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Question about fuel trim

posted Apr 07, 2016 03:40:30 by JosefMiller
If I had an exhaust leak causing a lean condition. Would my trim data increase with rpm. In other words my fuel trim go positive more?

Or would it get better similar to vacuum leaks.

I was thinking since the exhaust pulses are sucking the air in that more of them with higher rpm would increase trim.
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6 replies
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Noah said Apr 08, 2016 01:54:34
That's an interesting question. It would have to be a leak before the sensor to matter of course. I can't decide if at higher rpms the exhaust pressure would overcome the air's ability to enter at the leak, or if under higher rpms, the exhaust moving over the leak would create a kind of venturi effect and draw in more air.

Massachusetts, USA
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JosefMiller said Apr 08, 2016 21:48:16
I was curious because I learned that an exhaust leak can cause the o2 slow response code and wanted a way to go in the direction of an exhaust leak from scan data.
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Dylan said Apr 10, 2016 12:12:57
I was thinking since the exhaust pulses are sucking the air in that more of them with higher rpm would increase trim.


Nope. The outside air is drawn in after an exhaust pulse when the pressure is lower than the outside air. At higher engine speeds the pulses are close together.
When the engine speed increases the amount of time this can occur is reduced so the exhaust leak is most often seen at idle and low engine speeds.


Belgium, Europe
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Noah said Apr 10, 2016 13:19:24
Makes sense, like a vacuum leak.
Massachusetts, USA
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Tyler said Apr 10, 2016 15:16:18
Good question, never really thought about it that way before.

Trying to think of a good way to experiment with this, besides drilling holes in my exhaust manifold...
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JosefMiller said Apr 13, 2016 12:34:52
Thanks for the replies!
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