Hi, I'm interested in doing my own prom burning soon, since I have a really weird TPI combo (69 327) and I'm sure there is room for improvement over my current chip. I have no emissions equipment and no cats. The egr has been eliminated in my prom. Is there a performance/driveability advantage to running in open loop at all times (except maybe a cruise closed loop) for a fairly radical setup? Or can the O2 sensor trying to reach a target AFR in closed loop help performance more than open loop settings? What is the O2 sensor's role in open loop? It may just be my imagination, but I think my current setup pulls better throughout the rev range in open loop than it does in closed loop. I'm FAT rich everywhere, this cammed up 327 always was with carbs, and still is with TPI unfortunately. It seems that the long duration of this cam just makes for a stinking rich idle no matter what.

 

Try both.
It's a 3 sec change to go from closed loop to open loop. Some engines, like open loop, since they run better at rich or lean of stoich..
The thing about chip burning is that you can do alot of experimenting, with just using a few key strokes.
When you push the operating band of the engine up high enough, they just don't like to idle too well. There are compromises to all design issues.

 

open loop rules, closed loop drools.

 

327',
normal operation is:
light throttle --> closed loop
floored --> open loop
...........
in a car that is in good condition,
why would you NOT want to use closed loop for cruise?

 

That's wrong. If you are in closed loop and go WOT, you are still in closed loop. Whatever the BLMs are at when going WOT, that will be your WOT fuel correction (if blms are above 128). Open loop=no WOT fuel correction based on fuel trim.

 

Meaning that the computer looks at the row of BLMs right below it goes into open loop and then uses those correction factors for the WOT values, but doesn't reference the O2 sensor? Is this true for all of the GM ECM's (specifically the 8746?)

Tony

 

Not all cars run best at Stoich.. And some will last a fair bit longer at richer then stoich.. I carry more oil pressure by running a tad rich, with a light decrease in timing. Less timing, slightly rich, drops the load on the rods, so the oil isn't worked as hard.

 

The last statement is not entirely true (Open loop=no WOT fuel correction based on fuel trim. ). Open loop fueling will use any learned in BLM terms. Whether in WOT or not. The qualifier of the BLM needing to be at 128 or higher for it to be used in WOT is true.

Some Masks ($61) reset the BLM terms to 128 at every key on. Others ($8D) only learn in and save 2 cells (of the 16) to be used at the next key on. Other masks ($31, $42) will adjust the upper and lower learned in BLM cells at key on. There are key on init min/max terms that are used to limit how far from 128 the cells are allowed to be at key on.

RBob.

 

What do you mean? Open loop can't use learned blms. The computer will go through the cells but no correction is going on. That is why every TPI car I have scanned reads 128 blms in every cell while in open loop.

 

Tim, I was under the impression that Open Loop will use the stored BLM values also (for $8D). And in WOT as the ECM will make a correction if the ECM detects a "lean condition" (> 128).

I've always considered WOT/PE as a "special" closed loop myself.

RBob, great info as always. Never realized the different strategies for different calibrations.

 

.
Interesting thread.
GM seems to have a looser definition of 'closed loop'.
than I do. To me, if you're not looking at the
O2 sensor, that should not be called 'closed loop'.
In the future, I am going to call the following condition...
...normal, warmed up, WOT, commanded PE of 12.5:1 .....
as
the open loop part of closed loop operation.

 

Not sure which of the four TPI Masks you are referring to. However, on $8D check the SAM Cells calibration area starting at L8523. If the two SAM cells are not getting learned in check the coolant qualifiers. The other item that will prevent the re-use of the SAM cells is a battery cut off.

RBob.

 

So you are saying that if the car goes back into open loop while running, the blms will not reset to 128. Instead, they will stay at the learned value untill closed loop is "restored" ? That doesn't sound right. So if I go unplug my O2 right now while in closed loop, the BLM value will stay where it's at? I really thought the ecm resets the value to 128 when going back into open loop.

 

IMO, it's a mode independent of open or closed loop.

 

Not sure what would happen if the O2 was disconnected while running to force open loop. It isn't something that I've looked at. My previous response was for normal mode operation and open loop due to an engine restart.

It would be interesting to try an O2 disconnect while running. Just to see what happens.

RBob.

 

I'm fairly sure the ecm will do what I said. It would be interesting to know for sure. I think grumpy would label this as useless research though.

 

S_AUJP = Same thing. I NO'd the code for allowing the ECM to do WOT fuel corrections.

Tim