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Discussion Starter · #1 ·
Hey I have a 2001 Lexus is300 that is now throwing a misfire code after I did some work on it. the car was leaking pretty bad out of the valve covers so I changed those and figured I’d do the plugs well I’m at it. I put NGK iridium ix plugs in and also replaced the wires with Ngk wires. The car started but ran rough and had p0300 and p0302 and p0305, it also threw a code for the catalytic converter. I went and checked the spark plug gaps aswell as replaced some vacuum hoses running from the plenum to the evap system, after I did that the p0302 and p0305 went away and same with the cat code but I’m still throwing the P0300 random misfire code. I’m really not sure what the issue could be, before I started working on it the only issue was a code for a faulty power steering pressure switch that I haven’t changed yet, if anyone has any ideas what may be causing the misfire or common mistakes I may have made that could lead to this the advice would be appreciated
 

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Did you check the coil connection wires? If they are not in well. It will cause it. Also how old re the coils?
 

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Discussion Starter · #3 ·
I’m not sure how old the coils are they came on the car when I bought it but they seemed to be working good before.
 

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One of the coil pack connectors were broken so I replaced it, thanks! That got rid of the P0300 but it still idled rough and these icons were blinking red on my OBD, I’ll attach a photo. I’m not sure what issue these could point too, can someone let me know.
Sustained misfire will cause the MIL to flash. This is because misfire allows raw fuel into the exhaust, the catalysts will then ''burn'' that fuel and get hotter. Is this continues for long, the catalysts will fail.

If its running rough and MIL is flashing, a misfire code will arrive soon.
 

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Discussion Starter · #8 ·
Sustained misfire will cause the MIL to flash. This is because misfire allows raw fuel into the exhaust, the catalysts will then ''burn'' that fuel and get hotter. Is this continues for long, the catalysts will fail.

If its running rough and MIL is flashing, a misfire code will arrive soon.
So how do I fix this issue?
 

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Discussion Starter · #9 ·
It started rough then ran ok for a little while but the P0300 came back after that and the trac control off and abs lights blinked for a bit too. but it wasn’t consistently throwing the misfire code and the blinking dash lights stopped
 

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The things that come to mind are coil connectors, the coils themselves or a vacuum leak.

It surprises me you only needed to replace the connector on one coil. I've done plugs on two of these cars, both 2002 models. Every coil connector was f*cked.

If you're going to go back in there again, you should Ohm-out the coils to check them. Perhaps it's a decent time to replace them, but I'd Ohm them first.

Speaking of going back in there - reading through your posts, you've been in there twice so far. Are you absolutely certain you've created good seal between throttle and Y-pipe to the plenum, as well as reconnected every hose? A slight vacuum leak could cause a lean condition and cause some random misfire. In fact this seems more plausible because you are getting random MF and not MF on specific cylinders. A bad coil or connector or wire or plug would be cylinder-specific, whereas a vacuum leak would effect all 6.
 

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Discussion Starter · #11 ·
I replaced the coil packs aswell as the connectors, still have a misfire. I torqued the y pipe to plenum bolts down to 20 nm and I’ll check the spec for the throttle body to the y pipe. Does Anyone know which vacuum lines could cause this because I’m pretty certain I have them all connected and I even replaced a bunch of the cracked hoses running from the purge valve into the evap and theres no kinks.
 

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Discussion Starter · #13 ·
I didn’t torque down the bolts connecting the throttle body gasket to the y pipe, but I tightened them pretty hard with a wrench so could that be the issue?
 

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Discussion Starter · #14 ·
I’m getting the p0300 and the power steering code, the power steering code was around before the job and the steering is just fine so I’m planning on dealing with that later. Haven’t checked the fuel pressure yet but it was working before so I’m not sure how anything could’ve changed in regards to that since I haven’t touched the fuel system. I’m not sure what the igniter connector is though
 

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Ignitor is the unit bolted to the right side strut tower with wires going off to the coils.
 

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Discussion Starter · #16 ·
The igniter seems fine, I never disconnected it and the clip is still good. The gasket surface behind the throttle body seems pretty rough though, there’s some small scratches and pits. not sure if that’s a big deal on the throttle body gasket or not
 

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Discussion Starter · #17 ·
Also one of the new spark plug wires running from cylinder 1-6 seems too short, it fits really tight but I’m not sure if this is an issue. I’ve heard of people getting misfires with aftermarket wires, do you think this could be an issue on my car?
 
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