Went to do a fluid flush on the '94 FD (stock except cat-back and radiator) and noticed in the air box that there was a black tar like substance (it is tacky and extremely hard to get off the plastic but on skin it comes off with just water and rubbing) inside on the bottom and around the air filter. Not sure where it is coming from as none of the holes in the box or pipes have that substance coming from it. Oil inside the engine is still amber colored. Any ideas?

 

looks like the rubber seal on the air filter melted. high chance that could be a shanghai special filter and it did what it do

 

Seal melted K&N Intake (93+ RX-7)

 

Cheap filter affected either by the high heat or a chemical reaction if you oiled the filter or some combination of the two.

 

cheap filter, they just do that sometimes. i had a pair of shoes do it!

 

Any idea of what to use to get it out? It's really hard to scrap out and it still leaves residue. I've tried Gunk Heavy Duty Degreaser and Goof Off.

 

Every case of dissolving plastic stuff is different, so it's hard to say. In general, I've found that chemicals don't work at cleaning the results of these kinds of melting plasticizer reactions. Running water and aggressive rubbing with something similar in make-up to what it was to begin with, has worked best at lifting it off as it would tend to want to bind to a similar type of rubber/plastic moreso than whatever surface it's on. Think something like a big rubber eraser or something. Gonna be pretty hard to get into the corners of that airbox though.

 

A heat gun to soften it a bit might help you peel or roll it off.

 

Diesel fuel is usually good for removing rubber/tar like substances.
Good stuff to have around for cleaning parts also.

 

I installed a K&N filter when they first came out for the 1992 Mazda MX3 1.8L V6 cars.
My filter melted and I sent it back to K&N for a replacement.
So obvious when you look at it.

 

I ran K & N panel filter in the stock air box for several years with absolutely no issues. And I’d bet it got a lot hotter under the hood than a MX3.
And it didn’t look like it was a K & N filter that melted in the OP’s case.