MrLeadFoot

Anyone ever heard of this? According to the data log at the tuner's, logging began at idle at 19 degrees. As we started to run it hard, the computer switched to the low octane table before any KR was even detected. Then, KR appeared and the computer pulled even more timing. Then the computer seemed to stay in the low octane table, and we couldn't seem to get it out of that mode. No codes, and all sensors "appear" to be working. The computer seems also to be adding 12-14% fuel.

We looked at the timing numbers and compared them to the tables, and saw that the low timing we were logging matched up to the low octane table's numbers, and that's why we're pretty sure the car is "stuck" there.

For what it's worth, on the first visit to the tuner, we saw the same thing. I even left and twice ran the gas to down around 1/8 of a tank and refilled it, hoping to get the computer to pull out of the low octane table. I even babied it after the second refill, just to make sure my driving habits weren't contributing to the problem (who knows if that even matters).

The tuner and I are at a loss.

Now, I don't know if this means anything, but the car previously had a very noticable audible ping when equipped with stock stat and stock fan settings, when "normally" taking off from a stop. The ping could also be heard at tip in. Car ran easily up to 230 coolant and 230 oil temps. I recently switched to a Hypertech 160 Stat, and now the car runs 178 - 185 coolant and 200 - 205 oil. Surprsingly to me, just lowering the temps appears to have eliminated the audible ping.

As expected, after a hard run today at the tuner's, and much idling to try and diagnose the low octane table problem, we saw coolant temps climb into the 225 range over that long period of idling. After temporarily giving up to mull over the problem, I drove home. Since the car was really hot then, I was disappointed to disccover that the pinging was back with a vengeance. But, once the car cooled to around 190 coolant and low 200s on oil temps and lower during the drive home, the ping stopped completely. In fact, since changing out the stat and lowering fan temps, today was the only time I've ever heard the ping again.

The only mod I have installed is a Halltech Stinger (with stock air screen MAF screen installed). Please bear with me because I'm starting to grasp at straws here, but I'm wondering if the larger Halltech airbridge could be contributing to the problem, although the tuner doesn't think so because he's not seeing any significant turbulence.

I'm running Denso Iridium plugs. While I don't think they could be a contributing factor, should I even bother trying to switch plugs... maybe even to a lower temp range?

Fuel filter was replaced today as a stab in the dark, since the car has 37k on it and it's never been replaced. Knock sensors have also been replaced very recently. O2 sensors appear to be operational, as does the MAF sensor.

What else can possibly cause this, that even an experienced tuner might not be aware of? I am aware that someone would have to pay for the time he spends trying to diagnose this problem, and stabbing at things with a process of elimination can get expensive, no matter how honest and fair he is, so I'd really appreciate some feedback and input here.

I am quite resourceful, so I would rather try to rule some things out on my own here. I'm even wondering if some kind of fuel injector problem could cause this, but I'd hate to buy new ones only to find out that they aren't the problem, because I can start getting into some serious money on replacing things just out of desperation, you know what I mean?

With that said, could dirty injectors cause something like this? If so, should I try cleaning them? How, and with what? Something easily available, I hope.

I should say that under SOTP driving, I don't notice anything unusual. No stalling, hesitation, surging or anything like that at all.

I also recently did a debarb with GM Top End Cleaner, and installed a catch-can to prevent adding the oil back into the intake, so I'm pretty sure carbon buildup can't be the culprit here... or can it?

The only consolation for me thus far, is that I now know for a fact no matter what anyone tries to tell me is that high temps without a doubt can definitely cause a LOT of pinging.


Thanks in advance!

'01 MN6 Coupe, using LS1Edit for tuning

Mike Mercury

it is my understanding that once the lower octane table is selected, the only way to reset it up higher again is to add at least 5 gallons of fuel to the tank.

Occasional KR will cause a temporary reduction in spark advance. Once the PCM determines it's happening more than just "occasional", then it selects a lower octane table an stays there until you fill up again (5 gallons or more)

BTW, no need to post this in multiple forum sections. We all read every C5 section anyways.

MrLeadFoot

Which I've done more than once, only to have the PCM go back to the low octane table.

That's the puzzling thing. The PCM switches to the low octane table BEFORE and KR is logged. And even in the low octane table, running 14 degrees advance, we see it pulling another 6-8 degrees from there!

Ok, sorry about that. After posting under Scan and Tune, I was just thinking that more people read the General Fourm.

MrLeadFoot

Is that it? Any other opinions? Would someone who understands how the PCM really works, or someone familiar with tuning be so kind as to chip in their $.02? If you want to help a fellow C5er, but don't want to give away too many secrets, I understand. Maybe you can PM me? I promise not to blow your cover.

c5inphila

My car had false knock after a cam install. My tuner ( kevin" nogo " from east side performance) changed the numbers in the low timing table to the same numbers in the high table.

MrLeadFoot

What do you mean by "false knock"? Is false knock the same as pinging, or detonation? In other words, could you actually hear it, or see that the knock sensors heard it, and pulled timing as a result of it?

c5inphila

No, you could not hear the knock. He said it could be a number of things ,vibration, exhaust system touching something else etc. But i also had the problem of it staying in the low table, thats why he switched the numbers.

Red C6 Z06

I was going to suggest that it could be the routing of the Knock sensor wiring that allowed EMI to induce some false signals into the knock sensor wiring, but you said that the low octane table was selected before any knock signals, so that probably isn't an issue.

Is there ANYONE who really knows how the PCM works? Does anyone really understand the software (even GM Engineering)!? Obviously tuners can learn from experience how things they do affect the operation of the PCM, but does anyone have an understanding of the intent of the basic operating code or the FPGA or microcontroller coding in the PCM? I sort of doubt it.

MrLeadFoot

Did you notice the difference once he copied the numbers into the low table? Did your drivability and performance increase? And, I assume you haven't had any problems because of it, right?

In my situation, I'm kinda wondering what would happen if the low table was made the same as the normal numbers. I mean, if I'm stuck in the low table as it is, and the PCM is still pulling timing, then what would happen if the low tables timing numbers were the same as the normal? Would the PCM still pull timing? If so, how far would it go? Would it still reduce it all the way down? If so, what use would copying the numbers be, you know what I mean? But, if there is something positive about that maybe I'll give it a try.

c5inphila

My car ran better after he altered the timming table, he also reduced the timming in some areas by 2 degrees ( 27 degrees peak ). The timming will still be reduced in case of a real knock condition. He said to always use at least 93 octane. The stock low table is there if you use a lower octane it will not knock and pull timming all the time, it just goes to the lower table till you fill up with better fuel. Kevin said all engines are diffrent as far as timming requirments, and 2 degrees would not be noticeable. I checked a few times afterwards and there is no more knock retard and 2 weeks ago ran 11.86. 116 mph . Sounds like the heating up of your engine is causing your pinging , mabey to lean a/fr ?good luck, walt

MrLeadFoot

I wonder if all the hand-held programmer devices, like Hypertech, Predator and Vinci also copy over the low table. Might explain why nearly everyone who gets them says they "see" a performance increase. Hmmm...