strange ecm action - LS1LT1 Forum : LT1, LS1, Camaro, Firebird, Trans Am, Engine Tech Forums
 
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post #1 of 3 (permalink) Old 05-18-2014, 12:00 PM Thread Starter
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strange ecm action

I had just cleaned the fuel injectors on my 90 camaro 3.1 v6. Doing a running data scan to confirm if cleaning injectors fixed lower gas mileage problem.

Car was idling fine at around 800 rpm and would suddenly jump up to over 3000 rpm. Data stream would go blank during 3000 rpm run time.

After anywhere from 10 to 30 seconds of 3000 rpm run, the engine would suddenly drop back to 800 rpm and my scanner would reboot and again show data stream.

Freaked me right out that engine could get that high of rpm without opening the tb butterfly at all. I concluded my ecm had a stroke as it would do the 3000 rpm thing on a fairly regular basis.

Changed ecm to a spare I had on hand, used original memcal from old ecm. Everything is doing fine.

And for the first time in a long time my o2 is switching back and forth instead of staying in one range. And my rich- lean monitor is also switching rapidly. It was staying rich all the time.
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post #2 of 3 (permalink) Old 05-18-2014, 12:55 PM
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sounds like when that happened it was going into limp mode.

the idea behind limp mode is to force the engine to run in the event of a major failure. it runs a very primitive calibration that has very little input or output. memcal calibration is disabled entirely, iac is opened wide, fueling is defaulted to a primitive map, timing is fixed fairly low, datastream stops working, etc.

3000rpm in limp mode can happen (usually not that high though?); since all that extra fuel compliments the extra air from the iac.

what i dont understand is why it would come out of limp mode. it's not supposed to do that.

definitely wouldnt be using that ecm again...
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post #3 of 3 (permalink) Old 05-18-2014, 03:29 PM Thread Starter
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I've read these older GM ecm's main problem is soldering coming loose over time. So I will check it out at my leisure. In the mean time I have 2 more good ecm's just in case.
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