← Back to list

1999 Ford Grand Marquis P0171 and P0300/P0301/P0303 Misfire Codes - How to Fix Vacuum Leak Issues

Model: 99 Grand Marquis Fault Code: P0171 Posted: 2015-05-26 09:48

Hi, I'm experiencing persistent engine issues with my 1999 Ford Grand Marquis. After recently replacing the intake manifold (a common failure point), I've been consistently getting fault codes P0171 (System Too Lean), along with misfire codes such as P0300, P0301, and P0303. I've already cleaned the MAF sensor—initially brown, then grayish after cleaning—and believe it was dirty, but the problem persists. I'm now at a loss on what to do next. The misfire codes indicate specific cylinders (P0301, P0303) and others that appear randomly (like P0308), while P0300 suggests multiple cylinder misfires. I've also had P0174 in the past, which I believe was triggered when I cleaned the MAF sensor but forgot to reconnect it properly. I'm looking for clear, actionable advice—especially regarding vacuum leaks, fuel pressure, and potential causes behind these recurring codes. Any insights or troubleshooting steps would be greatly appreciated.

Related fault codes

P0171

Comments (3)

Anonymous 2015-05-26 10:57

P0171 (System Too Lean) is a common fault in Ford vehicles—often caused by vacuum leaks. Try the 'spray trick': spray carburetor cleaner around suspected hoses and connections while the engine is running. If the code clears, you've found a leak. PCV system leaks are especially common in older Fords. Check for any loose or cracked hoses. For more details on P0171: https://www.obd-codes.com/p0171 The P030x codes indicate misfires—check if spark is present at the affected cylinders and inspect plug condition. If no spark, it could point to ignition or fuel delivery issues. If vacuum leaks are ruled out, check fuel pressure next.

Anonymous 2015-05-26 11:26

Thanks for the helpful tips! I replaced my fuel pump last fall and installed new spark plugs during the intake replacement. I'll try the spray trick on suspected cylinders to test for vacuum leaks. I understand that P0301 is a consistent misfire, while others like P0303 or P0308 appear less frequently—this suggests some cylinders are more prone to issues than others. The presence of both P0171 and P0174 codes (lean mixture) has been frustrating. I believe the P0174 occurred when I cleaned the MAF sensor but didn't reconnect it properly—a likely cause for that specific code. This is really driving me nuts, so any additional troubleshooting steps would be extremely helpful. Thanks again!

Anonymous 2015-05-26 11:42

Always start with vacuum leak testing—use the spray trick on suspected areas (especially cylinders 1–4, which are passenger-side). A vacuum leak can cause both P0171 and P0174 codes. Fords from this era are well-known for recurring P0171/P0174 issues. If leaks aren't found, check fuel trims using an OBD2 scanner to assess actual air-fuel ratios. Misfires can stem from faulty injectors or low compression—start with vacuum leak checks first. While you're troubleshooting, adding a bottle of Techron or Seafoam fuel system cleaner may help clear carbon buildup and improve performance. Note: Cylinders 1, 2, 3, and 4 are on the passenger side.