Silly questions about 3x00's with Untunable PROMs

[Mysteriphys]

after reviewing a few of the threads, and some prior knowledge of electronics, I'm just wondering, for all these projects that people may want to do involving their 3x00's.If you can't tune the prom.

Is it possible to fool them?

i.e. say you installed a higher flow TB with a good high flow intake, and you want it to run slightly leaner than it would normally allow, (due to it detecting higher air flow, thus wanting to jam tons more fuel into the cylinders, wouldn't it be possible to calculate a resistance value, (or at least install a potentiometer in line) that would allow you to define your fuel flow, or would it start throwing everything else out of whack?

this goes for other areas as well considering I can imagine certain sensors would be way too complicated to try to control specific aspects.

[RobertISaar]

if you want to do it that way, go check ebay. you'll find everything you need to fuck up your vehicle by sending your PCM false signals...

 

or you can do it the right way and either convert to OBD1 or OBD2.

 

BTW: a TB alone won't require tuning.

[Mysteriphys]

understood, guess it doesn't pay to live on a shoe-string budget.

 

I'm aware that the TB isn't the only thing, it would be sensors, valves, ect, I was just trying to get an idea as to if it was possible, without spending a months pay to get a harness from the local rip-off yards...

 

thank you for that most definite answer.

 

p.s. if you read this "i.e. say you installed a higher flow TB with a good high flow intake, and you want it to run slightly leaner than it would normally allow, (due to it detecting higher air flow, thus wanting to jam tons more fuel into the cylinders, wouldn't it be possible to calculate a resistance value, (or at least install a potentiometer in line) that would **allow you to define your fuel flow**, or would it start throwing everything else out of whack?"

 

I'm not being specific to the TB, I'm being general about the fuel delivery system.

 

either way I got my answer

[RobertISaar]

not only did i post a guide to do it yourself, i'm also offering the service build OBD1 conversion harnesses.

 

i only posted this initially on 60*V6 since i got a rather cold reception here.

 

and i'm not trying to be an ass, but people go to ebay looking for this exact type of thing you're describing, and serious fuck over their motors. i feel the need to convey the consequences in a very serious manner, for they are serious consequences.

[Galaxie500XL]

I guess in a theoretical way it's possible...

 

For example, the PCM schedules fuel delivery based on O2 sensor output voltages..if you wanted to deliberately run you car richer or leaner, you could play with the O2 sensor output voltages the PCM sees.

 

However, that being said, it would be difficult. Let's say, for instance, you wanted your car to run leaner...so you increase the O2 sensor output voltage the PCM sees to fool the PCM into thinking the mixture is rich. AT THAT MOMENT, the PCM will command the injectors to supply less fuel.

 

All well and good, but now the real fun starts. Since the PCM is monitoring the O2 output continuously, the O2 sensor will detect the now very lean exhaust, and vary the output from the O2 sensor, in this case LOWERING the output voltage. The PCM now sees this new value, and enriches the mixture to correct the lean condition, undoing all the careful work you just did.

 

Theoretically, you could bounce the A/F mixture back and forth over and over, because the balance has been upset. That's the nature of a closed loop system...

 

 

 

 

 

 

[Mysteriphys]

ya, reading into Rob's conversion, he's right about the cold reception, tbh it looks like backwards is the way to go with my cutty, unless I can get an intrigue with the 3800 and a decently low mileage tranny.

 

oh and a dynamic oscillator running the resistor value timed to the pcm response would probably solve the lean/rich issue, but then if i'm going to have one of those controlling a/f I might as well just forget what I said about the shoestring budget lol...

[RobertISaar]

its also the flaw of a closed loop system that uses a narrowband O2... it goes richer and leaner than commanded on purpose, thats why a car with proper tuning you can watch a graph of the O2 voltages and its a constant wave going up and down.

 

as for modifying the O2 signal, don't do it. changinging it is one hell of a gamble, because if you start leaning it out by telling the ECM its running rich, your knock sensor is going to start tripping very quickly due to all of the extra heat you just caused in the combustion chamber thats causing preignition.

 

also think about this: lets say you change it just 5%, thats a 15.4:1 AFR, 10% is ~16.2:1. thats about as lean as you can go without actually losing fuel economy/causing some melted pistons.

 

as far as the oscilator: the ECM reads the O2 signal 80 times a second.

[Galaxie500XL]

Good point...I wasn't suggesting actually monkeying with it, but used it as an example of how difficult it would be to change only one small thing.

 

I'll admit that I'm mostly self-taught on how the whole thing works...but hopefully, I gave a fairly accurate description of the problem.

 

Melted pistons will ruin almost anyone's day..

[Mysteriphys]

well.. tbh if I had 300 bucks, I'd have a parts car and a 3800/4t60E with half the mileage of my 3100/same combo, and I wouldn't bother asking :-P I guess i'm just bored and want to do something to my car other than scrape off pin stripes, or rubbing out the key marks from the previous owner.

[RobertISaar]

thats about how i felt for the longest time, then i took a gamble bought a bunch of parts, put them together and i have what is likely the first W-body that converted from OBD1.5 to 1. i cannot complain about it, its much easier to deal with now. that and tuneable, which is always a plus.