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Discussion Starter #1
Hey folks,

I've been browsing the forums for a while and have learned a lot. I currently have a p0171 code on a 2001 Impala LS with the 3800 series II engine.

Background: About 8 months ago my car would start bucking at high-way speeds. It was pretty scary stuff. I would be accelerating from ~45 mph and all of a sudden the car would just lose power. I didn't feel safe driving it. I fixed that problem by replacing the Mass Airflow Sensor (MAF).

I've also been having problems with my coolant disappearing as well as my engine oil, with no evidence of leaks on the ground!! I park my car in the same spot everyday. I took it to a shop and they couldn't track down any oil leaks, they recommended that I go to conventional oil rather than using synthetic. They did replace the plastic coolant elbows that run through the tensioner pulley with aluminum ones.

Current Issue:
The battery and starter are working excellent, but the initial start of the car is always an issue. The engine cranks over fine, but will NOT stay running unless I give it gas for about 5 seconds after starting. This happens regardless of the state of the engine; hot or cold.

I also have the p0171 code indicating that the engine is running lean. The idle does not like to stay at 750, it instead likes to hover around 1000 rpm. I have a bluetooth OBDII device and the torque app. I monitored the Short-Term Fuel Trim (STFT) and Long-Term Fuel Trim (LTFT) at idle speeds and at 2500 RPM in park. The STFT and LTFT are 20 and 16, respectively, at idle. At 2500 RPM the STFT goes to 0, and the LTFT decreases the longer I stay at 2500 RPM.

From what I've read it sounds like a classic case of a vacuum leak. However, I used a propane tank and rubber hose 3 different times around the Intake Manifold and all sources of vacuum lines with no effect on the engine.

Is there anyway for me to definitively conclude that there is an internal vacuum leak? I know the Intake Manifold gaskets are notorious in the 3800 series II, but I do NOT want to just start throwing parts.

Thanks!

Jason
 

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You MUST check fuel pressure at trhe rail. It should be around 43 lbs, non supercharged....if not, bad fuel pump or clogged gas filter..
 

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The lean condition may also be from a "DIRTY" mass air flow sensor. This is located on the throttle plate housing. I clean these very regularly at tuning intervals. Easy to do but must be GENTLE! If you unplug, this sensor, does the engine run better or not?? If not, check for a plugged up fuel filter. If that's ok, test the fuel pressure. If you are looking for vacuum leaks in the intake area, USE CARB and CHOKE cleaner. This chemical is very good for finding any intake leaks. Best of luck....Jake L.
 

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Discussion Starter #4
Today I tested the fuel pressure by plugging in a Fuel Pressure gauge at the Fuel Pressure Regulator port.

Here are the results:

1. Initial Key ON - 51 PSI

2. Engine Start # 1- Dropped to 45 PSI, then went above 55 PSI, engine stalled out

3. Engine Start # 2- 45 PSI and engine stayed running

4. Key OFF - 40 PSI

5. Key back ON - 55 PSI while pump was running, dropped to 45 PSI after pumped stopped running

6. Keyed OFF - 50 PSI

About a month or 2 ago I actually disconnected the Mass Airflow (MAF) Sensor and noticed the car did run better without it. I contacted the supplier of my part and informed them that it must have been a faulty MAF sensor. They shipped me a replacement free of charge. After I started using this newer MAF sensor I experienced the same problems I had with the other one!

Could I have gotten 2 bad sensors? I use the torque App and the MAF sensor readings seem to respond fine with different engine speeds
 

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Whenever I had problems with a MAF, I would switch to a different company.

There is only a few very thin wires passing the readings back.

I would definitely try another manufacturer. Make certain you have electrically cleans connectors. Consider DeOxit for cleaning.
 
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