I have a pro prominator,7747 ecm, holley 670 tbi, holley fuel pump, projection 2" manifold, camel hump heads and the old l-79 327 cam (.447 lift, 272 dur). I got it to run pretty damn free and sounds awesome, blms are nearly all 128 (but I have yet to get a full datalog because it won't idle). It runs great in park, it will idle for the first couple minutes after it starts up then it just dies from too much fuel... and it tries to run in gear at idle but it can't quite manage it and wants to die. I was reading up and found that a cam with more overlap will cause the computer to read leaner when it actually is richer from more oxygen due to more overlap. To fix this you need to lower the median voltage for the o2 sensor to offset the larger overlap but how do I do that in my $42 code 7747? Or is there other ways to fix my idle problem? I can watch it jump from .332 to .640 Mv. right before it stalls. under throttle it seems to do okay. I asked a local speedshop guy and he says that cam is not big enough to affect the idle but honestly he knows jack about fuel injection. He automatically wanted to get my a new iac and egr valve and I just shook my head in frustration... nobody knows anything about this stuff locally.


Little help from the tbi gurus please.

 

consider idleing in open loop with no feedback from 02 sensor.

 

What's the cam's duration at .050 and what's the LSA? What timing are you running at idle, it's going to want more timing than a stock cammed motor, also, where is the IAC at when idling, ie. how many steps?

 

Here are a few suggestions that help get mine to idle initially, once you get it to successfully idle at a stable rpm in closed loop then you can tune from there.

- be sure your IAC is wired correctly. If you do a search you will find info on how to test the IAC operation.

- make sure your MAP sensor is working correctly. You motor should idle at about 45-50 kpa

- make sure your TPS sensor is operating correctly.

- Your cam will probably want quite a bit of timing at idle. I would set the initial idle at least at 10 degrees and reflect this in the prom. (be sure to disconnect the bypass wire when setting timing)

- you want a large flat area in your timing tables set to about 25 degrees. Set all the values from 400-1200 rpm, 35 to 60 kpa to about 25 degrees.

- do the same with the VE tables. A large flat area around idle will help to stabilize idle.

- set the target idle rpm to about 800 rpm

- set the stall saver to 600 rpm and exit stall saver to 825 rpm

- set the idle adjustment on your TBI initially so that a strip of paper just freely slides thru along the throttle blades.

Hope this helps. good luck.

 

Actually that cam will idle about where my L82 cam idled in my 10.5:1 305. That is around 750 RPM and 60 KPA MAP.

You are going to want about 25* of timing up to about 1,000 rpm from 20-60 KPA MAP. It is OK to set the timing to 25* as low as 400 RPM, your cam will want it.

Good tip on the stall saver. It does no good to command idle at 750 rpm and have the stall saver trip at 480 RPM, by then it is too late with this cam.

I have already gone over setting the initial timing, I said 12* then set it at 12* in the .BIN.

I also went over setting the IAC to maintain idle speed out of gear. You might also bump up the steps from Park-Drive as well. IIRC, my 305 wanted about 15 steps added to maintain idle speed when the transmission was shifted from Park/Neutral into Reverse/Drive. Stock it is set at like 1-3 counts, this might induce a stall when you go to shift into gear.

You are only going to pull about 13-14 in/hg of vacuum @ 750 rpm unloaded with that cam which is 55-60 KPA. The second you drop the shifter into Drive from Park, the vacuum is going to PLUMMET down to 9-11 in/hg (65-70KPA) and the RPM should then be around 750-850 RPM. I recomeend another 50-75 RPM added to this, should you plan to run A/C when idling. This will drop the vacuum down to about 7-8 in/hg (70-75 KPA) idling in gear at 900 RPM or so with a nice sound to it. If you run A/C with an automatic transmission, you MUST idle at about 900 RPM in Drive with the A/C on or face stalling and very rough running when it is on. Especially the second you go to turn the steering wheel and load the engine with the power steering pump.

Almost all of the .BIN calibrations from my 305 are on my Old laptop which recently crashed (haven't had time to try to revive it) or I would post some timing tables/ VE tables up.

Also like stated above, disable closed loop idle as the constant AFR swing can cause you a list of problems, especially around idle.

http://www.geocities.com/ecmguy.geo/bruce/prog_101.html



Here is proof it can be done, 10:1 305 with 224/224 @ .050, .450/.461 (.480/.490 w/ my 1.6:1 rockers), 114* LSA installed on a 108* ICL. It idled at 65 KPA in Drive @ 700 RPM

EDIT- Also look at the attached screenshot. I would try to reduce the highlighted cells by 5 points, then burn again, no good still, reduce by 5 points, try again, at some point it will become lean enough to idle. Then you can work on the VE around that. Make sure that you are getting 25* of timing at idle in these cells, then try the slowly reducing the fuel. It will idle. RBob wrote a good paper about the spark control in a 7747.

http://www.diy-efi.org/gmecm/papers/747spark.txt