1999 Ford F-150 5.4 Triton Misfires and Rough Running Issue
I bought a 1999 F-150 5.4 Triton with 158k for $700. It had misfires on cylinders 3 and 4 and looked like an intake manifold gasket leak. I replaced the intake gasket and swapped coils, but it got worse. No hard codes now, but live data shows misfires on cylinders 1, 3, and 7 (first three in the firing order). It runs rough and has no power. I’m about to throw injectors/plugs/coils at it.
You’ve got a classic Ford modular V8 misfire, but the pattern here—misfires on 1, 3, and 7 (all on the same bank, all odd cylinders)—points away from random coil or plug failure. Since it got worse after the intake gasket job, I’m leaning toward a vacuum leak or a problem introduced during reassembly. Swapping coils didn’t move the misfire, so throwing parts won’t help yet. If it’s a lean misfire, you’ll see it on the same bank, and that fits your symptoms.
Most likely culprits
- Vacuum leak at intake manifold or runner gasket — All misfires are on the same bank, and it got worse after intake work.
- Injector harness or ground issue (bank 1) — All affected cylinders share the same bank; harness pinched or not seated after intake job.
- Fuel rail not seated or injector O-rings pinched (bank 1) — If you disturbed the rail, could be leaking air or not delivering fuel.
How to narrow it down
- Spray carb cleaner or brake clean around the intake manifold (bank 1 side) while idling. If idle smooths out or RPM jumps, you’ve got a vacuum leak.
- Check fuel injector harness connectors on cylinders 1, 3, and 7. Make sure they’re fully seated and not pinched under the intake.
- Pull spark plugs on 1, 3, and 7. Look for signs of lean burn (white, blistered), fuel fouling, or coolant.
- Check for pending codes and freeze frame data. Even if no hard codes, pending P0171/P030x can point to lean or misfire root.
- Scan live data for long-term and short-term fuel trims (Bank 1 vs Bank 2). If Bank 1 is pegged positive, you’ve got a lean condition on that side.
- Listen for hissing or use a stethoscope around the intake runners (bank 1). Obvious hissing means a vacuum leak.
- If you have a noid light, check for injector pulse on 1, 3, and 7. No pulse = harness or PCM issue.
Possible parts (if needed)
- Ignition coils (only if you confirm a dead coil with a test or swap)
- Spark plugs (if fouled or damaged)
- Fuel injectors (if no pulse or dead with power/ground present)
- Intake manifold gasket (if you confirm a vacuum leak after your repair)
A few questions
- Did you use a torque wrench and follow the correct pattern/spec when installing the intake manifold?
- Did you disconnect or disturb the injector harness or fuel rail during the intake job?
- Are you seeing any fuel smell, coolant loss, or white smoke from the exhaust?
Don’t shotgun parts yet. Find the root cause—most likely a vacuum leak or harness issue on bank 1. If you find a big vacuum leak, fix that first and recheck misfire counts.