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Old 02-19-2010, 12:33 PM   #1
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Default Dead spot on datalog

I have a 94 Miata that has a Megasquirt PNP. The other night while driving around doing a datalog I had a miss, or a dead spot that hit right before a spirited acceleration run. Datalog attached the spot happens at 2080.606s. Its a new engine, and I'm trying to rough in a map before I go to the dyno. I did a search on spikes but mine is dropping and not spiking. Any clues, please review the attached datalog?
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File Type: xls datalog201002162158.xls (719.3 KB, 55 views)
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Old 02-19-2010, 12:38 PM   #2
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did you feel it or just notice it on the log?
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Old 02-19-2010, 12:46 PM   #3
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Felt it. The car bucked. Felt like a dead miss. Grounds are all clean and tight. Connectors have been checked. This is new to the car. I've run MS on the car all last season with no problems.
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Old 02-19-2010, 01:56 PM   #4
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How very interesting.

My first thought when I saw this was "Oh, looks like a standard CAS glitch", but then I noticed something rather curious. When it happens, RPM, TPS, IAT and CLT all drop to minimum, however MAP remains unchanged. The reason I find this interesting is that the MAP sensor is internal to the MS, whereas all these other sensors are external to it.

So I'm trying to figure out what all of these things have in common.

TPS, CLT, and IAT all share a common ground. Specifically, a black/blue wire that goes into the ECU at position 2D, loops through the ECU, and then goes out to actual ground. The CAS has its own ground, although it looks like this might be paired into the ground return for the aforementioned. Specifically, it's a black wire with a light green stripe that hits pin 2F of the ECU, as well as the MAF sensor and the CAS, and goes to ground at a point which I believe to be somewhere on the intake manifold.

Of course, if the TPS ground were lost, I'd expect it to spike to maximum rather than minimum...

A fault internal to the MS is also a possibility. The schematics for the PnP board are not published, so I can't speculate too much here. On a standard R3.57 board, a fault of the Vref circuit would also cause a dropout in the MAP reading, but it's possible that they're doing something different on the PnP. Without knowing where they take the pullup for the CAS inputs from, I can't say whether a fault of Vref would cause a dropout there. If it turns out that Vref is a likely candidate, then it could also be an external fault, such as the green/white wire of the TPS, EGR, or TCM shorting to ground.
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