Negative Manifold Gauge Pressure. ??

[Mxr695]

2008 f250 6.4 Spartan 275v4 57k miles
Lately for the past few weeks I have noticed I have negative reading( -.5 to -.7)in dashdaq while idle its never been this way always would read (.7 to.9) at idle .truck drives fine sounds normal but the readings have never read that I've had it tuned since brand new . From my under standing there should not be vacuum on our trucks but not sure so that's why I'm posting . Just need some ideas as to what this could be. Fuel filters brand new just changed oil and air filter afe 7 that's clean as well. Any ideas I'm open couldn't find to much in the 6.4 forums

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[drewactual]

that is an environmentally variable thing.. temperature has a big effect on it.. cold air entering is already denser than warm air- heavier because is colder/denser, too..

that is a small movement from what you're accustomed to seeing, and then there is the question of parameters the sensor is capable of reading as well..

I'd not sweat it, unless it is drastic- or unless you can compare apples to apples in terms of environmental conditions.. i.e.: write the readings down, reference the temp/humidity, and compare it to another day with same temp/humidity..

[Mxr695]

Alright I will start keeping logs. I know lately here we have had some humidity with cold weather which we usually don't so maybe that's it. Just odd I thought because it has been consistent up until now . Truck was just in shop last month for bed plate oil leak. Other than that that's the only problem I've ever had major repair anyways I just always watch my gauges and keep a eye on things. What would be drastic readings in your opinion?

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[drewactual]

I would guess a few whole points would be worthy investigating.. MAP's will tell the tale, at idle, of leaks.. vacuum isn't used as much as it should be to isolate problems, imHo..

How to Use and Interpret a Vacuum Gauge

^that is a good tool to have, but you gotta figure, we're running diesels and boosted at that.. the manifold hovering around center is normal- drawing any vacuum (and please others better versed correct me if I'm wrong) at idle would represent an obstruction, and any positive draw (in whole numbers, now) would represent a turbo pushing it..

it's the bounce and the way it bounces that is concerning.. solid and steady is what you want.. a leak will make it bounce.. the way it bounces will also tell you if it is a valve seat, a rocker, and even if an injector is sticking..

[drewactual]

huh.......

MAP sensors measure absolute pressure. Boost sensors or gauges measure the amount of pressure above a set absolute pressure. That set absolute pressure is usually 100 kPa. This is commonly referred to as gauge pressure. Boost pressure is relative to absolute pressure - as one increases or decreases, so does the other. It is a one-to-one relationship with an offset of -100 kPa for boost pressure. Thus a MAP sensor will always read 100 kPa more than a boost sensor measuring the same conditions. A MAP sensor will never display a negative reading because it is measuring absolute pressure, where zero is the total absence of pressure (it is possible to have conditions where negative absolute pressure can be observed, but none of those conditions occur in the air intake of an internal combustion engine). Boost sensors can display negative readings, indicating vacuum or suction (a condition of lower pressure than the surrounding atmosphere). In forced induction engines (supercharged or turbocharged), a negative boost reading indicates that the engine is drawing air faster than it is being supplied, creating suction. This is often called vacuum pressure when referring to internal combustion engines.

In short: most boost sensors will read 100 kPa less than a MAP sensor reads. One can convert boost to MAP by adding 100 kPa. One can convert from MAP to boost by subtracting 100 kPa.

what's your filter look like?

[Mxr695]

My readings don't bounce the numbers are solid either positive or negative but stable . Now my air filter has seen its better days the dust here in the dessert has took its toll on it even though cleaning it with the kit afe sells it just doesn't look up to par . Going to pick up a new filter this weekend I was thinking that might be effecting it

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