Howdy!

Posting this in hopes it helps someone with similar issue.

Vehicle: 1999 Blazer, 2dr, with 4wd

Problem/Symptoms:
Several weeks back SES light came on with P0171 and P0174 at same time. At that time the only symptom I had (but didn't know at the time) was a strong fuel odor from exhaust. Over the next week or so, I started having subtle drivability issues, starting with hesitation/surging on acceleration from stop/low speeds (<30mph). Over the next week or two problems progressively got worse and started idling rough but never stalled. Researched and found fixes ranging from low oil, dirty air filter, dirty MAF, vacuum leaks, to bad O2 sensors, spider assembly, FPR, or fuel pump.

Troubleshooting:
Based on the fact that both codes tripped at same time, I ruled out O2 sensors for the time being. Didn't have fuel pressure tester and research showed that fuel pumps rarely die a slow death, so started with the easiest/cheapest culprits first. Changed: oil/filter and air filter (needed to be done anyway) and cleaned the MAF sensor. Result - slightly better performance and DTC became intermittent, but drivability issues persisted and SES mostly stayed on.

Looked for vacuum leaks best I could but didn't find any (but I don't have the tools/knowledge/experience to thoroughly test). Used scan tool to monitor fuel trims which confirmed lean condition (both STFT avg ~25-50%, both LTFT avg ~25%) . Researched more and found many articles/post for many different vehicles having a vacuum leak at the PCV elbow. Checked mine and it was soft/pliable and distorted; also had a small (2mm) cut in the accessory line coming off the PCV valve cover on the upper intake plenum. Replaced: both valve cover grommets, PCV valve, PCV line assembly (with elbow), and PCV valve cover for a total of 35$ (OEM from dealership). Replaced everything last night and went for a test drive. Results: DTCs went away, drivability issues gone, reduced noise from engine compartment, improved fuel economy, no fuel odor from exhaust.

Note: After getting the old PCV parts off and inspecting them, I found two large holes in the back of the PCV elbow. Also, prior to replacement, I was able to easily remove the PCV valve and fresh air tube from the old valve cover grommets. After replacing the grommets and other components, the new PCV valve and fresh air tube fit very tight and took a bit of force to fully seat. Even if you don't have the P0171/P0174 code(s), for 35$ it might be a good idea to check the PCV system components if you notice idle issues, poor fuel economy, fuel odor from exhaust, or excessive oil on valve covers around the PCV valve/fresh air tube/grommets.

 

As it looked before removal.


Back of PCV elbow. Holding PCV valve cover which attaches to plenum.

 

Old post but you just made my day.

 

Glad it has helped someone. The forum has been a great resource for me so it feels good to contribute when I can.

Crazy thing about those codes/issue was it was recommended I replace the fuel pump (based on codes and symptoms, but no diagnostics). Glad I followed my gut and found the problem instead of throwing parts at it.

 

Yeah I did replace my duel pump. Ground was my problem. I suspect I have more vacuum leaks to hunt down.