Thank you for raising the MAP possibility being cause for a chronic rich condition (-ve LTFTs). The existing MAP is pictured in this thread above along with a few comments there, as a precursor to arriving at this point. I had also been thinking about the MAP for awhile. I might have replaced it before, but haven't because of uncertainties as to how to confirm it good/bad and if bad then which is the right one to put on. When I look up replacements, I uncover different numbers and pricing ranging from 30$ to 150$. In other words, I am not confident on what to replace it with? Numbers given in phone calls to dealer and autoparts stores vary. 56041018AC 56028562AB 56041018AD 68199324AB ... and numerous generic brands too ???

Also, can you advise if elevation makes any difference to the stated 17-21 inHg target range for idle intake manifold pressure? My elevation is 690m, 2250 feet. If the MAP sensor is reading low, would that not cause a lean condition? (low pressure, less air, less fuel)

 

If you had a $20 independent vacuum gauge, you could validate on your own. But a quick web search yields you are still pretty far out of spec:
: https://www.motor.com/magazine-summa...-vacuum-gauge/
Inches of Altitude Vacuum:
Sea level-1000 ft. 18-22

1000-2000 ft. 17-21

2000-3000 ft. 16-20

3000-4000 ft. 15-19

4000-5000 ft. 14-18

5000-6000 ft. 13-17

I haven't bought a new MAP for a Ram.... I'm usually working on vehicles that are 10 years old and under, MAP is a rubber diaphram, and rubber lasts a while - but not forever. Typically I'm replacing EGRs, o2 sensors,,and maybe a temp sensor. Once I had to buy a good used PCM for my 04 1500. Now that the kids are driving, I've bought a couple much older used cars and to combat insurance costs - I'm buying used and 10+ years old with 100k miles or more.

A map sensor isn't very special, but anyone can generate poor quality. I'd go with a name brand, or the OE supplier for Dodge.

I'm still recommending the independent vac gauge!!!. You could confirm your suspicions and check out the replacement.... Plus its an easy and fast way to check out an engine. Plug it in and look at the dial face. I'd print the Gauge reading ouija board out and put it on a poster in your garage Looking at Vac can tell you a bunch about engine condition.

 

Other things could be causing a low vac signal.... have a look a that vac ouija.... I'd hope its just a bad MAP sensor. My 04 2500 5.7 has 165k miles on it and torquePro reports it as 21 in/HG at idle. We are about 1300 feet above sea level.

 

I've suspected an aging MAP giving skewed readings for a long while. This is leading down the path of probably confirming it (or something else). I can get a vacuum gauge onto the manifold and direct compare.
Interesting that as of right now:
- local weather station ambient barometric pressure is: 104 kPa ( 30.7 inHg )
- MAP reading on the scanner with engine off is: 98 kPa ( 28.9 inHg )

So right off the get-go the MAP is reading low, by 6%. This just happens to correspond near 2x the value of the negative LTFTs in the range -8% to -14% which we are seeing in the logs. What's the connection? I am not clear, yet, whether low MAP reading would correlate directly or inversely to the fuel trims. Please advise/correct the following thought: If MAP is showing low pressure, means less air, the result should have PCM give less fuel, it follows then exhaust gas result should end up lean, the O2 pick up lean, sending the STFT LTFT to +ve to demand more fuel. Assuming that logic is correct/true; If this MAP is reading low then the test drive log data LTFT's should be more on the +ve side? The opposite is occurring, we are seeing -ve's.

I believe it to be original 15 year old sensor. I have never changed it and I knowing the previous owner personally, surely he never would have either. Acknowledge that until I hook up a gauge and direct compare, there is no knowing what the rest of the pressure curve of the MAP sensor looks like when the engine is running. Being an old aged sensor, it may be either overly stiff or overly bendy. Kinda like Grandma when taunted with broccoli vs chocolate. It is simple enough to get a 35$ sensor off of Amazon. I have to order some other stuff anyways. Do I go for SMP AS321 or the SP MP114 ?

 

MAP is so skewed, I wouldn't try to guess the stock setting. Gotta watch analysis paralysis. I'd focus on validating you have good vac and the MAP is bad. If you find the opposite, time for a new thread.

I'd probably buy both, the cheaper one and OE. Try the aftermarket copy first and if it works, return the OE. Low risk, high return.

Good luck!

 

O2 sensor replaced. Fixed that erratic one. Good voltage trends on all 4 sensors now. LTFT (Fuel trims) have stabilized and seem to be happier, smoother trends, but still constantly in the -6% to -12% range, never +ve. The negative trim is worse at low throttle. It is less negative at higher speeds, higher throttle. I think I have it figured out. The numbers line up and make sense.

Ambient pressure today is about 102 kPa. The MAP with engine off reads 97 kPa. So MAP is reading 5 kPa low at the start. What I cannot know is if it is a constant 5 low or is it a 5% low through the range. Doesn't really matter for this exercise. Either/or the fuel ratio error will be more at low speed (high vacuum) and less at high speed ( low vacuum).

When engine is running at idle MAP reads 34 kPa. I do not have a vacuum gauge, yet. One is on the way. Without the gauge I still can specu-calculate what the idle vacuum range could be. The "vacuum" in the manifold at idle would be the ambient pressure minus the MAP reading. If the MAP was perfect then 102 - 34 = 68 kPa ( 20 inHg ) vacuum. If the MAP is constant skewed 5 kPa, then 102 - (34+5) = 63 kPa ( 18.5 inHg). If MAP error is 5% across the range then 102 - 34*1.05 = 66.3 kPA ( 19.5 inHg ). Referencing the "on all cylinders" vac gauge pin-up above, I can deduce that the manifold vacuum range of 18.5 to 20 in Hg is just fine. There are no engine condition concerns.

Now if I throw in that the MAP is reading at 5 kPa or 5% low, it means that at any time the actual vacuum in the manifold is LESS than what the MAP infers it to be. So what is happening with the fuel ratio from the PCM as a result? Well, actual vacuum in the manifold is less, therefore less air density in the manifold than the sensor is telling the PCM there is. Less air needs less fuel. The PCM is giving air per the MAP sensor but since that device is skewed the result is a rich fuel mix being given. Following that through out the exhaust, it is going to give a rich reading at all of the O2 sensors, and ultimately the feed back loop is going to be constantly negative fuel trims. Putting numbers to it based on the data to see if this is in range of an explanation : At idle the MAP is inferring there should be a 68 kPa vacuum. The actual can be 5 kPa off. 5/68 = 7.4%. Meaning at idle the fuel ratio the PCM feeds to the engine can be as much as 7.4% rich. Guess what am seeing in the datalogs? Negative LT fuel trims at idle of -8% to -12%. Coincidence? As speed, rpm, and load increase there will be less vacuum in the manifold; so can expect there will be less of an effect of the 5 kPa skew in the MAP reading. At around town steady driving speed the LTFTs in the datalogs reduce to -2.3% to -7%.

It definitely correlates. The MAP is reading low, resulting in constant rich condiition, constant -ve fuel trims. That said, things are not as bad as thought.
A new MAP sensor is on order. I will update when that is installed.
If the new MAP sensor is any good, the effect should be the LTFTs shift up and operate in the +3% to 3% range.

 

Your MAP sensor is off by about a factor of 2 at idle. Weather reported pressure is supposedly adjusted to sea level when reported, so that might be your 5% change when the engine is not running. I'm not following your math, but I also haven't been in a physics course studying gas laws for a really long time. Make sure you have some assorted sizes of rubber hose and a plastic T so you can get your vacuum gauge hooked up when it arrives, I'm interested to see your empirical findings

If the map sensor construction includes a rubber diaphragm, I'd assume yours is torn or has a hole in it - not allowing it to pull the diaphragm as far as it should towards the intake. You indicated you were cleaning it annually, did you use rubber safe solution? You should crack that bad sensor in half and have a look at the internals when you're done with it.

 

PV=nRT

Question; does pump fuel octane rating have any bearing on fuel trims?

 

Maybe, but, not very much. Octane rating is just the fuels tendency to resist pre-ignition, so with higher octane fuel, you can run higher compression/more timing advance, and not blow your pistons apart. You *might* see a VERY slight increase in fuel mileage using a higher octane gas, but, the difference won't be enough to make up for the additional cost.

 

OK. I have received the gauges and the new MAP sensor. Hope to do some testing over next couple days and have something to report back.
Will get to test the old and new MAPs on the Ram tomorrow evening. Then some test driving to see where the PCM fuel trims go as it relearns based on the new sensor.