False knock from lifters and rockers = lean

[KANE]

Yeah- like the title says.


So I was finally able to get my '82s lean condition sorted out. The ECM was taking fuel out as the result of perceived knock. Turned out the valvetrain needed to be adjusted- false knock from noisy lifters and rockers.

Why? Apparently they weren't readjusted after the bigger cam installed a few years ago had been broken in. :duh




Runs like it should now!





What's the lesson here? 31 year old car valvetrains shouldn't be overlooked.

 

[Hib Halverson]

Yeah- like the title says.


So I was finally able to get my '82s lean condition sorted out. The ECM was taking fuel out as the result of perceived knock. Turned out the valvetrain needed to be adjusted- false knock from noisy lifters and rockers.
Why? Apparently they weren't readjusted after the bigger cam installed a few years ago had been broken in. :duh

Runs like it should now!

What's the lesson here? 31 year old car valvetrains shouldn't be overlooked.

"What's the lesson here?"

Ah, well....engine control systems DO NOT change fuel delivery due to false knock. When the engine controls detect knock–be it false or real–they retard spark.

 

[KANE]

"What's the lesson here?"

Ah, well....engine control systems DO NOT change fuel delivery due to false knock. When the engine controls detect knock–be it false or real–they retard spark.

Yes- pulled timing... and then pulled fuel (which we talked about).

Your system is rich so the ECM is pulling out fuel. Block learn (today it's called long-term fuel trim) is below 128 and your integrator (or short-term fuel trim) is just a tad low.

Here was the progression...

• engine with mild, low throttle hesitation; valvetrain noise intermittent- suspected lifters losing prime or out of adjustment
• datalogged; scans revealed excessive knock counts and lower than 128 values for BLM
• adjusted fuel pressure; set base timing, idle, and TPS position; all values matched factory specs- no change in BLM or knock counts
• lifter / valvetrain noise went from intermittent to nearly constant
• adjusted valvetrain
• BLM and integrator values back to normal

false knock-> retarded timing -> pulled fuel -> poor performance

I could see the knock counts and the lower BLM- just had a devil of a time figuring out the source.

Hib- if you remember... you and I talked about the values- 118 BLM. I also had 120+ knock counts within three minutes. Once I adjusted the valvetrain- it went back to ~128 BLM.

 

[Hib Halverson]

Valvetrain noise will cause false KR and, as a result, the system retards timing until the KR incident ends.

Engine controls do not lean the fuel delivery in response to KR.

I'm not disputing your scan data, I'm just stating that for whatever reasons the ECM is subtracting fuel, it's not in response to a knock signal.

But forget the scans...let's just think about that in theory. The main reason for feedback control of spark is to eliminate detonation when the engine is under load. The KS "hears" knock and the ECM retards the spark until the KS stops hearing knock. When the engine is detonating, the LAST thing you'd want to do is lean the AFR as lean AFR makes detonation worse.

Again, the ECM subtracting fuel is not a strategy the ECM uses in response to knock.

As for why the fuel trims are moving with the KR? Your guess is as good as mine but since the fuel trims normalize when there's no KR, why worry?

 

[KANE]

As for why the fuel trims are moving with the KR? Your guess is as good as mine but since the fuel trims normalize when there's no KR, why worry?

 

[KANE]

As for why the fuel trims are moving with the KR? Your guess is as good as mine but since the fuel trims normalize when there's no KR, why worry?

Hib- The only thing I can think of is that somehow... maybe... uneven lash between cylinders was creating the issues not just with knock- but also with combustion. The number 2 cylinder was the worst culprit- it was seriously out of adjustment.

Like I said, the cam was replaced with a higher performance cam several years ago and I'm sure that it was never readjusted after break-in (this all predates me). Then... after I swapped out the intake, re-tuned it, and drove it up to and around a higher redline, I think just exacerbated things that may have already been in motion. There had been some noise when cold for a few years from what I understood- and it seemed to have been getting worse from what I had heard.

I'm probably really lucky nothing broke or got bent up at this point.


Whatever... it runs like a dream now.