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

[DesertBMW]

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.

I have lots of experience tuning stock engine management and after all these years the best tune that I have found is stock tune. You and me live in hot dry climate, chips made somewhere in humid and cold states will simply ping here in AZ.

There is no power increase with bigger injectors at all, no power with bigger TB, it is just waste of time and money. Best mod that I have found is running M50 injectors with 2.5bar regulator, this makes the fuel spray better at the same exact rate and stock so you can run stock chip. Adjust valves slightly loser than recommended spec, old valve springs and worn valve guides will sometimes make the valve float and engine misfire with up to stock spec valve adjustment.

[6SPD]

Good read but few solutions.

I've had a similar issue on my m20... A 2.8L 10:1 build with a 272 cam.

Nothing has seemed to work for me except looking running a standalone or bumping the octane using octane booster

[Hoffbrohaus]

Update. I have put in a stock chip and eliminated the detonation. However....

Cold starts now are ridiculous. I need to give it some gas as I'm starting, sometimes it stalls right away. Once the car is warmed up it idles ok, a little lumpy. Took it to a trusted mechanic for a smoke test and he said he wouldn't do it because he knows it's not a vacuum problem since my AFRs are good.

I put in some stock 14# injectors recently cleaned by MEPEH, and still have the starting problem with lumpy idle.

Just to recap:
New wires, plugs, coil, dizzy looked fine
Put in Stock chip, injectors
Valve adjustment
New fuel filter
Tested fuel pump flow was ok
Fuel pressure is right, although it doesn't hold after pump turns off
ICV is buzzing but resistance is not per bentley manual and when I disconnect it nothing happens to idle

[Hoffbrohaus]

Ordered a stock chip from a R3ver so we'll see what that does. In the meantime I checked my ICV based on the Bentley. It buzzes in the on position. But when the car is running the idle does not change when I disconnect it. The resistance is 24ohms when Bentley says 9-10. What does that mean? Should I clean it out or just replace?

[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.

[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

[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?

[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.

[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]

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!

[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.

[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]

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.

[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.

[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.