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reverse vacuum load phenomenon
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Post reverse vacuum load phenomenon 
what could be causing the load to read in reverse i.e. ~100kPA at idle and at acceleration & wot MJLJ reads at ~30 to 20 kPA?

is map sensor bad? a circuit component soldered in reverse?

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Try measuring the vacuum manually with the engine off using a squeeze bulb ( or your mouth. Smile ) and verify the MAP readings move in the direction they should.

If it behaves and reads correctly, then try running the engine with the MAP disconnected so it always reads atmosphere.

If you get the expected behavior then it would seem something mechanical on the engine is causing the problem. At this point you could compare it side-by-side with a standard vacuum gauge attached to the same reference point as the MAP sensor.


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I'll give those things a shot brent.

so, there's no chance that an incorrectly installed circuit component would cause that?

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No hardware component failure would cause a reverse reading, correct.


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ok, here's what is occurring:

-disconnect vacuum from MJ and load stays on 100 through-out rpm
-connected an infant nose suction thing-a-ma-bob onto the map sensor of the MJ and when pulling vacuum, load guage reads from 100KPa to 90>70...etc.
-when blowing air into the map sensor with the same device it climbs from 0 to 20>30...etc

-load reads at idle from the 100 load cell (bottom left hand corner) and goes upwards through the decreasing load cells.
-what i decided to do then is reverse the timing entries so instead of retard timing at the 80-100 load sections i entered those in upper cells.

could it be attributed to a firmware upgrade? was this ever observed by anyone else here?

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dr.occa wrote:
ok, here's what is occurring:

-connected an infant nose suction thing-a-ma-bob onto the map sensor of the MJ and when pulling vacuum, load guage reads from 100KPa to 90>70...etc.
-when blowing air into the map sensor with the same device it climbs from 0 to 20>30...etc



Line 1 makes sense, but I'm not sure about your line #2 above. Since you're trying to measure pressure, ensure the the configurator option is set to 'forced induction' so the gauge goes to 255 KPa, otherwise the gauge will keep spinning around back to zero. Smile

On your remaining points, do you have the map sensor disconnected with the engine running?

On the map, the load goes from 0 to max KPa, top to bottom on the load axis. Typically ignition advance decreases as load increases, but this is more pronounced on forced induction engines due to the increased dynamic range.

As a test, try reverting back to the factory default ignition map and run with that. It does work for an NA engine, you simply lose resolution on your load axis since you're not running forced induction.


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sorry for not being clear brent.

well, the application is N/A and when the car idles it reads at 100 KPa and on acceleration the red box goes up the tables i.e. 100KPa idle>accelerate>90...80...70KPa

shouldn't the red box be at around 10 or 20 KPa at idle and upon acceleration descend down the table to the higher load numbers e.g. 10KPa @idle>accelerate>20...40...100KPa?

correct me if i'm completely wrong.

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Sounds like the MAP sensor is reading vacuum correctly when you test it with the syringe.
Is your vacuum take-off fully *downstream* of the throttle butterflies? i.e. inbetween the butterfly and the cylinder head. A reading of 100kPa at idle can be due to the takeoff being upstream of the butterfly so it is seeing atmospheric pressure.

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I also would add that it sounds an awful lot like an exhaust restriction.

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So it seems you can produce the correct response in the MJLJ when manipulating the vacuum manually. Your problem may be caused by the vacuum take-off port in your carbs.

Are you able to fix a standard vacuum gauge to that port and monitor it manually?


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brentp wrote:
So it seems you can produce the correct response in the MJLJ when manipulating the vacuum manually. Your problem may be caused by the vacuum take-off port in your carbs.

Are you able to fix a standard vacuum gauge to that port and monitor it manually?


Wow, yeah is it hooked to ported vacuum?

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all the vacuum ports are tapped into the intake manifold per each runner between the mikunis and the head. just like this:


you know what, i'm an idiot...

finally just learned the conversion tables: 100KPa = ~30 inches of mercury
and also did some homework: upon acceleration manifold vacuum DECREASES

dunce cap for me Embarassed

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Right, the map sensor is measuring absolute pressure-. More vacuum- less 'pressure'. As you open the throttle, the vacuum reduces- WOT and you are near 100KPa, atmospheric pressure.

Don't worry- I have more of those moments than I care to admit. Smile Smile


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Yeh, the other one I had recently is the MJLJ is reading 72kPa and my vacuum gauge is reading 30kPa, so the MJLJ is completely inacccurate and unusable.

But...
MJLJ is reading absolute pressure of 72kPa
Vacuum gauge (clue is partly in the name) is measuring 30kPa of vacuum from atmospheric ,and 30kPa vacuum from atmospheric is 102-30=72kPa

...so they are both the same Very Happy

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so it is okay that my MJLJ reads 100KPa at idle and on acceleration begins to drop to 80 - 70KPa right?

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