I have a 98 GMC Jimmy. I maintain the truck very well. Backstory--I have put 52,xxx miles on it since I bought it at 148K miles, in Dec. 2011. I do not know the history of the fuel pump, but the check valve went bad during my time of ownership, so I have to cycle the key. Fuel injection spider was replaced (X VIN version IIRC) with metal Fel Pro lower IM gaskets about 2 years ago. In February of this year, I had a catback exhaust put together with mandrel bend tailpipe and Flowmaster muffler. I also replaced all of the oxygen sensors for good measure.

Fast forward to March of this year. When I got the rough running and P0300, I started with replacing the cap and rotor. At this time I discovered the front tab on the distributor was broken. I got the Dorman repair plate, and installed that too. Plugs were platinum Bosch and wires were Summit Racing, and still good at this time (30K or so miles on them). Still had the P0300 and rough running. I replaced most of the vacuum lines. I could not see or feel where the one that goes down by the distributor originates, so I cut that short and added a splice connector. I replaced the fuel filter, Seafoamed the engine through the brake booster, and cleaned the EGR valve. Still had the rough idle and code.

I decided maybe the plate did not let the cap and rotor seat properly, and given that the truck had 200K miles the distributor was worn. I replaced it with a new AC Delco unit along with the cap and rotor it came with. Still had the rough idle and code, plus one for cyl. #3 misfire. I replaced the plugs with AC Delco ones and that went away, but the P0300 came back eventually. The plugs that came out were very black and sooty, almost an oily black.

Now, I know I've been throwing parts at the truck here, but I have very limited time and diagnostic tools besides a scan tool app on my phone. My logic is that all of these parts can cause this issue and they will eventually die anyway. I would hate to have to go through this time and time again due to the failure of every component that can cause this issue.

I replaced the EGR valve and the MAF (Ebay parts). Truck ran much better but still eventually got the P0300 back along with a P0171 and P1133. Just yesterday I replaced what I found to be some small sections of vacuum line I missed. Now truck runs much much better! I also sprayed carb cleaner into a vacuum line and saw that o2 sensor 1x1 and 1x3 shot up to 1.0V. I also put a gauge on the fuel service port and got 48 psi with the engine running. Unless Im wrong, the o2 sensors are still good and my fuel pressure is in spec. My LTFT1 was in the negative the whole time and since I replaced the vacuum lines my LTFT2 went from +15 or so to less than +10. I know, I probably should have started with these tests.

Now, I am left with a confirmed P1133 and pending P0171. I am stuck at which direction to go.

 

Fuel trim is a problem. The PCM is trying to richen the mixture. Zero is ideal, anything more than 5, plus or minus, is a problem worth looking in to. The P1133 and the P0171 are both indicating a problem on bank one, (driver side) The P0300 is likely being set due to the other DTC's. Running fuel pressure at 48psi doesn't tell us diddly squat. GM does not publish any "engine running" fuel pressure specs for this engine. All readings are taken with the engine OFF.

Assuming B1S1 oxygen sensor is good, (these things like AC Delco & Denso, Bosch is not recommended) low fuel pressure would be the first thing to check. If or when fuel pressure is low, a borderline oxygen sensor could easily set a lean DTC, which you have 2 of.

When you test fuel pressure at the service port by the distributor, the gauge will indicate regulated pressure, or maximum fuel pump output pressure, whichever is LOWER. For example: If the pump can only provide 50psi, you have no idea what regulated pressure is, and volume will definitely be insufficient. Follow the steps in this link, and post all 4 readings: https://blazerforum.com/forum/2nd-ge...eakdown-88305/