As I pulled the injectors to take to the shop, I noticed that the side runners of my LIM are a lot browner than my center runners, and they're also full of tiny black crumbs. Swiping my finger into the runners left it oily and with a bunch of crumbs stuck to the finger. They're kind of crushable with a bit of pressure. Mind you center ports are clean.

I look inside the runners of the upper intake manifold, and the outer ones have some kind of black stuff caked onto their walls. I reach in to touch it and it's brittle and crushes under my finger with a bit of pressure. Perhaps some of that broke off and fell down into the LIM runners. I don't know what the hell it is, but I think I know where it came from. Basically when I took the engine out in the winter, I had my manifolds cleaned by a machine shop, then blasted, and then powdercoated, and I think most likely during the cleaning process, not everything got rinsed well from the inside, and when later on the powdercoat was curing in the hot oven, this residue burned and turned into charcoal. Since it was inside the runners, it was almost impossible to spot.

Best I can describe it is when you cook pizza in the oven, and some cheese drips down and burns to charcoal, this stuff is basically very similar, and probably just as brittle.

Now what worries me is that some of this obviously broke off and got sucked into the engine, and I'm afraid it left some scratches on the housings... Is that possible? Or would it need to be a fairly hard substance to cause scratches?


And onto the second issue. Oil in the runners and ports. I pulled off the LIM, and all the ports were covered in dark oil, that instantly dripped out and down the gasket. Especially side ports. I don't think it's engine oil, because I just changed it recently and it's clear. Only oil that's dark that I can think of is the 2-stroke, but that should be mixed with fuel and burn. Why is it inside the LIM and how did it even get into the outer ports? The injectors only spray into the center ports. Is it possible the mixture is getting blown back into the ports and LIM, and then the fuel evaporates, leaving oil behind?

 

That's what I'm thinking. Those little black "crumbs" are deposited oil and unburned fuel over time. My theory is that it's some of the unburned gas and oil going into the intake ports; there's a little overlap between the exhaust and intake stages with peripheral exhaust ports.

 

Yep, he's right ^^^

The reason why you have the black 'crumbs' is because of the Dynamic Effect Intake (DEI) system designed into the SE's from the start. This uses the airflow from one rotor to affect the airflow to the other rotor resulting in a mild supercharger effect (no, I'm not kidding - google it). The net result is that you get some blow-back of the exhaust gases back into the intake on the secondary side (the outter runners, which feed the '6'-ports, the auxiliary intake ports which revolve to open under exhaust backpressure) and this gets deposited on the relatively cool insides of the aluminum intake manifold.

By the way, the manifold has that rough texture to promote surface adhesion of the airflow for better laminar flow through the center of the air channel, along with helping to prevent backfiring by slowing the intake flow somewhat. It's designed that way.

Easy cleaning? Take off the UIM and LIM, get some oven cleaner safe for aluminum oven trays and your garden hose. You got it - spray that thing down with the oven cleaner, let it form a slurry inside the intake passages, and then hose it all off. It'll be sparkly-shiny when you get done and good for another 100k miles.

Try not to break off to many crumbs which will go down the intake passages and enter your engine. You don't want anything in there that's harder to compress than air/fuel mixture.

Also, while the LIM is off - do a search here for my username and 'cleaning 6-ports', and you'll see my tutorial on how to clean the port passages, turning sleeves, and air actuators.

 

So I take it oil in the ports is something to be expected, then?

Manifolds are pretty clean, I just need to scrape out the charcoal stuff. Will look up port cleaning, thanks.