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Ive searched this before but really couldnt find what i was looking for. Can anyone tell me all the things that come into play when it comes to vtec kicking in. I know it has to do with the proper oil level/pressure and temperature... but what else is involved? when i first bought my ex, the speedo didn't work, so naturally the vtec didnt work. Well when i fixed the speedo, the vtec still isn't kicking in. I have enough oil and the car is running at the proper temp. I even switched out the vtec solenoid with one that I knew was functioning... any thoughts?
 

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ECU must see warm coolant and sufficient oil pressure (I'm not sure, I think it was 165 degrees and 65psi respectively on my y8, but I haven't looked at that stuff in over a year). Be at wide open throttle, going over the minimum speed, what is that 30mph maybe. Engagement on the y8 is 5500rpm but the your z6 may be lower.

Then if the ECU gives the signal, the solenoid must switch (not stick closed), the oil must be able to pass through the solenoid valve gasket's screen and build pressure without leaking across the gasket, and finally all the little vtec lifter lockers gotta slide over. After all that, it's working but you won't hear it on a stock intake.

To get my vtec to work every time:
*Oil pressure- cheapo oil filters do not flow well enough for a fast transition, especially if your motor is higher-mileage with worn bearings and doesn't exactly have oil pressure like when it was new. K&N oil filters work well at this due to the bypass valve. My Motorcraft FL-1A (MASSIVE) filter that I remote-mounted does work well without a bypass due to its relatively enormous amount of filter element surface area.
*VTEC solenoid- my gasket screen wasn't plugged (this does happen sometimes tho), but it was cracked and leaking between its inlet and outlets. It's cheap to replace from the dealer, but make sure you buy the three bolts that hold it on too. It's a pain in the butt when you break one off back there and there's no room for a drill unless you remove the intake manifold! (I got lucky, the bolt core shaft snapped, but the thread barely pulled the end out.) Also, don't overtighten....
*Oil- I used to use Valvoline max-life, but even with everything else working, it would sometimes not engage. My one day on the track, my first pass I had VTEC in all 3 gears, my second pass I had vtec in 3rd gear only, my third pass it was 2nd only, etc etc. Also on occasion I believe I had a vtec lifter stick in the ON position (BAD idle, poor low-end) until I revved up and actuated VTEC again. Apparently the seal conditioners in that oil worked too well. I have since switched to Mobil1 and believe it or not, I burn less and VTEC is totally reliable. You don't have to use Mobil1, any typical oil will work as long as it doesn't have that funky goo in it.
*Sound- Any aftermarket intake will make it noisy, but a velocity stack in the filter is the ticket. I've tried both, and the stacked unit is SO loud in comparison.

Hope this helps...

Holy I typed a lot
 

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you sure it's not kickin in?

when vtec engages it's nothing dramatic, it's not like feeling boost from a turbo or anything.

hook up an LED light to the vtec solenoid to see if it's sending the signal.
 

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since you fixed something with the speedo I would check the vss
 
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