: Onboard computer and the octane rating of the fuel being used.



therodman
12-05-08, 10:38 PM
I used to check my onboard diagnostics once in awhile to see what octane it indicates is being used. I've been using regular for a bit when gas was up over 3 bucks, no problems, everything okay. I checked the onboard computer and it would indicate 87 octane. Recently I've filled up with premium which is 93 octane and added some very expensive octane booster. To my amazement, when I checked it again today, it still indicates 87 octane.

Any thoughts as to why I'm still getting a reading of 87 octane when it should be up over 95.

I checked the timing and it's right at 10 degrees advanced at idle and goes up to about 25 with a little tap on the accelerator.

The vehicle is a 94 STS with a Northstar of course.

therodman
12-05-08, 11:02 PM
Rapid Roger, I read what you wrote about octane on another thread which was the following,

"Cadillac engineers have developed the 275 HP Northstar engine to give peak performance on 87 octane gasoline.This is done by limiting the range of the knock sensors and the timing advance.Using a higher octane in this engine will not result in better performance as various engine controls(mentioned above) are not calibrated to take advantage of higher octane fuel.

The 292 HP Northstar engine,while able to run fine on lower octane fuel,delivers peak performance on 92 or higher octane as its engine controls are able to adjust to higher octane fuel and deliver better performance.

As another poster mentioned,I to have noticed MPG (http://www.cadillacforums.com/forums/#) variances on fillups at various gas stations.I am wondering if some vendors are charging for 92+ octane and dispensing 87-89 octane.I intend to look deeper into this and if I find anything of interest I will start another thread......"



I have the 300 HP Northstar and was thinking exactly what you wrote, maybe the supplier is putting 87 octane into the 93 octane tanks. Seems my car should've noticed a difference by now and should be telling me that I'm using 93 octane!!

IFD158
12-06-08, 08:21 AM
How does your car diagnostics tell you this info. i have a 96 eldorado. Am i able to run the diagnostics and find out my octane reading? if so how. thanks

Submariner409
12-06-08, 10:24 AM
The car diagnostics do not tell you or are even able to calculate the octane of the fuel in the tank. Maybe OBD-I (pre-1996+) had the ability, but I don't think so........

"Very expensive octane booster" is snake oil and hype. Read the label -"Raises octane as much as 3 points" as in : .3, as in 93 to 93.3. If you believe the hype, I have a bridge in Brooklyn that I'll sell, cheap. Don't forget that increased octane better controls the flame travel in the combustion chamber (it slows the burn rate under adverse conditions) so "octane", in and of itself, supplies no more power in the ignition process.

For IFD158, read the stickys at the top of several of the forum main pages. There are several which refer to pulling and reading Diagnostic Trouble Codes (DTC's) from the instrument panel. You do not need an aftermarket scanner.......they miss a lot of system codes anyway: too generic.

Some gasoline marketers still dye their fuels, but its difficult to keep the brands and grades apart, because no dye color is standard across the market for a specific fuel grade. (Old 80/87 avgas was red, while 100/110 (? 120) was greenish. BUT, used in very, very different aircraft engines.)

FYI, My 2002 STS timing is set to supply a total advance of 34 degrees at road speeds. The timing varies from slight retard to 34 degrees under driving conditions, but will always run at 32-34 degrees on the road, regardless of the fuel octane in the tank. The PCM pulls timing only when it receives a knock sensor reading indicating excessive timing advance for the conditions. Timing normally varies all over the charts under road conditions, so it's fun to be able to watch it on the ScanGauge, which, btw, only works on OBD-II, post-1996+.

therodman
12-08-08, 07:15 PM
IFD158, Submariner may be partially correct because OBD-1 does have the ability to calculate octane, When you scan for codes, If I remember correctly, after scanning it defaults to PCM and displays PCM data, you then access PCM data ( to do that you depress your up or down fan speed controls ) and go to PD42, that is your octane reading. When you start reading the PCM data it starts out with PD01 and then goes up to around PD97, it increases by 1 every time you depress the fan speed up or down control. Not every number from 1 thru 97 is displayed, there are some skips but PD42 is where you want to look for your octane reading.

To start your onboard scan, depress the Off and warmer or red button button at the same time on your climate control. It will then go into the onboard diagnostics, takes about 1 minute to complete a scan! If there are any components that aren't quite up to snuff you'll see a code go by when you do the scan. If you do this and write down the codes, either myself or someone on this board will help you with those codes.

Have a good night, sorry it took so long to get back, I was at some football games over the weekend!

therodman
12-11-08, 09:58 AM
Checked again today and the PCM is still telling me that I'm using 87 octane. I'm almost finished with a second tank of gas so the next fill up I'm going to use another station and see what happens.

Blown & Injected
12-12-08, 04:53 PM
Wonder how it calculates octane if it does so. I'm guessing it looks at advance at the onset of knock and estimates the fuel...but why would it display the info if it did not determine it unless there is a different program one may select based on the type of fuel???

I can tell you my experience with boosters is nothing good. While monitoring knock retard I could see no difference between 93 octane and 93 octane with a double dose of 104+ Octane booster. It did however turn my spark plugs red/orange :( The difference was clear, as in zero knock retard with a load of Sunoco GT-100. It smells so good too. - this was obviously not in the DTS

Submariner409
12-12-08, 05:14 PM
Anybody with an OBD-I car have the ability to read what octane gas is in the tank ???

AJ, ewill3rd ??? How does the PCM/ECU determine fuel octane rating ?? ....... from analysis of long-term knock management ?? Somehow that approach just doesn't seem feasible because driving habits, terrain, temperature, altitude, towing, .......all would introduce artificial knock reading interpretation. OR, is the supposed octane readout actually that value which the PCM/ECU is programmed to allow the car to run with....?

B&I is correct on "octane boosters". Snake oil. Ethanol (!) is a better and cheaper octane enhancer.

STSj90
12-20-08, 03:42 PM
Would this work on my car? Could i do this i wonder?

And if i tried it. Could i mess anything up in there? Like change somthing on accident?

Let me know..If anybody knows...Ill try it.

Submariner409
12-20-08, 05:41 PM
We're still waiting to find out how the OBD-I ECU/PCM determines what octane fuel is in the tank. Has anyone besides therodman determined octane rating by viewing PCM output ?

med
12-20-08, 10:54 PM
My 1994 FSM says:

PD42 Octane Level Of Fuel

`

med
12-20-08, 11:01 PM
oops!

My 1994 FSM says:

PD42 Octane Level Of Fuel
Display shows the octane level of the fuel based on recent knock sensor activity. This parameter will only display 0, 87, 90, or 93 indicating the fuel octane calculated by the PCM. A parameter value of 0 indicates the PCM has not had enough time to calculate the octane level. Display will always read 0 for vehicles with LD8 (270 h.p.) engine.

I fill up with premium here in Milwaukee (MOBIL) and my car indicates 87 octane for PCM data PD42 also. Maybe it's a winter fuel thing!

Submariner409
12-21-08, 11:30 AM
med, Thanks for the info. From your and therodman's experience, it sounds as if this is not the most accurate way to determine fuel octane, and that the readout is also restricted to the VIN 9 (300 hp ?) engines.

STSj90
12-21-08, 04:40 PM
So i have a question. Will this work on my car? And how long does it take to determine the octane level usually?

EDIT:

Did some searching around and found out it will work on my car. I have a question> WTF?!? It kinda sounds like people are doing this with the engine running!?? The checking timeing at WOT and stuff like that. Am i not understanding somthing. OR Can i go into a scan with the engine running?

STSj90
12-22-08, 12:27 PM
I checked my stuff out. With the engine running...Ive away's just checked the codes and scaned the car with engine off. Didnt know you could go into that stuff with it running lol.

Anyway, I went to pd42. It says 0. I guess it hasnt had enough time to calculate? I went threw lots of other cool stuff. But i had know idea what it was. Lots of nice pretty numbers bouncing around lol. Its pretty awsome to me. I almost couldnt stop playing around with crap. :D

Does anybody know what each one of those PD.. means? Or have a link that says whats what? Cause it would be cool to be riding around and check timing and all those other cool figures. I wonder if oil pressure is in there somewhere??

therodman
12-27-08, 12:49 AM
STSJ90, here is the info you asked about, or then again, maybe you know what everything is below and are asking what the values for easch should be.

The Powertrain Control Module (PCM) Data selection allows viewing of inputs the PCM uses to control powertrain functions. Each data parameter is displayed on the Driver Information Center (DIC) in appropriate units. The values displayed are non-fail safe.
PD01 (ED01) TPS (Throttle Position Sensor) in degreesPD02 (ED02) Manifold Absolute Pressure Sensor in Kilo Pascals (kPa)PD03 (ED03) Barometric Pressure in kPaPD04 (ED04) Engine Coolant Temperature (ECT) sensor in degrees CelsiusPD05 (ED05) Intake Air Temperature (IAT) sensor in degrees CelsiusPD07 (ED07) EGR Pintle PositionPD08 (ED08) Spark Advance in degrees Before Top Dead Center (BTDC)PD09 (ED09) Ignition Ground VoltagePD10 (ED10) Fuel Pump Feedback Voltage (battery voltage)PD11 (ED11) Engine Speed in revolutions per minute (rpm)PD12 (ED12) Vehicle Speed in miles per hour (mph)PD13 (ED13) Pintle PositionPD15 (ED15) Ignition 1 VoltagePD16 (ED16) PCM Output Fault Status APD17 (ED17) PCM Output Fault Status BPD19 (ED19) Power Steering Sensor (kPa)PD30 (ED30) Front Bank Injector Pulse Width in milliseconds (ms)PD31 (ED31) Rear Bank Injector Pulse Width in milliseconds (ms)PD32 (ED32) Front Oxygen Sensor VoltagePD33 (ED33) Rear Oxygen Sensor VoltagePD34 (ED34) Front Oxygen Sensor Cross Counts (per second)PD35 (ED35) Rear Oxygen Sensor Cross Counts (per second)PD36 (ED36) Front Short Term Fuel TrimPD37 (ED37) Rear Short Term Fuel TrimPD38 (ED38) Front Long Term Fuel TrimPD39 (ED39) Rear Long Term Fuel TrimPD40 (ED40) Knock Sensor Spark Retard in degreesPD41 (ED41) Knock SensorPD42 (ED42) Octane Level of Fuel will display 0, 87, 90, or 93PD69 (ED69) Cruise LashPD70 (ED70) Cruise Control Feedback (percent)PD71 (ED71) Transaxle Pressure Switch (PRNDL Code)PD72 (ED72) Transaxle Input Speed in rpm (PRNDL P/N Sw)PD73 (ED73) Torque Converter Slip Speed in rpmPD74 (ED74) TPC Solenoid CurrentPD75 (ED75) Current ERPD76 (ED76) Transaxle Gear RatioPD77 (ED77) Transaxle Oil Temperature in degrees CelsiusPD78 (ED78) Transaxle Shift Adapt in pounds per square inch (psi)PD80 (ED80) Transaxle Garage Shift Adapt in secondsPD81 (ED81) Torque Converter Clutch (TCC) Solenoid Duty Cycle in percentPD82 (ED82) Transaxle Shift Time in secondsPD83 (ED83) Non-drive Wheel Speed in kphPD84 (ED84) Brake Booster Vacuum in kPaPD97 (ED97) PROM Calibration-TransaxlePD98 (ED98) Ignition Cycles (key cycles on/off)PD99 (ED99) PROM Calibration-Engine

therodman
12-27-08, 12:54 AM
When I entered all the data in my last post it was in a nice column, I see when I submitted that post it put everything into a line, sorry. It looks to still be readable though. SLSJ90 you go by the PD prefix, the ed I believe is for a later year. For example PD98 is ignition cycles and if you had alater year PCM it would display ED98, does that make sense?

Hope this helped a little!

STSj90
12-27-08, 04:20 PM
Thanks for the info. Ill def be checking everything out. Should be fun to see....:)

Ranger
12-27-08, 10:18 PM
You really need a FSM to understand those numbers. It will tell you what the actual numbers are as well as what the parameters are for comparison. Without the parameters, the actual numbers are pretty useless.

Sub, I'm just guessing here, but I would suspect that it is looking a spark retardation, knock sensor readings and maybe TP, MAF, MAP and O2 sensor readings. Bottom line is that it is likely using several different things to "calculate" (estimate) the octane

AJxtcman
01-16-09, 07:29 AM
I used to check my onboard diagnostics once in awhile to see what octane it indicates is being used. I've been using regular for a bit when gas was up over 3 bucks, no problems, everything okay. I checked the onboard computer and it would indicate 87 octane. Recently I've filled up with premium which is 93 octane and added some very expensive octane booster. To my amazement, when I checked it again today, it still indicates 87 octane.

Any thoughts as to why I'm still getting a reading of 87 octane when it should be up over 95.

I checked the timing and it's right at 10 degrees advanced at idle and goes up to about 25 with a little tap on the accelerator.

The vehicle is a 94 STS with a Northstar of course. Read at the bottom for the explanation.


The car diagnostics do not tell you or are even able to calculate the octane of the fuel in the tank. Maybe OBD-I (pre-1996+) had the ability, but I don't think so........

"Very expensive octane booster" is snake oil and hype. Yes all GM cars have the ability to calculate OCTANE


Wonder how it calculates octane if it does so. I'm guessing it looks at advance at the onset of knock and estimates the fuel...but why would it display the info if it did not determine it unless there is a different program one may select based on the type of fuel???
Mostly correct, but on the right track


Anybody with an OBD-I car have the ability to read what octane gas is in the tank ???

AJ, ewill3rd ??? How does the PCM/ECU determine fuel octane rating ?? ....... from analysis of long-term knock management ?? Somehow that approach just doesn't seem feasible because driving habits, terrain, temperature, altitude, towing, .......all would introduce artificial knock reading interpretation. OR, is the supposed octane readout actually that value which the PCM/ECU is programmed to allow the car to run with....?

B&I is correct on "octane boosters". Snake oil. Ethanol (!) is a better and cheaper octane enhancer.
I am getting closer to revealing the logic


oops!

My 1994 FSM says:

PD42 Octane Level Of Fuel
Display shows the octane level of the fuel based on recent knock sensor activity. This parameter will only display 0, 87, 90, or 93 indicating the fuel octane calculated by the PCM. A parameter value of 0 indicates the PCM has not had enough time to calculate the octane level. Display will always read 0 for vehicles with LD8 (270 h.p.) engine.

I fill up with premium here in Milwaukee (MOBIL) and my car indicates 87 octane for PCM data PD42 also. Maybe it's a winter fuel thing!
Nice to hear from you Matt.


med, Thanks for the info. From your and therodman's experience, it sounds as if this is not the most accurate way to determine fuel octane, and that the readout is also restricted to the VIN 9 (300 hp ?) engines.
Second that

OK the logic for the Displayed Level.
This is based off of 2 tables on most cars. The Low Octane and the High Octane Tables. If the PCM is in running in the High Octane Table the DIC will display 93. If the PCM is running in the Low Octane Table it will display 87.
Now Matt says that we have a 90. Hmmmm. That might make some sence. I never looked for a third table yet. If you knew what you were looking at all GM OBD II car would tell you the octane level.


The Logic on Determining the Octane Level. The PCM calibrates or learns the knock value on each trip. The "trip" is the key. I am would need to look up what the trip is on this. If you start the car cold drive 2 block to the gas station and then shut it off. Then drive back home I would guess it would not meet the criteria to learn base knock value. I have seen on Northstar's a valve train noise set a DTC for the base knock value not learned.
Example: If you had piston slap on a 2000 and it was gone in under 2 minutes the PCM be in the high octane table. I would think that the engine would need to be at a set temp before it would learn the value. Maybe above 158? You can't learn CASE below that

HPTuners
If the engine has been operating for a moderate time without any Knock Retard then Knock Learn is zero and the interpolation favors the High Octane table, if Knock has been detected then the interpolation will move towards the Low Octane table.
Source: GM > Engine > Spark Control > Spark Advance @ http://www.hptuners.com/help/


JET DST
To determine what fuel is being used the PCM will switch to the high octane spark table each time the fuel tank is filled. If too much knock is then detected the PCM will switch back to the low octane spark table.

Source: Jet Help file


LS1_EDIT
When the PCM sees excessive Knock, It assumes you are using poor gas, and shifts to the Low Octane Spark Advance table.
Source: LS1_Edit User Guide.