I recently purchased a 1998 Camry 5SFE 4-cyl with about 100k miles on it. The seller could not provide maintenance history aside from indicating he followed the prescribed maintenance schedule which included changing the timing belt around 85k miles. He also mentioned using full synthetic oil for a number of years, but couldn't remember when he switched from dino. A mechanic who looked over the vehicle for me before purchase indicated no major problems.
Without detailed maintenance history, I decided to spread cleaning and/or replacement of various parts into affordable pieces over several months just in case they weren't done according to the schedule or simply overlooked/ignored. The first piece involved cleaning the throttle body, changing the plugs, and replacing the valve cover gasket. According to the mechanic, the throttle body looked like it was never cleaned. He also mentioned that the long-term fuel trim (LTFT) was rather high at 15 to 17%. Not high enough to throw a code, but definitely something to ponder in the near future.
Determined to save some money by attempting to solve the LTFT problem myself, I began monitoring live data on my OBD2 reader to collect fuel trim information in a variety of driving conditions. After several runs, I've observed the following:
1) At cold start, the ECM runs in open-loop (OL) mode which uses LTFT to determine injection pulse width. During this period, my ODB2 reader shows a LTFT of about 16%. Once the O2 sensors warm up, the ECM switches to closed-loop (CL) mode and begins using them to adjust the short-term fuel trim (STFT) to maintain the ideal 14.7:1 air/fuel ratio. At this point, my OBD2 reader shows a STFT of about -16% which completely negates the LTFT. As the engine warms up, the ECM slowly reduces RPM and the STFT slowly drifts towards zero while the LTFT remains about 16%. Once relatively warm, the STFT hovers around zero and the engine idles around the specified 700 RPM, but seems to subtly hunt which I notice as slight fluctuations on the tachometer instead of obvious rough operation. Aside from the latter, the engine runs smoothly at all RPMs with no hesitation.
2) When driving at common speeds, the LTFT returns to acceptable values between 0 and 6% except for a hump between 1300 and 1500 RPM where it jumps to around 20%. I haven't noticed any negative values for LTFT, but that's almost expected from an engine with 100k miles on probably original fuel injectors. After engine temperature stabilizes, I no longer notice any slight fluctuations on the tachometer and my OBD2 reader shows a solid 700 RPM.
Based on the above information, here's my thoughts on what might be causing the problem.
1) The fuel injectors may need cleaning or replacement, but I'd generally expect to see a high LTFT at all RPM if that were the case.
2) The fuel pump may need replacement, but that would likely manifest itself as hesitation and/or stalling at higher RPM.
3) Unmetered air is getting into the cylinders. This air might come from a vacuum leak perhaps caused by slightly malfunctioning idle air control (IAC) or PCV valves. The mechanic supposedly cleaned the IAC valve, so I'm hoping to leave it as a last resort due to high replacement cost. The PCV valve probably got cleaned during the valve cover gasket replacement, but I don't know for sure and haven't checked it yet. A slightly malfuntioning MAP sensor could cause the ECM to miscalculate fuel trim, but I'd expect problems such as misfires at other RPM if that were the case.
4) After warming up, carbon deposits on the first O2 sensor may cause it to measure improperly during periods of low exhaust pressure such as idling. The voltages seem within specifications, but I haven't pulled it out for further examination.
Can anyone here provide some insight or a solution to this problem? I find these types of problems very interesting, but I'm starting to lose sleep over this particular one.
Thanks,
Matt
Without detailed maintenance history, I decided to spread cleaning and/or replacement of various parts into affordable pieces over several months just in case they weren't done according to the schedule or simply overlooked/ignored. The first piece involved cleaning the throttle body, changing the plugs, and replacing the valve cover gasket. According to the mechanic, the throttle body looked like it was never cleaned. He also mentioned that the long-term fuel trim (LTFT) was rather high at 15 to 17%. Not high enough to throw a code, but definitely something to ponder in the near future.
Determined to save some money by attempting to solve the LTFT problem myself, I began monitoring live data on my OBD2 reader to collect fuel trim information in a variety of driving conditions. After several runs, I've observed the following:
1) At cold start, the ECM runs in open-loop (OL) mode which uses LTFT to determine injection pulse width. During this period, my ODB2 reader shows a LTFT of about 16%. Once the O2 sensors warm up, the ECM switches to closed-loop (CL) mode and begins using them to adjust the short-term fuel trim (STFT) to maintain the ideal 14.7:1 air/fuel ratio. At this point, my OBD2 reader shows a STFT of about -16% which completely negates the LTFT. As the engine warms up, the ECM slowly reduces RPM and the STFT slowly drifts towards zero while the LTFT remains about 16%. Once relatively warm, the STFT hovers around zero and the engine idles around the specified 700 RPM, but seems to subtly hunt which I notice as slight fluctuations on the tachometer instead of obvious rough operation. Aside from the latter, the engine runs smoothly at all RPMs with no hesitation.
2) When driving at common speeds, the LTFT returns to acceptable values between 0 and 6% except for a hump between 1300 and 1500 RPM where it jumps to around 20%. I haven't noticed any negative values for LTFT, but that's almost expected from an engine with 100k miles on probably original fuel injectors. After engine temperature stabilizes, I no longer notice any slight fluctuations on the tachometer and my OBD2 reader shows a solid 700 RPM.
Based on the above information, here's my thoughts on what might be causing the problem.
1) The fuel injectors may need cleaning or replacement, but I'd generally expect to see a high LTFT at all RPM if that were the case.
2) The fuel pump may need replacement, but that would likely manifest itself as hesitation and/or stalling at higher RPM.
3) Unmetered air is getting into the cylinders. This air might come from a vacuum leak perhaps caused by slightly malfunctioning idle air control (IAC) or PCV valves. The mechanic supposedly cleaned the IAC valve, so I'm hoping to leave it as a last resort due to high replacement cost. The PCV valve probably got cleaned during the valve cover gasket replacement, but I don't know for sure and haven't checked it yet. A slightly malfuntioning MAP sensor could cause the ECM to miscalculate fuel trim, but I'd expect problems such as misfires at other RPM if that were the case.
4) After warming up, carbon deposits on the first O2 sensor may cause it to measure improperly during periods of low exhaust pressure such as idling. The voltages seem within specifications, but I haven't pulled it out for further examination.
Can anyone here provide some insight or a solution to this problem? I find these types of problems very interesting, but I'm starting to lose sleep over this particular one.
Thanks,
Matt