Source of knock?

[q92awd]

This may be long, so I appologize in advance.
Pre knock problem: Added a 190 fuel pump, 550 injectors, and 2g mas at the same time and the car ran great. No knock that I was aware of.
Knock problem begins: I bought and installed the maft and 3" gm maf and tried to tune using the maft. Would peg the knock count (43) for a normal drive, but could also stop the car and start again and have no knock count.
Currently: I bought a s-afc and went back to the 2g mas. Still knocked a lot so I replaced the knock sensor and reset the ecu to start tuning again. Car would run fine but would need a little extra fuel in the low throttle settings of the safc to keep the fuel trims at 100%. Now it's at +4@1k, +15@2k, +18@3k and then I can drop it back down after that to keep the fuel trims close to 100.
Every time I start the car and let it warm up for a minute and then drive, the knock count pegs at 43 again. It then settles down for a while and then starts to knock again after about 10 minutes of driving.
Any help would be greatly appreciated. Let me know if you need more info.

 

[kpt4321]

When does the car knock? Does it knock under part throttle, and then when you lift of and reapply throttle the knock goes away?

 

[q92awd]

Mostly knocks under part throttle.
When I lift off and give it gas again to try to stop the knock sometimes it will stop. Other times there doesn't seem to be anything I can do to stop it - lift off and gas it, lift off and floor it - nothing works.
Seems to do it mostly under load, like going up hills but it isn't exclusive.

 

[Zenja]

My car does the same thing. There's probably some source of phantom knock but it's really hard to give a helpful suggestion to check like this.

 

[q92awd]

Any reason why I have to set the safc to +15? Seems very odd to be that high with 550's and a 2g mas. Possibly a bad O2 sensor? I replaced it about a year ago (only a couple of thousand miles), but I'm grasping at straws to get my fuel trims and knock under control.

 

[kpt4321]

Those fuel curves are weird. It's fairly normal to have a more positive correction at 1k, but your taper is in the opposite direction.

I would check for boost/vacuum leaks by pressurizing the intake system, before anything else.

 

[q92awd]

Originally posted by kpt4321
Those fuel curves are weird. It's fairly normal to have a more positive correction at 1k, but your taper is in the opposite direction.

I would check for boost/vacuum leaks by pressurizing the intake system, before anything else.

I did find a boost leak. Don't know if I got the only one because my leak tester failed on me. I'll take it for a drive later and figure it out.
Thanks for the help! I'll keep this updated.

 

[talon1979]

Originally posted by q92awd
Any reason why I have to set the safc to +15? Seems very odd to be that high with 550's and a 2g mas. Possibly a bad O2 sensor? I replaced it about a year ago (only a couple of thousand miles), but I'm grasping at straws to get my fuel trims and knock under control.

Those setting actually sound about right to me. 1g's love fuel.

 

[DSMWillyb]

I'm basically experiencing the same thing. My car was running great with the stock 1G MAS and 450's - and as soon as I switched to the MAFT I haven't been able to figure it out...

*3.5 GM MAF/ MAFT
*255lph/780cc
*FMIC/1G BOV
*14B (mildly ported)

Fuel Trims:
Low - 86%
Mid - 94%
High - 96%

I get knock (full count usually) as soon as I get into boost (~1 - 2psi) and it's driving me crazy!!!

I've checked/replaced:

*Knock sensor
*Boost leak test - found leaks, but did not fix knock problem...
*O2 sensor - it cycles normally on the logs/jumptronix AF guage...

I don't know if it's phantom knock or what... I'm planning on dumping in 2 gallons of Toulene as soon as my tank is >1/4 full... If that doesn't cure it I don't know what the heck it is...

 

[Coldandafraid]

ok, this is just an observation... but these people that are now having phantom knock problems, started having the problems AFTER the GM MAFT install...

I too USED to have nice 20 degree timing advance at wot before the GM maft install, and now, can only manage 10 degrees, and am seeing partial throttle phantom knock.

could the GM maft somehow be related, or causing the phantom knock???

 

[kpt4321]

WillyB: Your situation in particular has several things which stand out with me.

Basically, you are not pulling a lot of airflow (14b), and the ECU is seeing quite a bit less airflow then you are really pulling. Since you have 780 cc/min injectors, that small amount of airflow you have to begin with is getting cut in half by the time it gets to the ECU. As such, the ECU is going to be VERY libaral with the timng adance.

1g's in particular (as opposed to 2g's) tend to have more advance and less fuel enrichment in the boost onset region. This will generally make the car feel better, but it also makes it more likely to knock. The MAFT allows you to pull more airflow here, and the huge injectors just give you more timing advance.

As a start, I would strongly consider turning your CAS to retard the ignition timing a little bit.

I would also consider getting a chip with injector compensation. That will put the timing back much much closer to normal/safe values.

Definately toss some GOOD race gas in there too, to see what happens. I'd be willing to bet that the problem goes away, at least some, because I think it is real knock.

 

[kpt4321]

could the GM maft somehow be related, or causing the phantom knock???

If the knock is phantom, then a device like the MAFT will have zero effect on it whatsoever. Phantom knock is named as such because it occurs when the motor isn't really knocking, caused by some other source.

If the MAFT is causing knock, then it's pretty darned likely that it's real knock. Like I touched in the aboe post, anything that opens up the intake (or exhaust, really) is increasing the VE of the motor which allows you to build more boost and more load faster and sooner.

 

[Coldandafraid]

ahh, well I had just noticed that a number of peoples "phantom knock" problems started after the gm maft install.

I have a stage III chip installed now, thats how I know im knocking around 50% throttle... but I dont knock at idle, or under 30% throttle... so maybe mine IS real knock.