Misfire from valve seal ?

compression readings for that cylinder stayed the same hot or cold. but the misfire is only when warmed up like when the thermo is open. I scoped the injector and see a good waveform , when it starts getting rough the injector on that cylinder goes offand the check light comes on saying misfire on that cylinder flashing. I know the injector ecu is doing that because if I shut off the car and restart it the waveform is back to normal with a rough idle , than the injector goes flatline battery voltage. on every restart the waveform is back with pintle hump.

The thermostat opening should not change much? Given that the engine is full of water all the time. If anything water pressure might drop a bit once it opens. A head gasket leak could be hard to catch on a compression test if it is intermittent.

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What vehicle is this? Does it have a 2-wire or 3-wire coil? Do you have an amp clamp to check coil primary side current. If it has a 2-wire coil, you can check for control on the negative wire with an incandescent test light.

Anyone feel free to correct me, but wouldn't a low compression miss typically be worse on a cold engine.

boxster 2.7 liter . 3 wire coil. the coil continues to fire during the misfire. with the spark tester its a good spark. its the injector that the ecu turns off.

wondering why the gauge somewhat jumps while increasing pressure.

the right side gauge jumps oddly while adding pressure

I don't see any images. Are you talking leak down gauge?
Im baffled, if you have good spark, good fuel/air, and good compression, you have good combustion.
The rough idle is definitely before the computer shuts down the injector. But not when the engine is cold. You also notice fluid droplets in the exhaust manifold that you suspect are water.
Im thinking cylinder issue. I guess temperature rise could cause the spark to change or the injector, but you graphed the injector and it looks good until it goes flat. Did you graph the spark?

Did you try looking for hydrocarbons in your water? I saw this a while back but never tried it myself.



Good luck.

Edit. I don't see the link i shared?
youtu.be/MCo0SLeMEfk






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I preset the leak down gauge and connected the hose and Im showing 40psi on both gauges seems like less than 1 percent leakage , however you could hear it from the crankcase.

You preset the gauge to what psi? 100psi right? Don't read the difference between gauges but the difference from static to loaded on the down steam gauge.
If it went from 100 down to 40 that's a big leak. When you go to the other cylinders what do you get? 10-15% leak would be normal and it should be similar accross all postons

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From the first video I preset it to 30psi before connecting hose to cylinder. Once i connected the hose both gauges dropped to about 15psi.
The other was about the same . You could still hear it from the oil cap. But only that one cylinder is misfiring.

are you setting all the cylinders to top dead center when you perform the test? You have a 50% leak if it goes from 30 to 15. my set is a bit different. it has percentage on one side and psi on the first gauge. I have to set it to the max and then connect.

it's set at top dead center. From the box im told its 15 percent leakage. I only tested it at low pressure because someone sugguested it might be hiding the leakage at higher psi.

Regardless if i increase psi or preset the psi before connecting to cylinder you could hear air coming from oil cap. Even tho a standard compression test have close numbers across all cylinders

Do you get the same noise from the oil cap with all cylinders? sounds like valves leaking. It could also be a head gasket leaking into a passage connected to the valve cover area?

No sound from intake or exhaust only from oil cap. All does cylinders are doing same sound but only one cylinder is misfiring

Go by your chart, I'm not familiar with that set. What is the leak rate on the good cylinders?

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The rate was about the same but no mosfire on other cylinders

What about the radiator/water? Any bubbles? Im out of ideas

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no bubbles. its just coming from the oil cap and if you disconnect the air separator pcv you also hear it blowing into manifold. im not sure why the leak down is the same number on both gauge. im thinking about pulling the head big job on mid engine

what should the compression psi be for this engine ? anyone knows the specs ?
2.7h boxster 987