I need some input on a troublesome P0171 in my daughter's 2001 Toyota Celica. This code indicates "system too lean" condition. This is a 4-cyl, 1.8L engine.

Problem arose shortly after replacing MAF sensor w/ new sensor to fix a MAF code (P0101). This code has not returned.

Car runs fine and got close to 40mpg on long trip. My Obdscan scanner indicated the Long Term Fuel Trim (LTFT) reached the limit, +38% which sets the CEL. Short Term Fuel Trim was negative (lean condition). Monitoring the primary o2 sensor shows higher voltage (high 02) condition much more frequently than low voltage.

The list of faults leading to P0171 are: Air Induction system leaks, Injector blockage, Mass Air Flow Sensor, ECT sensor fault, Fuel Pressure, Gas leakage in exhaust system (ahead of O2 sensor), open/short in o2 sensor ciruit, and O2 sensor performance issue.

I checked exhaust system carefully for leaks and found nothing.

I checked the air intake and vacuum lines for leaks and initially found nothing suspicious. I cleaned PCV and cleaned new MAF. Scanner output indicates stable MAF readings at idle and linear increase in air flow w/ rpm.

I ruled out injector faults as car idles fine, accelerates fine, and generally runs fine.

ECT temps from scanner are in line w/ expected fully warm temps (200-205F) so no problem here.

Fuel pressure is a possible problem, however there is no place to tie in and check pressure. It requires rigging a tee w/ special fittings to measure pressure. One of my checks for intake air leaks was to remove the fuel rail and inspect the injector/manifold seals. In doing this I established there is subjectively good fuel pressure (not unexpected since it starts and runs OK). However low fuel pressure would result in a lean condition, leading to high O2 sensor readings. This scenario seems unlikely but I don't have much left to investigate. Sherlock Holmes said something along the lines "When all other possibilities have been eliminated, the remaining possibility must be the solution no matter how unlikely". I'm there,... I think.

I replaced O2 sensor since it had quite a few miles w/ a Denso universal. No detectable change in O2 sensor readings from before to after replacement.

A closer inspection of the PCV hoses found some cracked ends. I cropped the ends, and now notice that LTFT seems to have stabilized at around 25%, high but not enough to set CEL.

I performed a further intake air and vacuum system leak check using first throttle body cleaner sprayed on potential leak sites. Nothing found. I next tried injecting smoke using air siphon to pull smoke from a cigar into a vacuum line. Plenty of stinking smoke that only found its way out of intake air inlet. No other smoke indications.

I've now reached point that either I try to rig fuel pressure measuring system. A difficult task requiring sourcing of special quick connect hose fittings. It appears that replacing fuel pump, filter, fuel pressure regulator would not be much more difficult or costly, so that is my plan. Fuel pump is accessible through a removable cover beneath rear seat.

My question is does anyone see anything I've overlooked or something else to investigate other than fuel pressure?

Thanks for reading. As you might suspect, I'm a little frustrated.

Regards

 

Does the celica ecu have some sort of reset to use the correct readings of the new maf?

If you reinstalled the original maf sensor, what happens to the fuel trim?

Maybe hook up a vacuum gauge to the intake to see if the map sensor is providing the ecu with the proper readings?

Maybe try unplugging possible culprits, so the ecu has to use some default data. Then see if the fuel trim comes back to normal.

If you pinched the fuel return line, does the fuel trim come back to normal? Or unplugging the vacuum line to the fuel pressure regulator?

Are celica ecu's prone to failure?

I'm not familiar with Toyotas. I'm just trying to throw out some simple suggestions.

 

Update: I took a long ride today to see if LTFT would go to 38% limit and set CEL again. CEL did not come on, and LTFT stablized around 20%, still high. I performed a smoke test on exhaust and confirmed no exhaust system leaks. Injected Air/Smoke just pushed out around the foam stopper in exhaust tip.

I've ordered fuel pump assy w/ fuel pressure regulator. I think this may be the problem, unless fuel screen or fuel filter are severely blocked. I also ordered new injector seals, but there's no indication of leak around these after repeatedly spraying throttle body cleaner at base of each injector.

Q:If you reinstalled the original maf sensor, what happens to the fuel trim?
Response: I don't know. I installed in parking lot where my daughter works and had to leave shortly. So I didn't get to confirm MAF sensor worked beyond car started and ran normally. I reset ECU and left. She reported the CEL came on again shortly thereafter. I assumed the MAF problem was present, but when I finally got to scan, the code was P0171 and no MAF code (P0101).

Q:Maybe hook up a vacuum gauge to the intake to see if the map sensor is providing the ecu with the proper readings?
Response: No MAP in this system. I should check vacuum level to see if this indicates anything.

Q:If you pinched the fuel return line, does the fuel trim come back to normal? Or unplugging the vacuum line to the fuel pressure regulator?
Response: P0171 is system too lean. If fuel delivery is problem, it's low fuel pressure. I haven't tried pinching. The fuel pressure regulator is in the fuel pump assy (non-return regulator) and there is a pulsation dampner on the fuel rail.

Q: Are celica ecu's prone to failure?
Response: ECU would be a suspect if I was measuring good O2 sensor reading and LTFT was high. Everything appears good w/ ECU. This car has had very few problems and she has driven about 7 years.

Thanks for input. I'll post results of fuel pump assy replacement. It may be some time as I have to travel on business for a bit.

regards

 

Probably the number 1 reason most of Toyotas/Lexus set DTC P0171, System too Lean (Bank 1)/Fuel Trim Malfunction was due to a dirty MAF sensor. I learned first hand that if the air cleaner was not installed correctly (i.e., entirely closed, as many Toyotas had plastic tabs the air cleaner cover had to slide under first, before attaching the clamps, etc.), then the MAF sensor gets dirty pretty quickly. With P0171 set, I noticed the car drives with a lack of power, once cleared, it runs normal.

Toyota list the following trouble areas for P0171:

• Air induction system
• Injector blockage
• Mass air flow sensor
• Engine coolant temperature sensor
• Fuel pressure
• Gas leakage in exhaust system
• Open or short in heated oxygen sensor (bank 1, sensor 1) circuit
• Heated oxygen sensor (bank 1, sensor 1)
• EFI relay
• PCV valve and hose
• PCV hose connection
• ECM

 

Thanks, I have shop manual and have been through these. Last avenue is fuel pressure, which I've decided to replace fuel pump, filter, and fuel pressure regulator (all in tank) rather than attempt to come up w/ a fuel pressure measurement rig.

regards

 

I've purchased a replacement fuel pump, filter, and fuel pressure regulator, but haven't installed due to business travel.

I may also replace the MAF, as it's relatively inexpensive and problem originated after replacing MAF. I have looked at MAF output and numbers appear to be "reasonable" compared to calculated mass air flow rates based on RPM and estimated engine efficiency. Since I don't have any "real" efficiencies my numbers are based on avg engine values.

I'll post again when I have installed parts.

regards

 

After some time, effort, and money the Celica is running w/o P0171, at least so far.

I replaced 02 sensor, fuel pump, PCV hoses (this was at least part of problem), and MAF (3 times).

My first repair attempt was to replace the MAF w/ an aftermarket unit. After replacing other parts noted above, the engine was running better but LTFT was running +20-25% w/o P0171 (35% sets the code). I should have left well enough alone and driven for a while, but I was returning the car to my daughter in PC, FL and would not get a chance to do anything else for a while, so I decided to replace the MAF a second time (again an aftermarket part). This time the car ran about the same, but LTFT was higher and P0171 returned shortly. During all this work, I also tried to replace the valve cover gasket set, but the valve cover had been glued down w/ RTV and I could not break it loose (absolutely no pry points).

By this weekend I was in FL to return the car, and did not have any tools. I decided to try Toyota. They identified the valve cover was leaking (worse than before I started), and said the aftermarket MAF sensor was the problem. A new sensor was $229, but not in stock. They said it was running OK, and we could drive until next day Tuesday and they would install MAF when it came in.

When we tried to leave Toyota of PC, it would not accelerate at all and was difficult to start. I returned and complained that car was worse than when I arrived. They worked a short time and declared the car was good to drive. It now ran exactly as when I brought it in, but CEL came on shortly.

I had to fly back to Houston on Tuesday and my wife stayed behind to pick up the new MAF. She will fly home later.

The new MAF appears to have fixed the problem. Mechanic said toyota's MAF uses fine platinum wire and aftermarket use cheaper materials that don't work as well. They are probably right, but it make take a while to be certain. BTW, the gave us the MAF as compensation for the initial bad experience. I still dropped $330 for valve cover gasket replacement, air filter, and spark plugs.

Regards

 

CEL came back on again this AM w/ my wife driving in PC, FL. She said it started rough but smoothed out w/ warmup. Same performance as w/ other MAF's.

Now I know why Toyota PC gave away the MAF. They knew problem was not solved and did not want any further effort to diagnose.

There must be an intake leak somewhere despite all my efforts to find. I did a smoke test, but my smoke (cigar smoke) was not heavy and possibly I missed leak. It may be an intake manifold leak.

Fortunately it drives OK for most part and she'll be able to use.

regards

 

Last summer my buddy dumped hi girls 01 toy celaca in my lap after 3 shopes tryed to fix it But this one was a lot less trouble to fix becous it had a pluged inj. when cold and a missfire code with lean code. So can you put in some cleaner in it or take it to a shop with a scope. A good scope tech may be $ well spent. I just got a pico scope 6 mo. ago and I am still trying to use it well. But I can tell you this a good scope in the hands of a good scope guy can save you a lot of $ and time good luck

 

I agree, but finding a "good guy" is not easy. As you see Toyota punted when faced w/ complex problem.

Useful to summarize all I've done:

1) MAF replacement
2) O2 sensor replacement (no difference)
3) PCV hoses replaced hoses w/ cracked ends (helped reduce LTFT from 30-40% to 20-25%)
4) Fuel Pump replacement (no difference)
5) Injector/engine seals and injector fuel rail orings replaced (no difference)
6) MAF again (no help, slightly higher LTFT)
7) MAF by Toyota (no difference, running about the same, don't know code at this point but suspect same P0171 and P0300, 0301,..etc).

I don't have the car and may not for some time. I will look for a tech who may be able to diagnose. I'm guessing this must be an intake system leak and was hoping to find someone w/ a smoke test machine to perform a rigorous test.

thanks