Trouble Diagnosing - Rough Idle, Cold Start issues

JonTheRed · Dec 18, 2019

This issue started back during warmer weather and seemed to taper off as fall came into full effect. Engine has to crank and crank to start but will eventually turn over and run. Once up to temp it seems to run relatively normal. When in park it idles at around 1200 but drops to 700-800 when in drive and will bog down when coming to a stop at red lights. The cold weather (~low 30's) seems to also be making it worse as it runs better when its warmer out. Replaced spark plugs, coil packs, air filter and ran sea foam to try and determine if there was a carbon buildup issue. Finally replaced the throttle body as it seemed to point to a TPS issue. The only moment that seemed to improve how the car ran at all was after the spark plugs were changed. Fuel efficiency seems to have suffered slightly which now leads me to think that the O2 sensors are the culprit.

Heres the list of codes:
P0300 - Random Multiple Misfire detected
P0303 - Cylinder 3 Misfire Detected
P0304 - Cylinder 4 Misfire Detected
P0420 - Catalyst System Efficiency Below Threshold Bank 1
C1201 - Engine Control System Malfunction
C1241 - Low or High Power Supply Voltage
C1246 - Master Cylinder Pressure Sensor

Any thoughts?

75aces · Dec 19, 2019

Have you tried to unhook the battery to see if the relearn will help?

When it's warmer, does the starter exhibit the same issue as cold weather?

What's the current battery voltage?

In the cold, the rubber hoses will shrink and can be brittle at this point. Try using a hand vacuum gauge to see if the engine stumbles. If it does, then there's a vacuum leak in the system.

What brand plugs did you install? Are the coil packs Denso?

DR650SE · Dec 19, 2019

Check the resistance of all your fuel injectors with a multimeter. I recently had something similar with my van tested the resistance in the injectors and all of them had a resistance of 14ohms. The only difference is my mis firing injector had 30ohms. Replaced it and problem solved. It's worth a shot to see. Injectors may be going bad.

My money is in bad injectors.

Sent from my SM-G965U using Tapatalk

Mgeorge · Dec 21, 2019

if all else fails, do a compression and leak down test for a bad head gasket. I had two misfires in adjacent cylinders and the cause was the head gasket was broken between those cylinders. I also had a rough idle, poor mileage and pinging even when running super before the gasket blew completely and the car developed quite a noticeable miss.

Trouble Diagnosing - Rough Idle, Cold Start issues

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