Thread: 3.8L SC Random stalls, faulty intermittent start!

The problem is you need to know the engine's forward (not back firewall) cylinder head (bank 0 I think) temperature! These cars (like most) have no "head" temperature sender, only the one that measures the temperature of the circulating coolant liquid.

At low RPM not enough coolant is circulating quickly enough to prevent heat spikes in the heads nor to measure the true head temperature, especially the front bank head which has no air cooling headroom up to the hood, (header heat is trapped there by the plastic engine cover thingy) due to the low hood-line.

Just like in water and air, in an engine (cast iron, steel or aluminum) HEAT ALWAYS RISES!

If you heat a bar of steel with a torch it melts more quickly with the torch flame applied at the top than it will if you try to heat (melt) the bottom! You have to get the whole thing red hot (saturated sink) to start melting the bottom.

So what is happening here is "heat riser sinking" up into the cylinder heads (valve covers, blower) and the ICM mounting platform! We need to add some added auxiliary air cooling for this riser heat that may not show up in the coolant temperatures until AFTER the (water pumped) RPM rises! Also when the coolant is not circulating with ignition off the ECM has no clue how hot the engine (particularly the forward cylinder head) gets afterwards AFTER the car is parked. (we are talking 45 minutes of serious 200˚+ oil-after-baking, even with fans)

All you need is a piece of wire connected to the upper right terminal (2000 GTP) of Relay 12 to activate both fans on Low. Simply ground the wire (turns on Relay 12) through a switch or thermostat on the head or ICM platform! (will not interfere with ECM should it ever need to activate Both/High Mode either)

Even for regular daily use in heavy summer city traffic I would recommend adding an ignition powered relay to turn the fans on Low (Ground Relay 12 return) FULL TIME (if you don't have or can't find a suitable button thermostat to only do so automatically) while the engine is running, especially if you use lower grades of gas and wish to keep the decorative top plastic cover thingy fitted, blocking upper over the engine airflow convection from the front cylinder bank/headers. These 185˚ button thermostat switches can be grabbed at any HVAC dealer, look up Hunter (cottage/RV) wall furnace "Blower Kit". Those cooling (close on high temp) button thermostat switches are around $15 so a $3 relay might be cheaper but wont help you (or your oil) when the car is parked. We need a furnace "forced-air heat exchanger blower motor" type thermostat switch that only turns "on" when the temp rises above it's value/setting.

Since the ECM only has two transistor outputs dedicated to running the fan relays, one does what I mentioned above (grounds Relay 12) and the other turns on (grounds) both Relay 9 and Relay 10 (hard wired together, so grounding either does both) for high speed mode. Note the polarity use of pins 85 and 86 on GM/Siemens relays is reversed on some relays! Use a meter to find out which side has 12v before adding-in a wire to ground the other coil "return" terminal pin! (to test or activate it)

The big Siemens relay's coil pins are at the lower left and the upper right socket holes. (which is the coil power and which is coil return varies - see above)

To run the fans on high with a switch all you need are 2 IN4005 'blocking" diodes connected to the one wire (again) connected so that the wire can ground 2 (all 3) relays at once as the ECM does (with 2 outputs). The two "blocking" diodes prevent the ECM from ever having to risk running ALL 3 relays with only one of it's output transistors. (Probably not any big deal, but worth noting and paying attention to, do not "jumper" Relay 12's coil return to either Relay 9 nor Relay 10's coil return(s) - use 10 cent diodes!)

This way, (with diodes) even though you have all 3 relays effectively "jumped" for your "fans on high" grounding wire/switch, the ECM can still turn on Relay 12 alone (Low speed) whenever your thermostat switch is not also "calling for" High speed mode (if you wish) cooling! No diodes are necessary to run Relay 12 (both fans Low) alone.

The fans can run in 3 possible modes: (ECM uses 2 fans/both together only!)

Relay 12 ON - both fans on Low (series connected 6v Mode)
Relays 9+10 ON - ONLY driver's side fan runs on High (!2v Mode) (#error - very bad, see below#)
All 3 Relays ON - Relay 12 runs passenger side fan on High (12v Mode)

The middle "driver's fan only" on High mode (R9+R10) above is bad/useless! It just blows (and traps) BOTH THE CYLINDER HEADS HEAT over to the ICM/Belt Battery passenger side and slightly cools the transmission and one cylinder a bit. If you're stuck in bumper to bumper traffic, one fan alone is useless, it just blows heat around like a convection oven, trapped across the rad/grille clearances.

Doubtless even if this was reversed so the passenger/ICM side fan was the single (R9+10) default fan, we'd be "convection baking" the ATC/ABS, tranny and cruise control, it was not a good idea either! You always need both fans on Low or High ONLY, creating positive air pressure sucking/blowing in cold air evenly and forcing the coolest hot air out the bottom of the engine compartment.

Running both fans on High is noisy and definitely not easy on your battery for post-cooling after you've parked the car!

Dude. This is some pretty good info. Do you have any pics of what you did to the coil and fan relay area? Better yet wanna come mod my car since you live near me? Id pay ya.

Thanks agentbluescreen. My car is a N/A 3.8 2001 GT, and I do not have stalling, but I was looking at doing this as a preventative measure. I do appreciate the pics, I will be tackling this myself very soon.

Do you think the high temps of the ICM/coil area is whats causing the fires?

Originally Posted by SuperSport

Do you think the high temps of the ICM/coil area is whats causing the fires?

That the 3800 community was getting recalls for? No...it was oil leaking onto the manifold and sparking a small fire which led to electrical stuff and that...goes up quick engulfing the whole motor...

Originally Posted by bluegtp91

That the 3800 community was getting recalls for? No...it was oil leaking onto the manifold and sparking a small fire which led to electrical stuff and that...goes up quick engulfing the whole motor...

Kinda hard to imagine oil spontanoulsly combusting...but if thats the case, then so be it. Just kinda thought it would be something electrical overheating shorting out and causing a fire, like the ICM area...

Originally Posted by SuperSport

Thanks agentbluescreen. My car is a N/A 3.8 2001 GT, and I do not have stalling, but I was looking at doing this as a preventative measure. I do appreciate the pics, I will be tackling this myself very soon.

Do you think the high temps of the ICM/coil area is whats causing the fires?

No extraordinarily high oil damaging temperatures, primarily caused from using lower grade premium or regular gas (in city driving) cause the overheating of the confined front cylinder head and the warpage/melting of the plastic valve cover and failure/damage to the frontmost edge of the valve cover and it's gasket, which cause a slow but steady oil leak (that gets worse after parking) out over the center of the cylinder head, that drips oil onto the front exhaust manifold. On factory engines a flammable, decorative snap-on (two lower tabs - one on the drivers side will be already melted) plastic "ignition wire hiding thingy" catches fire (there is not another one on the rear cylinder head, this was just a front-view decorative-dress-up ignition wire guide-hider) from the burning oil, thereafter igniting the flammable decorative top plastic intake manifold cover and then the gas lines. (Supercharged Series II engine covers)

The oil fire ignition danger is especially bad if the car is thus driven for a significant (20 minute distance on such poor fuel) trip on very hot days in slow stop and go traffic and then parked in a garage. The fire will start after about 20 minutes, as soon as the header heat has sinked back up into the now-uncooled cylinder heads opening the small (running) oil leak further.

As I pointed out before the ECM/dash temp indicator NEVER has any/much clue about how hot the cylinder heads ever get, except when the motor is running! It is mis-located way, way down in the coolest middle part of the V block on the nice cold transmission side, down under the rad thermostat-coolant port. Thus the ECM (and you) merely deduces an ASSUMED average running-engine temperature by guessing how hot it might be from the re-circulating coolant fluid's temperature!

As a result of this defect, (poor/improperly too cool location of the coolant temperature sender) the ECM will never turn your cooling fans on when the engine is off and the car is parked even if the engine is literally on fire, not even to blow the little initial flames (starting on the wire guide) out! If you haven't gone in for the recall remove and discard this front HV ignition wire guide immediately, use some tie wraps to hold your longest two front bank HV ignition wires out from the block hanging loosely off the Rad harness! (just a temporary anti-fire fix)

GM will not replace the thus easily damaged/melted plastic valve covers but do freely install and supply a fatter more temperature resistant front valve cover gasket and they remove and discard the dangerous plastic front bank ignition wire guide/hider with some new standoff clips of some sort, that keep those two wires from also being so close to the manifold heat problems. They also do not replace baked wires.

Their excuse for these recall heat damage repair limitations is that "you must have deliberately used bad gas".

I maintain it was a poor design, poorly tested that failed to take into account real long term everyday daily driving situations and conditions beyond the owner or driver's control that inevitably results in damages and deteriorations that they are avoiding responsibility for.

My gas lid does not specify premium fuel only!