Pre-ignition, detonation at wot in 3rd & 4th gear

[Hoffbrohaus]

I'm hearing detonation around 4.5k-5k rpm at full throttle only in 3rd and 4th gear (maybe 5th too but don't want to go to jail). 1st & 2nd are fine all the way to redline.

I have done the following while chasing this...
New FPR
New o rings on injectors and cleaned. (19#ers btw)
New plugs, new wires, new coil
Checked cap & rotor looked good (no carbon anywhere and tips were fine)
Installed wideband o2 and afr gauge, afr looks good - 14 and gets richer at higher rpms 12 or high 11s when the Pre-ignition occurs.

What do you guys think?

I'm also having a long crank issue but not sure if it's related. Fuel pressure checks out fine just not holding when pump is off.

[digger]

what engine mods and specs etc if applicable?

[Hoffbrohaus]

what engine mods and specs etc if applicable?

M20B25 sorry I always forget about the 4cyl brethren
M20 MAF, Big bore throttle body, 19# injectors, custom chip for 19#ers and 91 octane.
Not sure if it matters, but catless supersprint exhaust.
Also forgot to add to the list of recent fixes a valve lash adjustment and new fuel filter.

[LJ851]

WOT switch is a possibility. If the switch doesn't close, you are running with the higher advance numbers of the part throttle map. You might blast through that area fast enough in the lower gears to not really notice it.

[Hoffbrohaus]

WOT switch is a possibility. If the switch doesn't close, you are running with the higher advance numbers of the part throttle map. You might blast through that area fast enough in the lower gears to not really notice it.

So sure enough I am getting detonation in other gears. It looks like it is load related. Since I heard nothing in 1st or 2nd gear I tried it going uphill, then I heard a little detonation. 3rd & 4th gear still much more pronounced and while going uphill it starts sooner, like 4500 rpm with an AFR of 13.5

I will check Bentley for the WOT switch and see if I can test resistance or something.

[ak-]

Your AFRs seem pretty okay.
13ish @ peak torque for an NA engine isn't harmful and I think near optimal.

You can test the TPS yeah, and I have a new one from the dealership if yours ends up being a dud if interested.

[dewme5]

Is it a new problem? It's possible the chip tune is not great for your engine. Each individual engine has it's own temperament. It's also possible you have a bad tank of gas. Cheap station, bad gas, extra ethanol, not enough ethanol, winter blend, etc.

[digger]

its unlikely to be a mixture thing on a basically stock NA engine. the timing could be out, try a stock chip?

does the engine have an oil control issue and burn oil? if you get oil into the chamber it lowers the octane substantially and you can get heavy carbon buildup. ive had success with seafoam and subaru upper engine cleaner which helps clean some carbon out

make sure you arent drawing in hot air from the engine bay as this also increases tendancy to knock

[dewme5]

its unlikely to be a mixture thing on a basically stock NA engine. the timing could be out, try a stock chip?

He has a tune, so does the chip push the timing limits on great gas? When mixed with bad gas, can easily lead to knock issues. Knowing when the problem started, along with the mods before and after will help to narrow the issue down.

[Hoffbrohaus]

Nothing new, I have been chasing this gremlin since I bought the car in Oct 2013. I have tried a bunch of different gas stations Shell, Chevron, Texaco, all 91 octane all result in same issue. I have tried 3 different chips, tuned by 2 different people same results. At one point I thought it was the M30 AFM I had, that's when I put the AFR gauge in, saw how lean I was. Put a stock AFM in and now my AFRs are good but still have pinging.

I will check the TPS/WOT when I get back on Sunday. Thanks guys!

[digger]

He has a tune, so does the chip push the timing limits on great gas? When mixed with bad gas, can easily lead to knock issues. Knowing when the problem started, along with the mods before and after will help to narrow the issue down.

someone with more experience might be able to chime in as to whether a bone stock m20 is knock limited on pump 93 or 91 etc

[Hoffbrohaus]

Ok I took off my TB and checked the resistance per the Bentley manual. All looked ok, adjusted the TPS a tiny bit. Still have the problem. At this point I'm thinking of going straight stock, that is...put in stock injectors and ECU and see what happens. Unless you guys have other ideas.

[Hoffbrohaus]

its unlikely to be a mixture thing on a basically stock NA engine. the timing could be out, try a stock chip?

Can I use a stock chip w/19# injectors?

does the engine have an oil control issue and burn oil? if you get oil into the chamber it lowers the octane substantially and you can get heavy carbon buildup. ive had success with seafoam and subaru upper engine cleaner which helps clean some carbon out

Yes it burns oil but I don't think in the chamber since my plugs look ok. Thinking it was just leaking on the outside of the block since the undercarriage is covered. I tried seafoam early on in the process with no luck. I did notice a thin coating of oil in the intake manifold when I took the throttle body off, is that something?

[digger]

stock chip will work with 19lb for purposes of diagnoisis, oil in the intake is pretty normal since the valve cover breather gets routed back into the intake

[Bullhead]

I have ran into something similar to this many times. Someone thinks they can advance the timing through the belt or chain and slip the cam a tooth or 2 in advance to try and "advance" the ignition. These cars do all that themselves internally and on a cpu interface. It is possible it has "jumped" time or someone has permanently advance or retarded the timing. The latter is done almost exclusively on purpose for NOS. I am saying this could be your culprit because it definitely sounds like a timing or vacuum issue to me.