Q: Idle random misfire, very light but not passing smog

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I had just done a head gasket job and started up great ran it for a bit and got misfire code and variable valve timing code and knock sensor #2 on snap on tool. tried to pass smog and it passed 25mph test but failed idle HC:263 , CO: .02% , Co2: 7.4% , O2: 10.0% , NO ppm: 32

i have smoke tested before those numbers and fixed all leaks but still cant find anything if someone could push me in the right direction because 3 other mechanics have taken my money and left me in the same spot. i have replaced o2 sensor, checked coil ohm readings, new spark plugs, new oil, valve seats are good, compression 180 across the board, pistons looked great with no wear during rebuild, camshaft position sensor, fuel filter, fuel pump pressure at 50psi on 3.5 bar system, only thing i could think of maybe injectors or coolant temp sensor but i have obd1 system so am having a hard time without a snap on scanner please help.... Robert

My car has 200000 miles.
My car has a manual transmission.

Hello, thanks for writing in. Random misfires can be caused by many things, and can sometimes be difficult to diagnose for this reason. If you’ve already checked out the listed components then it may be possible that the coolant temperature sensor or injectors are causing the issue. If the coolant temperature sensor or circuit have an issue, then the signal may be compromised and throw off the engine computer’s calculations. A bad sensor can result in an excessively rich or lean condition, which can cause misfires. It may also be possible that one or more of the injectors may be dirty or clogged, and needs to be cleaned or replaced. This is more likely if they are the original injectors, and if the car ever sat for an extended period of time. I would start by testing the coolant temperature sensor, and then going from there.

If you would like further help, consider having an expert automotive technician from YourMechanic come to your home or office to inspect and diagnose this issue for you, and make or suggest any repairs as needed.

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