Hot-soak engine starting failure

[justdave]

Seeking specific testing / troubleshooting info for the IAC (Idle Air Control) valve. Did not see anything in the Techinfo section under Electrical or any other heading there. My parts-store aftermarket Manual has very limited info (covers years 89-to-05; too many to be comprehensive).

Have an intermittent problem: after 99 Bonne/L36 is driven, parked, 'hot'...it will start right up IF started within next 1-to-2 minutes. If we wait 10 or more minutes, engine cranks and may or may not start...if it does, idle is ROUGH, and RPMs wander between 1000 and 500 RPM...usually dies within 5 seconds, UNLESS i lightly tap throttle repeatedly....IF i can successfully keep engine running, it will suddenly start running smoothly and normally within about 10 seconds of that 'babying'. Then can be driven on highway normally and runs well for it's age / mileage of 162K.
Diagnostic info: NO DTC codes were in Memory of PCM; has new battery (no problem cranking engine with starter); has a M.A.F. labeled "Japan" and "Reman" (have seen other posts mentioning one should avoid non-GM/A.C.Delco MAFs, but they do not say technically WHY, besides having a bad experience with such...); new spark plugs (only because they were 'due' for changing) made no difference to problem; when already running, engine "will" continue running if M.A.F. is un-plugged
(i.e. - the PCM "will" adjust things on-the-fly without the MAF sensor input...RPMs wander above and below 900RPM a bit, as expected).
Have scheduled installation of a new Catalytic Converter, thinking that it's old passages are likely clogged up from so many miles of use, and 'may' be shrinking when heat-soaked 10 minutes such that exhaust flow gets "too" restricted, perhaps exacerbating a borderline problem elsewhere. (Either way, the Cat is due for a change; it will get a new O2 sensor there also; sensor behind engine is new last year.)
Hmmm...just read the info about plastic intake manifolds' / gaskets leaking coolant into engine. Perhaps i have a slight leak of coolant into intake plenum, only when engine sits pressurized/hot for 10 minutes??...and that coolant is screwing-up my start-time Idle mixture causing stuttering...until it gets burned off??
Any thoughts from someone who has experienced my specific bizarre problem symptoms? Anyone have the FSM testing / troubleshooting info for the 1999 L36 IAC = Idle Air Control valve??

[00Beast]

Sounds like a bad ICM. Only real way to test is to buy a known good one (Junkyard AC Delco's are the best, generally), and see if it does it with the new one. Testing them doesn't seem to be reliable...

[justdave]

11-14 17:21 reply: Thanks for the ICM tip. Why exactly do you suspect that Module??...since the engine runs 100% normally 99.99% of the time; except when it exhibits the specific starting problem posted, and no DTC was thrown, i figured the ICM was fine. (Do i hear laughter among the troops at that last statement?)

[Mechanical Mike]

Next time it happens try starting with the gas pedal to the floor. This is known as the clear flood mode. If that seems to help then we'll have you check some things that could be causing it to flood.

[justdave]

11/15 12:52 reply: Thank you for your tip. Already knew about "Clear Flood" Mode; tried it just for hahas, and made no difference. Today we are going to heat engine fully, shut it off for 10 minutes (i.e. - try to reproduce the failure conditions), examine carefully for exterior coolant leaks, listen with a stethoscope for any hissing inside those wonderful Plastic Manifolds, then pull off the intake 'hose' from throttle body...stick a vacuum-cleaner hose in there, run vacuum, move hose around, and smell the outlet of the vac for coolant. Yeeha, what fun.

[00Beast]

These are the exact conditions in which we've seen ICM's fail numerous times, rarely ever with a Check Engine Light.

[justdave]

JUst so 00beast et al know...i AM paying attention to your tips...but since i am an 'escaped scientist', i usually require "more input" (as No. 5 would say), before i replace a non-returnable Part.
Are you telling me Ignition Control Modules have been observed in the wild causing sputtering and dying idle WHEN engine is started after sitting hot for 10-minutes, but do not cause those symptoms when engine sits hot for just 1-to-2 minutes??
I believe i read reports that the 92-to-94 ICMs were known to be junk and fail in the tens of thousands, and that GM released a revised Part. Have you guys found that the 99 ICMs are failing like flies all-the-time, or just in heat-soak conditions, or what? Did GM release a revised ICM Part Number??
Wouldn't be the first time an electronic module did squirrelly things when heated...but where i have a problem with the "replace your ICM" diagnosis, is the fact that once the engine gets past that 1st ten or fifteen seconds of stuttering/rough idle, everything is back to normal...and i cannot fathom how an over-heated module would get cooled-off, or, return to 100% normal function, within 15 seconds of being squirrely.
P.S. - my earlier plan to stick a small vacuum hose into the intake plenum through the throttle body received a little set-back; have never seen a hard screen across the front of a throttle-body before (non-removable too). That was a surprise...good back-up plan in case of junk getting past the air-filter though...or, is that a tiny honeycomb of air-straightening passages?...or an intake-noise muffler perhaps??

[00Beast]

I've seen 92-05 ICM's fail. I was honestly not aware that they were ever revised...

The MAF Screen (front of the TB) is removable, there's a snap-ring holding it in. It's to create laminar flow across the MAF sensor. Definitely no muffling affect from it...

[justdave]

Respectfully, since i am not getting any requested information, please remove this thread from your site. I will seek elsewhere.
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Update 12-11-2012: in case this info is useful to others, am posting the 'cures' eventually found after much testing, and elimination of multiple possible causes. 1.- Ignition Coil connectors (internal female) were found to be "loose" on their male blade counterparts (sticking up from the I.C.M). Fixed this by rotating the "tips" (only) of the male blades by 5 degrees, using "smooth-jaw" pliers (to avoid scratching the coating on the blades), then re-seating the Coils while wiggling them a bit. *Use silicone di-electric grease around all 6 connectors to keep moisture out* The previous simple procedure CURED "misfires" trouble code. (Apparently, engine vibrations caused intermittent "connection conniptions" at those very-important connections from the I.C.M. to the very-low-resistance Primary windings of the Coils; any increase of resistance in that circuit, especially dynamically-changing resistance, will obviously cause trouble. 2.- Fuel pump on this 163,000-miles car was tired, AND the Factory fuel-pressure regulator was sticking, AND the fuel injectors were gummed-up. New "Sorenson" fuel-pressure regulator, AND all-new parts running from it's very-important vacuum fitting on it's top, all-the-way to the intake's rubber connector, AND running several tanks of non-ethanol-blend gasoline WITH "Lucas" (or your choice) fuel-injector cleaner additive...allowed the injectors to spray well-enough that the engine now starts easier, and runs very smoothly on-the-road. (I.e. - as opposed to getting too-much fuel, the engine was actually not getting a fully-atomized spray of gasoline into the air-flow into the cylinders, as the engine turned relatively 'slowly' during starting; this is when full-pressure injector spray patterns are most important! Dribbles and half-circle spray patterns et al, just won't get the starting job done.)