04 4.2 Lean condition mystery

[Dieseldog]

Hello again! After a week long hunting trip running 4x4 through snow and mud my TB isn't running correctly again. I have P0171 🤮. So far I've replaced the TB gasket, intake manifold gaskets and injector seals. I already had the gaskets due to a shipping error on a head set I used previously to repair an issue. It starts fine and seems to idle fine as well but if I try to accelerate in gear it just bogs till it dies. This is what I know off the top of my head-
MAP 11 inhg at idle
O2 sensor flat line at idle
STFT 19% at idle
I can get exact live data tomorrow but these measurements were the ones that stood out.
I did a makeshift smoke test with a vapor smoking pen but I didn't see any smoke leaking. This leads me to believe it to be fuel related. The issue happened over the course of two days. It began as hesitant acceleration and progressed to not accelerating at all. The only code is the P0171. I plugged the evap line on the TB and the brake booster line at the manifold and double checked the vac line to the fuel pressure regulator for fuel and leaks. It still does the same thing. I don't have a MAF or air pump. The 02 sensor behaves The same way with a replacement. The CAT shows 1 psi of back pressure at WOT but that figure is from 1-2 months ago. The fuel system has had the pump replaced with a cheap Yita Motor pump and the filter is new. The tank has been plastic welded to repair the filler neck break. The fuel line from the filter to the rail was broke at the test port so I repaired it with a compression fitting and it has held up fine. Tomorrow I'm going to get the fuel pressure numbers but what do I need to look for to determine if it's the pump, regulator or injectors? I have an action cam I can use to record the fuel pressure as I throttle it. If there is any other information I can provide to assist in this diagnosis let me know. Also if you think it's could still be a vacuum leak where could I look again? I'll smoke it again if need be. Thanks again!

 

[mrrsm]

Even without having a Mass Air Flow Sensor on your 2004 SUV to get clogged and dirty enough to throw a P0171 Code... there IS an additional place where Unmetered Air can enter the Exhaust Stream and confuse the Upstream O2 Sensor:

The Secondary Air Injection System Solenoid:



Occasionally, this illustrated Solenoid Air Control Valve can STICK OPEN and allow additional Air to enter the Exhaust Stream and cause problematic STFT vs, LTFT issues to set a P0171 Code. Since the Item is conveniently located on the Passenger Side Middle area on the Engine Head... access and R&R are quite straightforward.

 

[Dieseldog]

MRRSM my TB doesn't have the SAIS. Is the 04 EXT version supposed to have one?

 

[mrrsm]

I thought the SAIS Pumps were only installed on the 2005 and Later Model GMT 360s... Too. But after watching THIS Repair on Youtube for the R&R of a SAIS Pump on a 2004 Trailblazer, I took a chance that perhaps yours was also equipped with it. If not... then the Lean O2 Mystery continues and more help should be on the way from other more informed Members:

There is always the dubious possibility that VOP (Video Original Poster) got the YEAR WRONG about his OWN SUV during filming, But I doubt it. It would also be good to know if GM made this SAIS Pump Design changeover in Mid-Year of 2004.

There is one possibility that might be evident on your Used 2004 Trailblazer after looking the Engine over:

Even without having a SAIS Pump Air Control Solenoid present and instead having a Block-Off Plate installed, which would normally eliminate ANY chance for this to cause a P0171 Code to throw from a malfunction here ...it might still allow Unmetered Air to slip by if that BLOCK OFF PLATE AND-OR HIGH TEMP GASKET HAS COME LOOSE.

 

[Dieseldog]

When I got the TB I had to replace the cylinder head and it didn't have the SAIS on the original head so I replaced it with a used head that didn't have the SAIS either. But that does tell me to check the gasket on the block off plate. I see where you are going with this and I didn't consider an exhaust leak could be an issue but I am going to double check the exhaust system now.

 

[budwich]

BUT... after all the work on the heads and stuff ... AND before your trip, was your vehicle running correctly? ie. trims were "reasonable" and no codes?
IF so, it is unlikely to expect an issue with a "missing sais" AND its more likely an issue with wiring. Since you have changed the o2 sensor with NO luck, then that points more towards the wiring which might have an issue... by wiring, it also means the connector. You need to check these. Further do an electrical check from the PCM towards the o2 sensor to help understand where a problem might be. Of course, also check for a cracked exhaust manifold near the o2 sensor / exhaust coupler area.

Lastly, check your MAP... both for leaks and electrical connection / operation.

 

[Dieseldog]

budwich the TB ran great and only started with these issues after I got back from the trip. Today I plan on checking the fuel pressure and I'm going to try and inspect the exhaust system as best as possible to rule out an exhaust leak.

 

[Mooseman]

The fuel system has had the pump replaced with a cheap Yita Motor pump

This would be my #1 suspect. Been there. Check your fuel pressure.