I know this has been asked a zillion times but the answer escapes me. I have read everything there is to read but I am apparently missing something.

I have an 84 Corvette with a Prominator (7747) Ported CFI Intake, Stock TBs and 1.6 ratio rockers. After I balance the TBs my idle ends up high (1000-1200 RPM) after I plug them back in after balancing. The standard procedure with the stock ECM (6026) is to drive it above 40 mph and the IACs will reset themselves and idle normally. I tried this but the idle is still high. My IAC counts stay at 50. So I end up mechanically adjusting the idle to get to the 750 RPM. What the f..... am I missing?

 

This is the thread to read:

https://www.thirdgen.org/techbb2/sho...hreadid=225984

Just about everything is covered. Don't know if it was mentioned, however, if the throttle blade adjustment screw is turned in with the engine running the minimum TPS% required for idle can be exceeded, then the ECM drops out of idle control until an ECM ignition off, wait ten seconds, ignition back on sequence.

RBob.

 

Thanks. I have been reading reading reading...............

 

Elk, have you considered reducing the IAC counts? i assume your 50 is closed loop at idle. i believe there is a post somewhere that states where they should be at idle, engine up to temp, closed loop, in gear. 50 seems way high? now your 2 single TB's are going to be different on counts. i think you are at 1/2 of fbody crowd on that in counts since you have 2 IAC's

 

And don't get too cared away with trying to use the 747 IAC cal numbers.

The dual pintles, don't bleed air at the same rate as a single would. The CFI pintles are a single taper, unlike all the 747 stuff that uses the dual taper design. What I did pre 747 hac days was change the taper, so the openings were more gradual. You can do that with code changes, I just mention this as a FWIW.

 

Ron - I have been able to get the IAC counts down to 15 or so. I have a flare up at idle which takes a while to decay out. I am going to work on that and set the park position close to where idle is when the motor is warm and see what happens.

Grumpy - Thanks for the suggestion.

 

this issue has been discussed b4. i think their was a lenghty post 30 days ago. analogy was kinda like a cat chasing its tail. the cell car is idling in maybe a couple possibly 3 cells. when this flare up occurs look at those cells it idles in. it seems the ecu is trying to do a correction by adding fuel, removing it, and cell to cell adjusting spark per the value in cell. maybe those cells it is idling in/across need the same fuel values and same spark values. not sure. maybe someone can chime in. also it was suggested to try to set commanded idle speed low and then with throttle stop screw set idle above the commanded value. then the idle is set physically and ecu may have less control over it. not sure. anyway i too had surging idle but not always. i have manual trans so a little diff. when i coast to stop sign the idle is high, sometimes surges BUT if i blip throttle it settles down nicely. could be linkage/worn cable. with the swap of TB's i may have new issues this spring. 2 weeks maybe. snowing here

 

I seem to be making good progress. I would like to thank everyone for chimming in and giving me stuff to think about.

Here is the latest. I can get the idle to come down to where it needs to be but I noticed this morning on a cold start the IAC counts drop to 40 and then stay there until I put it in gear. From there the IAC counts will drop to where they need to be ~18. Does someone direct me on what to look at for cold start? I have the 7747 hac but looking at assy code makes my head spin.

One other I almost forgot.... When my fans kick in the idle drops real low and then recovers. The voltage drops to 13.7v on the dash and I realize the dash reading is in accurate. I have measured the voltage with a volt meter and it stays at 13.9 to 14.1. I am running a 140amp alternator I know it will draw more current etc. so I think I have enough juice. I may wrong - won't be the first time. Is there a way to compensate? I have seen the voltage correction tables .. is that where I should concentrate.

Thanks in advance.

 

I HAVE NOT TOUCHED TUNERCAT SINCE 11/03 BUT I THINK I RECALL SEEING A COOLANT TEMP ENRICHMENT TABLE(COMMANDED) AS WELL AS THE TIME IN SECONDS VS IAC STEPS. NOT SURE IF THERE ARE ANY COLD START SPARK ADJUSTMENTS. POSSIBLY NOT. I NEVER ADJUSTED AS I GOT THE BIN FROM DOMINIC AND JUST USED IT AS IS.