DIY PROMDo It Yourself PROM chip burning help. No PROM begging. No PROMs for sale. No commercial exchange. Not a referral service.
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It appears reducing my idle SA raised my idle speed ? 800 to 875 rpm.
I saw a change of MAP(VAC) from 40 to 37. A/F stayed the same per WB.
So I believe the SA change affected the VAC. I understand the head desigh affects the flame speed so more efficient heads and or spark(2 plugs/HEI) may dictate a change in SA for idle and elsewhere.
Your now getting more torque (and power) from lowering teh SA. Since the idle speed increased and MAP went down, you had too much timing before. This causes the engine to loose power from having to compress a prematurely burned charge. this means that the engine had to run under more load to maintain speed. Ideally, the mixture should be fully burned by the time teh piston has gone slightly past TDC. Earlier = work lost to compressing the burned charge. Later = energy lost to the charge developing peak pressure too late.
OL idle for me was adopted a couple years back to help with surging(lean?). I since reduced prop gains, OL idle, took IAC activity out of equation setting idle RPM under rpm set with TSS, and added VAFPR(PW is about 1.9 msec and rock steady). I am at 13.0/1 on WB.
I dont know how RBob has the EBL set up, but you might try adjusting some of the gains. Much of the engine surging at idle can be caused by timing issues or the engine too lean, but at 13:1, the engines power output is fairly flat around that area, so it shouldn't be an issue.
Make sure that your low RPM, high MAP SA is properly set. For exaple: If the timing is off in the low RPM/high MAP area the engine will go into that area and want to stall, the IAC will open to increase the engine speed. When the motor does recover, it hits the area of the SA table that is properly set, which causes the engine to rapidly speed up. This causes the IAC to rapidly close down, which causes the idle to drop down into the area of the SA table that is off, leading back to the start of the cycle.
The original idle control logic in the TBI ECMs was rudamentry and non-linear, so it could possibly lead to surging under certain conditions, assuming its still in use in the EBL. The later PCMs ('93 and beyond) use standard linear PID control logic with both the spark and IAC airflow being used to control the idle, so the action is better, and is less succeptable to surging.
I am in SA tableat 600 rpms high MAP(90) 7 SA EBL is .50d higher SA.
low RPM high map? How would I see that area of mainSA? My stall saver prevents low rpm high map. I witnessed a stall this morning as I backed off on TSS. Typically I idle 850 rpms. At 600 rpms SS kicked in and I went from 38 map in log to 52 as iac opened.
Idle pulsated 600-850 or so.
This morn I closed the TSS a tad and I am back at 850 steady commanded to 650.
If you have a stick, then its easy. With the engine idling, let the clutch out and see how the engine responds. I had to set my closed throttle high MAP/low RPM SA to around 18 deg and the same cells in the open throttle table to 15 degrees, or my engine would not idle/run there. It would surge and stall as soon as the motor tried to idle down or I let the clutch out. Also, does the EBL have proportional idle SA correction? I seem to recall that being used in the 8746 ECMs. That table can be quite useful for controlling the idle speed. You set the SA at idle to be a few degrees less than what is required in the areas the engine normally idles in, and then set up the proportional SA to provide the right ammount of feedback. It responds much faster than the IAC.
If theres one thing Ive found, its that the SA is one of the more critical components to getting good idle quality. Under normal conditions, the SA should remain constant, or the changing SA can cause the idle to surge. But, the SA must also be reduced in high MAP areas to prevent going into detonation. If you only have one SA table, then its a balance between the needs when driving vs. the needs at idle.
Last edited by dimented24x7; 09-05-2009 at 04:58 AM.
Also, with that cam, you should be able to get the idle down to around 650-700 as it should pull good vacuum. I have mine set to 700 RPM with the engine pulling around 11 in Hg of vacuum. With the SA and idle gains set up right, the idle is steady and I can let teh clutch out to move the car w/o needing to use the throttle. Once you have everything set right, you should be able to get fairly good idle quality without the need for high idle speeds.
Ronny, when you backed off the TSS to get the ECM to idle off the IAC, did you also set the desired idle speed to 800 to 850 in the ECM? I would start there so that it is idling at the same RPM as it does when on the TSS.
If you are using the idle stablization SA, then while idling on the TSS that will be removing as much timing as in the tables. It will just sit on that value as the desired idle RPM is set to a lower value. The ECM will be trying to lower the idle RPM by pulling that timing.
This spark timing will be added back once the ECM sees that the idle is closer to the desired value.
Can also use the 'set idle SA' option. This will prevent the idle SA from following the main SA table. Set the option flag and then set the idle SA value to the same value as in the main SA table in the area that it idles. This is done for a smooth transition from idle SA to non-idle SA.
If you set both the 'use set idle SA' option AND zero out the idle stablization SA tables (RPM low & RPM high), the idle SA will be what it is set in the SA - Idle SA value. It won't move as long as the ECM is in idle mode.
This may be a good starting point in getting the engine to idle on the IAC. Then if the engine RPM tends to roll or bounce, add in small amounts of idle stablization SA to calm it down. Can follow the same curve as was in the tables, but with less SA.
To control the idle speed the IAC will run at two different rates. Once the actual RPM exceeds a certain threshold the IAC will step at a faster rate. The slower rate is used when the engine RPM is close to the desired idle speed.
The IAC action to control the idle speed is more of an integral action then proportional. As it: checks, steps, delays; checks, steps, delays; checks, steps, delays.
The delay time will be longer as the engine RPM gets closer to the desired RPM. And it is one step at a time.
