I finally finished the engine swap in the $600 Volvo. Yes, it runs. However, I’ve been ripping my hair out for the last 2 days trying to figure out why it idles like shit.

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When the car is cold and I fire it up, it will run and idle fine for a few minutes until it warms up a little. Then it starts to idle like crap. Basically feels like a random misfire. If I hold the gas pedal at around 1000 rpm it idles fine. Driving also feels fine until I start to coast and the rpms drop to idle.

Check engine light is not illuminated and there are no codes.

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So far I’ve tried to replace MAP sensor, air temp sensor, coolant temp sensor, TCV, cleaned and then replaced ETM with another one. Checked compression - 135psi on all cylinders. Replaced spark plugs. Replaced fuel filter. Got another ignition coil and tried swapping one at a time to isolate it - no change. I did notice that the coils have hairline cracks on them. Checked all vacuum hoses, checked fuses.

Next I was gonna order 5 new coils and replace them all to rule them out. Otherwise I am running out of ideas. Could flaky crank or cam sensors act this way? What about VVT solenoid? Unplugging the solenoid doesn’t seem to have any effect.

I want to start driving it but this issue is holding me back from getting it plated.