Persistent lean issue under load

[Park Ranger]

Hey folks,

I have an FZJ71L-RJMNK, general market (no o2, waste spark, no distributor, MAP based, i.e. the open loop 1FZ-FE).

Issue is that while at idle it runs fine, but under load it leans out.

Checked:
- Smoke test, no issues. Carb cleaner test also didn't cause any variation in RPM.
- Water temp sensor, voltage correct.
- Air intake temp sensor, voltage correct.
- Replaced the MAP and checked voltage, correct.
- Replaced the Fuel Damper.
- Replaced the Fuel Pressure Regulator.
- Checked the fuel pressure and fuel leak down and both are correct.
- Grounds are clean, no electrical issues.
- Checked fuses and relays, no issues.
- Blink test shows no codes.

I'm at a loss as to what else could be it, short of replacing the injectors.

Alternatively is there a shop near Santa Ana, CA that would know how to handle these imports? Or I guess the nearest shop I can always ship it out.

Thanks.

 

[Eurasiaoverland]

I was in a similar situation with my 3RZ-FE and a high idle, eventually it was fixed when I replaced the intake air temp sensor (I think the one it came with was aftermarket junk).

 

[OGBeno]

Hey folks,

I have an FZJ71L-RJMNK, general market (no o2, waste spark, no distributor, MAP based, i.e. the open loop 1FZ-FE).

Issue is that while at idle it runs fine, but under load it leans out.

Checked:
- Smoke test, no issues. Carb cleaner test also didn't cause any variation in RPM.
- Water temp sensor, voltage correct.
- Air intake temp sensor, voltage correct.
- Replaced the MAP and checked voltage, correct.
- Replaced the Fuel Damper.
- Replaced the Fuel Pressure Regulator.
- Checked the fuel pressure and fuel leak down and both are correct.
- Grounds are clean, no electrical issues.
- Checked fuses and relays, no issues.
- Blink test shows no codes.

I'm at a loss as to what else could be it, short of replacing the injectors.

Alternatively is there a shop near Santa Ana, CA that would know how to handle these imports? Or I guess the nearest shop I can always ship it out.

Thanks.

You are about an 8hr drive from our shop. Welcome to bring it here but we are currently accepting work for Q1/Q2 2027.

 

[Park Ranger]

You are about an 8hr drive from our shop. Welcome to bring it here but we are currently accepting work for Q1/Q2 2027.

Thanks I appreciate that.

The process of importing and everything has already taken over a year and a half, and after fixing the lean issue it'll still be another 3-6 months. Let's see if we can't get this resolved sooner, if not I'll let you know in the next few weeks and ship it over to your shop.

As you can imagine I'm desperate to get this LC ready. I knew it was going to be a long and difficult process but it's proven to be more than I expected.

 

[JuanJ]

Hello,

It would help to know what is the model year.

As mentioned above, sometimes the air intake sensor can give you trouble.

Because of its location, it often ends up over the exhaust manifold during maintenance. If the manifold is hot, it may damage the sensor.

There should be an oxygen sensor and a catalytic converter between the exhaust manifold and the muffler. Even in general markets, the engine needs both this sensor and the crank position (the exact name escapes me right now) to work properly.

Have you tried resetting the ECU? Remove the glove box, disconnect the battery, unplug the ECU, wait for 10-20 seconds, plug it in, reconnect the battery. Sometimes it helps.

This variant of the 1FZ engine is sensitive to loose connectors and dirty/damaged pins. It is worth checking, cleaning and replacing if necessary.







Juan

 

[Park Ranger]

Hello,

It would help to know what is the model year.

As mentioned above, sometimes the air intake sensor can give you trouble.

Because of its location, it often ends up over the exhaust manifold during maintenance. If the manifold is hot, it may damage the sensor.

There should be an oxygen sensor and a catalytic converter between the exhaust manifold and the muffler. Even in general markets, the engine needs both this sensor and the crank position (the exact name escapes me right now) to work properly.

Have you tried resetting the ECU? Remove the glove box, disconnect the battery, unplug the ECU, wait for 10-20 seconds, plug it in, reconnect the battery. Sometimes it helps.

This variant of the 1FZ engine is sensitive to loose connectors and dirty/damaged pins. It is worth checking, cleaning and replacing if necessary.







Juan

Thanks Juan, I'll have to check exactly what the tech did with the air sensor, because they said: "I have checked the air intake temp sensor and it is working properly." I believe they checked the voltage but I'll try to confirm.

> There should be an oxygen sensor and a catalytic converter between the exhaust manifold and the muffler. Even in general markets, the engine needs both this sensor and the crank position (the exact name escapes me right now) to work properly.

This one is an open-loop model, no O2 from factory. The VAF terminal has a variable resistor that lets you adjust the mixture.

> Have you tried resetting the ECU? Remove the glove box, disconnect the battery, unplug the ECU, wait for 10-20 seconds, plug it in, reconnect the battery. Sometimes it helps.

The battery did die at some point, I haven't tried just re-setting it. Could a dodgy battery cause these issues? I figured it would be more likely to be fuel volume, or that it's not kicking the pump into high gear. Seeing as it seems to work fine at low load (or free revving) but only shows the issue when under load (actual driving).

 

[Eurasiaoverland]

Out of interest, what exactly are the symptoms.of running lean and how are you making this diagnosis?

 

[JuanJ]

Thanks Juan, I'll have to check exactly what the tech did with the air sensor, because they said: "I have checked the air intake temp sensor and it is working properly." I believe they checked the voltage but I'll try to confirm.

> There should be an oxygen sensor and a catalytic converter between the exhaust manifold and the muffler. Even in general markets, the engine needs both this sensor and the crank position (the exact name escapes me right now) to work properly.

This one is an open-loop model, no O2 from factory. The VAF terminal has a variable resistor that lets you adjust the mixture.

> Have you tried resetting the ECU? Remove the glove box, disconnect the battery, unplug the ECU, wait for 10-20 seconds, plug it in, reconnect the battery. Sometimes it helps.

The battery did die at some point, I haven't tried just re-setting it. Could a dodgy battery cause these issues? I figured it would be more likely to be fuel volume, or that it's not kicking the pump into high gear. Seeing as it seems to work fine at low load (or free revving) but only shows the issue when under load (actual driving).

Hello,

Once again, it would help to know your truck's model year. It appears to be a LHD, hard top, LX grade, assembled under license (from a CKD) 71 Series.

The 1FZ-FE engine in a 70 Series is very sensitive to

1. Improper contact between coils and sparkplug cables.
2. Oxygen sensor.
3. Crankshaft position sensor (I cannot remember its exact name, this sensor is located between the crankshaft pulley and the oil pan.)

Incorrect input from any of these makes the ECU go haywire: it tries to compensate, fails to do it and then forces the engine into limp mode.

The catalytic converter is located next to the frame rail, below the passenger seat. You should check if it is there, and whether there are signs of its removal. It is expensive to replace, and chances are the previous owner chose to delete it.

There is no such thing as a variable resistor to adjust the mixture. The ECU does that based on the input from the oxygen sensor and other sensors.

You should check your harness for hacks or splices.

A dodgy battery does not affect the ECU. The EFI system and its ECU are well protected, and there are safety features in the electrical system that fail before the ECU does.

Hope this helps.





Juan