Griffin
05-15-2002, 10:11 AM
Just wanted to start a thread to explain and centalize info on lean codes, since they are such a common problem and I see people asking about them in general and Noob all the time.... If I say anythign that needs explaining let me know and I will do my best to clarify.
The code P0171 (system lean) is actually set when the Total Fuel Trim (combined long and short term) exceeds a preset value (20% positive). Why does this happen? Well it can be a number of things, but most common are intake or vacuum leaks and bad or misleading MAF values.
Vacuum / intake leaks are the easiest to understand: Unmetered air enters the engine and the O2 sensor notices the system is running lean. It then increases the amount of fuel provided by adjusting short term and then long term fuel trim to compensate. One easy way to find an intake leak is to take a can of carb clean (a combustible aeosol) and spray around the intame manifold and intake hoses, paying special attention to any joints or clamp areas. If when you spray the carb cleaner the RPMs increase you have located the intake leak.
The other most common cause (bad MAF values) if normally due to the coating of the MAF hot-wire element by oil or debris. This is especially annoying because it will pass a resistance check with flying colors. The best way to confirm it is to check the grams per second reading on the scan tool. on a 1.8 liter engine with the A/C turned off and no load on the engine in N or park you can expect to see about 2 grams per second at idle and like 7-9 grams per second at 2500 RPM. The best way to confirm is to find a vehicle that isn't having problems and compare teh results. If the number on the car with the P0171 is LOWER than the number on the known good car that is an indication that you either have a big vacuum leak or a MAF that is reporting falsely. Swapping a good maf out of a buddy's car into yours is an easy way to double check. If you swap it and the numbers get back to what they should be and teh fuel trim corrects, you have found the problem. You can they either clean or replace your MAF.
Griffin
The code P0171 (system lean) is actually set when the Total Fuel Trim (combined long and short term) exceeds a preset value (20% positive). Why does this happen? Well it can be a number of things, but most common are intake or vacuum leaks and bad or misleading MAF values.
Vacuum / intake leaks are the easiest to understand: Unmetered air enters the engine and the O2 sensor notices the system is running lean. It then increases the amount of fuel provided by adjusting short term and then long term fuel trim to compensate. One easy way to find an intake leak is to take a can of carb clean (a combustible aeosol) and spray around the intame manifold and intake hoses, paying special attention to any joints or clamp areas. If when you spray the carb cleaner the RPMs increase you have located the intake leak.
The other most common cause (bad MAF values) if normally due to the coating of the MAF hot-wire element by oil or debris. This is especially annoying because it will pass a resistance check with flying colors. The best way to confirm it is to check the grams per second reading on the scan tool. on a 1.8 liter engine with the A/C turned off and no load on the engine in N or park you can expect to see about 2 grams per second at idle and like 7-9 grams per second at 2500 RPM. The best way to confirm is to find a vehicle that isn't having problems and compare teh results. If the number on the car with the P0171 is LOWER than the number on the known good car that is an indication that you either have a big vacuum leak or a MAF that is reporting falsely. Swapping a good maf out of a buddy's car into yours is an easy way to double check. If you swap it and the numbers get back to what they should be and teh fuel trim corrects, you have found the problem. You can they either clean or replace your MAF.
Griffin