PCV valve pressure sensor...what does it do?

JP

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#1
What is the purpose of the PCV valve pressure sensor?
 

JP

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#2
I guess it's called the MAP sensor?
 

billr

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#3
Are you sure you mean MAP? The two, MAP and PCV are unrelated; and I have never heard of a PCV pressure sensor, although it could be a meaningful engine parameter to monitor. MAP, however, is a very common sensor and MAP is a vital factor in engine and trans operation. The acronym stands for "Manifold Absolute Pressure" and roughly is used the same way intake-manifold vacuum was used in the old days to control spark advance (vacuum advance can) and auto trans hydraulic pressures and shift-points (vacuum modulator). MAP is directly related to the load (torque) on an engine. MAP is probably used, in some way, to calculate fuel flow in any FI system; and is one of the prime factors in speed-density (no MAF) systems. MAP, in that case, is most of the "density" factor.
 
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#5
Positive Crankcase Ventilation employs just a check valve that permits air fluid flow in only one direction. Before PVC, a cylinder was never a perfect seal, blowby on the rings would always end up in the crankcase pressurizing it. So a breather tube was used, nothing more than a tube connected to a hole drilled in the block above the oil line.

Was a major problem, because when taking your foot off the gas, created a vacuum in the crankcase instead of pressure, sucking up dirt from the road that got into an engine. But now wondering why they never installed a check valve in that tube to prevent that. With PVC, for lack of a better word, what is put into the crankshaft is crap, unburnt fuel, carbon, exhaust gases, but instead of blowing it outside, returned back into the intake of the engine. Either at the air cleaner, at the base of the TB or carb, or directly into the intake manifold putting all the crap all over your injectors.

PVC only permits that crap to flow from the crankcase into the engine, but under different circumstance, can have more pressure in the intake manifold than the crankcase, PVC prevent that reverse flow or otherwise you will be adding even more crap to your crankcase. This crap could have either been filtered or with a pump, pumped directly into the catalytic converter, hey, that would hurt the stockholders. Pump pressure would have to be much higher than exhaust pressure.

The chief component of exhaust gases is water, H20, if any sulfur is in the gasoline with high combustion temperatures, output is H2SO4 or sulfuric acid that eats away your engine requiring frequent oil changes. The C in HC's is carbon that also builds up on the rings inhibiting their sealing ability with even more bypassing. This is a 130 year old problem, efforts to eliminate C in gasoline are constantly killed by various oil controlled governments that would save a ton of emission problems.

Carbon is also highly conductive killing your spark when built up on the center electrode. Also builds up on intake valves restricting air flow.

Intake vacuum was used directly to control a vacuum module to retard spark advance with low intake pressures, when accelerating the throttle vane is wide open reducing intake manifold vacuum with the final result being a slower burning air fuel mixture. Intake manifold vacuum is nothing but constant, more like a series of spikes in vacuum, to get around these spikes a simple restrictor was used. Major control functions are for spark advance retarding and EGR valve operation.

The MAP sensor replaces this once simple function, is a transducer that converts manifold pressure into an electrical signal, either a proportional DC voltage or a continuously variable frequency that can talk to a microcomputer or microcontroller as more commonly called. A solid state chip was developed back in the 70's whereas the conductivity of electron flow is based on pressure along with circuitry to output a proportional signal. This can talk to the controller where a computer like program determines the degree of spark advance based on mean parameters. In other words, an arbitrary set of values based on current knowledge, not the actually operating requirements of the engine. And substantially more complicated with a debate as to whether it is any better than the simple vacuum system.

Only one MAP sensor is used to save cost for this less than two buck item except when you go to buy one. So no comparison can be made as to whether its functioning properly or not. Best that they can so is to determine whether its open or shorted, but nothing to do with linearity. Typical failure is a misfiring condition when accelerating that tells you its not working properly, no scanner will show that. But can show a misfire, the rest is a guess on your part as to the cause.

That computer code was stored in a burnt in PROM, unchangeable, but with OBD II that code is stored in flashram, no different than your digital camera, and equally can be corrupted due to glitches or wide temperature variations. But its cheaper and can be upgraded via the internet if you have special passwords.

Was also determined that for peak performance, all these parameters will be fixed in the code, no more individual controlled adjustments like AF ratio or ignition timing and advance. Just like hubs replaced adjustable wheel bearings at extreme cost to the consumer. Is it better? Good question. Is it a headache to troubleshoot? You can bet it is. Definitely is a lot more complicated and the more complicated it is, the more problems we all can have. But also is the law.
 

kev2

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#6
I'll bite- what vehicle?
Maybe on a turbo or supercharged eng?
 

JP

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#7
Sorry, this is my '99 Lumina again. I didn't realize that it was the MAP sensor; seems like an odd location for it right above the PCV valve, and I thought it was related to the PCV.
 
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#9
The MAP sensor is mounted to the PCV valve cover.......................Description and Operation


The Manifold Absolute Pressure (MAP) sensor responds to changes in intake manifold pressure (vacuum). The MAP sensor signal voltage to the PCM varies from below 2 volts at idle (high vacuum) to above 4 volts with the key ON, engine not running or at wide-open throttle (low vacuum). The MAP sensor is used to determine manifold pressure changes while the linear EGR flow test diagnostic is being run (refer to DTC P0401), to determine engine vacuum level for other diagnostics and to determine Barometric Pressure (BARO) .

If the PCM detects a voltage that is lower than the possible range of the MAP sensor, DTC P0107 will be set. A signal voltage higher than the possible range of the sensor will set DTC P0108. An intermittent low or high voltage will set DTC P1107 or P1106 respectively....
 

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billr

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#10
No wonder there was some confusion... it looks like that MAP sensor is mounted right on the PCV!
 

JP

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#12
Yeah, that's where I got screwed up....I thought the MAP would be somewhere else.