P0300 code

Hi Mommabear,

Have you checked you actual fuel pressure? Two things come to mind with the fuel, first you have low fuel pressure from the pump. You test this by putting a fuel gauge on the rail and see what the pressure actually is. Second, your fuel pressure regulator is faulty and your fuel volume being delivered to the injectors is not sufficient.

Next is unmetered air is coming into the cylinders. This means you have an air leak past the MAF sensor.

The way to determine which of these it could be is to monitor the o2 sensor data and see what vehicle is actually doing. Is rich or lean. I am going to guess it is lean but remember this P0300 means that different cylinders are misfiring at different times. So it is something that effects all cylinders and not just one. That means most likely it is either air or fuel.

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

Okay. So we may be able to assume fuel pressure is okay but that does not eliminate the regulator nor the air leak past the MAF sensor.

As for oil leaking down on the spark plugs, that could cause it but that more than likely is going to cause a specific cylinder misfire not a multi cylinder misfire like you have. Remember P0300 codes are meaning that the PCM is seeing different cylinders misfiring at different times. That means a cylinder will misfire and then the same cylinder on the next compression stroke, it will not misfire but another one will and so on.

That means we have something wrong that effects all cylinders. The way we need to diagnose this so we are not guessing at parts is to monitor the short term fuel trim in the PCM and see what it is saying. If it is lean we may have a fuel issue or air leak issue. If it is rich then we have a spark issue. Once we know this info we can move onto the next step but at least we know whether we are chasing a lean condition or a rich.

You can go ahead and replace a lot of these parts in hopes that it fixes it but if they don't it would have been cheaper to have it diagnosed.

You need a scan tool in order to read what the sensor is saying. I know you mentioned you don't have a gauge to test fuel pressure or anything so I am assuming you don't have a scan tool? If not, I would suggest getting one or taking it to someone who can look at this stuff.

I know that may not be the answer you want but if we don't see what the engine is doing based on the sensors then we are just guessing.

Here is a guide that you can test the air leak or vacuum leak portion of this:

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

If you find no air leaks past the MAF sensor here is a guide that talks about the MAF sensor itself and how you can clean it. If this is sending bad info then the PCM will over-fuel the engine causing a misfire.

https://www.2carpros.com/articles/mass-air-flow-service

Here are some other guides that will go through some basic things to check as well:

https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

Yes. OBD scanners but just be careful because some only read codes and do not let you monitor live data.

Someone actual just reported that the Autel AL329 from Walmart was not bad and pretty cheap for what you can do.

I am not recommending this but just make sure if you are going to look into buying one that it not only reads codes but can look at freeze frame data and live data.

The one I mentioned is about $50.00. They range from about fifty dollars to hundreds or thousands. Clearly, you don't need the higher priced ones if you are just looking to read sensors and stuff.

If you get one, just start with sending me the short term fuel trim for both bank 1 and 2 and we can go from there. Thanks

Sounds great. Thanks.