Hello and Welcome to justanswer..
My name is XXXXX XXXXX Im happy to help you with this -
What I suggest you do is use an engine flushing agent for this - there are good
engine flush agents I can recommend - you cannot buy these in the shops - they are a trade only product , for just these types of issues as you have.
I will however give you a link to where you can buy the product - and that is HERE
..( I dont recommend
any supplier over any other and have no
connection with this / that or any other - its just that there are other "net" suppliers - but that is a decent priced one.)
I would suggest as it is the car it is - with that engine size , that you purchase two - and then use as directed for "stubborn" issues such as this. Basically that is to carry out the engine flush as directed - but an "extended flush" - which means running the engine as directed on the bottle - 20-30 mins or more with it in...
I have never known this product not to work even on the most stubborn cases so used as per - above, you should obtain proper results -
I know you say you have used "oil cleaners" but again, this trade only product is the best
for just such issues -
IF you were to not clear it - then the only way forward would be to strip the engine as needed to manually clean out the lines if that is found to be the issue. So its well worth doing the above at this stage to hopefully save all that work -Let me know if you need anything more on this at all - Meantime -- Please click on the OK rating to Excellent ratings service for me now, Thank you for that... and you can get back to me anytime if you need / wish..
Best Rgds, MIKE. Please note, you are rating the content of my "answer" - my help to you - not the "system" which can experience delays etc which are not under my personal control -or delays caused by "time -off" which we all have on occasion - Thank you.