Q: Rough idle falls on its face any throttle passed half 0p0420 0p0430 0p0300

asked by on January 02, 2017

I brought the SRX a few weeks ago it had a rough idle when I got it I scanned it and these came up 0p0300 and 0p0302 so I bought the coil packs because the guy I got it from had the spark plugs change thinking that what caused the idle problem it didn't I got it home installed the cool packs and I noticed the temp creeping up a bit but not much I check the fans and they weren't coming on checked all the relays they were good so I got a new cooling fan assembly put that on and the car ran really good for a couple weeks then it started missing again so I checked the plugs and coils all check out I began looking for vacuum leaks found the intake bolts were backed all the way out I got intake manifold gaskets and a little bit of vacuum line and a couple 90's but it all on sprayed the whole top of the motor with starting fluid and couldn't find anymore vac leaks now I'm going crazy can't seem to fix it I thought I was pretty good at diagnosing stuff like this but I guess Not with cadillacs

My car has 155088 miles.
My car has an automatic transmission.

First off, the P0420 and P0430 both refer to a catalyst efficiency problem, while the P0300 and P0302 codes refer to a misfire problem. If you are certain that the issue is not with the ignition system, then I would look at either the fuel supply system, or the catalytic converters. When a catalytic converter fails it can cause the types of issues you are describing. Basically, if the exhaust gasses cant get out, it will cause any number of problems. Then again, if there is a problem with a fuel pump or injectors, that don’t allow the correct amount of fuel into the engine, this will also cause these types of issues. There are just a lot of possible causes for an engine to idle or run rough. Some testing of the fuel and exhaust systems will be necessary to rule them out. If any of this is something you feel you could use a helping hand with, contact a certified mechanic, like those available at YourMechanic.com.

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