First of all, i'll introduce myself and my car since this is my first post.

I am a new Lexus owner from Spain, proudly been since christmas last year. The car is a silver 1991 LS400 (with nak!) Imported to the EU in 94. Unfortunately all of its previous owners didn't take much care of it (most likely because almost no one here knows about these cars).

​Thanks to its robustness it still runs good and smooth after 140k miles of being most likely beat on. I did have to do some work for it to pass inspection, the most important parts being replacing the PS pump and most of the exhaust since it was basically rotten. I try to do all of the work myself.

As you can imagine this is probably going to be a money pit, but since I got it for a fair price and I wanted a project to learn on I think it will be worth it. My intention is to try and restore it to the way a true LS should shine and enjoy it on weekend trips, no more than 4000 miles a year.

Now on the topic at hand. Ever since the test drive the car has had a very sporadic CEL (Code 25, running lean) I have read here that it usually means bad upstream o2 sensors. The thing is when I checked the inspection results the lambda number was in fact a bit higher than usual, at 1.011, which if i'm not wrong should translate to a 14.8 AFR, not right for this car, which should mean o2 are in fact telling the truth and the car is running lean.

I have checked the MAF, it looks good but i will probably replace it anyway, will check PCV and plugs when i get the time, ECU has not shown any signs of anything weird for now, I have yet to test it with Yamae's guide.
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Clearly I'm not an expert so I'd like some help, could it still be bad o2 sensors? Is it probably safe to drive sporadically for a bit while I diagnose it?

Thanks in advance!

 

Some more info about the car that might be useful:

-Fuel pump seems to have been replaced recently.
-When I changed the PS pump I deleted the idle-up valve since it did look like it had been leaking through the vacuum line for what seemed to be quite a while. I connected both intake porta with a tube.
-MPG is about 20-23 which seems about right

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This is beyond my knowledge to address, but I'm curious about the power steer delete.

You say you blanked the ports on the valve with a tube, connecting them both together. Did you do the same to the connections points on the intake?

It's also possible you could have a vacuum leak on this connection, or any other vacuum system to create a lean condition. I would suggest running a leakdown test.

 

The ports I connected were the ones at the intake, I just didn't put the valve on the new PS pump at all, I put a drain plug on it instead, no leaks and runs fine atm.

The leakdown test would probably be a smart idea even though the idle is constant and good. It'll probably have to the next payday though since the LS seems to have a liking to my wallet and I haven't got the tool to do it.

 

Welcome! Is your LS 400 US Spec?

There is a procedure to test the upstream O2 sensors from the diagnostic port with a volt meter. It's better with a scope, and requires someone to help you rev the engine up to 2500 rpm.

Attached is the diagnostic procedure I followed on my 1991.

I got a code 25 on my car last year and found that one of my original upstream O2 sensors had become lazy. It also had a bad heater circuit when I removed it and tested on the bench, which is why the code was thrown.

Most of the time, that is the problem with this code. It is easy enough to test with a volt meter though, so I always recommend that first

The Karman-Vortex MAF can get quite pricey for these cars and does not often fail (unless you try to clean it...). It would not hurt to have a spare, but you probably won't need it. Don't throw your old one away!

Regarding the ECU, you should definitely inspect it. Replacing the ECU capacitors would probably be a good idea if they have not been done, since the car is 30 years old.

 

My LS is indeed US spec! I will follow that exact procedure when I get the chance to get someone to help! In the meanwhile i will probably still use it a bit since it seems to be intermittent and not running far from stoich.

​​​​​​Thanks for the advice, I'll be sure to post my findings when I perform the test!

 

I will also take apart the ECU to check how its doing, if it needs more than replacing caps i'll probably ship it somewhere for it to be rebuilt. If anyone knows a place in the EU where they do quality 1uz ecu rebuilds i'm listening. If not i'll probably have to look into the states and be left stranded for a bit with shipping times and stuff.
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If you haven't already make sure your fuel filter has been replaced. A clogged filter will cause low fuel pressure which will manifest as running lean.

 

Sorry for being so late to update on this, but thanks to everyone that helped me. The CEL is either no more or very very sporadic ( 1500+ miles). It has gone away progressively either as I did more stuff or just drove it, don't know which. Here is what I've done that could have affected the issue so I can help anyone getting the same code.

-I did the procedure listed above to check the O2 sensors, everything within spec.

-I visually checked the ECU and it does not show any signs of capacitor leakage

-I did the timing belt and water pump since the latter was starting to make a bad chattering sound.

-Replaced the EFI water temperature sensor.

And the two things that most likely to have fixed my issue are:

-When I was doing the timing cover gaskets both PCV/ Ventilation hoses broke off because they were as stiff as the cover itself, so I had to replace them, I would not discard at all the option that they were already causing a vacuum leak.

-This is just amazing: When i replaced my distributors, caps, wires and spark plugs I saw NipponDenso markings on all eight spark plugs. Nippondenso became Denso in late 1996, which leads me to belive that those were the ORIGINAL SPARK PLUGS 155K MILES AND 34 YEARS LATER. I replaced them with same spec platinum Denso plugs. The old ones have earned a good rest in my display case at home among my model cars.

​​​​​I am still astounded how well this car has held up being treated so badly all these years. Honestly there has been a time i've been thinking about trying to sell it and get something sportier, but It's so good and comfortable I'm sure I'd regret selling it, so I intend to keep enjoying it for a long long time.

 

Valve cover gasket leaks are also vacuum leaks. This is something people are surprised to hear but you really want to resolve all oil leaks in the engine if possible.