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Rough idle? with minor knock

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Ok so here's the story, changed the thermostat back to stock because I just did a motor swap and the new motor didnt have a thermostat only one I had was a 160*. Checked the plugs and changed the oil. Then the alternator wasnt charging, and for some reason now it is, frickn weird. But now theres a rough idle...and yesterday when I was trying to test the alternator it didnt have this rough idle. So double checked the plugs again, checked the 02 sensor, was kinda black but figure thats from the 160* tstat running the motor rich. don't get any check engine lights or anything, so I figure its a vacuum leak, but all the vacuum lines I check are fine. What am I missing? Any suggestions? Thanks


This is a 97' 4.0L btw.

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  • 1 month later...

First replace the thermostat as previously recommended. Their are so many vacuum lines and gadgets that it is very easy to miss a bad one. First remove the vacuum lines, one at a time, from the manifold. See what the motor does. That should give you a clue as to which lines to look at closer. Block the opening on the manifold. With the importance vacuum plays in the system now a vacuum tester is nice to have. One with a hand pump to evacuate the line and a gauge.

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