I took my car out on the first long drive yesterday to my work. The drive is 35 miles one way with varying terrain - so it is a perfect drive for testing out my tune. The morning drive to work was perfect. BLMs were either rich in the mid 120s or on the dot of 128. Fueling changes were fairly minor with a VE learn (subtracting 1 or 2 in a few cells).

The drive home was a completely different story. I got stuck in traffic for 45 minutes and that is when fueling went out the window. Over the course of about 10 minutes, I watched my idle BLM steadily climb from 131 and topping out at 145 at idle. For the rest of the time stuck in traffic, I was idling with a BLM of 145.

Looking at the datalog when I got home, it looks like the CTS/IAT temperature rising were the cause of the leaning out issue. In the morning the CTS was around 192 degrees F, and IAT was hanging around 100 degrees F since I never hit traffic. BLMs were perfect for this drive. On the way home with my 145 BLM idle, my IAT was at 163 degrees F and my CTS was at 230 degrees F at it's worst. Me needing to change to a 180 degrees thermostat and turning the fans on earlier to get the CTS down aside, it appears the IAT affects fuel a ton. When the IAT is cool, my fueling is dead on. When the IAT is hot, my fueling goes lean. I've seen this other days as well but could never figure out why until I got stuck in traffic

I did some research last night and it looks like the fix is to replace the MAT with an IAT from a LT1 Camaro. Installing the IAT in the airbox by the radiator looks like the most popular option. Apparently this should give much more accurate air temperature readings.

This got me thinking: how did GM make the MAT work for TPI cars? Does a stock 305 go really lean when stuck in traffic? Or does their tune account for idling for long periods and heat soaking the MAT to keep the BLMs closer to 128?

 

On the MAF ECMs not at all, only used for EGR enable. On the SD ECMs it was used in the cylinder air charge calculation. And it wasn't perfect, it too moved the BLMs.

RBob.

 

I'm surprised with the SD cars they were okay with a such a wide swing of fueling values. Depending on the temperature I get wildy different BLMs.

 

Are you able to get your blm average over the entire log? What are you logging with?

 

Simply moving the IAT sensor isn't enough. You need to run the LT1 MAT tables as well.

 

I'm using the datalog function in the EBL P4 Whats up Display. I'm not sure how to get an average BLM out of that but I'm sure I could dump the data into Excel to extract an average.

Good call. I saw that as well. It seems like it helped a lot for the people that did it. Now only to figure out how to modify the Y air pipe somehow.

 

I've been working on a document that deals with how the o2 sensor controls fueling (pulse width) in AUJP $8d (speed density). While specific to just this code, it may also provide some insight as to what occurs in other versions. The study also examined the effect on PW when coolant/MAT temperature differential and airflow are eliminated from the PW equation and only MAT is used. Below is an extract from the draft document that addresses your observations.

The Effect of Switch 0x018 b4 on PW
This is a very important Switch because it significantly affects the calculation of PW.
When b4 is set, values from the following Tables are used to compute a PW Scaling Factor based on air flow and the coolant/MAT temperature differential:
-0x80E=COOLANT/MAT DELTA MULT .vs. AIR FLOW
-0x7FD=Air Flow .vs. MAT or Coolant/MAT Differential
When b4=0, these Tables are not used and only MAT is used to compute the PW Scaling Factor. The Table below provides PW values with the Switch set and not set:

Effect of Switch 0x018 b4 on PW (Coolant =200°F, Air Flow =32 Gm/Sec)
-----------------------PW (ms)---------
------------Factory Default----------------------If Bit Cleared
MAT (*F)--------Set=1--------Cleared=0------% PW Change
44---------------1.740----------1.938----------+11.4%
75---------------1.679----------1.816----------+8.2%
99---------------1.633----------1.755----------+7.5%
121--------------1.602----------1.709----------+6.7%
146--------------1.632----------1.645----------+0.78%

Observations:
- PW is considerably richer when only MAT is used to compute the PW Correction Factor (when b4 =0); versus using air flow and the coolant/MAT temperature differential (when b4 =1).
- The above example assumed 200°F coolant temperature. But the above PW changes would be “virtually identical” at any other coolant temperature because coolant temperature is not used in the calculation of PW when 0x018 b4=0. “Virtually identical” is necessary because coolant temperature is used to compute an INT update delay factor that could have a minor effect on PW.

The results show objectively what we intuitively know, that the engine will run richer at lower MAT and coolant temperatures and leaner at higher temperatures. This is why it’s important to minimize temperature fluctuations and tune in a relatively constant MAT and coolant temperature range where normal engine operation occurs. Otherwise there will be unwanted changes in INT and BLM.

It also shows that a more consistent tune can be achieved with a relocated MAT sensor that minimizes air temperature fluctuations. The factory GM MAT sensor is actually a solid core coolant temperature sensor which is subject to heat-soaking and more prone to reporting the temperature of the TPI plenum rather than actual incoming air temperature. This is why replacing the factory MAT sensor with a sensor which measures true air temperature will provide a more accurate MAT (for example, AC Delco 213-243 [GM 12160244]). The relocation is generally best in the air intake just before the throttle body to avoid the potential of heat-soaking. Note that MAT relocation will require reduction in VE Table values because the lower MAT will naturally provide more fuel as shown above.

It could also be argued that if the MAT sensor is replaced and relocated, Switch 0x018 b4 should be set =0. The theory being in this instance that coolant temperature will have virtually no impact on PW, and the relatively constant MAT will ensure values from VE Tables will produce a consistent PW unaffected by coolant temperature.

There is no downside to setting b4=0. In fact it is consistent with another factory default Switch setting of L8018 b0=0. When =0, MAT is used for Startup Idle Delay IAC Steps (Table at 0x6A1) and the Scaling Factor for Cranking Fuel PW (Table at 0x3AF). When set =1, coolant temperature is used.

HTH.