2019 Mustang GT - P0300 Code

[funks_5.0]

I recently purchased a 2019 Mustang GT Premium PP1 w/ 54k miles. Just picked it up Wednesday and it now has a check engine light. I am currently waiting for my OBD EX to be delivered otherwise I would check myself for what the code is. Ran it over to AutoZone and the only code it is showing is a P0300 random misfire code. I let the dealership where I bought it from know and they are more than willing to help out, but since its not a Ford dealer, they are unsure of next steps to take, especially on a vehicle with aftermarket parts.

Currently from what I know, the vehicle has a CAI, a ported 1320 Junkie Performance Intake Manifold, JLT Catch Can, long tube headers, no cats, H-pipe, and the stock active exhaust. I am told the car has no tune on it.

Symptoms I have experienced consists of a rough idle, RPM drops from around 600 to 200-300 and back up to 800ish and then to 600, and then it will happen again. This is the first vehicle I have had without cats, but it smells like it is running very rich at idle, it is very overwhelming to the point where I can park the car in my garage and close the door 15-20 minutes later and still smell it in the morning. I do not notice it when driving but that could be due to the fact that I am in motion rather than sitting still.

The dealer wants to start with the battery as research has shown these cars to be sensitive to low voltage, I am taking that to get load tested today. But reading it from a meter it appears its holding a charge, and also charging when the vehicle is on. I believe the dealer and his mechanic are under the impression that the vehicle needs to be tuned, which is fine by me I was planning to have it done before this came up. But would the issues I am experiencing and the CEL be caused by a lack of a tune with all the aftermarket parts on it?

If the battery checks out we were going to move onto the O2 sensors, but without a code specifically for those, I would prefer not to go chasing my tail replacing part after part.

Sponsored

 

[Call_Me_Bruce]

All those aftermarket parts and no tune, not very likely. Do some easy stuff first to eliminate those. First thing i would do on a P0300 is check for vacuum leaks around the intake manifold, especially since the one you have is aftermarket. I would also eliminate anything internally wrong with the engine with a compression test. If you have a strong smell of unburnt gasoline, is it being over-fueled? If you do a compression check you'll need to pull the plugs and if they are coated in gasoline that could be an indicator as well. With no CATS, not sure what O2 sensors would be doing. With no history on what tune may or may not be installed it might be a little difficult to determine the cause of the CEL.

If it has the factory calibration in the ECM, it will never run right with those after market parts. The factory settings are looking for up and downstream O2 sensors with signals within spec. Also, I think it would not like the aftermarket intake manifold either on the factory tune.

I would then go onto Motorcraft Online and get a 24 hour subscription to the shop manual for your car and see what Ford says to troubleshoot a P0300. That will give you some guidance as well.

Best of luck.

 

[Cobra Jet]

We see this over and over on this site.

Unfortunate but too many reasons why buying a used modern performance vehicle with mods - and no way of telling if the car had a tune ir was tuned…. should be a flag of caution.

2 questions:
Is your State one that requires emissions inspections or State inspection in general?

When you purchased the vehicle, was it an “as is” purchase or is there anywhere in the docs that the Dealership guarantees the vehicle will “pass inspection”? Because I can tell you it won’t pass for emissions (if tested) and in some states it won’t pass a visual if there are aftermarket LT’s on it. The other issue too - some Ford Dealers won’t touch a vehicle that’s been heavily modified especially if they don’t know if the PCM has been touched…. Because the PCM is a liability and they risk the chance of bricking it if there is an unknown tune and it can’t be flashed back to stock…. You’re also then on the hook for excessive diagnostic fees along with any parts/programming charges. With that said and IF the Dealership guaranteed the vehicle to pass inspection - you’re only recourse is to go back to them and tell them they need to fix all items to pass the vehicle OR somehow get them to buy it back….

Too often buyers of used S550’s with mods (hidden or not) end up having to work through myriads of issues due to the prior owner’s modding. Then you have Dealerships who bought the vehicle from an auction or took it on a trade in to flip for fast $$$ (but won’t back the car when issues arise).

Good luck and post back if the Dealership helps.

 

[funks_5.0]

All those aftermarket parts and no tune, not very likely. Do some easy stuff first to eliminate those. First thing i would do on a P0300 is check for vacuum leaks around the intake manifold, especially since the one you have is aftermarket. I would also eliminate anything internally wrong with the engine with a compression test. If you have a strong smell of unburnt gasoline, is it being over-fueled? If you do a compression check you'll need to pull the plugs and if they are coated in gasoline that could be an indicator as well. With no CATS, not sure what O2 sensors would be doing. With no history on what tune may or may not be installed it might be a little difficult to determine the cause of the CEL.

If it has the factory calibration in the ECM, it will never run right with those after market parts. The factory settings are looking for up and downstream O2 sensors with signals within spec. Also, I think it would not like the aftermarket intake manifold either on the factory tune.

I would then go onto Motorcraft Online and get a 24 hour subscription to the shop manual for your car and see what Ford says to troubleshoot a P0300. That will give you some guidance as well.

Best of luck.

Thank you for the advice, I will continue down this path. I did some further investigation online and realized the car has a PMAS CAI, not sure if it is the tune required option or not. Lets say it is the tune required option, without a tune could that be the reason it is throwing an engine code and having a rough idle?

We see this over and over on this site.

Unfortunate but too many reasons why buying a used modern performance vehicle with mods - and no way of telling if the car had a tune ir was tuned…. should be a flag of caution.

2 questions:
Is your State one that requires emissions inspections or State inspection in general?

When you purchased the vehicle, was it an “as is” purchase or is there anywhere in the docs that the Dealership guarantees the vehicle will “pass inspection”? Because I can tell you it won’t pass for emissions (if tested) and in some states it won’t pass a visual if there are aftermarket LT’s on it. The other issue too - some Ford Dealers won’t touch a vehicle that’s been heavily modified especially if they don’t know if the PCM has been touched…. Because the PCM is a liability and they risk the chance of bricking it if there is an unknown tune and it can’t be flashed back to stock…. You’re also then on the hook for excessive diagnostic fees along with any parts/programming charges. With that said and IF the Dealership guaranteed the vehicle to pass inspection - you’re only recourse is to go back to them and tell them they need to fix all items to pass the vehicle OR somehow get them to buy it back….

Too often buyers of used S550’s with mods (hidden or not) end up having to work through myriads of issues due to the prior owner’s modding. Then you have Dealerships who bought the vehicle from an auction or took it on a trade in to flip for fast $$$ (but won’t back the car when issues arise).

Good luck and post back if the Dealership helps.

Luckily I do not have to pass state inspections. I should have been more clear, I purchased this vehicle from a used car dealer, no affiliation to any specific manufacturer. He is more than happy to help out and we have been discussing next steps as well. I am just trying to get a step ahead and propose next options.

Emissions is not a concern of mine and I have a high performance shop near me that has worked on loads of Mustangs/Shelby's with fantastic reviews and great performance. As long as I can get these issue sorted out, anything else that comes up will be sent over to this shop for further review. Not a fan of the ford dealerships near me...

I mentioned in the reply above that the PMAS CAI could be a tune required one but I do not know how to verify, the only difference is the non tune required comes with a pre-calibrated MAF sensor whereas the tune required does not come with a MAF sensor. Could that contribute to the rough idle/CEL?

 

[Call_Me_Bruce]

Thank you for the advice, I will continue down this path. I did some further investigation online and realized the car has a PMAS CAI, not sure if it is the tune required option or not. Lets say it is the tune required option, without a tune could that be the reason it is throwing an engine code and having a rough idle?

With all of the mods, you listed in your first post, without a tune I would suspect it would be throwing a lot more codes than just a P0300.

 

[Goterr0r]

You either have a tune or the PMAS came with an updated MAF sensor to be used without a tune. The headers and ported mani don't need a tune to run. The car wouldn't run or really stay running with that PMAS without a tune or their MAF sensor.

Sadly, you are likely finding out why the car was traded in.

Start with the basics quick. Compression test, pull some plugs, check the IMRCs if they are not locked out so you have leverage with the used car dealer if something major is wrong.

And i would get a tune regardless so you can dial in what you have and know where you stand from the ECU standpoint.

 

[NGOT8R]

Check the gauges and tell us what the air fuel ratio is. If it’s 9.8 or so, you have an E85 tune loaded. If this is the case and you’re running premium in it, that would cause your symptoms.

I met with another member here in M6G, who had this very problem. He had just bought a ’21 GT which already had LTHs, a full exhaust and a CAI, two weeks prior to me meeting him. However, the car did not come with a tuning device and therefore, he was stuck with either finding gas stations that sold E85, or go to the dealer and have a factory tune flashed to the PCM and then buying a tuning device and have someone tune the car for the mods that it had.

 

[funks_5.0]

Check the gauges and tell us what the air fuel ratio is. If it’s 9.8 or so, you have an E85 tune loaded. If this is the case and you’re running premium in it, that would cause your symptoms.

I met with another member here in M6G, who had this very problem. He had just bought a ’21 GT which already had LTHs, a full exhaust and a CAI, two weeks prior to me meeting him. However, the car did not come with a tuning device and therefore, he was stuck with either finding gas stations that sold E85, or go to the dealer and have a factory tune flashed to the PCM and then buying a tuning device and have someone tune the car for the mods that it had.

The air fuel ratio when at idle is around 14, however, when the RPM starts to drop the AFR drops to 12.6 is what I recorded but I have seen it go lower, and then back up to 15 and starts to average back out at 14.2 or so.

 

[Hoofer]

We see this over and over on this site.

Unfortunate but too many reasons why buying a used modern performance vehicle with mods - and no way of telling if the car had a tune ir was tuned…. should be a flag of caution.

Good luck and post back if the Dealership helps.

^this.^

Hope it’s not the tip of an iceberg and the dealership helps out.

 

[NGOT8R]

The air fuel ratio when at idle is around 14, however, when the RPM starts to drop the AFR drops to 12.6 is what I recorded but I have seen it go lower, and then back up to 15 and starts to average back out at 14.2 or so.

Okay, with that being said, it sounds like you’re on a pump gas tune up, so that’s good, however, without a tuning device, you’re stuck right where you are with that tune. The only way around it is getting a reflash to the factory tune and finding a tuning device and tuner of your choosing to retune it.

Someone else mentioned checking for vacuum leaks too. I have a PMAS (no tune required) on my 2019 Bullitt which came with a new MAF sensor. My IMRCs are also locked out. Something else that comes to mind is, maybe the previous owner locked out the IMRCs and did not have the tuner turn them off in the tune. When I locked mine out, I had to pay an additional $150 to have the tuner turn them off in my tune. I never tried to run the car without the IMRCs turned off in the tune, so I don’t know if the car would or wouldn’t even run without them being turned off.

 

[NGOT8R]

Can you post pics of your CAI and intake manifold, so we can see if you have everything plugged up? If not, here are some pics of my PMAS for reference. I have red arrows pointing to the two ports on my PMAS that are capped off. Also, follow the blue line that I drew on the hose and placed an X on the CAI tube to show where it should be plugged into a 90 degree elbow on the bottom of the CAI tube. Lastly, if your MAF sensor has the same part number as the one shown in the last pic, you have the PMAS no tune required version.

 

[NGOT8R]

Something else I just thought of too is you should unplug the MAF sensor and remove it from the housing and confirm that the gasket is in place and you’re not sucking unmetered air. While you’re at it, spray the sensor down really good with MAF Sensor Cleaner before reinstalling it.

 

[funks_5.0]

Something else I just thought of too is you should unplug the MAF sensor and remove it from the housing and confirm that the gasket is in place and you’re not sucking unmetered air. While you’re at it, spray the sensor down really good with MAF Sensor Cleaner before reinstalling it.

I appreciate your help so far. I am currently at work and did not drive the car today, but I will post a picture of the engine bay with some close ups when I get home. In my original post I mentioned the dealer was concerned about the battery, but it tested to 78% life at 12.5v, sounds like its still in good working condition.

Adding on to your picture, the two ports you shown being capped off is not the same as mine from what I can remember. One of the ports has a connection with either a hose or something else connected. Not sure if that means anything or not, but figured I would throw it out there. Doing my best to learn as I go!

 

[NGOT8R]

I appreciate your help so far. I am currently at work and did not drive the car today, but I will post a picture of the engine bay with some close ups when I get home. In my original post I mentioned the dealer was concerned about the battery, but it tested to 78% life at 12.5v, sounds like its still in good working condition.

Adding on to your picture, the two ports you shown being capped off is not the same as mine from what I can remember. One of the ports has a connection with either a hose or something else connected. Not sure if that means anything or not, but figured I would throw it out there. Doing my best to learn as I go!

You should be able to configure the tube different ways if needed. It’s been over 3 1/2 years since I installed mine now and it seems like I might have removed a plug at the bottom and installed the supplied 90 degree fitting with hose barb to attach that hose to. That’s easy enough to check by looking on the bottom of the tube to see if it has a block-off plug. the main thing is to ensure you have no leaks in the system. My PMAS came with multiple fittings and I only used what I needed to, in order to avoid interference with my nitrous solenoids.

 

[NGOT8R]

I also just replaced the battery in my car a few months ago. It showed good (12.6V to be exact) with the quick handheld tester at AutoZone. I didn’t trust that, so I requested that they do a load test and when they did, the battery immediately failed, so they replaced it under warranty. I posted pics that I took of the battery on the machine at AutoZone somewhere on M6G, but I can’t seem to find them now.

Sponsored