Got the GM 3bar installed. It started up but idled really rough, gave it some gas and HELLO EXTRA FUEL. I hooked up the datalogit and I was way out of wack. So I ordered a brand new 3 bar (the other one was used).

Installed the new one and same thign jsut not quite as bad. I'm idling in the P7 row! The MAP responds to pressure changes when i open the throttle and such, it's liek it's baseline needs adjusted, but I can't go any less than the "0" offset recommended for a GM 3 bar.

Scale 41800
Offset 0



Any help would be greatly appreciated.

 

Try adjusting the scale until the correct P row is used for idle, or change the Setting 3 Map Reference Boost values.

I also suggest you calibrate the new sensor in order to kown what PIM ABS values corresponds to what PSI boost level. In fact you could calibrate both and can have a direct conversion so that all you have to do, is change the Setting 3 Map Reference Boost values to be equivalent.

Al you need is a DL and a regulated vacuum/pressure souce. Simpler to do than most think.

 

any more detailed info on doing this? or a link to a good thread?

The thing is though, that SOMETHING is wrong and it's not the MAP sensor or the values I entered.

 

I'm getting .4v @ idle which is about -15mmHg

 

anyone have a chart or soemthing that shows me what .4v corresponds to for a GM 3 bar?

 

here are a couple of logs for the GM 3bar

 

Think: hand vacuum pump, 0-30psi mini pressure regulator, and shop air compressor.

 

what does the scale number mean to the PFC? Like what would the graph of voltage to pressure look like for 41800? Then i can figure out if the sensor is giving the wrong voltage or if it's something with my settings.

 

I am using 44000 with an offset of 357 for my GM 3 bar. If you look at my log you can see what the pimV are for various vacuum readings. I don't know what your vac readings are at idle, they can vary depending on a number of things, porting, AFRs, rpms....

I do have a formula which allows you to calculate values but it is not readily available. From my log at 18'' of vacuum the PIMV is around 0.7V

You can always just do what Chuck is suggesting, in fact that is how the boost values were obtained in the log I supplied.

 

what does the scale number mean though, like if I adjust the scale to a lower number so that it matches up with the correct row for idel will it mess up "off idle" ? Or is it linear? It seems the offset value is used for those adjustments (which doesn't help me because I need negative numbers).

I'll go through whatever process i need to to set it accurately but should it really be off this far? I mean most guys throw on a 3 bar, plug in 41800/0 and there car drives fine, mine would not. This leads me to think it's something other than my settings.

 

Without the formula handy (meaning..I'm speculating)..vac/boost vs voltage is linear

basically what the scale is accomplishing is shifting the y-axis (vac/boost) to correspond with the voltage from a 3 bar sensor vs a 2 bar

in other words you have 3 Bars of values for 5 Volts vs 2 Bars for 5 volts

and finally depending what is the timing and fuel from one row and column to the next your idle can be seriously affected using the values you have entered
Without the formula handy ( meaning..I'm speculating)..vac/boost vs voltage is linear

basically what the scale is accomplishing is shifting the y-axis (vac/boost) to correspond with the voltage from a 3 bar sensor vs a 2 bar

in other words you have 3 Bars of values for 5 Volts vs 2 Bars for 5 volts

and finally depending what is the timing and fuel from one row and column to the next your idle can be seriously affected using the values you have entered

 

Here is the formula the PFC uses to convert the sensor voltage to a P- row value. As you can tell it is linear.

P-Row Value = (Sensor Voltage x Scale x 9828.8 / 65536) - Offset

On a side note; have you tried to identify what cell you are idling in, using the map trace function of the monitor?

 

OK, I have a hand vacuum pump with gauge and a mini air regulator. The best gauge I could find was a 0-60psi one with tick marks every 2 psi, but it's a diaphragm design so it's steady at all outputs.

What's my next step?

I figured feed the MAP with everything from -X mmHg to +30psi and write down the corresponding voltage. From this (if it's linear) I'll be able to calculate the equation of the line.

y= mx + b [p row value] = (9828.8/65536)(scale)(sensor voltage) - offset

which number does the slope of the line represent?