low compression

Hello, I followed your mechanics advice and carried out another compression test, these are the results:

Cylinders 1, 2 and 4 all at 15bar.
Cylinder 3 9bar.
I then put a tablespoon of oil in cylinder three.
Result Around 11 bar
My engine light stays on constant when car is running right.
When running on three cylinders the engine light flashes.

There is also a popping noise coming from the rear of the exhaust

Can your mechanic tell me what they think it is now I have carried out the second test, is it piston rings? Or problems with the head?

Hello,

If adding the spoon of oil raises the compression then it points to the piston rings.
You will likely have to change the piston ring in order for the compression to improve.

The popping noise you hear is possibly backfire, due to the un-burnet fuel:

https://www.2carpros.com/articles/engine-backfires-while-running

https://www.2carpros.com/articles/how-a-car-engine-works

I have attached the basic ring replacement procedure for your reference.

Please let me know of any questions.

Thank you.