Idle Surge help please
12-29-2011, 12:55 PM
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Idle Surge help please Can Anyone figure out why this thing is surging like it is. The base idle was nice and smooth. then I tried to tune the VE and ever since I swapped stuff over to do so it gets up to temp and surges till it stalls.
12-29-2011, 07:51 PM
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You have probably richened up the mixture by VE changes. Rich idle mix
makes the motor "soggy" and the RPM control loop goes unstable.
Now I have seen both basic value, and MAP-axis slope cause instability.
With a big cam, you have to expect that what the wideband says and
what the real inlet mixture is, are going to diverge. You may not want to
idle at 14.7 indicated. This is the irst thing to chase - what idle commanded
AFR (bidirectional controls, in the scan tool) gives you the highest idle RPM
and lowest idle MAP, with timing and IAC locked? Get that wideband AFR
and your average narrowband voltage written down, true up VE to get
the right open loop wideband number again and set the O2 switchpoints to
the recorded mV so closed loop seeks the right answer instead of something
that will put if fat (not forgetting that the headers are liable to turn that
sour in could weather, mistrimming you rich additionally).
makes the motor "soggy" and the RPM control loop goes unstable.
Now I have seen both basic value, and MAP-axis slope cause instability.
With a big cam, you have to expect that what the wideband says and
what the real inlet mixture is, are going to diverge. You may not want to
idle at 14.7 indicated. This is the irst thing to chase - what idle commanded
AFR (bidirectional controls, in the scan tool) gives you the highest idle RPM
and lowest idle MAP, with timing and IAC locked? Get that wideband AFR
and your average narrowband voltage written down, true up VE to get
the right open loop wideband number again and set the O2 switchpoints to
the recorded mV so closed loop seeks the right answer instead of something
that will put if fat (not forgetting that the headers are liable to turn that
sour in could weather, mistrimming you rich additionally).
12-29-2011, 08:24 PM
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Immmm you lost me. lol
Followed you about the running rich part, what does not make sense though is why did it idle good at base then then I swapped over to use the VE it started then. Only when hot and I have already did the multiply % of the VE 3 times. Could it still be that rich?
Or were you saying something else had to be changed to lean out the VE?
Followed you about the running rich part, what does not make sense though is why did it idle good at base then then I swapped over to use the VE it started then. Only when hot and I have already did the multiply % of the VE 3 times. Could it still be that rich?
Or were you saying something else had to be changed to lean out the VE?
12-30-2011, 06:26 PM
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Speed density mode loses you the stability that the slower MAF gives.
When I put mine back, idle cleaned up a lot. The behavior was a MAP
surge driving excess fueling, a fuel-loop oscillation if you will. MAP is too
jumpy and if the fuel reacts to every burp then you can set up a yo-yo
situation.
I've wondered about relocating the MAP to the far end of a couple of feet
of vacuum hose, thinking this could "filter" the action.
When I put mine back, idle cleaned up a lot. The behavior was a MAP
surge driving excess fueling, a fuel-loop oscillation if you will. MAP is too
jumpy and if the fuel reacts to every burp then you can set up a yo-yo
situation.
I've wondered about relocating the MAP to the far end of a couple of feet
of vacuum hose, thinking this could "filter" the action.
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