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Intermittent P0171 code

Tiny
RON555
  • MEMBER
  • 2002 TOYOTA CELICA
  • 1.8L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 181,000 MILES
Hi,

I bought this car recently and I have struggled with code P0171 illuminating sometimes associated with bad fumes around the car on idle and it gets poor MPG in city. I started tracking fuel trims and noticed that long term fuel trim is always in the positive side but sometimes in the range of 5-10% other times in the 15-20% and found that it is always higher with half tank and up. I saw it drop to 5-8% which is not too bad when there is only 25% fuel left in tank. Last time the p0171 appeared long term trim was 18% short term trim was 19.5%. I replaced spark plugs, gas cap, air filter, MAF and even the purge valve with no change. I don't hear loud exhaust sound. I never top off the tank. First two images are freeze frame rest just taken while driving. Any insights will be appreciated.
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Sunday, December 13th, 2020 AT 4:36 PM

15 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,164 POSTS
The code and trims indicate a lean fuel mixture. The high numbers (trims) indicate the computer is trying to compensate for this problem.

If there are no exhaust leaks, start by checking for engine vacuum leaks. Here is a link that shows how that is done:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

If you look at the attached pic, it describes what the code indicates and shows that it is related to a lean mixture. Therefore, checking for a leak is my first recommendation.

Let me know what you find.
Joe
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Sunday, December 13th, 2020 AT 9:57 PM
Tiny
RON555
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Hi Joe,

Thank you for taking the time to reply! I tried to spray around but nothing changed then I bought a cheapo vacuum test gauge and here is a video for the test. The clicking noise around 0:10 is the purge valve noise continuously clicking. I will do more spraying tomorrow.

Regards,
Ron
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Sunday, December 13th, 2020 AT 10:14 PM
Tiny
RON555
  • MEMBER
  • 9 POSTS
I did some more visual inspection for the vacuum hoses and everything is intact nothing broken and sprayed throttle body cleaner with no change in rpm of trims values. I forgot to mention before, the car feels little bit weak when going uphill and I will be depressing pedal for more power but the response is weak sometimes but maybe because I used to drive 8 cylinder Crown Victoria before this car while this one is 1.8L with high mileage. I never had misfire codes and replaced spark plugs too. The vacuum gauge in my Torque app shows 25 in/hg which is little high. Not sure how accurate these Bluetooth OBD2 adapters are. Do you think it is a fuel delivery issue or compression?

Regards,
Ron
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Monday, December 14th, 2020 AT 1:44 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,164 POSTS
Hi,

The vacuum is a bit high. There are a few things that can cause this. For example, a plugged or partially plugged air filter. Also, an incorrect PCV valve restricting flow. If you look at pic 1, it indicates possible causes.

Here is what I suggest next. Check the above things first. If they appear good, confirm there is nothing blocking airflow into the engine. If it looks okay, then check fuel pressure.

Here is a link showing how in general to check fuel pressure:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

____________

Here are directions specific to your vehicle. The remaining pics correlate with the directions.

___________________________________________

2001 Toyota Celica GT L4-1.8L (1ZZ-FE)
Component Tests and General Diagnostics
Vehicle Powertrain Management Fuel Delivery and Air Induction Fuel Pump Fuel Pressure Testing and Inspection Component Tests and General Diagnostics
COMPONENT TESTS AND GENERAL DIAGNOSTICS
On-Vehicle Inspection

pic 2

1. CHECK FUEL PUMP OPERATION
a. Connect the TOYOTA hand-held tester to the DLC3.
b. Turn the ignition switch ON and TOYOTA hand-held tester main switch ON.

NOTE: Do not start the engine.

c. Select the active test mode on the TOYOTA hand-held tester.
d. Please refer to the TOYOTA hand-held tester operator's manual for further details.

pic 3

e. If you have no TOYOTA hand-held tester, connect the positive (+) lead from the battery to terminal 4 of the connector, and the negative (-) lead to terminal 5.

pic 4

f. Check that there is pressure in the fuel inlet pipe from the fuel line.

HINT: If there is fuel pressure, you will hear the sound if fuel following.If there is no pressure, check these parts, Fusible link, Fuses, EFI Main relay, Fuel pump, ECM, and Witting connector.

g. Turn the ignition switch to LOCK.
h. Disconnect the TOYOTA hand-held tester from the DLC3.

pic 5

2. CHECK FUEL PRESSURE
a. Check the battery positive voltage is above 12 V.
b. Disconnect the negative (-) terminal cable from the battery.

c. Purchase the new fuel tube and take out the fuel tube connector from its pipe.

Part No.:
1ZZ-FE: 23901-22100
2ZZ-GE: 23271-88600

pic 6

d. Remove the fuel pipe clamp.

pic 7

e. Disconnect the fuel tube (fuel tube connector) from the fuel pipe.

CAUTION:
- Perform disconnecting operations of the fuel tube connector (quick type) after observing the precautions.
- As there is retained pressure in the fuel pipe line, prevent if from splashing inside the engine comportment.

pic 8

f. Install SST (pressure gauge) as shown in the illustration by using SST and fuel tube connector.
SST 09268-41047 (95336-08070), 09268-45014 (09268-41200, 09268-41220, 09268-41250)

g. Wipe off any splattered gasoline.
h. Reconnect the negative (-) terminal cable to the battery.

pic 9

i. Connect the TOYOTA hand-held tester to the DLC3. If you have no TOYOTA hand-held tester, connect the positive (+) lead from the battery to terminal 4 of the connector, and the negative (-) lead to terminal 5.

j. Measure the fuel pressure.
Fuel pressure: 301-347 kPa (3.1-3.5 kg/sq.cm, 44-50 psi)
If pressure is high, replace the fuel pressure regulator.
If pressure is low, check the fuel hoses and connections, fuel pump, fuel filter, and fuel pressure regulator.

k. Disconnect the TOYOTA hand-held tester from the DLC3.
l. Start the engine.
m. Measure the fuel pressure at idle.
Fuel pressure: 301-347 kPa (3.1-3.5 kg/sq.cm, 44-50 psi)
n. Stop the engine.

o. Check that the fuel pressure remains as specified for 5 minutes after the engine has stopped.
Fuel pressure: 147 kPa (1.5 kg/sq.cm, 21 psi) or more
If pressure is not as specified, check the fuel pump, pressure regulator and/or injectors.

p. After checking fuel pressure, disconnect the negative (-) terminal cable from the battery and carefully, remove the SST and fuel tube connector to prevent gasoline from splashing.
SST 09268-41047 (95336-08070), 09268-45014 (09268-4200, 09268-41220, 0926-41250)

pic 10

q. Reconnect the fuel tube (fuel tube connector).

CAUTION: Perform connecting operations of the fuel tube connector (quick type) after observing the precautions.

pic 11

r. Install the fuel pipe clamp.
s. Reconnect the negative (-) terminal cable to the battery.
t. Check for fuel leakage.

3. REMOVE REAR SEAT CUSHION
4. REMOVE FLOOR SERVICE HOLE COVER
5. DISCONNECT FUEL PUMP & SENDER GAUGE CONNECTOR

pic 12
6. INSPECT FUEL PUMP RESISTANCE
Using an ohmmeter, measure the resistance between terminals 4 and 5.
Resistance: 0.2 - 3.0 Ohms at 20 °C (68 °F)
If the resistance is not as specified, replace the fuel pump.

pic 13

7. INSPECT FUEL PUMP OPERATION
Connect the positive (+) lead from the battery to terminal 4 of the connector and the negative (-) terminal 5. Check that the pump operates.

NOTE:
- These tests must be done quickly (within 10 seconds) to prevent the coil burning out.
- Keep fuel pump as far away from the battery as possible.
- Always do the switching at the battery side.

If operation is not as specified, replace the fuel pump or lead wire.

8. RECONNECT FUEL PUMP & SENDER GAUGE CONNECTOR
9. REINSTALL FLOOR SERVICE HOLE COVER
10. REINSTALL REAR SEAT CUSHION

____________________________

Let me know what you find.

Joe

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Monday, December 14th, 2020 AT 3:54 PM
Tiny
RON555
  • MEMBER
  • 9 POSTS
Hi Joe,

I actually replaced the both air filter and PCV valve with no change. I wonder what else could be blocking the air flow to the engine? I will try to gauge fuel pressure although they don't make it easier on these engines with no testing port. I highly appreciate the time spent and knowledge you are providing.
Thank you!
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Tuesday, December 15th, 2020 AT 4:09 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,164 POSTS
Hi,

You are very welcome. The best I can do is try. I would suggest the fuel pressure test. They make nothing easy. LOL When you check it, pay attention to fuel pressure drop off. That is when you turn the key off. If it drops fast, it could be related to a leaky injector.

Let me know.

Joe
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Tuesday, December 15th, 2020 AT 5:48 PM
Tiny
RON555
  • MEMBER
  • 9 POSTS
Joe, now you mentioned the leaky injector, the engine actually starts little bit harder when it is hot or warmer and that is a symptom of a leaky or bad injector right?
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Tuesday, December 15th, 2020 AT 6:09 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,164 POSTS
Hi,

It certainly can be. Often times, you will see a small amount of black smoke from the exhaust. Try the fuel pressure test. When the pump is turned off, watch how quickly the pressure drops.

Let me know.
Joe
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Wednesday, December 16th, 2020 AT 1:46 PM
Tiny
RON555
  • MEMBER
  • 9 POSTS
Hi Joe,
This slightly higher vacuum reading at idle (24-25 range) is concerning me. Do you think the high vacuum reading at idle is caused by wrong timing? Meaning the timing chain skipped one tooth maybe? The engine rattles at cold starts. Or is it related to Idle Air control valve?

Thank you!
Ron
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Sunday, December 20th, 2020 AT 11:52 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,164 POSTS
Hi,

I don't see how it could be timing related. However, it could be related to a partially plugged catalytic converter. Also, (in theory) the IAC, if restricting too much air flow at idle could increase it.

Do me a favor. Just for the heck of it, remove the oxygen sensor before the catalytic converter and see if the vacuum drops to normal or drops at all.

Here is a link that explains how to check a catalytic converter, but it indicates removing the catalytic converter. What I suggest should show a difference if it is the cause.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

We are still dealing with a lean mixture (P0171). That is the part that is confusing me.

Let me know.
Joe
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Sunday, December 20th, 2020 AT 9:40 PM
Tiny
RON555
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I will try to measure temperatures around the catalytic converter first. The engine burns some oil and not sure how much that effected the catalytic converter. I will be switching to full synthetic instead of the conventional tomorrow. Will report back after measuring the temperatures.
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Monday, December 21st, 2020 AT 10:08 AM
Tiny
RON555
  • MEMBER
  • 9 POSTS
Hey Joe,

So forget about the vacuum reading cause I hooked the actual vacuum gauge to brake booster hose and it was reading in the normal range while the Torque app. Reading 25! So it must be some calibration error inside the app or something. So now back to the one issue which is high positive fuel trims. This a snapshot from the app after some driving. It is very erratic and unpredictable.

Thank you!
Ron
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Monday, December 21st, 2020 AT 1:21 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Let me know what you find with the catalytic converter. Also, the high positive fuel trims are likely a result of what we discussed. Although the vacuum is normal, the fuel pressure could be weak. Have you had a chance to check it? We discussed it earlier but I'm not sure if you have the chance to do it.

Joe
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Monday, December 21st, 2020 AT 8:45 PM
Tiny
RON555
  • MEMBER
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Hi Joe!

Sorry I got a lot going on and did some extra work on the car so I wanted to post an update. Since your last reply, I replaced the fuel injectors with used re manufactured ones from eBay cost was $25.00 for set of four! And replaced coil packs for cheap from junk yard ($25.00 for almost new coil packs) but all of that didn't bring the high trims down until I replaced the O2 upstream sensor which it was also used from junk yard. The trims are not perfect but in the range of positive 3.9-10% Total but i'm suspecting that the cat is bad too cause after disconnecting the battery terminals when I have done all the work mentioned, the catalytic converter monitor stuck at catalytic converter test incomplete for the last 100 miles. Would a bad catalytic converter cause that? Or it should throw a code right?

Thanks Joe!
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Saturday, January 23rd, 2021 AT 5:53 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,164 POSTS
Hi,

In most cases, the catalytic converter will throw a P0420 code indicating it's operating below threshold. It will likely take a few trips for that to set.

By the way, the trims look really good now. However, like you, I feel the catalytic converter was damaged.

Joe
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Saturday, January 23rd, 2021 AT 6:32 PM

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