A/F runs rich for warmup based on conditions. With higher ambient temps (air through the filter), the PCM assumes the car will warm up more quickly, regardless of coolant temp. There are several factors that affect A/F ratio on warmup:

MAF is typically NOT reading. The IAC is metering air past the Throttle Plate.
O2 sensor is IGNORED just as it is at WOT
IAT
ECT
MAP (for Series 2 cars starting in 1996, not factored in with Series 1 L67'*).

The PCM uses a 'default rich' setting rather than relying on all the sensor inputs in normal (warm) modes of operation.

It'* a combination of all these inputs that determines open or closed loop modes in the PCM on cold start.

Ok, so when the car does warm up, it doesn't need IAT readings? And thats why simply using a resistor as a ricer trick to think the car is receiving colder air than it really is doesn't work?

EDIT: I'm sorry, I am terribly off topic...

It'* vaguely on topic.

After the car warms up, it uses the IAT, MAP (if equipped), MAF, O2, and TPS to determine proper fuel ratio. The IAT input is also used to determine timing advance.

The resistor trick won't work on OBD2 cars with MAP sensors, as the PCM will learn the trick very quickly based on MAP sensor readings. Remember the MAP sensor isn't used on OBD1 cars for PCM input. It'* ONLY used on SSEi'* (not even SC SSE or SLE) as a boost indication for the gauge. So the ricer resistor trick works SOMEWHAT with the OBD1 cars, but can have the negative effect of advancing the timing into KR realm, particularly with pulley'd L67'*. Remember my 'magic box' I rigged up? The most I could get out of the IAT translator I built was 2 degrees, or about 5-6 horsepower. But I was running VERY high octane fuel to protect against too much timing and KR.

Right, that all makes sense, but now I feel like I am going in a circle. So on a 2000+ L36, if your ECT reads consistantly below 176 it will throw a code. One fix is to remove your IAT and replace it with a 18k resistor. Now you have no IAT, what does the computer think of this? Clearly it can't use the IAT for AF...

So does it just make do without using other sensors?

The resistance simulates a higher IAT temperature. And most of the AF calculation comes from the MAP, MAF, and O2 at that point.

I went with the 180* t-stat this summer and it hasn't changed anything about the performance of my car. I agree with what willwren said on the first page regarding LIM and UIM.

It'* only $10 and you probably spend that much in a week on drinks and stuff. Go for it.

...and to throw a couple pennies into the ring, here....

Neither bonneville or oldbuick mentioned anything about the FSM....at all.

Bear in mind guys (bonneville and oldbuick) that we are open to new information that supercedes what we already (think we) know, but that information should be presented with recognizable backup data from good sources and not just from heresay are "common knowledge".

While we may not be the most advanced mechanical website out there, we do try to provide information, for the good of the membership, that has been tested and verified to work without harming our cars or causing undue stress. In many cases, which you'll see as you read many past performance and brainstorming articles, we stress that a certain mod or other will likely cause undue stress on a transmission resulting the eventual replacement or rebuild.

Just to let you know that this is where we're coming from.

My info comes from GM, I had major pinging and detination problems on my 2000 after putting in the lower T-stat on the advice of a board member herein a post I read. It threw a check engine light the same day i did the change and the flush and bleed process mentioned here, and I took it back to the dealer w/o telling him of the t-stat chg because I thought that it had nothing to do with the problem. I forget what the code came out saying but the tech tracked it down to a temp problem and I watched him use a laser type temp gageon my t-stat housing to check the gage and he said " your t-stat is stuck open" He then chgd the stat and told me it was the WRONG t-stat. He then put in a 195 and vulaa, problem was gone. I did notice the temp only got to about 175 and it now goes to about 200 degs. I then told him the deal with the idea of the lower t-stat from my reading here. He told me and again I didn't question him as he is a certified GM tech with 18 plus years that " by chg the stat to the lower one you are NOT reducing the temp of the stove pipe and we have prooved that with wireless digital thermos when tested at GM school" " the stove pipe comes from the exhaust gasses and will run the same temp give or take a plus or minus 2deg chg with either t-stat" So.. Mark me down as the first with the problem I guess but he said he has seen problems from using the lower temp t-stat, oh and let me know where to send the T-stat for the gold plating !

buick, my dad was a former GM Tech, and while there are certain things that you can do to a vehicle, and GM says not to...dosent mean that you cant do them and have a perfectally running vehicle...and the thermostat issue is a perfect example

and we are not trying to reduce the EGR stove pipe temps.....we are reducing the COOLANT temp that is flowing in the passages in the UIM

all in all i believe your 180 Tstat being that it was stuck open was the problem....your code was caused by a false reading that the PCM was getting because it was not functioning properly.......NOT because it was a 180

Nope, sorry, he thought the stat was stuck open only before he removed it and saw it was working and was the wrong temp. I have the stat here and have boiled it in water with a digital thermo and it works as it should. All I can tell you is I saw it all with my own 2 eyes, and maybe others here have had luck doing the change , for me i will stick with what works and the 195 works just fine. The UIM still gets hotter than the 195 degs anyway esspecially around where the pipe goes thru. Keep in mind if you shoot the temp of the engine block and the head area and the EGR they are all over 195, at least mine were when he did it in front of me. I guess you use what you feel works right and my comfort factor right now is with the GM tech mostly because he prooved it !