MAP signal noise

[thedrew]

Hey Folks,

2276cc, MS3x... CB 40mm throttle bodies. Everything is grounded at one spot, so I question weather is electrical noise.

I'm seeing my MAP signal oscillate considerably.....is this normal or do I have something wrong?

The MAP signal bouncing up and down causes my VE to go up and down....(Red & Yellow lines)

I have a crossover tube between both sides and it goes to an external GM MAP sensor.

Thanks!

graph3.png

[Piledriver]

First, determine if its real or actual "noise".
Start it up, let idle stabilize and then pinch off the MAP hose.
(unless the hose or sensor leaks it will keep idling)

If the small oscillation goes away, you need to add a damper of some sort, I sample off a 1L catch can off the plenum that is also part of my PCV system. A lot of folks use a $1 plastic fuel filter.

The MAP lag can of course be reduced (lower number=more averaging) if needed, and 99% of the time you will want to use the "event average" MAP option.

If pinching off has no effect, then its electrical noise.

Curious---where is your "single point" ground?

Make absolutely certain all the sensor grounds are at the ECU mainboard, not the common ground the ign/inj etc drivers all go through.

[thedrew]

That my be my problem, my coils all ground at the same spot my ECU and sensors do.

[Piledriver]

That my be my problem, my coils all ground at the same spot my ECU and sensors do.

Ah.




Still,pinch off the hose and see.
Sensor grounds go to mainboard.
Greatly reduces the potential weird, and IIRC is the setup used in the manuals.

I disagree with the manual as to using the engine as the common ground due to alternator/ignition noise , but the sensor grounds go to the mainboard. In a front engine/front mounted battery setup it makes sense, not usually for ACVWs.

If you have an external ignition driver, use the engine to ground the power side of those.
(LS2 coils have a separate logic and power ground, the power side goes to the block, logic ground goes to mainboard or perhaps the common, not as sensitive as an analog sensor)

[4agedub]

If the engine is non turbo, would it not be better to run TPS as the primary input and map only for altitude compensation?

[jhoefer]

That to me looks like a combination of MAP variation from only sampling a single runner and a sampling rate in the ECU that's too low. The pattern is too regular for high frequency electrical noise (though there's still the possibility of a low frequency bias). And the semi-regular variance between single spikes and double spikes looks like a sampling rate issue to me, that is, it's not sampling fast enough to follow the actual waveform. MAP variance from a single runner at 2400 RPM should show spikes 20 times per second, yours seems to bounce between 20 or 10 depending on where the sample aligns.


If pinching off the vacuum hose does prove it's real MAP variance, you'll need to add an accumulator to smooth out the vacuum pulses. As mentioned, there's various ways to make one, e.g.: http://www.msextra.com/forums/viewtopic ... 66#p333566

[Piledriver]

If the engine is non turbo, would it not be better to run TPS as the primary input and map only for altitude compensation?

Speed-density is far easier to tune, allowing sufficient vacuum signal.
If the ITBs are properly sized you can usually just use SD with some mechanical filtering.
(allowing you are getting a signal from all holes)
There are various sampling modes to cover most needs on ms2/ms3, but the (edit) "event average" mode works best in most situations.

Many systems do support a SD+Alpha-N blend mode for wild setups.

[thedrew]

I'm running the ITB mode, I have a balance tube that runs between both throttle bodies and then a T to my MAP sensor and FPR. I got another external MAP sensor I might hook up so I can watch both sides independently, I'm just not sure if I want to bother wiring it yet.

I might grab a inline fuel filter and use that to try and smooth out the pulses.

I do agree, its too regular to seem like eletrical noise, and its consistent regardless of the ignition pulses....

[jhoefer]

If sampling from the balance tube, the variance could be a result of the firing order and dual carbs. The two cylinders on one side (same carb) pull vacuum in sequence before moving to the other side of the engine and the airflow in the tube changes direction. An accumulator is still the solution there.