ECM knock test/ timing?

[ccarley]

I'm just curious, and have a theory.

So, the ECM does a knock test by adjusting timing to make sure the knock sensor works after you fire it up, within a certain amount of time, yes?

I found my Suburban would die or almost die while warming up, and then throw a check engine light and not run well. Sometimes it would "hiccup" and flicker the check engine light, but continue running. On re-fire, it would run fine. I never checked the code, but figured my timing was off, because it would ping under load.

Sure enough, the timing was off by a few degrees. I re-adjusted to zero, and it's been running fine since, with no pinging. I'm guessing that during the knock check, with the timing off, it would just push it too far off to the point where the engine dies and throwing a code. I never checked the code before fixing the timing...

 

[dyeager535]

Is this just with the motor idling? It probably depends on what setup/application, but I just learned that the test on some at least is run under load (which makes sense) and that it increases timing until it knocks. Premium gas can cause lack of knock sensor feedback as well apparently.

How far off was your timing? I know people run easily 2* advanced, and I'm sure some are even at 4*, never heard what you describe as a problem.

I also saw some pretty smart folks recommending to just turn off that test when tuning, as it can happen at inopportune times.

 

[ccarley]

It's hard for me to say how far off it actually was. My timing pointer has a large V-notch next to a few smaller V-notches. The large one (I believe) is 0, but I have no idea what the smaller ones are marked at. It was 3 or 4 notches away from 0.

Over the weekend it was with the motor idling; idle was high, then it "kicked" low and flickered the light. I was driving under mild load yesterday when it set the light & died.

Today it has been fine, so far.

Oh yeah, here in CA, I sure haven't used anything other than regular 87 octane in the tank Regular gas is finally getting under $4.50/ gallon...

Thanks for the reply! Maybe this is a different issue than what I think it is...

 

[dyeager535]

I've been playing with the idle timing on mine (26*), and I haven't heard it ping at idle...without a load, apparently it won't happen, or you've got to try REALLY hard to make it happen.

You very well might be dealing with something else. I don't know as I've checked my Camaro manual to see if it defines when the test takes place.

 

[eagle mark]

It's not the ESC test. That is done warmed up at highway speed at steady state.

You are setting timing with bypass wire disconnected, correct?

If it's happening during warm up I would suspect an O2 sensor issue causing it when going closed loop. Really need to check DTC codes. There will be a Code 42 from setting timing with bypass wire disconnected. Look for others. After reading codes dissconnect batterey to clear them and see if they come back.

 

[ccarley]

Hi eagle_mark,

Indeed I am setting timing with the bypass wire disconnected, warmed up to operating temp.

After setting the timing, I haven't seen the light at all, and although there is some hesitation during acceleration when cold, it's been doing that for a while (and I suspect it may be EGR related after reading another thread). I replaced the O2 sensor when I got the truck a few years ago... with a Bosch unit I believe. I haven't seen the O2 sensor code at all since replacement, but that sure doesn't mean it couldn't be bad.

Thanks for the info on the ESC test too! That helps get my head wrapped around this.

 

[eagle mark]

CEL will not be on after setting timing, but the code will be stored.

You should have another code too with what is happening. It will point you in right direction.