1228747 ECM $4f

[chris989]

Hello,

Is the 1228747 ECM a know headache? I have it in my 89 burb and once again I have idle issues. Nothing i change will affect the IAC counts. One minute i have zero IAC and others I have 40+ IAC counts and the idles remains at 950.

I will go back over the TBI unit for leaks and obvious issues. I am not a wiz at TBI but with the tables and scalars what is it that really determines IAC. When I change something like the RPM vs coolant temp, nothing happens?

Thanks,
Chris

[dave w]

Check out the following link for information about "Tuning the TBI" : http://www.gmcmidwestclassics.org/We...0the%20TBI.pdf

dave w

[chris989]

Dave,
Thanks for the link, I have done all of that before and again tonight. I set my RPM to 525 (engine hot) with the IAC zero'd out and unplugged. Plug in the IAC with a stock bin, wait for all the decay garbage and it idles at 676 w/20 IAC step. Turn motor off, restart, Wait again, and it idles at 850 w/38 IAC step. Nothing changed but turning off and restarting.

I get GM needed the thing to perform for all people in all enviroment but how can I eliminate every IAC command besides coolant temp/RPM and AC? I remove everything I think would work ( guessing) and nothing affects a change. I think somethingmust be a miss in the open/closed loop scalars????

Any help is appreciated. I am considering tossing it all, getting new heads and an aftermarket EFI but i hear of headaches with those too.

[tayto]

perhaps the throttle shaft is worn and needs to be bushed. my caprice always had a quirky idle until i buahed the throttle shaft and put correct injectors in.

[chris989]

I bought a TB from "Performance TBI" (aka tbi parts). I was first impressed but my injectors started leaking after a few thousand miles and when i called them i was told, "it happens, hard to get good injectors for the unit". Went to Summit and ordered new injectors in under 60 second. Not hard at all. Then i saw he had linked to Headbytes.com and I was worried.

The unit idles low enough to stall with the IAC zero'd and disconnected. I set it to 525 RPM base line. I feel my issue is somewhere in the idle logic and just don't know enough to understand which is giving me a fit.

I found last night "idle open loop- idle TPS threshold" set at 2%. I idle at 2.6%. Not sure if i am jumping in and out of logic and sometimes my tuning takes over and sometimes it is the pre programed idle.

[dave w]

Sometimes I use the "TBI Tuning" procedure I posted as a course tune. Then I break all the rules and fine tune with all electrical connectors plugged in (fully warmed up engine), and adjust idle mixture screw for about 20 steps (+ / - 10 steps). In some cases 40 steps is acceptable. It's real fussy "breaking all the rules" because adjusting the idle mixture screws changes the TPS voltage. Typically I need to slot the TPS mounting holes so I can get the optimal 20 steps and 0.55 TPS volts.

For TBI injector service, I have the best results with the original factory injectors serviced by Mr. Injector http://www.mrinjector.us/

For off the shelf remanufactured TBI injectors, I have the best results with GB REMANUFACTURING that I get from a local parts store.

It's very challenging to get a "good set" of TBI injectors, because most TBI injectors are over 20 years old. Perhaps new aftermarket TBI injectors do not meet the Original Factory Quality standards?

Very seldom will the IAC parameters need adjustment.

dave w