Have an '89 IROC 5.7
For some time, not long after I had my injectors rebuilt, and without any Service Eng codes , the idle was a bit sporadic - 500-700 rpm intermittently whether cold
or hot. Finally decided to tackle this problem and purchased some new ones. When I pulled the old ones off I noticed 2 of them having some type of hard black
crusty stuff on the casings. It looks as though (see photo) it seeped out of the area between where the plastic and the metal part meets. Never seen this before
so wondering if any one has also experienced this?

 

Those look like they were up against something that got REALLY hot, and melted them

Which positions were they in the engine?

 

Unfortunately as I removed them I didn't pay mind to do a visual inspection of them. Just tossed them in the box. Later as I was taking them out of the box did I notice those only 2 with the crust. There is nothing in the lower intake that looks as though it'd go anywhere near any injectors. No vacuum hoses or any rubbery types. If it were rubber, there would def be a familiar odor of burnt rubber. I checked all the hoses that I would suspect have melted but found everything ok.

One of the inj, has the crusty black stuff on back side toward the intake and other on the left side.

I described this to a person at Southbay inj. where the new ones were purchased and he said it's rare but it does happen where the seal breaks between the plastic and metal area. If it were melted rubber, it would have the soft rubbery feel to it but this feels like a crusty type from that of hard plastic.

 

It looks like something flat and hot was pressed up against them. The one on the left in the photo especially.

Doesn't look like something "on" them at all. Looks ALOT more like their plastic shell is just melted and crustified and turning to dust.

I'd suspect something like, in a past life, an AIR line or something like that, was forced against them, for a rather long time. Probably not anything rubber. Looks like it was hard and flat and almost certainly metal, and left no trace of itself. All of that burnt garbage is the injector itself. No, I can't begin to imagine what it could have been, how it got there, or where it is now.

 

The photo is a bit deceiving - what you think is a plastic shell is actually steel. I did clean one of them and it looks just like it should.

If you have a TPI system you should be able to see where the injectors are in relation to the runners and the lower intake. There is nothing
that I can see as far as any rubber or plastic that can go near them.

 

Ummmm... no.

What I think is a plastic shell, is a plastic shell. It's black. There's metal underneath it. It's rusty. The "corner" of the cylindrical metal object, the main body of the injector that actually is the valve that handles fuel, is peeking out in both of those. Particularly the left-hand one. You can clearly tell that material is MISSING from the exterior plastic coating (no doubt related to the electrical part of it), not EXTRA.

Something melted or burned them. Something nearly flat.

You're right I think, I agree, I also can't think what could possibly have done that; but they're clearly and obviously MELTED and BURNT, not built up with crud. SOMETHING did it. Something METAL, not "rubber or plastic". What, when, who, where, how, ... is a mystery. But that stuff is not "built up" on them; it's the remains of the plastic housing part of the injector itself, that's disintegrated.