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High Long-Term Fuel Trims at Cruising Speed on 1998 5.7L Vortec — Diagnosis & Troubleshooting Tips

Model: 1998 5.7L Vortec Fault Code: P2177 Posted: 2014-06-21 20:19

Hi everyone, thanks for the helpful insights—I've learned a lot from this thread. I'm currently troubleshooting a 1998 Chevrolet Z71 with a 5.7L Vortec engine (128,000 miles). My OBD2 scanner shows long-term fuel trims (LTFT) between +10% to +14% at 1750 RPM and while cruising at 65 mph. When accelerating or climbing hills (above 2000 RPM), the LTFTs rise to 18–22%, but drop back to -5% to -8% when idle at 700 RPM with no load. The MAF sensor appears to be reading accurately based on my graphs, so I'm considering a cleaning first. Upstream O2 sensors are functioning normally (oscillating as expected), and downstream O2s look stable. Short-term fuel trims (STFT) remain within ±5% of zero throughout all conditions. What's puzzling is why LTFTs are significantly elevated during cruising—especially since idle readings are lean-negative. I'm looking for a focused, cost-effective troubleshooting path to identify the root cause without spending too much time or money. Any suggestions on what could be causing this? Is it a vacuum leak, low fuel pressure, MAF contamination, or something else? Thanks in advance!

Related fault codes

P2177

Comments (9)

Anonymous 2014-06-22 07:09

The Powertrain Control Module (PCM) adjusts fuel trim based on load and RPM. In your case, the PCM is adding extra fuel to compensate for a lean condition during cruising. Start by cleaning the MAF sensor. Also consider using a product like Techron Total Fuel System Cleaner—available at AutoZone—to clean intake valves, injectors, and carbon deposits. Check for vacuum leaks and verify there are no misfire events in your scan tool. Always check the misfire counter to confirm if any cylinders are experiencing issues.

Anonymous 2014-06-22 09:11

I occasionally see two random misfires on the total misfire counter, but I haven't identified which cylinder(s). I'm planning to use Seafoam as a fuel system cleaner to remove deposits and improve combustion efficiency.

Anonymous 2014-06-23 20:56

Today I noticed my spark advance drops nearly to zero while cruising or accelerating. This could be contributing to the misfire symptoms I’m experiencing—especially during uphill driving or hard acceleration. I'm still investigating whether this is related to a timing issue or faulty sensor readings.

Anonymous 2014-06-24 10:04

The PCM controls ignition timing based on inputs like MAP, TPS, and crankshaft position. What specific symptoms are you seeing—like rough idle, hesitation, or loss of power? This can help narrow down the issue.

Anonymous 2014-06-24 17:07

After cleaning the MAF sensor, my long-term fuel trims have improved significantly. The misfire has reduced but still occurs during acceleration or cruising—more like a brief RPM dip than a full misfire. It happens in both forward and reverse drive. I also notice that spark advance dips during these moments, which may be linked to the issue.

Anonymous 2014-06-25 07:17

The PCM uses inputs such as MAP (Manifold Absolute Pressure), TPS (Throttle Position Sensor), and crankshaft position to calculate ignition timing. Any irregularities in these signals could lead to incorrect timing or fuel trim adjustments.

Anonymous 2014-06-25 07:37

Is there a scan tool parameter that can indicate wear on the distributor? For example, any abnormal correlation between the crank and cam position sensors?

Anonymous 2014-06-26 10:48

A detailed misfire diagnosis suggests that if the issue occurs during uphill driving, towing, or hard acceleration—and resolves at idle—it may be related to cylinders 3, 4, 5, or 6. In such cases, inspect valve guide clearance. Remove the valve cover and check for binding valves. If a binding valve is found, consider re-honing the valve guides to achieve a clearance of 0.002 inches per guide. Reinstall with new valve seals. A good scan tool should show current and historical misfire counts by cylinder. What are your LTFTs like now? Also, check crankshaft and camshaft waveforms on an oscilloscope during the event for deeper insight.

Anonymous 2014-06-26 13:48

Valve guide clearance issues could be a factor in older 1998 Vortec engines. Before disassembling any components, perform basic checks: verify fuel pressure, conduct a vacuum leak test, and review the misfire counter data. Is your vehicle stock? Are you using original AC Delco parts or aftermarket components? These details can help determine if the issue is due to wear or modifications. A worn distributor would typically show abnormal CMP/CKP sensor correlation—though this might not trigger a code immediately. Still, it's worth checking for such anomalies in the scan data.