Engine is sputtering

Just to clarify your engine is sputtering and you unhooked the manifold and now it runs better? If that is the case then yes. It is most likely clogged.

If this turns out to be true and a muffler shop will have to do a back pressure test to confirm it, we will need to make sure the engine is running properly and there are no other issues. The reason is, clogged exhaust system can be from being worn converters but most likely caused by other things like oil and coolant going through the exhaust. This means we need to check for a head gasket issue. This is the most common cause of things like coolant and oil going through the exhaust.

Here is a guide that walks through that:

https://www.2carpros.com/articles/head-gasket-blown-test

Let me know if you have questions but the first step is to get this confirmed with the proper test and as I stated, exhaust shops are normally the only ones that have that equipment. Thanks