: Has anybody bothered to check their Calculated Octane Rating on their OBDI Northstar?



peteski
09-03-05, 11:40 PM
I have a 93 Eldo with a N* engine. It specifies Premium gas only. I've been putting premium in it since I've owned. I've also read all about the knock sensor and that it is ok to run regular gas.

As the premium has price has climbed to close to $4/galon around here, I decided to switch to regular.

Lets go back a bit here. About a year ago, I checked the PCMs Data value (IIRC, P042) which is Calculated Octane Rating. As far as I can figure this out, that is showing what the PCM thinks you have for gas in the car (and controls the timing accordingly). So, when I checked it at that time, it showed 91 (which is Premium, but still a bit low, as I was putting 93 octane in the car). But, no big deal...

But last week, before I filled my tank with reglar gas, I read out the Calc. Oct. Rating again. I found it at 87!! I've been buying Super in a no-name gas stations and from what this tells me, I've been cheated! Seems that they have been selling me regular gas as premuim!!

So, I've been driving on regular gas for probably a while without even knowing.

I'm curious as to how many of you (with OBDI) have checked the Calculated Octane rating value, and what yours is showing.

We might have more dishonest gas stations than we think (not that it surprises me that much).

And, if you are using regulag gas and the PCM shows 87, then your engine has already properly adjusted to it, so it will fine.

Funny, not many cars can actually tell you the octane rating of the fuel you're using - this one does! But, it probably wouldn't hold up in court...

Peteski

eldorado1
09-03-05, 11:59 PM
Wow, that's interesting. I didn't know they did that.

One thing to take a look at, is if you filled up premium at a station that had one pump/nozzle for the 3 different grades. You can easily get a gallon of regular when you fill up with premium. I only go to places that have 3 nozzles. They're not making me pay extra for regular.

also - other things besides octane can change that value. Mechanical problems mainly, for instance, if your egr sticks, you're leaning out, and will probably get a few knock counts at idle, which will probably update that octane value as a lower quality fuel. Worn spark plugs could probably do the same.

peteski
09-04-05, 12:11 AM
Out of curiousity, if you have OBDI car - check your value.

My car could use new plugs and wires.
But I think that the knocks were only monitored when the engine was running at higher RPMs (not idle). But I'm not sure.

And I always buy gas from 3-nozzle pumps (not by choice - just by chance).

Peteski

mcowden
09-04-05, 01:22 AM
I don't know if this makes any difference, but FWIW, I have an AutoTap scan tool and software. You hook it up to the OBD-II connector and a laptop and it will take readings from the sensors at frequencies that you specify and chart the various parameters (such as timing) being calculated by the PCM. Anyway, point being that the timing chart varies wildly, at least on my car. It might be -20 degrees while cruising along and then +20 under hard acceleration (hypothetical numbers only, not real readings). Depending on various factors, such as perhaps learned IAC and TPS values, the timing could be offset from normal if, say, the TPS resistor has degraded a bit or the throttle body and blade are full of gunk or <insert x factor here>. If the display uses the timing or learned values numbers to calculate the octane number, it could be thrown off by all sorts of stuff. Just out of curiosity, you might try disconnecting the battery for 60 seconds to clear out some of those values, then checking the computed octane value using the same fuel you did when you got the 87 value.

My point is, I don't think there are any sensors to detect octane ratings directly from the fuel. The value you see in the computer is one the computer pulls out of its tiny digital a$$ based on several data sources such as the knock sensor, current timing settings, etc., and those factors can be influenced by other things that have nothing to do with the actual octane value of the fuel.

The other thing I wanted to throw in here is that the 91 value you saw before is what the manual considers to be "premium" fuel. It might not distinguish between 91, 92, 93, or 120 octane fuel. If it detects no knock, it might assume it's premium and just say 91 because that's what it thinks premium fuel is. That's probably also why there seems to be zero performance gain from using very high octane fuel. Anything above 91 and the computer is using its maximum performance values to set the timing, etc.

Wow that's a lot of typing for this subject. I'll shut up now. Anybody have any further information about how the system actually works? <sigh> I sure miss Rob.

Ranger
09-04-05, 02:05 AM
Mike,
I recall "Rob" mentioning this before. From what I remember it does not detect an actual octane rating but makes an assumption based on knock sensor reading or something of that sort.

davesdeville
09-04-05, 08:32 AM
Ranger that's what I remember too.

I have read that the outputs for PD42 are 0, 87, 90, or 93. I guess that's wrong if some of you are getting a 91. At this altitude, regular is 86 octane, that's all I use, and PD42 has always read 87. I suppose if what I read is indeed wrong and it can show other numbers, if I brought a tankful of 86 down to sea level it would read 86.

eldorado1
09-04-05, 12:05 PM
Anybody have any further information about how the system actually works?

I think, and this is digging back into the memory banks, what happens is the computer advances timing to a known knock condition. If no knock is detected, it sets the knock sensor code... The stock timing table is more than enough to create a knock when using low grade fuel. It probably "measures" octane by keeping track of knock counts and spark retard.

In the olden days of OBDI, the ECM would keep trying to advance the timing until it got knock. Then it would retard timing drastically (15+ degrees), and then start creeping the timing up again until it starts knocking (in 0.2 degree increments).

The northstar OBDI probably has some logic to keep the retard, and not advance it if frequent knocks are detected... and that retard can be looked up in a table which corresponds to "octane"

Jack Ammann
09-04-05, 05:24 PM
Out of curiousity, if you have OBDI car - check your value.

Peteski

Peteski....could you please give me/us a "cookbook" or a step-by-step procedure to manipulate the A/C controls to be able to read the value on the OBD I for the octane reading?

Thanks, Jack

peteski
09-05-05, 02:18 AM
Guys:
I didn't mean to imply that the ECM has an automagical way to sample the gasoline's octane rating. I realize that the value is derived from various inputs (major one of which is the knock sensor).

I just wanted to point out that there is such a parameter on OBDI cars and it might give us a clue what the car is doing to the ignition timing. I'm still not fully convinced that it is being totally fooled by other possible issues in my engine. I accept that possiblity (as it makes sense) while I still think that I might have been cheated at the pump (the current gas prices might have something to do with my attitude). :D

I do think that this parameter is telling me in what stage of ignition advancement the engine is currently running.

If the gas prices ever drop, I'll fill up in a brand name station with super (couple of tankfuls) and see what the Calculated Octane rating reads then. If it is back to the higher values (even though I didn't do anything to the engine), I'll be sure that I was swindled.

Also, since someone brought this up, I might be a little fuzzy on the 91 octane display. As you mentioned, it doesn't suppose to display that. I could have been 90 (even though I was buying 93 octane gas). Yeah, I know, I know....

Jack:
you can get to it doing the following (on my 93 Eldo):

Press and hold "off" and "warmer" buttons until the display lights up like a Christmas tree.
Release them.
Display will go through displaying codes for all monitored systems. Wait.

Eventually, it will display "PCM?".
Then you hit the "Hi Fan Speed" button (which means "yes")
Next it will display "PCM DATA?". Again hit the "yes" button (see above).
It then starts displaying the PCM data values starting with "P000" (or similar).
The "yes" button is then used to scroll up through the various values.
So, keep hitting "yes" until P042 is displayed. (I don't have my manual handy, but I think that
the Octane Rating is P042).
If you go too far, use the "Fan Speed Lo" button (aka. "no") to scroll back down to a lower parameter.
When you're done, hit "AUTO" to exit diagnostics.

Peteski

davesdeville
09-05-05, 06:56 AM
2 things... calculated octane rating is indeed PD042, but I believe that on my car if I use the "fan speed lo" button, the system exits the outputs and reads PCM? again. I'll check this tomorrow. Anyway if that's the case you can keep pressing hi to scroll through all the outputs until you start over.

Jack Ammann
09-05-05, 08:14 AM
Jack:
you can get to it doing the following (on my 93 Eldo):

Press and hold "off" and "warmer" buttons until the display lights up like a Christmas tree.
Release them.
Display will go through displaying codes for all monitored systems. Wait.

Eventually, it will display "PCM?".
Then you hit the "Hi Fan Speed" button (which means "yes")
Next it will display "PCM DATA?". Again hit the "yes" button (see above).
It then starts displaying the PCM data values starting with "P000" (or similar).
The "yes" button is then used to scroll up through the various values.
So, keep hitting "yes" until P042 is displayed. (I don't have my manual handy, but I think that
the Octane Rating is P042).
If you go too far, use the "Fan Speed Lo" button (aka. "no") to scroll back down to a lower parameter.
When you're done, hit "AUTO" to exit diagnostics.

Peteski


Thanks a bunch. The code on my '94 ETC from its OBD I is "PD42". And I really appreciate your giving me the information. :highfive:

Jack Ammann
09-05-05, 08:07 PM
Petski....I checked my octane rating on my OBD I on my '94 ETC and it read 87 Octane. I, like you, have NEVER put anything but 93 Octane Exxon in the tank :hmmm: Now I'm really wondering if I'm getting screwed or what :sneaky: ....AND I know I'm not gettin' kissed...LOL. I just hate to get screwed without gettin' kissed. I do use single hose pumps and I've often wondered how much regular fuel I have to take before getting to the Premium fuel. :hmm:

I read and recorded all the values for every item on PCM Data and I noticed my initial spark advance was 10 degrees BTC at idle....sooooo...

Do you or anyone else have any ideas? I thought about putting some Octane booster in with the gasoline...AND I can get some 100 Octane fuel to try if anyone thinks that's appropriate.

Ideas please.

davesdeville
09-05-05, 08:31 PM
Next fillup I'll come as close as possible to emptying the tank, then run a couple gallons of trick 101 and see what it reads...

eldorado1
09-05-05, 09:33 PM
Those of you running high test fuel, try this... Check the octane value after driving around town for a bit (with a light pedal). Then do a WOT run. See if it changes after that.

It'll be interesting to see if it 'checks' just after startup, after "cruise" conditions, or only after a full throttle run.

peteski
09-05-05, 11:55 PM
From all your responses it seems that we need to do more testing and get more people to contribute.

It is quite possible that (like mccowden said) if the engine is not in the top form (fresh plugs, wires, etc.) and all the sensors aren't 100% accurate, then maybe the PCM gets fooled and goes into degraded mode. That would mean that we all aren't getting full horsepower out of the engine.
Like I said - I remember mine being at 90 (or higher than 87) before. But that is also inconclusive.

Bring on your findings.

Jack: if you have access to 100 octane fuel and don't mind spending the money - give it a try. I don't think it'll hurt anything.

Too bad that "Rob" isn't here to take the mystery out of this....

Peteski

93ELDORADOETC
09-10-05, 03:28 AM
I only run Chevron, Shell V-Power, or 76 @ 91. When it's over 100 degrees outside I add octane booster...


Mine says 90

peteski
09-13-05, 02:05 AM
Thanks for chiming in with your findings.

Peteski