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Old 03-12-2015, 12:54 PM   #1
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Default MS3X Erratic MAP signal

New problem just cropped up that has me chasing in circles. This isn't a turbo issue, as the engine is a 1.6 MX5 engine with full race cams and high compression pistons. It is running a MS3X system.

Newly installed Sequential Spark and Fuel using a new 24/1 trigger wheel. Tested/tuned on dyno with very good results. Limited driving on road (its a race car) and it went like stink. Very impressed at this point.

First session on the track and it ran for a short time and then began breaking up throughout the rev range until about 6k. From 6ish to 8k redline it pulled cleanly. With stock gears, it was hard to keep it from falling below 6 during upshifts.

Back in the shop, it seems the MAP signal is showing 100 kpa at idle on the VE table (which has upper limit of 100 kpa set) - which makes for a very rich mixture. Checked MAP logs and it showed map values hitting 140 at times - and this is a naturally aspirated engine.

Was all set to replace the sensor but I decided to check the sensor itself, and it shows 1.7 volts on terminal 1 (the one with the notch), 0 on 2 and 4.87 volts on terminal 3 which as I understand it is normal for engine off, key on. 1.7 volts on the signal output is supposed to be the correct reading at normal atmospheric, 100 kpa pressure.

Anyone ever experience a similar issue? Ideas on what to check next?
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Old 03-12-2015, 11:54 PM   #2
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This is a 2.5 bar sensor I presume. With key on & engine off (or on a Stim), does applying positive or negative pressure on the MAP sensor change your MAP reading? Does it change your measured voltage?

A short datalog and copy of present tune would also be helpful.
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Old 03-13-2015, 10:41 AM   #3
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There was a variation in voltage (unsure of the values, was working on different project) when the MS was bench tested with Stim.

We can't ever seem to get the logs to upload to the forum. Can we send it by email?

Thanks for the help.
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Old 03-16-2015, 05:51 PM   #4
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I tried once more to attach data logs and a tune file, but the site gave an error message "Security Token Missing". Clicked on the "contact administrator" link and it went exactly nowhere. Whats up with that?

Took out ecu and disassembled case to get access to MAP sensor. Voltages good at sensor.

Reassembled and tested - MAP sensor responds to vacuum signal at MAP.

Started and seems to run normally. Not at all sure what happened because I don't know what caused it to start working. No ground wires were removed as they were cleaned and tested before we pulled the ecu out. Bad connection at the big plug on the back of the MS? That thing is heavy, and its the only wiring connection in the car that was undone when ecu was removed.
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Old 03-19-2015, 05:21 PM   #5
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Sounds like a faulty vacuum line. Plugging it back in probably fixed it- for now.
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