I have recently aquired yet another IROC, a relatively rare 1990 IROC-Z 5.7TPI. When I got the car I noticed several odd things about it, one of these is that it had a chronic miss and ran way too rich at all times. Once I got it home I began to diagnose this problem. Here is how it went;

1) Replaced all spark plugs, #7 was black with oil and the remaining plugs were severely black with soot (rich). Engine still had a profound miss.
2) Replaced cap and rotor as they were so bad and beyond anything I had ever seen. Still a profound miss, and way too rich.
3) So I do a very detailed search on this board for "running rich". My condition is "running rich at all times". As I continue the search that comes closest to my condition I notice a common thread, fuel pressure!
4) Fuel pressure! It breaks down further to reveal that many members posting this "running rich" question, have not checked fuel pressue. Kinda basic really.
5) These many fuel pressure problems seem to focus on the fuel pressure regulator, ruptured diaphram, wrong one, unhooked vacuum line, etc.
6) So I do a real good physical inspection of the fuel pressure regulator area to locate the vacuum line that connects it to the plenum. My original intention was to disconnect the vacuum line and smell it to see if I could detect the gasoline odor that would indicate a ruptured diaphram in the fuel pressure regulator.
7) Well waaaddayaknow! The damn thing was not even hooked up to manifold vacuum! So I not only had maxed out fuel pressure at "all times", but a vacuum leak as well.
8) I reconnected it to the original manifold vacuum sounce under the MAP sensor and it now purrs like a kitten! No more rich mixture.

Thanks to all those board members whose posts I read on my way to solving this problem, without posting the question. andyZ28

 

Further investigation;
I removed the TPI setup today to repair an oil leak from the rear of the manifold wher it meets the block. During the disassembly process I discovered that the small port fitting that supplies manifold vacuum to the fuel pressure regulator was pretty much plugged up with gum and crud.

All of the injector tips look nice and clean except for #7 cyl which is soot black and the O ring that seals the injector into the manifold appears to be burnt away. Why is that?

Black injector tip and burnt O ring! Somethings going on.

Also there is a small hard plastic line which goes from the bottom of the throttle body under the plenum to the EGR control valve near the distributor. This line was pinched completely closed between the plenum and the fuel rail connector pipe. Does anyone know what this vacuum source is for?

 

Just a WAG, but minimal spark, poor cumbustion could upset that particular cylindaer...may have been from what you've fixed, or a sputering injector...might also wnt to run a copression check to see if the issue is a valve problem.

But for a one in a million, check e coil resistance of that injector, make sure it not too low and overheating...doubtful, but possible.


The vacuum line...I think...is what controls the opening/closing of the egr diaphram.

 

Yeah, do a compression check and see if that cylinder is low. If it is squirt a bit of gear oil into it and retest it to see if it goes up. If it does the problem is the rings. If not it's the valves.

It could also be a bad injector that just pisses fuel all the time.

 

Compression 160 on that cyl.

 

Yeah, but that by itself doesn't tell you anything. What are the readings on all the other cylinders?