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Fuel leak on 1AZ-FSE T25


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I have a curious fuel leak that I am trying to figure out on my brother in laws car. It is visible from underneath the car and it seems to be dripping from around the area near cylinder 1, but it is very hard to locate exactly where it comes from with the intake manifold mounted. I can not find any visible leaks from the engine bay. It leaks quite a lot, 1 drop each second when the car is running on idle and it needs to run for a minute before the leak starts.

My first assumption was the injector seal to the fuel rail. So I removed the rail and inspected the O-rings. They had no visible cracks or damages. So I cleaned the seal surface on the fuel rail, put everything back together and found that the problem was solved. The car was driven for a couple of days with no fuel leak until the fuel leak suddenly started again. He says that the leak is intermittent.

I read fault codes P0175 (system too rich bank 2) and P0136 (O2 sensor circuit bank 1 sensor 2). All the O2 sensors have very recently been tested, measured and they are OK.

Does anybody have any idea what could cause this? I have been searching for an answer online for several hours over a couple of days now, but do not seem to find anything that fits these symptoms. Can the external leak be connected to the too rich code and the O2 sensor and if so how? Can an injector be leaking externally and internally at the same time intermittently? And would this be caused by the injector itself or other things in the fuel system? Can the fuel relief valve on the fuel rail be causing the system to get too high pressure so that it blows the fuel out of the seals? If the fuel pulsations damper was leaking this should have been visible from the engine bay, although it is possible that I missed it. But would a leak on the  pulsation damper fit the description of the issue?

Also, the starter will need to be turned for a bit longer than usual for the car to start after this problem occurred. Once it starts the engine runs fine.

Any suggestions would be very much appreciated.

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Hi Sigve,It sounds like you've done some thorough troubleshooting already. Given the symptoms, it's possible that the fuel injector for cylinder 1 could be intermittently leaking both externally and internally. This could potentially cause the fuel mixture to become too rich, triggering the fault codes you've seen.

Issues with the fuel pressure regulator or fuel pulsation damper could also contribute to fuel leaks and pressure irregularities, which might affect starting performance as well.

Since you've already checked the injector seals and they appeared to be fine initially, it might be worth looking at them again them to ensure there are no tiny cracks or defects that could cause intermittent leaks.

If the issue persists, you might consider asking a mechanic who can perform more detailed diagnostics, such as a fuel system pressure test, to pinpoint the exact source of the problem.

The fault codes you've received, P0175 (system too rich bank 2) and P0136 (O2 sensor circuit bank 1 sensor 2), could be related to the fuel injector issue. If the injector for cylinder 1 is leaking intermittently, it could cause an imbalance in the fuel mixture, leading to a rich condition in bank 2. Additionally, a fuel leak near cylinder 1 could affect the readings of the downstream oxygen sensor (bank 1 sensor 2),potentially triggering the P0136 code.

Addressing the fuel injector leak and ensuring proper fuel delivery to cylinder 1 may resolve these fault codes. However, it's also important to double check the O2 sensors and their wiring to rule out any other issues contributing to these codes.

an external fuel leak can cause the engine to run rich, triggering a corresponding engine code, and it can also affect the performance of the O2 sensor, leading to further complications with the air fuel mixture.

Hope this helps:smile:

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The 1AZ-FSE is D4 direct injection engine. Unlike indirect or port injection which is low/mid pressure, the system works at a much higher pressure using a high pressure pump. Those seals need to be in good shape.

Also, AZ engines, E10 is not recommended for use with these engines! 

Just a reminder and info for those reading and not aware.


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