started slightly vibrating then later the check engine light started blinking?

This is a video I took of the engine while running after replacing the spark plugs, you may not notice it too much on video, but it is vibrating more than normal. It was also doing this before I replaced them when the check engine light originally flashed.

Thanks for the images and video. Yes, I can tell there is a slight vibration. Did you clear the codes and if so, did they come back? You may have an intake valve that is leaking slightly. We can do a compression test to confirm the issue,

Please follow this guide and get back to me:

https://www.2carpros.com/articles/how-to-test-engine-compression

Please go over this guide and get back to us.

I’d need to buy that tester and wait for it to arrive so I will try that. Do you think it could be the ignition coils?

I also cleared the codes and they came back.

Yes, it could be, I would try replacing the coil on #6, also remove the spark plug to see how the color on the plug looks, please upload an image which can help me see what's wrong.

I think we can rule the ignition coils off, I replaced all of them with a brand-new set. Started the car, and it is still vibrating with the check engine light flashing. However, this time it is only showing one check engine code: P0306 (Cylinder 6). So, I went ahead and cleared the check engine, swapped the coil on cylinder 6 with another and still have the same issue. So, after installing brand new spark plugs and ignition coils, I still have a misfire. I called my mechanic, and he will be checking it today, he will be doing the compression test you mentioned.

Sounds good, if the compression is good you will need to replace the fuel injector, which is why I wanted you to remove the spark plug and see if there is no fuel being burnt due to a clogged fuel injector. Check out the images (below). Please let us know how it goes.

If the compression is not good, what would need to be replaced?

I left it at a shop earlier today since my mechanic was a no-show. They just called me and explained that the fuel injectors are bad, but the O-rings are also worn due to it, and they need to do a valve adjustment since the compression is not consistent as he said. Going to cost me a whopping $850.00.

Yep, sounds about right, you can save money if you do the work yourself, please check out the instructions above to see if it is something you want to do.

So, I am getting really upset at this point. I spent $1,500 last Thursday on the repairs from that shop. I have posted the invoice for you to see the “work” they did because I have lost trust in them if I am being honest. They told me they did a valve adjustment, cleaned the fuel injectors, replaced the o-rings and replaced one of the fuel injectors along with the valve timing solenoid sensor and the intake manifold gaskets. After picking up the car, it drove fine to work in the morning. Once I left work and got in the car, I noticed the engine vibrating again and the P0306 code came up. I took it back, they “fixed” it and told me they swapped the fuel injectors. After that, the code P3497 came up while I was driving on Saturday and the mechanic told me it must be the injectors, that it needed the stock parts because aftermarket ones can sometimes cause a problem. When I picked it up at that time, they told me the VVT sensor connector wasn’t properly connected. Just today, the same code P0306 pops up but I feel no vibration from the engine, what is up??

So yesterday I went ahead and cleared the check engine light and let the car run a bit, the RPM is at normal level (between 500 and 1,000) and the engine is not vibrating like it was those last few times and after running the car for a bit, the check engine has not returned. One thing I just realized this morning though is that one of the head bolts is missing. Could this be causing an issue?

Yep, that can cause a vacuum leak, the fact that you are getting the run around and the car is still not fixed I would try another shop or give them one last chance to fix it. Ask the shop to have their top mechanic do the repairs and to use factory parts. Also, you can request them to give you the old parts back in the replacement part box to ensure they did in fact use factor parts. If they did a compression test it would be nice to get the readings.

I went to a different shop just to get the bolt put in, he noticed the vacuum hose on the left side of the engine (it is in the image as well) was not fitted all the way and the hose clamp was a little further back than it should be. Right after driving off from that shop, I scanned it twice and the same P0306 code came up, but there was no check engine light. Yesterday afternoon after I left work, I scanned it twice and no codes came up. I’ve been driving it around today and scanning it, no codes at all. It is driving well, no weird RPM readings, should I leave it be until something happens?

Yep, it sounds like the problem might be fixed, keep driving it and if the problem comes back let me know and we can continue.

Well, the problem is back, haha, unfortunately. However, I am getting no check engine lights and no codes when I have scanned the car. I have scanned about 6 times, yesterday and today together, but what I noticed this morning is that when I’m accelerating at high speeds on the freeway, the car is jerking hard. When I accelerated to switch lanes, I was hitting close to 50 MPH and the car jerked very hard and some of the dash lights came on temporarily but quickly went away. What is going on??

It sounds like one of the new spark plugs is cracked, I would remove them all for inspection or you can allow the problem to continue, and the computer should pick up the cylinder that is misfiring. Does your scanner show misfire events?

I took the car to the same mechanic that same day I had the engine jerking, I scanned it before arriving to the shop and I got a new code, P0685 which is referring to a Power Relay Control Circuit open.

Did you check the spark plugs, is the engine jerking fixed?

I left the car with the mechanic on Thursday, I have yet to receive it and he has not answered the text messages I have sent. So, this situation has become a lot more complicated than it should’ve been. I will keep you updated, I will probably go and pick up the car because who knows if he’s even checked it, and take it elsewhere.

Sounds good please let me know.