I have recently started work on a 85' C4 with a L98. When i got to the car many hand had been on the motor and no success for them. I recenly Installed a MSD Ignition 6AL Hei Conversion, 8mm super conductor wires, and two stage colder over stock AC Delco Plugs. Comp Cam 1.5 Roller rockers and set the valve lash to 1 full turn. Set the Ignition timing to 6 deg btdc. Accell Chip and matching temp sending unit. The car runs a hella better and smoother but i am having cold start idle issues. Heres what happens.. The car starts fine two strokes and fires idles up and runs smooth at 800 to 900 RPM ignition timing steady the the IAC backs the idle off and idle gets to around 500 - 600 rpm and the Ignition timing starts freaking out. Car starts running rough then eventually dies. Also driving is affected, before engine temp reaches 130-140 you depress the accellorator and have to wait a good 10 seconds before the engine builds power. once everything heats up the engine runs like a Cadallac great power and great throttle response. Help Please

 

sounds like my 86 when the ECM went bad. once the ECM warmed up, it was fine. Anything in the middle from like 100 degrees to 160, it hardly wanted to run. I borrowed an ECM, and it quit doing that.
you might also try unplugging the ignition control module while it is having the problem.

 

I disconnected the ETS or ESC it when setting the timing .I read somewhere that the MAF can be fouled by K&N Oil based air filters by over oiling and the car has an K&N on it. If cleaning the MAF out doesnt work the ECM May be it but thats why i put a Hypertech Chip in it in case the PROM was fouled somehow. Thanks for the input

 

The MAF would set a code........are you getting any?

 

No MAF codes i have a Snap-On Modis scanner and i get no codes. I cleaned the MAF this morning with no success. I pray its not the ECM. Im going to try setting the idle position screw next the cap has been removed.

 

I had similar problems when the coolant sensor started to fail, try changing it out.
Barry

 

Set the screw and it doesnt die, surges a little. EST code appeared.

 

Changed the coolant sensor with the one that came with PROM

 

Mine has done the same thing for a while now. It just wont sit still and idle at 600-700 rpms like yours. I've replaced a bunch of sensors, new fuel parts, ignition parts, still have the problem. I bet it is the ECM, thats why i've been considering a carb swap. Of course it'd probably be easier and cheaper to buy the ECM for you.

 

EST code cleared, Hypertech chip calls for 10 deg base timing. Have to wait till tomorrow when engine is cold again. Any suggestions on what to check untill then. I also checked temp sensor output using MODIS and the readings where fine. I may check injectors.

 

I cant disagree with carburation because you dont see many dragsters with dry manafolds. But I wouldnt go that route unless the motor was stroked and had some major flow to itthat would require a 1100 cfm carb

 

the bottom end is the only thing I havent touched on mine. Its just i'm tired of driving it for a week then having it down for a few months because nobody can figure out the crappy electronics. I've looked at aftermarket EFI systems too but they're mostly for converting carb to EFI.

 

It's Got to be Something wrong...

False knock signal driving the timing back? Does the timing retard happen as a Result of the rpm's dropping or is it timing Causing the rpm's to drop?

BTW - the ignition module will switch back to controlling the timing advance itself (taking control away from the ECM - same as if the bypass wire was disconnected) at about 400 rpm - give or take. If the idle ever drops down to near that I'd suspect any timing weirdness to be caused by that happening. Just leave the bypass wire undone to eliminate anything changing in that regard.

I'd also suspect something wrong with the IAC motor if the rpms actually Do drop that low - ever. Shouldn't happen if the IAC is working, unless the idle target rpm was changed to be that low in the PROM program. Or - maybe if the ECM wasn't getting a correct rpm reading from the ignition module/pickup coil.

Try this:
- unplug the ESC bypass line
- set the "base" timing to something reasonable, 10° or so
- ground the A and B pins of the ALDL and turn on the ign, wait a few seconds for the IAC to wind all the way closed, then ..Leaving the ign on.. unplug the IAC.
- Now unshort the ALDL pins and turn the ign. off
- start it up and use the mechanical min idle stop screw to open the throttle plates enough to have a resonable idle speed - if warm then about 800 rpm should do it.

You have now eliminated anything the ECM does to control timing or idle speed. See what it does cold like that.

Disclaimer - I guess I don't really know what might be different about the timing control scheme with the MSD 6AL wired in there...

 

repaired certified ecm's are about $100 at checker, auto Zone.
some members here sell them for about $65. not like they used to be when they were $300 - $500