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2000 Pontiac Grand Prix P0171 Code: Fuel Efficiency Issues and Troubleshooting Guide

Model: 2000 Pontiac Grand Prix Fault Code: P0171 Posted: 2013-12-31 08:35

Vehicle: 2000 Pontiac Grand Prix with 3800 Series II K engine, approximately 140,000 miles Issue: OBD-II Diagnostic Code P0171 (System Too Lean) is present, resulting in poor fuel economy (~18 MPG). Steps Taken: - Identified a known issue on the 3800 series II engine involving a 'stove pipe' that can cause coolant and oil contamination. Performed a full internal inspection and replaced the UIM plenum, LIM gaskets, valve cover gaskets, spark plugs, wires, and PCV system. The engine ran smoothly with no visible leaks. - Conducted vacuum leak checks; found two deteriorated fittings, which were repaired. No improvement in fuel trim or code resolution. - Replaced the upstream oxygen (O2) sensor and mass airflow (MAF) sensor without success. Diagnostic Testing: - Used an OBD-II scanner: At idle, LTFT was stable at 16.4%, while STFT reached a maximum of 20% before plateauing—indicating a rich condition that triggers P0171. - When the upstream O2 sensor was unplugged, the STFT dropped to zero and LTFT remained steady at ~16.4%. This confirmed normal O2 feedback behavior. - When the MAF was unplugged, the STFT reversed direction and drove LTFT to -30%, with the code disappearing temporarily—indicating a potential MAF-related issue. However, after replacing the MAF sensor, the same behavior persisted: full richness when plugged in, and -30% trim when unplugged. - Tested driving with the MAF unplugged; fuel economy remained unchanged, which contradicts expectations of significant improvement. Next Steps: - Considered an exhaust leak before the upstream O2 sensor (difficult to access). - Planned to inspect fuel injectors and verify that o-rings are properly seated and not damaged. Question: What other diagnostics or repairs could resolve P0171 in this 2000 Pontiac Grand Prix? Any recommendations for exhaust, fuel pressure, or injector-related troubleshooting? Thanks for any insights or advice.

Related fault codes

P0171

Comments (5)

Anonymous 2013-12-31 09:12

Perform a fuel pressure test and share the readings. Have you tried using carburetor cleaner to detect vacuum leaks? Note: This method is effective but requires caution due to flammability. Is your vehicle stock configuration? This significantly impacts performance and diagnostic behavior.

Anonymous 2013-12-31 19:08

The vehicle is stock, though previous owners may have made modifications—I don’t believe so. I initially tried the carb cleaner spray test and didn't detect any leaks. However, after applying more pressure, I found a leak at the back of the plenum. I repaired it with T-fittings as best as possible. No RPM increase was observed afterward, so I consider it fixed. Finding exact rubber elbows has been difficult. I’d love to find an affordable vacuum hose kit for this model—though generic kits are hard to locate online. I have not tested fuel pressure yet. Given the poor fuel economy and the engine running rich (not lean), I assumed a fuel delivery issue is unlikely. Please correct me if I'm mistaken.

Anonymous 2013-12-31 23:50

V-mech wrote: ...I’d love to find an inexpensive vacuum hose kit for this car and just replace everything—but only saw generic kits online... I replaced all vacuum hoses on my T-Bird last year. I measured the existing hose diameters with a digital caliper and purchased bulk hose in the correct sizes at an auto parts store. It’s simple—just buy the right size, not a pre-assembled kit. I’ve seen 'kits' listed on eBay, but they often just contain lengths of properly sized bulk hose bundled together. So you’re paying more for someone to package it. A generic kit works fine—but buying from a local auto parts store is usually cheaper and easier. For specific vacuum elbows or parts, check out Dorman’s lookup tool at http://www.dormanproducts.com/. Most Dorman components are available at local stores. Note: If one hose is damaged, the rest likely are too—inspect all hoses during maintenance.

Anonymous 2014-01-01 07:47

Carburetor cleaner is flammable—I once used it excessively and caused a fire. Before I had a smoke machine, I used propane to run along intake gaskets while monitoring O2 sensor readings. If the O2 reading shifted toward rich, that indicated a bad gasket. Fuel pressure affects both fuel banks. What are your current fuel trims after fixing the vacuum leak? Sometimes it's hard to find specific elbows—dealers often require buying a full set of five. I’ve experienced this before. For reference, check out Dorman for parts with accurate compatibility.

Anonymous 2014-01-02 21:33

The P0171 code is now gone—at least for today. I’ll monitor if it returns. Here’s what I did since my last update in case others face a similar issue. I was advised to inspect the injector o-rings, as air ingress could cause misfires and incorrect fuel trims. While removing the fuel rail from the front of the plenum, I found frayed wires connected to the Delphi solenoid valve. I cut them and spliced in new wire with the same length. I also applied fusion tape to a few vacuum fittings as a precaution against tiny pinhole leaks. All injector o-rings were replaced—though the old ones appeared intact (as they were previously replaced), so I’m unsure if this was the actual fix. After reassembly, I cleared the code and started the engine. No check engine light or P0171 present. During driving, LTFT remained between 3% and 7%, which is within acceptable range—slightly high but stable. This allows me to pass a DE inspection. Thanks for all the helpful suggestions.