Hi I've got a 91 camaro rs with 305 and it runs rich and idles like it's got a big cam. When driving it's fine at full throttle but the moment I got to just cruise at a speed. It starts sputtering and what not. What cold be the problem.

 

Does your SES light work and if so, any codes? Stock TBI ecm is speed density and rather sensitive to sensor data. If I were to list a bunch of reasons why your engine is running the way it is with the amount of information you've given it would be a very long read and no offense, a waste of both our time. Give the local mechanic a $20 for diagnostic work.

 

The light does work and I had it read about 2 months ago and all it said was o2 sensor was saying it was running rich

 

Does it run well on first startup or always running rough? Try resetting the ecm blm memory? Black smoke? Any smoke during deceleration? Spark plug wires old with issues causing misfire that shows up as rich? My recommendation is still to get a mechanic involved for diagnostics. If you pull all of the plugs they might tell you something about a misfire. Our cars have only 1 O2 sensor on one side of the engine... should tell you where to start looking. Are you sure it's running well at WOT or just better?

 

HOW do you know the engine is running rich?

Misfire should show up as lean on an O2 sensor, not rich. It will also produce eye-watering, rich-smelling exhaust fumes...but the engine could still be too lean.

Never mind the codes...what is the data stream telling you? Engine temperature? MAP vacuum? Short- and long-term fuel trim? Knock-sensor activity?

What is your fuel pressure at idle, and at WFO?

When was the last time you checked plug wires, cap 'n' rotor, and the pickup coil?

 

Misfire allows unburned oxygen to get to the oxygen sensor. ECM interprets this as not enough fuel, compensates and adds fuel globally producing a richer running engine than is commanded in the cylinders that are firing (or trying to).
How do you figure the engine is running too lean? Often a lean engine will not want to rev-up let alone run well at WOT. I can't think off the top of my head a condition in which that would happen.

I still think a mechanic and diagnostic job is the best solution unless you want to upload a video to youtube and share it. If you do please edit it and keep it short and to the point.

 

We are mostly saying the same thing. We have no idea whether this engine is actually rich, or perhaps lean-misfiring, or misfiring for some other reason.

There's no diagnostics to go on in the original post other than his "rich" claim (unverified) and his claim that the engine runs rough--which could be rich, lean, spark, or internal engine failure.

No Youtube video will replace a few simple tests and the use of a scan tool.

 

I've had a false ECT reading cause this more than once on my car. A bad connection at the sensor made the ECM think the motor was at below freezing temperatures. Although, ambient temp was closer to 80F. But yeah...a data log would let you know what the ECM is seeing!