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496 mag strange intermittent problem

Old 06-23-2022, 02:56 PM
  #11  
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Originally Posted by bobz71 View Post
so i switched computer from working good motor to running bad motor, did the same problem to the original running bad motor thus i believe ruling out a computer issue. also test pressure at rail turn key on 43 psi, then slowly drops then start motor stays steady at 36ish psi both motors do exactly the same numbers.
Just a heads up, it's not recommended to put a good ecu on a bad engine. If something damaged the ecu, you will have 2 bad ecu's than. Always put the bad ecu on the good running engine.

Just my 02
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Old 06-23-2022, 03:47 PM
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Just a couple of stray thoughts from working on cars years ago:
If the coolant temp sensor comes unplugged (or wire breaks or bad connection, etc) the ECM will interpret that as coolant being at -55 degs F and enriches the mixture significantly.
Bad connections for MAP sensor (or a bad sensor) can cause a similar issue. ECM thinks eng is under a serious load. (A split vac hose for MAP sensor can cause the same issue but wouldn't go away after a power cycle)

Anyway, something to look at while awaiting the code reader results.


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Old 06-24-2022, 09:10 PM
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thanks for thatinfo i agree, but i think switching them and the problem occurring again ruled out a computer causing it.
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Old 06-24-2022, 09:14 PM
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so had the motor scanned, no codes at all, also checked fuel pressure 43 at startup, after running stayed at 38ish, both motors exact same readings, still happening
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Old 07-27-2022, 04:24 PM
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thanks to everyone and their suggestions i might have figured it out , i replaced plugs and wires, plugs looked good but did them any way, i think wires were original , i think at high rpm the plug wires were arcing somewhere and messing with the computer, seems to be fixed but more test runs with no issues and ill be convinced. thank you all again. bob
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Old 07-27-2022, 04:27 PM
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.060” is too big a gap. Plays havoc with everything.
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Old 07-27-2022, 07:19 PM
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Originally Posted by SB View Post
.060” is too big a gap. Plays havoc with everything.

What gap is best?
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Old 07-27-2022, 09:08 PM
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Normal plug gaps. .035”-.040”
.060 was derived from gm for emissions in trucks. Too much for engines working under lots of load and higher rpm (boats) ,all the time.

I believe (double check) the factory plug has .709” reach. If that’s the case, as example, the ngk tr-5 and tr6 has .039” gap. It is conventional with vpower electrode. Who cares about platinum or iridium plugs in boats, especially with such high gaps they need changing as often, or more often, than conventional plugs anyway.

These smaller gaps will make your wires and coils last exponentially longer too. :thumbs

Last edited by SB; 07-27-2022 at 09:11 PM.
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