M4 Error Manifold Pressure

Discussion and support for MoTeC's earlier generation ECUs

M4 Error Manifold Pressure

Postby senordos on Mon Aug 09, 2021 10:54 pm


Can you advise what the error "Error Manifold Pressure" is on a Motec M4? I'm assuming it's set if there is an issue with the sensor.

See attached file... we are getting pops and bangs, similar to when you hit boost cut, so I assume the Motec assuming we've hit the limit.

Things which back my theory:
- you can see the MAP jumps to 300 kpa quite often, and this is when we feel the popping.
- it jumps there, no build up, so looks like an error MAP reading thrown by a sensor issue

Things which are odd:
- sometimes it shows the error when there is a perfectly acceptable MAP reading
- it doesn't throw a "Error Over Boost" (not shown in attachment) but maybe this isn't logged?

Note that this car is mapped to 2 bar (300 kpa) so the boost figures when not maxed out look normal.


august_pop_and_bang_log.png (74.23 KiB) Viewed 2212 times
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Re: M4 Error Manifold Pressure

Postby David Ferguson on Tue Aug 10, 2021 12:14 am

Did you notice the error happens under vacuum when the MAP goes to zero. I suspect that the voltage goes below whatever minimum voltage is defined for that sensor calibration -- I see that the M4/M8 don't allow this to be set like we can with the hundred series ECUs. I suspect the reason you see the 300 kPa spike is because that is the default value you have set that is used when there is a sensor error. Have you tried changing the sensor?

It might be worth trying the 310kPa calibration so see if that works any better for you.
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Re: M4 Error Manifold Pressure

Postby SDean on Tue Aug 10, 2021 8:58 am

I agree with David on this one, it looks like the engine is going below the minimum value for the calibration and the M4 is reading this as a failed sensor, this causes the M4 to use the Sensor Default value that has been set un Sensor defaults as the MAP value to be used.
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Re: M4 Error Manifold Pressure

Postby senordos on Tue Aug 10, 2021 8:23 pm

Hi David / SDean,

Thanks very much for response. Your thoughts make sense, and so this would mean that "Error Manifold Pressure" flag is for a perceived sensor issue.

I don't see a setting for "minimal voltage" or anything in the M4, but I did check the calibration table for the sensor and the lowest value was for 40kpa for some reason, so I've added some cells lower (right down to 0 volts), so let's see if that fixes it.

I've also changed that the error value (previously 300kpa) to 296kpa, as I assume that given the overboost limit is 300kpa, when the sensor fails it was triggering the fuel cut!

Thanks again for your help,

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Re: M4 Error Manifold Pressure

Postby SprinterTRD on Wed Aug 11, 2021 2:39 pm

You cant set the trigger levels for errors in the M4.

The simplest way we used to to fix the issue is to put a 1mm or smaller restricter in the line to the MAP sensor.
From memory this problem mainly happened on the Delco sensors, one can only guess how good the knockoffs are these days.

The issue was particularly bad using the 1 bar sensors on large displacement engines using standard cams so the software was changed to ignore the 0KPA error if you used the 1 bar Delco sensor. The change was not implemented on the 2 & 3 bar sensors.
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