I retested the MAP because my teacher wanted to verify that was ok. I did both a manual test on voltages by backprobing and looked at the computer and compared the numbers to all-data. They were ok. I cleaned the grounds, still runs rough. I tested the fuel today. It was at a consistent 15-16 PSI while idling. I can test the timing soon to verify that is good as well. But other than that, I am unsure where to go next. Everything appears to be in working order, but it still isn't working correctly. Could it be the pressure regulator?

The scan tool also shows 128 on the fuel trim.
EDIT: Forgot to mention that the scan tool also says it it's running in open loop mode.

 

15-16 psi is higher than I’ve heard of in these years, usually people are struggling because fuel pressure is low. How/ where did you measure?
I’ve not experience on high fuel pressure and whether it can be caused by a stuck fuel pressure regulator or not. Hopefully someone with experience will respond on that one.

Did you look at spray pattern from injectors when idling? Should be no drips, spray pattern uniform.

trim will always be 128 when in open loop. Does it ever go into closed loop when driving or after warmed up?

 

Right, I've only heard of issues like this when it is running lean with low fuel pressure. I'm not sure if too high of a pressure would cause this. Regardless, I pulled the gas line off the filter and measured fuel going through the line there with a cool adapter thing we had in the shop. It didn't give me a pressure reading from there while the truck was off however. I was told it is a little harder to get a reading at the TBI system itself.

Spray pattern looks fine to me. There isn't any drip and it is a consistent flow.

I will have to drive it around a little more to see if it goes into closed loop ever or not.

 

The reason I was curious if it ever gets into closed loop driving is that to stay in closed loop the ECM has to be able to drive the O2 between rich and lean. I don’t have experience with a too rich condition but if the fuel pressure regulator was messed up causing a too rich situation, I would guess it would go into closed loop and then try to lean it out. You’d see trim/blm go down and either it’d be able to drive it lean and you’d have low trims or it wouldn’t be able to get it lean enough ( below something like 400mv) and it’d drop out of closed loop.
If, during idle when it doesn’t go into closed loop, if the O2 is always high - perhaps that is another indication.

typically, you’re looking for ~11-13 psi which is set by pressure regulator. So I would suppose a good fuel pump would be able to deliver more if regulator was frozen or ?
if you are always too rich and no one suggests something based on actual knowledge or experience - I’d probably open up the TBI.

 

Poked inside the TBI, cleaned everything I could, verified these were the correct injectors for the engine and examined the spray cone. Everything fuel-wise seems to be in perfect order. This is where I've had the breakthrough. I hooked up a timing light to wire #1 to test the timing of the engine. I noticed that occasionally the light wouldn't flash when it was supposed to, corresponding with when the engine would stumble slightly. I tried putting the light on the other wires too and observed the same thing. It would appear that the engine is essentially randomly misfiring.

 

You might look at pickup coil/reluctor. I don’t have experience with that but have read posts where they are intermittent causing problems. You may want to search for posts on that to see if similar.

 

I looked at that, the resistance is within spec. I almost wonder if it isn't the distributor itself.