2004 dodge ram 1500 suddenly getting only 5mpg

[NCRaineman]

Old school trick you can use to detect a leaking intake.... with the truck idling gently spray some starting fluid around the intake manifold. If the engine surges or revs up you know some of that is getting sucked in.

 

[shelby5041]

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[Dean2]

hey rainman..whats got me confused is the manifold pressure per live data is about 15psi with engine off key on....and once i start car its goes to about 20psi then drops to 5psi within few seconds...

how can a running engine manifold pressure drop below the engine off manifold pressure value.......even if there is a intake leak...this makes no sense to me.

The data us wrong. An engine that is not running has ZERO vacumn. It has to be running to produce vacuum. The 5 psi running, if accurate says you have a big leak. NC gave you good advice re old fashion way to check for intake gasket leak. Try it. You can run the scan tool all day, it won't fix the problem and you don't know enough about what you are doing to interpret the data any how. You need some experienced help.

 

[NCRaineman]

Physics lesson time!

At sea level, atmospheric pressure is about 15psi. When the engine is off, the absolute pressure inside the intake equals atmospheric pressure, so the sensor will indicate about 15psi.

When the engine is running, the downward motion of the pistons create a vacuum inside the intake manifold. With a running engine, intake manifold vacuum should be around 20"Hg . At 20“Hg, the sensor will show about 5 psi. This is because the sensor is measuring absolute pressure, based on a perfect vacuum, rather than atmospheric pressure.

So totally normal at idle to see 5psi.

 

[Dean2]

Physics lesson time!

At sea level, atmospheric pressure is about 15psi. When the engine is off, the absolute pressure inside the intake equals atmospheric pressure, so the sensor will indicate about 15psi.

When the engine is running, the downward motion of the pistons create a vacuum inside the intake manifold. With a running engine, intake manifold vacuum should be around 20"Hg . At 20“Hg, the sensor will show about 5 psi. This is because the sensor is measuring absolute pressure, based on a perfect vacuum, rather than atmospheric pressure.

So totally normal at idle to see 5psi.

This is accurate, but depends on whether he is measuring vacum or air pressure. Vacum gauge at idle should be reading 20, air pressure sensor would read 5.

 

[NCRaineman]

This is accurate, but depends on whether he is measuring vacum or air pressure. Vacum gauge at idle should be reading 20, air pressure sensor would read 5.

I'm assuming his scanner is pulling data from the MAP sensor and he's not using an old-school vacuum gauge.

 

[rzr6-4]

Physics lesson time!

At sea level, atmospheric pressure is about 15psi. When the engine is off, the absolute pressure inside the intake equals atmospheric pressure, so the sensor will indicate about 15psi.

When the engine is running, the downward motion of the pistons create a vacuum inside the intake manifold. With a running engine, intake manifold vacuum should be around 20"Hg . At 20“Hg, the sensor will show about 5 psi. This is because the sensor is measuring absolute pressure, based on a perfect vacuum, rather than atmospheric pressure.

So totally normal at idle to see 5psi.

You beat me to it by 3min. When OP said 101kpa absolute pressure, I anticipated and sure enough, people missed the absolute part. Can't forget the atmospher.

 

[Dean2]

I'm assuming his scanner is pulling data from the MAP sensor and he's not using an old-school vacuum gauge.

I am old school just based on how I learned to diagnose and fix issues, but I have found an old fashion vacumn guage is actually a lot better for diagnosing hose cracks, vacum leaks etc. Far easier to isolate issues using the gauge than relying oon the readings from remote sensors.

 

[shelby5041]

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[shelby5041]

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[shelby5041]

update...i think issue fixed...after i replaced all 4 o2 sensors i drove about 20 miles @ 35mph road and now my economy avg readout shows 14-15mpg...not 5-6mpg...and my check engine light so far has not come back on...from what i have been told its hard to believe bad o2 sensors can affect gas mileage that much but in my case it seems that it did.....thanks for everyones input.

 

[Dean2]

Change thee 02 sensor with OEM replacements. It is the easiest part to fix and givem the error codes you have and voltages they need doing. Might also have to fix the connectors. Once you do that clear the codes and see what you get. I am still betting root cause is the intake gasket but no point in digging into that till you have fixed the easier stuff that is also wrong.

Glad you got it figured out. You have also learned the value of following a disciplined process to diagnose issues. Pull codes, follow diagnostic tree, replace any parts that are clearly defective. Don't rip into stuff or start theowing parts at an issue on a guess. Follow the process every time and your fixes will usually cost u a lot less.

 

[shelby5041]

yep...as you stated at beginning of thread...however when i kept hearing intake leak it made me a bit worried..and that bad o2 sensors cant affect gas mileage from 15mpg to 5mpg .and
hearing all the othere possible causes wasnt too pleasant but once i did some basic diagnosis and using my obd2 scan tool with live data i was able to eliminate the unpleasant possible causes...swapping the 2 rear sensors and seeing the 0v follow the sensor and verifying the map sensor values were normal basically told me there was probably no intake air leak ). i hope.

 

[Dean2]

yep...as you stated at beginning of thread...however when i kept hearing intake leak it made me a bit worried..and that bad o2 sensors cant affect gas mileage from 15mpg to 5mpg .and
hearing all the othere possible causes wasnt to pleasant but once i did some basic diagnosis and using my obd2 scan tool with live data i was able to eliminate the unpleasant possible causes...swapping the 2 rear sensors and seeing the 0v follow the sensor and verifying the map sensor values were normal basically told me there was probably no intake air leak ). i hope.

Like I said in previous posts, I too am surprised bad O2 sensors were enough to drop the mileage that much, but if both sets had failed, and the ECM is reading over lean on both banks, then it will dump a lot of extra fuel trying to get the reading back where it belongs. This is the same overly lean condition caused by a intake gasket leak, but in this case, a false reading caused by sensor failure. All that extra fuel I am surprised you weren't getting backfires in the exhaust.

I am REALLY glad it worked out so well. You will have to update us in a few weeks, once you know for sure the fix worked.

 

[shelby5041]

both sets did not fail...just both on bank 1 failed reading 0v...thanks again..

 

[shelby5041]

update
have driven for a few days ..no check engine light and mpg good.

 

[shelby5041]

update
all still good