There might be an explanation for why the computer swap shifted the symptoms to another engine even though there is nothing wrong with either ECM. It could be a simple sensor issue and you should definitely consider that option before replacing an ECM. The MEFI 3 ECM has 'sensor replacement' logic that allows it to identify a failed sensor (ie: consistent readings outside of what would be considered normal) and replace the actual sensor data with a default value. The idea is that pretty much any sensor can fail and the engine can still limp home. Pretty nice feature really when you consider that a poorly running engine is a LOT better than a dead engine.
If your ECM determined a sensor was outside of normal, then it would continue to run in this 'sensor replacement' mode until a significant amount of time had passed where the sensed value was within the normal range OR the ECM was reset via the MDTC scan tool. It might be possible, that this sequence of events happened:
1. A sensor value fell outside of the normal range for some period of time. Probably the fuel pressure, coolant temp, intake air temp, or MAP sensor if I had to guess.
2. Your ECM determined that sensor had failed and replaced the reading with a default value that made the engine run rich. Remember a rich default is better than a lean default.
3. You swapped ECMs to your other engine without resetting it and it still thought that same sensor (on the different engine) was bad because not enough 'good' reading had been read in order for it to reset itself.
I don't know how long the ECM needs to see a 'good' reading before it starts using the real sensor data again. I also don't know if it is based on the number of engine starts or runtime. I believe it takes quite a while of normal running before it will reset that sensor channel and use the measured value again. I also don't know if disconnecting the ECM for a short period of time would reset it automatically. I don't think so. The nice thing about these ECM's is that the MDTC tool will tell you: 1. if this condition is active and 2. exactly what sensor is creating the problem.
My advice: I would definitely have someone scan the ECM and read out any DTCs stored in it prior to replacing it. You might just have a simple sensor issue that can be solved for less than $50 in parts. Get a code reader and rule this simple fix out before you throw big money at it. Here is one I Googled up for $59:
http://www.michiganmotorz.com/codemate-tester-mefi-code-reader-p-513.htmlGood Luck!
_________________
Jim
Richmond, Virginia
2005 Horizon 230 w/ Volvo 5.7GXI Duoprop
Towed by: 2007 Chevy Tahoe 4x4 LTZ
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