I'm still trying to track down the source of the problems with my 3.4 DOHC. Lack of power, bad exhaust smell on a '95 crate engine. I'm using CarBytes to monitor the engine.
I think that the MAP sensor is not working correctly. What symptoms would that cause?
When running at a constant 55 mph the manifold pressure is about 72 KPa and the BLM is 119. But then there is the occasional step in the MAP down to 42 or 43, followed by a step back up. At the same time the BLM steps down and the INT ramps up. There is no increase in speed or RPM, .
When the engine is not running the MAP is reading 116. Can this be? One atmosphere is 14.4 PSI or 101 kPa. Running downhill, about 2kRPM, 52 MPH, the MAP is reading 103. Later on, accelerating, MAP is 106.
The formula that I'm using for MAP is 28.1 + 0.369*sensor value. Is this correct? Should I go out and replace my MAP sensor?

Eric

Before you replace the sensor itself, check all of the wiring. YOu might have a bad connector or bad wires somewhere. Monitor the reference voltage coming out of the ECM and make sure its a steady +5V. It sounds like you may have a bad connection somewhere. I had a similar problem with the TPS circuit where the TPS voltage would drop momentarily and cause some weird problems since the ECM sets 0% throttle as the lowest TPS voltage observed (as long as it isn't zero; in the case of which it resets). This was because I had a slightly corroded or loose pin in the miniharness connector for the TPS reference voltage. It was erratic so it was very hard to track down. What I ended up doing was hooking up the scanner and turning on the ignition, then observing the TPS reading while wiggling wires until I saw it jump. If your scanner or laptop only updates once per second you probably won't catch it; in that case you should just disconnect the MAP sensor connector and measure the reference voltage yourself. The problem is you can't test the output voltage of the MAP sensor that way unless you tap into the wire right at the ECM where it receives the signal.

If I recall correctly the general formula for conversion from MAP sensor output voltage into pressure for the 1 bar map sensor is:
(voltage) * 20.26 = absolute pressure in kPa
This is true if MPA sensor responds linearly to pressure from 0 kPa (pure vacuum) to 101.3 kPa (atmospheric pressure on Earth at sea level) with a voltage ranging from 0 to 5 volts, which is my understanding of it.
When the motor isn't running you should see atmospheric pressure in the intake manifold, so somewhere around 5 volts on the MAP sensor. Depending on what altitude you are at you might see as low as 4.5 volts.

what's carbytes? I've never heard of it. Are you sure it's definition file is okay?

Eric, this is the part ofthe xml doc I used to obtain that calc (I can't recall where I got it from now tho). There's a table also associated with the definition. I don't really know which is more accurate but maybe looking at the raw data and using the table may give you a better idea of the values.
If the MAP value is jumping erraticly though I'd look into why that is, rather than worry about exact values. Do you have the egr and/or evap can purge solenoid connected at all. maybe one or the other of those opening (particularly the egr) is causing the variations.

<ROWAttrs AID="11" ACode="MAP" ADesc="(A/D)" Start="29" Bits="8" AType="1" Visible="TRUE" Chart="2" Series="3" Dash="14" DashFmt="0" Comments="Manifold air pressure">
−
<Scale>
<ROWScale SCode="kPa" SDesc="Kilopascals" Factor="0.369" Offset="28.06" Invert="FALSE" Reverse="FALSE" Units="KPA" Signed="FALSE" Hex="FALSE" AlarmMin="101" AlarmMax="104" DisplayMin="10" DisplayMax="104" DescMin="" DescOk="" DescMax=""/>
</Scale>
−
<Table>
<ROWTable RawByte="1" Value="10.7"/>
<ROWTable RawByte="2" Value="11.1"/>
<ROWTable RawByte="3" Value="11.5"/>
<ROWTable RawByte="4" Value="11.8"/>
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</Table>


------------------
Dave E

www.ltlfrari.com

[This message has been edited by ltlfrari (edited 01-02-2007).]

Hello Dave... Wouldn't the formula for the MAP be 10.7+0.369*value? You had 28.+0.369*value in your ald file. BTW, I made a change in the macro that eliminates the discontinuities in the data. I'll send you a copy as soon as I can find my data stick.

OK, so that change gets the range of the MAP correct. Now what I see is that the manifold pressure goes UP when I'm accelerating hard, and down when I take my foot off the gas. At idle it's at 60 kPa and when I'm running a constant 60 mph / 2150 RPM it's way up at 87 kPa. Shouldn't it be the other way around? Could the wires be reversed? HELP!

eric, I took the calc from the xml file, it's the factor and offset stuff. I assumed it was correct, never really thought about it though. Maybe the table overrides the calc on whatever it was I got the file from.
You'd think it should be the way you describe but it's not. When the throttle is closed, there's no way for air to get in to the plenum so there's a BIG vacuum in there = low map reading. When you open the throttle butterfly, the air can get in so the vacuum reduces, pressure increases towards atmospheric and you get a higher map reading ! So based on that it sounds like things are ok wiring wise at least.

If you want an unexpected vacum leak, check around the throttle arm. The black plastic piece that the throttle cable hooks to, mine was leaking like a SOB. The only way I found it was blocking the TB and pressurizing the engine, then dumping buckets of water on it.

I hope that my graph was posted... it shows

(top) speed, MAP and RPM (RPMs are in hundreds)
(middle) BLM and Int
(bottom) O2 sensor voltage and O2 counts.

The section that begins about 1/3 of the way across the graph was recorded while coasting down a hill in neutral. The BLM cell was always 0, 1 or 2. Aren't there more than 3 cells in the table? I was expecting to see some of the other cells when the MAP was low. There was no WOT in this test.

One other thing that seems odd... I thought that a value of Int > 128 would cause the BLM to be adjusted up. Have I got this backwards?

Comments?

[This message has been edited by eajohnso (edited 01-03-2007).]