Hi, I have a 1993 LS400 which has recently had a lot of work done to it. In December, my dad and I replaced all the engine seals and gaskets as well as the starter and a bunch of other things. It ran fine but then started giving me engine code 31 which we traced to the MAF sensor. So we replaced it (from another 93 LS) and tested it, and it appears fine. I drove it back from their house to where I live, which is about 2.5 hours on the highway. It drove great - no more codes or issues. When I got it back, it still worked great, The next day I was going somewhere and all of a sudden I get a CEL. Checked later that day and its code 31, again. This time we assumed maybe the ECU. Last week we took it out and turns out it was replaced by the previous owner in 2018, which has a lifetime warranty. Just heard from the ECU manufacturer this morning that they see no issues with the ECU. Now I'm at a loss because I genuinely have no clue as to what would cause this anymore. Any help would be greatly appreciated.

A little more about the issues it's giving me - it only gives me the code when I'm not on the accelerator. When slowing down or foot off the gas, I get it but as soon as I start speeding up it goes away. It sometimes dies at idle, it often has an erratic idle, it sometimes has a high idle, and after driving about 30-45 minutes on normal streets it will do this weird thing where it jerks and bucks when at idle or when I am below 20 mph. It feels like something is about to explode. Since the CEL only comes on when I'm not accelerating, maybe it has something to do with air flow? Maybe AFM? Oxygen Sensors? I don't know anymore. I really love this car but it keeps giving problem after problem and I am out of ideas of what to fix next.

 

I believe your car has a Karman-Vortex type MAF. Do not spray any kind of cleaner into these type of sensors. Because you replaced the existing with a used unit, you can't know if the replacement unit is faulty as well.

You can try this to narrow down if it's the MAF or the electrical circuit for the MAF that's the issue: https://easyautodiagnostics.com/misc...-sensor-test-1

Yamae might chime in soon with some good advice.

 

It needs to use an oscilloscope to precisely diagnose the Karman Vortex air flow meter. Without it, it would not be easy to judge since you need to observe the oscillating waveform and measure the change of time interval of pulses. Is it possible for you to observe the waveform?

A simple checking method is to replace to a good working one. But it seems to me that you already have done it. Was the one you used a genuine one and confirmed good when it was used at another 93? It is the often case that many aftermarket ones are not fully compatible and many owners are not happy with those.

 

On my old 93 the AFM was bad. ( cuz I was stupid and cleaned it before I knew not to) Inside the AFM is the Intake Air Temperature (IAT) sensor which sometimes wont throw a code when it's bad. Mine only threw a code 31, not the 24 for the IAT, which is what the problem turned out being. I tried 3 different used AFMs which supposedly were all good, but they weren't. All of them were bad. I forget the procedure to test it cuz that was over 10 years ago. But I never did get it fixed. I had the bucking and high idle like yours until I unplugged it. (NOT recommended) It drove awesome unplugged, with the ECU in limp mode, but ended up ruining other parts.