Hopefully this will help someone with a P030X code. During a hard acceleration I felt something happen and got an immediate CEL, P0303 and the car would frequently go into limp mode. I had recently put JCW 380cc injectors in and suspected #3 was defective. With the exception of the frequent limp mode mostly triggered at idle the car actually ran very well almost as if nothing was wrong and the CEL would almost never come on at highway speeds so with my being busy and having other cars to drive I continued to drive the car without too much worry but put off digging into it for many months. Then when doing an oil change I added STP and the lit CEL immediately went out? Now there's a clue! That did not seem indicative of an injector issue so last weekend I popped the intercooler and valve cover off and sure enough I had a broken exhaust valve spring. I had an R53 warped head complete with springs on the garage shelf that was in the car when I bought it for cheap a few years back so I used a spring out of that and my car is back to 100% again. **A word of caution** I was not very careful when I set the exhaust rocker assembly on the garage floor and ended up breaking both plastic keepers on one of the rockers and had to order a new exhaust rocker because I discovered that I could not find the keepers for sale by themselves anywhere. $30 something bucks from Rockauto for the one new exhaust rocker and a few days waiting for shipping didn't hurt too much but lesson learned.........carefully set your rocker arm assemblies out of the way on a rag or towel facing up when you remove them because it doesn't take much to damage/break the plastic keepers. I should have known better because I was careful with them when I had them out when I replaced the warped head when I purchased the car 3 years ago. If you're replacing a valve spring for the first time please study some posts, watch a few YouTube videos and use the correct tools and your life will be a lot easier. I learned my lesson(s) the hard way decades ago.

 

Glad you fixed it! Any pics of the broken valve spring? Stress cracked?

 

It sure is nice driving without the annoying yellow CEL constantly reminding me of yet another thing that I have to deal with. My assumption is that it is a stress fracture resulting from a manufacturing defect. If another spring ever goes then they all get replaced. I'll add a pic of the offending spring when I get home from work.

 

Everything is good now? If so great, but I would get the STP out of your engine.
Remember these little engines work hard, so higher heat builds up.
I worked in the auto industry and worked on a few engines that turned the STP and oil into a very thick goo! So thick, that the oil pump could not move the oil. Two of them I worked on, one oil would not drain out of the drain plug and another took two weeks to drain!
I really never could see a benefit, as the oil should have been all you ever need!
Do what you want, but that is my experience!

 

R53 exhaust valve spring fractured @ 120k miles

Under the bonnet. 15% pulley, jcw 380cc fi, CAI, DT bypass valve, one-ball exhaust mod, ready for a tune.

 

I'm changing the oil this weekend and taking your no STP advice. Thanks