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Code 25 engine lean

Tiny
DRED1990
  • MEMBER
  • 1994 TOYOTA SUPRA
  • 3.0L
  • 6 CYL
  • RWD
  • AUTOMATIC
  • 202,000 MILES
Need help/advice Toyota Supra 1993 GE.
I will keep the story short.
Blew head gasket and warped head, replaced both and also replaced spark plugs and leads and the car runs perfect no leaking and does not miss or any power loss.
Check engine light with code 25 engine running lean happens each time I drive it and warms up and each time I turn the car off and turn the car on again and start driving for about a minute or so the code comes back on, but still no missing or no power loss. Cleaned the o2 sensors overnight in petrol then drove the car for about an hour to get replacement o2 sensors. Removed the old o2 sensors and one is black while the other looks normal. Drove it home and took the new o2 sensors out to check em and the same problem one looks normal and the other one is black.
Still getting the running lean code so decided to check all vacuum lines for air leaks and did not discover any leaks. Just hunting for some advice please, because I am close to hitting the wall and running out of ideas.
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Tuesday, September 26th, 2017 AT 2:26 AM

22 Replies

Tiny
STEVE W.
  • MECHANIC
  • 13,014 POSTS
I would start with a fuel pressure check, low pressure could cause a lean condition. Next would be any exhaust leakage above the sensors (even a tight exhaust vale could let in enough to cause a problem).
Clean the AFM to be sure it is not sending the wrong data.
Check all of the wiring to the sensors, a bad signal wire will cause this code as the ECU substitutes a value if it does not get an actual signal from either sensor. The black is likely soot from oil burning as the engine has a lot of miles and wear.
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Tuesday, September 26th, 2017 AT 4:00 AM
Tiny
DRED1990
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Thanks for the reply. I will go through the steps you said and see how it goes.
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Tuesday, September 26th, 2017 AT 6:52 AM
Tiny
STEVE W.
  • MECHANIC
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Keep us in the loop.
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Tuesday, September 26th, 2017 AT 7:42 AM
Tiny
DRED1990
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So far I have applied exhaust gasket sealer to the entire exhaust ( exhaust gasket, o2 sensors gaskets ) so now it is 100% glued down and no possible leaks, Next step I will check intake side for any leaks and I will clean the air flow meter and check wiring for the o2 sensors as well, after applying exhaust sealant took it for a drive and still no change the check engine code comes on about two to three minutes during driving. This code coming on after around two minutes of driving is steering me to believe that it is the wiring to the o2 sensor one of them being damaged because the ECU uses the coolant temperature sensor for the air fuel measurements until the o2 sensors warm up to operating temperature. That is why I am thinking it is an o2 sensor wiring that is faulty damaged.
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Tuesday, September 26th, 2017 AT 10:58 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 43,070 POSTS
It sounds like you are on the right track. Here are some engine wiring diagrams (below) and a guide to help you do some testing to confirm the problem.

https://www.2carpros.com/articles/how-to-check-wiring

I gave you both turbo and non turbo engines

Please let us know what happens and upload pictures or videos of the problem.

Cheers, Ken
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Thursday, September 28th, 2017 AT 12:52 PM
Tiny
DRED1990
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Ok progress. The 2nd o2 sensor had 0 volts and was dead that for both the old one and the new one replacing it. Turned out to be not the o2 wiring itself but the o2 connector wiring back to the ecu that had some wires broken on it, now both o2 sensors are working and volts on both sensors. I conducted a vacuum test using a vacuum/fuel pressure tester and tested every vacuum port and all were showing 21hg which is in the green zone on the gauge so there's no vacuum leak on any part of the intake atleast. I reseted the ecu and took it for a drive but code still pops up after warming up and driving it down the road. I have a fuel injector sealer kit coming in tomorrow so I will be replacing all seals on the injectors so hopefully that sorts the problem out and also will conduct a compression test. I don't think spark or fuel is the problem due to no missing at all or no power loss but I don't know. Tomorrow I'll replace all injector seals and do a compression test then I don't know if it doesn't fix the code
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Thursday, October 5th, 2017 AT 3:31 AM
Tiny
STEVE W.
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How did you repair the O2 wiring? Some vehicles are VERY picky about sensor signal wiring, splicing a wire can introduce just enough resistance that the sensor works but falls just outside the proper range and you get a code.
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Thursday, October 5th, 2017 AT 9:45 AM
Tiny
DRED1990
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Cut both ends and exposed fresh wires on both ends then crossed both ends and twisted them together, I might solder them together then just to be sure then
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Thursday, October 5th, 2017 AT 6:12 PM
Tiny
STEVE W.
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Strip, clean, apply shrink tubing, twist wires axially, then solder, place shrink tubing over joint and shrink in place.
Simply twisting the wires = failure in a few minutes do to vibration and loss of contact.

Not the best video but it works.

https://www.youtube.com/watch?v=9ErNVJytyNs
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Thursday, October 5th, 2017 AT 7:10 PM
Tiny
DRED1990
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Hahaha Eric the car guy watch him all the time yeah sweet I'll redo it properly and I'm doing the injector seals later today and I'll let ya know how it goes
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Thursday, October 5th, 2017 AT 8:32 PM
Tiny
DRED1990
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Redone the wiring and soldered them and also replaced all seals and injector filters on the fuel injectors and still got the code, I can't do a fuel pressure test on the fuel rail due to not having a correct fitting for it so I'll try and find somewhere else to tap in try in front of the fuel filter or somewhere to get a fuel pressure. I even double checked vacuum pressures on all intake vacuum ports and again no leaks even pulling a vacuum line off the revs go up and then go back to normal once the vacuum line is back on so I can rule out vacuum leak
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Saturday, October 7th, 2017 AT 12:00 AM
Tiny
DRED1990
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How in depth knowledge do you have on o2 sensors particularly with voltage, one goes from 0.36 up to 0.65 roughly and the other one goes from 0.43 up to again 0.65 surely the 0.36 and 0.43 on low volts wouldn't throw a code unless both o2 sensors have to be exactly the same voltage, tomorrow I will solder the airflow sensor wires and do a compression test and then try to get a fuel pressure reading if I can find a spot to tap into. But the wall is closing in on ideas lol
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Saturday, October 7th, 2017 AT 6:54 AM
Tiny
STEVE W.
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O2 sensors are an interesting item. As a quick instructional, Early single wire sensors were platinum based and used the exhaust system as the return path for the sensors. They worked but you could get some strange readings if the exhaust rusted.

Next came the narrow band 3 and 4 wire zirconia/platinum types that start generating voltage at around 300 degrees F. And have heaters in them to get them operating faster. These "normal" zirconia type will run between.15 -.90 volts. Most generate around.45 -.50 at a "perfect" 14.7 :1 and will cycle high/low as they operate and the ECM uses that signal as proof the sensor is working and as the control input for fuel mix.

These evolved into the 5-6 wire wide band units used on some vehicles. These are VERY sensitive, They also operate differently. Instead of the wide swings in voltage they put out a relatively constant voltage and only swing up or down if the mix is badly controlled.

Then you have the Titanium dioxide based lambda sensors. (Commonly called an Air/Fuel sensor instead of an O2 sensor) These don't generate any voltage. Instead they act more like a thermistor in operation. These will be fed with a 5 volt reference source and the oxygen content of the exhaust changes the resistance that the reference signal is fed into. That changes the voltage the ECM reads as it's control signal. These don't show up as.1-.9 volts but instead they will show as 1.5 - 3.5 volts depending on the make and model vehicle they are on.

Your buggy runs a 4 wire zirconia/platinum sensor..34-.65 are on the lower end of the scale and could trip the system lean code because they are that low and not running closer to the normal range.
Low fuel pressure or a vacuum leak could cause that.

Factory test -
Connect the positive (+) tester probe of a voltmeter to terminal VF1 (for No.1) or VF2 (for No.2) of the data link connector (DLC1), and negative (-) tester probe to terminal E1.
Then follow the chart included.
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Saturday, October 7th, 2017 AT 8:49 AM
Tiny
DRED1990
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Mine is a 1993 model non turbo and its old school single wire o2 sensors, I did that test and that's how I discovered that one o2 sensor (the engine wire to it ) was dead and not producing any volts at all I then followed from the manual how to monitor the volts using ox1 and ox2 on the diagnosis port to monitor each o2 sensor
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Saturday, October 7th, 2017 AT 5:09 PM
Tiny
STEVE W.
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HMM that would make it different. Didn't see anything about it being the turbo unit in the description. For some reason they used the older tech on the turbo than the non turbo.
Not a big difference though. The signal is just carried back through one wire and it use the exhaust pipe as a return. With the voltage numbers you have you might try grounding the sensors directly to the chassis.
Be careful not to seal the back of the O2 sensors. They actually bleed air into the housing to make the electrodes work. One side is in "clean" air while the other is in the exhaust stream. The difference between the oxygen contents of those samples generates the voltage in the sensors.

Eric used to have a pretty good channel until money from outside sponsors seemed to make him forget how he got there.
I watch a bunch of the auto folks, plus a lot of the technical sites, sometimes you pick up a trick or remember something you haven't used for a while. It's also nice to see others fight with the same issues at times, makes it feel a bit less "lonely".
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Saturday, October 7th, 2017 AT 6:53 PM
Tiny
DRED1990
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Ok I'll give it a go and yeah exactly, I'll try and ground em/clean the bolts to ensure grounding, I'll try and find a spot to tap in to get a fuel pressure but I really believe it can't be anything with spark and/or fuel because it has no misses and drives perfect even full throttle high rpm no power loss at all
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Saturday, October 7th, 2017 AT 7:00 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 43,070 POSTS
Too me this sounds more like a MAF problem, have you tried to replace or clean it? Here are a couple of guides to help see what Im talking about.

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

and

https://www.2carpros.com/articles/mass-air-flow-service

If it has the mechanical kind I would see if the door moves freely. I have seen where the door pivot wheres out and the door drags inside the housing.

Please let us know what happens.

Cheers, Ken
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Tuesday, October 10th, 2017 AT 11:30 AM
Tiny
DRED1990
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I don't have any of those types mine looks ancient and is a 2 wire one, the clip is missing must of been broken by previous owner so it's pretty much 2 wires each connected to a metal clip that clips directly onto the sender where as the wires would be connected to the female plug to plug onto the sensor. The second picture is the best photo I have ATM where you can see on the intake that the wires directly connect to the sensor itself not through a female/male plug connectors so I might fix that up but at the same time wouldn't that set off a code straight away, the code comes on after around 1minute of driving when the o2 sensors warm up and start operating, one o2 sensor works fine volts jump and and down but the second one doesn't it stays high and doesn't move, I tried to rewire a few days ago and it started jumping like the other one but has failed again so I will have to really cut and connect fresh wires together and solder it and see what happens
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Tuesday, October 10th, 2017 AT 4:31 PM
Tiny
STEVE W.
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Yeah if the O2 sensor signal sticks it will cause codes and poor running.
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Tuesday, October 10th, 2017 AT 9:22 PM
Tiny
DRED1990
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Yeah one jumps bolts like it should but one is stuck on high volt, so I will get a new air sensor and also rewire the stuck volt O2 sensor and see how it goes but can't do that for a month due to going away for a month for work but I'll let you know how it goes when I do it
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Wednesday, October 11th, 2017 AT 1:01 AM

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