False knock? 3.8*

L

l3lue

Member
Joined
Jun 19, 2005
I'm wondering if I am getting false knock at the shift, currently car has no exhaust just a down pipe cut hanging (I know) ..Car runs good, just replaced fuel pump with racetronix unit an replaced fuel filter an reran fuel line in new braided hose..Fuel psi rises with boost level...

I got 3.8* @ 148o2 on scanmaster...Is this real? Or false knock from lack of exhaust?
 
I doubt the open down pipe would cause that. Mine never did. The first place you're going to see real knock and even false knock will be on the 2-3 shift. That's when the engine is subjected to some if its heaviest loading.
If the timing is a bit too aggressive or octane a bit too low, you'll see real knock at the shift and possible during the 3rd gear pull.
It could also be false if the 2-3 shift yanks on the engine hard enough to roll then engine over far enough that something is physically hitting somewhere.
 
l3lue said:
I'm wondering if I am getting false knock at the shift, currently car has no exhaust just a down pipe cut hanging (I know) ..Car runs good, just replaced fuel pump with racetronix unit an replaced fuel filter an reran fuel line in new braided hose..Fuel psi rises with boost level...

I got 3.8* @ 148o2 on scanmaster...Is this real? Or false knock from lack of exhaust?
Click to expand...
Really need to see Afr where the knock comes in.what mph on the scan aster recall does the knock happen?and does it go away throughout the pull or increase?fp rise is good but its the fp at wot that keeps things from melting.what fuel are you running?boost level?timing?
 
Mr.Spool said:
Really need to see Afr where the knock comes in.what mph on the scan aster recall does the knock happen?and does it go away throughout the pull or increase?fp rise is good but its the fp at wot that keeps things from melting.what fuel are you running?boost level?timing?
Click to expand...

Car is bone stock, stock injectors, stock turbo , Running 93octane and the boost is at about 15-16PSI, running a turbo tweak chip & LT1 MAF w/ Translator

Haven't checked the MPH on the recall (was not going very fast estimating probably 60MPH?), just saw the 3.8* and 148 - 02 milivolts..what exactly are '02 milivolts'? Please excuse my ignorance, newbie
 
O2 millivolts are you oxygen sensors readings. .148 millivolts is showing way lean.


Next time you see knock like that hold down both buttons and it will show the MPH where they both happened. Often, they will match in cases like yours. That's a hint that knock was caused by a lean condition.
 
earlbrown said:
O2 millivolts are you oxygen sensors readings. .148 millivolts is showing way lean.


Next time you see knock like that hold down both buttons and it will show the MPH where they both happened. Often, they will match in cases like yours. That's a hint that knock was caused by a lean condition.
Click to expand...

Well I just put in new fuel pump & reran fuel line and replaced fuel filter. Inside the tank looked really clean...what are some things I can look at that would be causing lean conditions? Thanks
 
First things first, you should verify fuel pressure is rising 1:1 with a FP gauge taped to your windshield. If your base setting FP is say 43psi, then you should see 58 psi at 15psi of boost.
 
Base fuel pressure reading with a tt chip should be 43 line off as well.
 
l3lue said:
Car is bone stock, stock injectors, stock turbo , Running 93octane and the boost is at about 15-16PSI, running a turbo tweak chip & LT1 MAF w/ Translator

Haven't checked the MPH on the recall (was not going very fast estimating probably 60MPH?), just saw the 3.8* and 148 - 02 milivolts..what exactly are '02 milivolts'? Please excuse my ignorance, newbie
Click to expand...
Check the recall when the knock happened.does it go away or increase.fp at wot?
 
l3lue said:
yes, new fuel pump w/ hotwire, new fuel filter , new feed fuel line braided from tank forward
Click to expand...
That good, now you need to check the pressure to make sure it's raising 1 to 1 with boost.
 
Do a third gear pull 50 to 80mph while keeping a close eye on o2 millivolts and knock. O2 # should be in upper 700 low 800's. Post back readings and don't forget to get the MPH reading.

Rick
 
Last edited:
Ok got to around 50mph an floored it, new numbers, 679o2 and 2.8 * ...the next set of numbers on recall are 082 and 074 (what do those mean?) looked to me the boost may be little higher than I thought (17-18PSI) can removing the exhaust make the boost raise at all? at WOT the fuel pressure seems pretty good up up close to 60PSI
 
It means you were running 82MPH when you saw the leanest O2 number of .679V


and you were running 74MPH when the highest knock retard reading of 2.8 happened.


If you have a restrictive exhaust, unbolting it can make the boost level rise. The more it flows the less rise you will see.
 
l3lue said:
Ok got to around 50mph an floored it, new numbers, 679o2 and 2.8 * ...the next set of numbers on recall are 082 and 074 (what do those mean?) looked to me the boost may be little higher than I thought (17-18PSI) can removing the exhaust make the boost raise at all? at WOT the fuel pressure seems pretty good up up close to 60PSI
Click to expand...
Drop the boost 2/3 lbs and do another pull see if your O2 goes up into the high 700s and the knock goes away
 
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