3rd & 4th Generation (1992–1996 & 1997–2001)Toyota Camry Discussion for years: 1992-1996 & 1997-2001
Topics of discussion range from fuel economy, safety, modifications, performance all involving America's favorite family car, the Toyota Camry.
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.
On page 14 of the diagnostics module, the suggested tolerance for both the STFT and LTFT on the 5s-fe are +/- 20%. Considering this specification, along with the fact that no engine codes are being recorded on the scan tool, it would seem that there is not presently a problem. ... Might be interesting to see what difference new oxygen sensors would make to this parameter .... maybe just remove, clean, and replace.
I personally think that you've covered most of the possibilities right there...I was going to basically list the same things you just said there...
a fuel injector cleaning is always a good idea at this point when you are unsure of what type of gas the previous owner used, etc. Also, problems with the fuel delivery by the fuel injectors don't always happen consistently throughout the rpm range since the performance of the injectors are also dependent on the actual real-time fuel pressure as well as how the ecu wants to open/close them
I also doubt that it's the fuel pump in your case as well, and I also doubt that it's the MAF sensor either. a malfunctioning maf sensor will cause many more problems than you're having- and most of them would be a lot less subtle.
The fact that the engine is trying to find a steady idle rpm also points to a slight vacuum leaks somewhere...the small ones are the hardest to find since it doesn't cause an obvious idling problem
another possiblity that you also mentioned and I think could also be a big possibility is the pre-cat o2 sensor that the ecu uses to tune the fuel trim with. it could be covered with carbon and not working as it should, but after this long (and with no maintenane records, again) it wouldn't be out of the question that the sensor itself is going back
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"Life is a deep sleep, of which love is the dream..." Ripped...and the girls are loving it.
The article seems to suggest that injectors that are not providing a good spray pattern will give the most problems for the PCM to compensate for rich/lean. From experience on my OBDI vehicle (92 138K) (not Camry) a used oil analysis showed increased fuel in the oil. My engine operation was excellent and I had no idea I was running rich. On a scheduled timing belt service I pulled the intake and the injectors and sent them out for analysis, cleaning and correction. They were returned to me with performance graphs. My last oil change with a UOA showed returned levels of acceptable fuel, near zero.
Not sure if this helps, but I find the discussion interesting.
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1995 Camry Wagon LE. 2.2 4cyl, 5S-FE, Auto, 187K
Informative article .... points out the importance of a uniform injector spray pattern. Also, O2 sensors, and MAP or MAF.
.... Most shops have an oscilloscope. Maybe they could diagnose injector pulse width non-uniformity by measuring the length of the time that the injectors receive a signal, at some given rpm .... comparing that value for the injector set, between cylinders, might determine if injector maintenance or replacement was required. The injector control wires ought to be accessible, somehow.
... Not completely sure I understand the injector pulse width control. Does the control module adjust the pulse width of each individual cylinder, according to the instantaneous O2 sensor reading? .... Or is it some sort of longer term averaging? ... Or does the control module adjust the entire cylinder bank according to the O2 sensor?
The article above didn't seem to go into much detail explaining short term or long term fuel trim.
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98 Camry LE, 2.2L, automatic
50k miles, drop in K&N A/F recent timing belt, water pump
Wow, that's a great article and definitely sheds some light on my issue! What's everyone think about trying a fuel system treatment like BG44K before spending considerably more money on professional injector cleaning or replacement?
I had been using a fuel addictive from time to time - still the injectors were not operating as new. There are companies that do nothing but test, clean, repair, re-test injectors, replace if necessary. If additives worked they would not be in business. That said, what the heck, it can't hurt. I would keep track of your present OBDII values, try the fuel additive, clear the codes and reset the computer so that it has to learn all that stuff over again and see what values you get.
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1995 Camry Wagon LE. 2.2 4cyl, 5S-FE, Auto, 187K
"The fact that the engine is trying to find a steady idle rpm also points to a slight vacuum leaks somewhere...the small ones are the hardest to find since it doesn't cause an obvious idling problem"
I have to agree with Eye8Pussies on this one. Check all vacuum lines for leaks, especially the large tube leading to the throttle body. That can move a bit as the engine flexes forward and back when you are on the throttle and then back off the throttle. As the car ages this flexing can cause cracks.
Let's also not forget one of the most basic items that can effect the air/fuel ratio - the air filter. Check it visually and then look down in the air filter box for leaves and twigs and dead animals, along with acorn nuts (I have heard of them being down inside there) that can move around down there.
I have to agree with Eye8Pussies on this one. Check all vacuum lines for leaks, especially the large tube leading to the throttle body. That can move a bit as the engine flexes forward and back when you are on the throttle and then back off the throttle. As the car ages this flexing can cause cracks.
I checked all fairly accessible vacuum lines including the intake hose and didn't notice any obvious problems even during flexing. However, I found that the previous owner or mechanic managed to snap the EVAP inlet air hose connector off the air cleaner while changing the filter. I replaced the top half of the air cleaner, but didn't expect nor observe any changes in idle speed stability or fuel trim.
Considering the 5SFE engine uses a MAP sensor located on the throttle body instead of a MAF sensor placed closer to the air cleaner, how would a leak in the intake hose affect metering of air?
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Originally Posted by Mike Gerber
Let's also not forget one of the most basic items that can effect the air/fuel ratio - the air filter. Check it visually and then look down in the air filter box for leaves and twigs and dead animals, along with acorn nuts (I have heard of them being down inside there) that can move around down there.
I took a good look at the entire air cleaner and filter while replacing the top half and only found minor dirt. Glad to find no dead animals!
... Not completely sure I understand the injector pulse width control. Does the control module adjust the pulse width of each individual cylinder, according to the instantaneous O2 sensor reading? .... Or is it some sort of longer term averaging? ... Or does the control module adjust the entire cylinder bank according to the O2 sensor?
That's a good question. The ECM derives long-term fuel trim from the adjustments needed to make short-term fuel trim average around zero for a particular set of conditions. Best I can tell, long-term fuel trim applies as an average to all cylinders. I'm not sure if short-term fuel trim applies to individual cylinders considering the single O2 sensor used to calculate it probably sees a mixture of exhaust from all cylinders. However, the ECM knows which cylinder it's firing and with useful crankshaft/camshaft position sensor data can probably make a rough estimate of when the majority of that particular cylinder's exhaust might pass the O2 sensor. Perhaps someone can provide more detailed information for us.
holy crap, an incredibly intelligent tech conversation! I haven't seen one in years!
Ionosphere, the pre-cat o2 averages all four cylinders exhaust outputs as it sits in a common chamber at the bottom of the exhaust runners. to accurately calculate exhaust trims for each cylinder, you need an o2 for each runner. There's a manufacturer that does it, I can't remember who it is though. However, this average isn't a problem because the equipment for each cylinder is the same. When that changes (say, an open injector wire or hole in the spark plug boot) it will suddenly stop firing altogether and will (hopefully) throw a misfire code.
Toyota put the 20% tolerance into effect because the engine ecu is able to tune the fuel trims to an acceptable level for performance and emissions as long as it's within 20%. Outside that window, I doubt the 1v o2 sensors could accurately depict what's going on. Plus, that's a huge air leak.
On to cleaning of the injectors! I've used the BG44k-did nothing to help my wife's car. I remain thoroughly convinced that BG is nothing but a bunch of chemical slingers in the aftermarket. I have used and been very impressed with the effects of a MotorVac cleaning. the fuel pump is disabled or fuel line looped back to the tank, and the Motorvac unit hooks to the fuel rail or supply line and feeds pressure to the fuel rail. there's a return line, if your car has a return fuel system. The cleaner is strong enough to melt paper fuel filters, and is run straight through the rail and to the injectors. I've always run it for a half hour, and my shops charged around $100 for the service. I've seen fords with walnut sized chunks of carbon on the valves fixed with it.
I truly believe the MotorVac is the top system for in-car injector cleaning. It's also cheaper and quicker than injector removal and professional injector servicing, but not quite as effective.
on injector pulse width:
four things control the flow of a fluid through a valve: pressure differential across the valve, orifice size, time open, and since we're talking about a pintle injector, valve height.
valve height is set because the injector is essentially an on-off switch with no in controllable in-between pintle height. Orifice size is set with the flow ratings of the injectors and is a design measurement. pressure differential across the valve simply means fuel pressure. In our application, we can usually expect to see 30-45psi, with a 5psi variance depending on manifold vacuum. So, we vary the amount of gasoline injected by changing the fuel pulse width (pw). This can be expressed as pw length (milliseconds) and be on the order of 2-50 ms depending on rpm and load, or as duty cycle (% on time). Duty cycle is more useful for the guys engineering fuel systems, so we just use ms.
All this said, I would recommend a motorvac service, make sure your tune-up is up to date, and don't worry about it. why? because your car is designed to tune itself on the fly for economy and emissions, and as long as your ltft stays reasonably within the 20% trigger point for fuel trims, your car isn't considered broke. cardinal rule, don't fix it if it ain't broke.
Just my opinion tho. I'd be happy to talk more about it because my 96 runs a 15% ltft and I'd like to see it more back to normal, too.
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1996 Camry Sport (that's a DX Coupe with decals) with 186k
european cars are just like japanese cars, just heavier and more expensive.
7 ASE's.
If there is a trend for ltft of +15% or so, then what component is likely to cause this?
Positive LTFT infers lean condition sensed at the pre-cat oxygen sensor. ... Insufficient fuel for correct stoic. combustion ... too much O2. ... consequently ECM adjusts injector pulse width for more fuel, therefore the observed positive fuel rich LTFT. .... Maybe just remove the O2 sensor at the front engine exhaust manifold and check for carbon deposits .... could also easily check resistance value and voltage output. ... Carbon deposits on O2 sensor might also affect its operating temperature, giving an incorrect output signal value.
O2 sensors are probably expensive. ... Pre-soak for a day or so with rust penetrant spray.
If there was some sort of problem with the injectors .... spray pattern or uniform fuel delivery, it seems like there would be a difference in combustion between the cylinders .... maybe some sort of vibration. The power delivered from the deficient cylinders would necessarily be less than the ones with good fuel spray patterns and complete fuel combustion.
Any power stroke differences between cylinders might require advanced instrumentation to detect .... maybe an accelerometer mounted to the engine block, or a high resolution crankshaft velocity sensor.
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98 Camry LE, 2.2L, automatic
50k miles, drop in K&N A/F recent timing belt, water pump
Ionosphere, the pre-cat o2 averages all four cylinders exhaust outputs as it sits in a common chamber at the bottom of the exhaust runners.
With fuel trim applying to all cylinders, what happens to those which operate properly or require different treatment? In other words, if one cylinder runs lean due to a clogged injector and the ECU compensates by increasing the fuel trim, do the other cylinders run slightly rich? If so, that's part of the reason why I'm concerned although the long-term fuel trim isn't high enough to throw a DTC. Assuming the injectors aren't clogged evenly, cylinders running rich due to increased fuel trim might generate considerable carbon deposits leading to future problems. Eyeballing the O2 sensor might shed some light on this theory.
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Originally Posted by 96CamrySport
Plus, that's a huge air leak.
Wouldn't clogged fuel injectors also cause the ECU to increase fuel trim? I sure hope I don't have to start hunting down an obscure vacuum leak, but I definitely haven't eliminated it from the equation yet given the subtle hunting at idle before the engine reaches normal operating temperature.
Quote:
Originally Posted by 96CamrySport
On to cleaning of the injectors! I've used the BG44k-did nothing to help my wife's car. I remain thoroughly convinced that BG is nothing but a bunch of chemical slingers in the aftermarket. I have used and been very impressed with the effects of a MotorVac cleaning. I've always run it for a half hour, and my shops charged around $100 for the service. I've seen fords with walnut sized chunks of carbon on the valves fixed with it.
I already have a can of BG44K, so I'll probably just try it. Live data mode on my OBD2 reader should inform me of any improvement from using it. Even a small improvement would probably verify dirty injectors as at least a portion of the problem. I'm also planning to clean the O2 sensor, but I'll wait until a significant amount of BG44K has passed through the system so it doesn't quickly regain a coating of carbon. I'd also like to avoid multiple variables in the equation. If I do wind up needing a professional injector cleaning, I'll try to find a shop with a MotorVac. I suppose they'll tell me if I ask about it specifically.
Quote:
Originally Posted by 96CamrySport
cardinal rule, don't fix it if it ain't broke.
As an engineer that loves troubleshooting, I tend to obsess over this sort of problem. I just can't help it! Also, I couldn't find any threads tackling my specific problem, so I figured an informative technical discussion with progress updates and perhaps a detailed solution would help other obsessive people in the future.
Quote:
Originally Posted by 96CamrySport
I'd be happy to talk more about it because my 96 runs a 15% ltft and I'd like to see it more back to normal, too.
If there was some sort of problem with the injectors .... spray pattern or uniform fuel delivery, it seems like there would be a difference in combustion between the cylinders .... maybe some sort of vibration. The power delivered from the deficient cylinders would necessarily be less than the ones with good fuel spray patterns and complete fuel combustion.
Any power stroke differences between cylinders might require advanced instrumentation to detect .... maybe an accelerometer mounted to the engine block, or a high resolution crankshaft velocity sensor.
After having the throttle body cleaned, the engine hesitated excessively under load while warming up. Aside from minor vibration, it idled fine in park or neutral but not in drive or reverse. The engine simply wouldn't stabilize or respond when idling or accelerating under load. However, bringing the engine to high RPM in park or neutral worked fine. The ECU did not log any misfires nor throw a DTC. The hesitation went away once the engine reached normal operating temperature. I suppose a chunk of buildup from the throttle body got into something and eventually exited because the problem disappeared after several days of driving which included bringing the engine up to 6000 RPM. I still notice some vibration at idle, but nothing I'd consider out of the ordinary.
After having the throttle body cleaned, the engine hesitated excessively under load while warming up. Aside from minor vibration, it idled fine in park or neutral but not in drive or reverse. The engine simply wouldn't stabilize or respond when idling or accelerating under load. However, bringing the engine to high RPM in park or neutral worked fine. The ECU did not log any misfires nor throw a DTC. The hesitation went away once the engine reached normal operating temperature. I suppose a chunk of buildup from the throttle body got into something and eventually exited because the problem disappeared after several days of driving which included bringing the engine up to 6000 RPM. I still notice some vibration at idle, but nothing I'd consider out of the ordinary.
Matt
I've encountered this exact same symptom while cleaning the TB on a corolla recently. My diagnosis was that I had a little liquid TB cleaner sitting at the bottom of the IAC motor (since the IAC is on the bottom) that I hadn't completely blown out with air, and it must have dried out in a few days, because now everything is back to normal.
Then again, this is just a guess of mine. Oh, and I wasn't hesistating on cold accel, just having trouble idling, so yours might have been different. Either way, sounds like something that happens when you clean the TB of these cars. Good to know Im not the only one.
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01 BMW 330Ci - 5spd / 93 BMW 525i 5spd-SOLD/ 98 Camry LE
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1998 Camry 5SFE 4-cyl idle long-term fuel trim
recent timing belt, water pump
Linear Mode
