Cold start misfires bank one only

Welcome to 2CarPros.

The P0300 is a random misfire and could come from either bank. The vehicle has low mileage, but has anyone checked fuel pressure? Has anyone checked engine compression specific to those cylinders? Fuel pressure that is too high or low can cause a random misfire or even to the cylinders you mentioned. Also, there could be a head gasket issue causing lower compression in those cylinders.

Once the vehicle is up and running, does the misfire go away or do you continue to feel it?

Let me know.
Joe

Welcome back:

You know, I question a couple things, but I have no idea what they have already done. Has anyone checked fuel pressure? Also, has the vehicle been checked for simple engine vacuum leaks?

Take a look through the following. When you have a misfire related to one bank (which is questionable when you have a p0300) here is a list of possible causes.

- Electric fault or loose contact in the control line to solenoid valve of camshaft adjustment
- Solenoid valve of camshaft adjustment electrically/mechanically faulty or clogged
- Hall sender rotor (sensor wheel) faulty/kinked
- Secondary air
- Intake camshaft adjustment too slow
- Intake camshaft timing incorrect
- Exhaust camshaft timing incorrect
- Fault in the valve lift control
- Oxygen sensor faulty (also: loose contact in wiring harness)
- Exhaust system blocked/narrowed
- Foreign bodies in intake tract, this can lead to sporadic misfires on changing cylinders.

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Are the codes you listed originally the only ones that were ever set? Also, and don't laugh, are you using premium fuel? Low octane (below 91) will cause driveability issues and misfires on this vehicle. It's just a thought.

Let me know.

Joe

Welcome back:

Did you feel a misfire when driving? Also, clear that code and see if the same code comes again.

Let me know.

Joe

Based on what you said, I would recommend checking two things. First, when the engine is cold, steel contracts. If there is going to be a vacuum leak, that's when it would happen. I would check for a leak.

Next, the engine coolant temperature sensor for the computer indicates the coolant temp. If it isn't sending a correct signal, the fuel mixture could be too rich or too lean. That can cause a misfire. Interestingly, if the sensor is wrong, as long as it sends a signal, the computer has no idea it's wrong. You could check what it is reading with a live data scanner if you have one. Just check what it indicates the temp is when the engine is cold and hasn't been run. Does the temperature seem to mirror what the ambient temperature is?

Here is a link that shows how to find a vacuum leak:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

I suspect they are missing something simple. To have it run properly and then have that many misfires is most likely a simple fix. (at least we would hope.)

By the way. If you have a live data scanner, can you provide the short and long term fuel trims when the engine is at an idle? It may help me figure things out easier.

Let me know what you find.

Take care,
Joe