it was running will do that tomorrow. I was just wondering the values i was thinking that 29 was about right.

 

[MChrist]

I would assume 15, since MAP stands for Manifold Absolute Pressure. At 20psi boost, it should read 20 psi on top of atmospheric (14. 7 at sea level), so 34. 7??? I'm just guessing, so somebody can feel free to correct me if I'm wring here.

 

[Alan Reagan]

If it is giving the value in inches, 29 is about right with the engine off. 14. 7 is psi at sea level. That would be between 29 and 30 inches of depending on where you are and what the atmospheric conditions are at the time.



For instance, if you are in the eye of a hurricane at sea level, you would read about 28 inches or in a high pressure system about 30 inches.



I don't understand why it would read 58 inches in the off position unless it is faulty. Was that with the engine off but the key on? Here's a table of boost to voltage values:



Boost - Voltage

0 - 1. 045

6 - 1. 48

10 - 1. 75

15 - 2. 05

18 - 2. 27

20. 5 - 2. 44

 

[hcole]

Alan, I chased this rabbit before. I thought the reading should be app. 30 inches Hg with key on engine off. No, according to my Actron CP9145 scan tool. It will give this reading, 59-60" Hg on any map sensor plugged in. It will give this whether running at idle or just key on engine off. I have concluded that the base line for this sensor according to the Actron is 59-60 Hg. Now, when you accelerate down the road it will go right on up to 80"Hg. I think the value the sensor puts out equals a certain amount of boost value on the Actron. It is accurate if you over look the baseline. I'd really like to know others thoughts though.

 

[THoltz]

I would assume 15, since MAP stands for Manifold Absolute Pressure. At 20psi boost, it should read 20 psi on top of atmospheric (14. 7 at sea level), so 34. 7??? I'm just guessing, so somebody can feel free to correct me if I'm wring here.

At 20 psi boost it will read 20 psi not 34. 7. The term psi really means psig or pounds per square inch gauge and is relative to atmospheric pressure. So 14. 7 psia will show 0 psig on a pressure gauge calibrated for psig. PSIA is pounds per square inch absolute, which means pressure measured on a gauge is relative to 0 pressure. So, atmospheric pressure on a gauge calibrated for absolute pressure would indicate 14. 7 psia. Hope this makes sense. I learned this a long time ago in Naval Nuclear Power School, so I am sure I have forgotten some of the details but hopefully I was semi clear on the concept.

 

[Bob4x4]

The odd ball readings are due to the odd ball scanner,

The factory scanner will give psi readings that we are all familiar with.

I seriously doubt a map sensor issue if it only shows up at 2krpm

 

[Bob4x4]

At 20 psi boost it will read 20 psi not 34. 7. The term psi really means psig or pounds per square inch gauge and is relative to atmospheric pressure. So 14. 7 psia will show 0 psig on a pressure gauge calibrated for psig. PSIA is pounds per square inch absolute, which means pressure measured on a gauge is relative to 0 pressure. So, atmospheric pressure on a gauge calibrated for absolute pressure would indicate 14. 7 psia. Hope this makes sense. I learned this a long time ago in Naval Nuclear Power School, so I am sure I have forgotten some of the details but hopefully I was semi clear on the concept.

The later 24v trucks do in fact show map numbers,so yes on the scanner you will see atmospheric plus boost-not straight boost pressure

 

[THoltz]

The later 24v trucks do in fact show map numbers,so yes on the scanner you will see atmospheric plus boost-not straight boost pressure

Ahh, so it does show absolute pressure. Good to know. Thanks for the info.

 

[Alan Reagan]

Alan, I chased this rabbit before. I thought the reading should be app. 30 inches Hg with key on engine off. No, according to my Actron CP9145 scan tool. It will give this reading, 59-60" Hg on any map sensor plugged in. It will give this whether running at idle or just key on engine off. I have concluded that the base line for this sensor according to the Actron is 59-60 Hg. Now, when you accelerate down the road it will go right on up to 80"Hg. I think the value the sensor puts out equals a certain amount of boost value on the Actron. It is accurate if you over look the baseline. I'd really like to know others thoughts though.

That's good to know. I have an Actron but haven't looked at it concerning the boost.

 

[KGrotte]

So i finally took the truck for a drive with the scan tool hooked up. I indeed am using a cheap scan tool it is a innova 3130. it displays the MAP values in (inHg) With scanner plugged in, engine off and map plugged in it reads 58. unplug the sensor and it goes to 29. Driving down the road map values go up from 58. at 15 lbs of boost it reads 75in. I have recently installed a new HRVP44. I was running one before that but it was built in 2002. I am not to shure if there is newer technology to advance the timing in the pumps if so that may be my issue as i am running a toner ring on the cam to already advance the pump. it just seems like the computer and the pump are not talking right ...