Engine surging at idle

[webb1624]

Engine cranks and runs but is surging at idle. I have tried several fuel maps and ignition maps but I can't seem to get the "hunt" out of the idle. Engine will rev great until I try to get the afr down then it stumbles a little. The afr is showing rich but if I try to lower the fuel it will not stay running. When idling it will surge from 900 rpms - 1100 rpms. If anyone has a similar combo as mine I would appreciate a tune or advice that might help me get rid of this idle issue.

1998 Ford Explorer block 347 ci
AFR 185 heads
Holley Systemax intake
Ed Curtis cam
Mac 1 3/4 shorty headers
75 mm TB
42 lb/hr Bosch injectors
Ford Explorer ign coils
MS3 w/ MS3X expansion V3.57
LC 2 wideband (2)

[slow_hemi6]

Is that exactly the msq that matches the log?

[webb1624]

Yes, that should be the msq that I was using when I made the log

[jsmcortina]

I helped tune a similar engine, the "trick" was to flatten out and reduce the timing and work on the VE table so that the fuel pulsewidth isn't bouncing around too much. So in the idle region you want the VEs to reduce as you go up the table.

Why? Well, the fuel equation is PW = DT + (ReqFuel * MAP * VE * ...) as the MAP increases (the dot moves up the table) more fuel is commanded, by reducing the VE you can undo this effect in that area.

James

[Rick Finsta]

I *just* got it. Now I understand a post I read over on the Holley EFI forums (cross-posted YB.com) about a diagonal pattern to the lbs/hr (or in our case VE) fueling numbers to stabilize idle.

So as you go up and right, because the VE is not an absolute injection value (but part of a calculation) if you have the same numbers for VE in all your idle areas you will get wandering PW and surge. If you reduce them in a diagonal manner then you will get stable PW over a few hundred RPM in the idle and off-idle areas.

For instance:

20 | 19 | 18 | 17
21 | 20 | 19 | 18
22 | 21 | 20 | 19
23 | 22 | 21 | 20

Is this what you're saying, James?

[webb1624]

Making some progress on my project. I took the advice of jsmcortina and modified my fuel map and it made a good bit of difference. I now have the car idling decent. Around 800 to 950 rpms. Idle still hunts a little, but much better than it has ever been. The car revs good but when coming back down to an idle it wants to die. Sometimes it cuts off and sometimes it just struggles really hard but stays running. What is my next step to resolve this issue ??

Thanks

[Matt Cramer]

Can you post a fresh data log of how it's doing with the updated tune? That'll show everyone what the tune is doing in the real world.

[webb1624]

Matt, I have a new log but its telling me the log is to big. How can I shorten it so I can upload it ??

[Matt Cramer]

Put it in a ZIP file. They compress pretty well.

[webb1624]

Here is the log with my lastest tune

Thanks

[Matt Cramer]

Try changing the first two columns of your spark advance table to give you 18 degrees timing at 500 RPM and 14 degrees at 800 RPM.