growthroughlife

I have a 1992 LS400 that I have been doing tons of preventative maintenance on over the last year. About 1 week ago the car started dying at stop lights. Would usually start right back up but lately it will not start unless my foot is on the accelerator. It feels like the car is idling at around 200-400 RPM (my RPM gauge is not working so I cannot tell exactly what it is at). If I don’t keep my foot on the accelerator the car dies.

I have just replaced the ISC with a remanufactured one 2 days ago, which seems to have made it worse. I also replaced the fuel pressure regulator at the same time.

In the past 3-6 months I have also replaced the fuel pump, fuel filter, PCV valve and hoses, rebuilt ECU from LsCowboyLs, timing belt/water pump, spark plugs, spark plug wires, distributor caps/rotors, ignition coils, radiator + hoses, thermostat, coolant temp sensor, all O2 sensors, alternator and battery (& much more this is just the related engine parts)

I cannot track down what could be causing this. It is worth noting there is an EGR delete, which caps off several vacuum lines. I have no vacuum lines coming off the intake, and I believe the vacuum line coming off the fuel vapor canister is capped off as well (doesn’t make a difference if the cap is on or off it seems). Also the BVSV broke off during one of the installs.

The next few parts I will be installing are the fuel pressure dampener, throttle position sensor, and the crankshaft position sensor. Besides that can anyone point me in some direction for how to diagnose this. I don’t think it is the injectors, but it could potentially be the AFM or something funky with my weird vacuum lines. There is no check engine light on either.

CLLEXUSS

I have no idea how that works when there is an egr delete and capped lines etc. These cars utilize computers as you are well aware of. I just think its interesting and wonder how you make sure the computer is okay with these modifications because there are signals and parameters and if something has been modified how do you make sure your computer is happy with the absence and deletion of components? I mean if your fuel canister is capped off too. Yea I don't get how you are trying to tackle all of these other different things when you have fundamental issues you know about but are not trying to correct them. Maybe there's something I am missing though that certainly could be.

growthroughlife

I bought the car for cheap from some young kid knowing most of the issues it has had, which mostly were not detrimental. If I could do it again I’d opt for a better maintained first gen LS400, but I have enjoyed my time working on it. This is the first time the car has behaved this way and it’s really making me scratch my head as so many components are new. I’ve spent the last week researching this forum and many other forums to see any possible cause for it, which I have mostly eliminated the possibility of several different causes.

I do know there is a resistor in one of the EGR plugs which tricks the ECU into thinking the EGR is always open. I bought the car like this and it has been running great for almost a year now. I actually just discovered that the fuel evap canister vacuum line wasn’t connected to anything, so I tried capping that one off without realizing what it was connected to (I have since reverted that). The other lines that are capped are on top of the throttle body, which usually connected to EGR components, as well as the one vacuum line at the back of the intake plenum near the firewall. It does make it difficult to reference the vacuum line diagrams to identify if it is a vacuum leak.

These problems I’ve been experiencing only started showing around a week ago. I figured since it was idling poorly it was the ISC. After installing the ISC the car was idling very high until I reset the ECU by disconnecting the battery for an hour. Then it started to idle very low again, and stall when stopped or the revs suddenly drop. I can’t even figure out how to check the fuel pressure since there is no schrader valve on the fuel lines on the first generation 1uz.

The only thing I can think of going wrong recently is that the BVSV broke during the install of the ISC, though I’ve heard people saying this doesn’t have a huge effect on engine performance. I also had this stalling issue before that broke. Maybe I didn’t install the ISC correctly which seems unlikely. Or the problem lies with the TPS, AFM, or crank position sensor. It does seem like a fuel issue though.

LeX2K

I use this pressure gauge on various Toyota's yes it's not the best quality (and probably not very accurate) but good enough.

You've replaced so many parts that it could very well create even more problems making diagnosis very difficult especially if you didn't use OEM parts (nothing on Amazon or Ebay can be trusted to be OEM even if it is stated as such). Are you SURE the ECU was rebuilt properly? Do you have a vacuum gauge?

growthroughlife

Ive made it a point to only use OEM parts or OEM manufacturers for this vehicle such as Denso, Aisin, and NGK. I do understand that does complicate diagnosis. I hadn’t replaced anything for about a month and it was running very healthy, until it started idling poorly a week ago. I have full confidence that Bill (LSCowboyLS) rebuilt the ECU properly, but it is a possibility that it wasn’t, in which case he does offer a lifetime warranty. Though I’d like to rule that out until I’ve determined that’s the only possible link to these problems. Considering everything I had replaced is OEM, and done as per the factory service manual specifications, and I haven’t had any issues directly as a result from any of the new components I replaced. It leads me to believe it is something I have not replaced yet. Though I will not completely disregard the chance that I messed up installing something, or have a faulty part somewhere.

I do not have a vacuum gauge but I was going to rent one from Autozone, before I discovered there is no schrader valve. Apparently on these first gens you have to use a specific banjo bolt to connect the vacuum gauge to, but I cannot figure out where to get it or what size I’d even need.

LeX2K

Vacuum gauge is easy there are multiple places to hook it up. Fuel pressure gauge you can go to the fuel filter or one of the pipes on the fuel rail. Did you buy your parts from a legit source? Amazon and Ebay are not they sell mostly fakes unfortunately.

Fuel pump failure on a Toyota is rare, it happens but almost always because the car has sat long enough where the fuel turned to sludge and varnish.

growthroughlife

After taking apart and replacing several more components of the engine (TPS, Air Filter, Power steering IACV, various hoses, crankshaft position sensor, serpentine belt tensioner/pulley, and idler pulley), I discovered that the timing on my right hand cam appears to be off by one tooth. I replaced my timing belt/water pump and related pulleys in April, and the car ran almost perfectly for months. It is only now that I am discovering the timing being off, and the idle being so low.

Below is a picture of where the right hand camshaft sprocket sits when the crankshaft is set to TDC. I do not believe the left hand cam is out of time, but it is possible as well. This was my first time doing a timing belt job and I was very diligent to triple check all my resources and ensure I installed everything correctly, is it possible the belt slipped a tooth? Why would I not notice this idle issue for the few months after the belt was replaced otherwise?

LeX2K

Your timing belt has marks I'd use those instead of the ones on the back plate I know that means removing the crank pulley but you'll be 100% sure the timing is correct. Marks on the gears/backplate can be iffy.

growthroughlife

The marks on the belt itself did not line up once rotating the engine twice over from TDC to TDC. I was told to verify the marks on the sprocket and bracket itself as per instructions on LexLS and the Ratchets and Wrenches video on 1uz timing belt. I haven’t heard anyone mention those marks not being reliable. I’ve already had the crank pulley off twice, so that shouldn’t be an issue. But why would I have to do that to line up the belt marks?

How do you suggest I go about retiming the engine? I’m obviously going to have to do the majority of the timing belt job over again, but how would I ensure it’s timed properly in just using the belt marks?

growthroughlife

I have finished reinstalling a new timing belt and all the pulleys while I was in there. I made absolute sure that the timing was correct. After putting it all back together it still has the same issue of idling too low. The car stalls while cold and you have to apply some pressure to the accelerator with the AC off until it has warmed up.

I took the car into my local Lexus dealership for a diagnosis because I could not figure it out. Well they told me that because the car does not have an OBD2 port, they were not able to hook up their scan tools and couldn’t figure it out either. How did we diagnose issues with cars before these fancy scanner tools? Problem is this car is older than most of the techs working there so it’s hard to find someone who knows what they are looking at. I’d expect more from the dealership, however. The master tech there thought it was either the TPS or ECU, both of which are new.

I’m kind of at a loss here. Not sure if I should take it somewhere or what else to look at. This weekend I will likely find a way to hook up my fuel pressure gauge, which I am used to using on a schrader valve, so not exactly sure how I’ll do that. I’ve searched this forum countless times, and could not find anyone with a similar issue, and those that I did find, my car is not affected by (such as new IACV for power steering).

growthroughlife

For the first time today, I actually have discovered 2 engine codes present by connecting Te1 and E1 on the diagnostic port. These codes did not appear before, I have tested at least 5 times this month, but strangely I still do not have a check engine light and the light does function as it should. The codes I got were 24 - Intake air temp. sensor signal, and 31 - Air flow meter signal. Of course, the AFM is the culprit, the part I've practically turned a blind eye to in this diagnostics. Though it is strange I never got a check engine light and only now do I see codes in the diagnostic procedure.

Now I actually did test the AFM when these problems first started occurring, albeit not thoroughly enough. I only tested the resistance on the THA - E2 terminals on the AFM as per instructions in the repair manual for my car. The resistance was in spec so I wrote it off. I am fully convinced that this is what my problem is, especially considering how much of the engine bay is brand new and OEM, and I'm sure this is a 32 year old part at this point. I did discover this post: https://easyautodiagnostics.com/misc-index/lexus-maf-sensor-test and will be following it this weekend to fully confirm if my issue is with the AFM itself or the connector/harness. I have already purchased a new AFM from Amayama, which will not be here till the end of the month, and I also ordered a new pigtail for the MAF to be on the safe side (though electrical and wiring is still very new to me).

I wanted to confirm today before my commute to work that the AFM is faulty by disconnecting it and seeing if the engine still runs. I have read many posts on this forum about the car running better without the AFM connected because the ECU will default to predetermined values for the A/F ratio. I figured if the car runs fine without the AFM connected, that must be the issue (I would not drive it disconnected long term). However, I tried letting the car warm up with the AFM connected, turning it off, disconnecting the AFM, and turning the car back on, but it died almost immediately. I tried to reset the ECU with the AFM disconnected and tried starting again, same issue. For whatever reason the car will not run if the AFM is not plugged in. Though the engine will stay running if my foot is on the accelerator, it reeks of fuel making me believe its running rich even with the AFM connected, and disconnecting it makes it run even richer. I've read about some people's cars dying with the AFM/MAF disconnected and some people saying it runs much better, so I'm not entirely sure what determines that.

I wish I had more time to thoroughly test this but I will not be able to until the weekend. I'm glad to have most likely discovered the cause of this problem, and I am hoping that a new OEM AFM will fix my issues, but I do fear the issue lies within the engine harness. Or at the very least the wiring for the AFM, which I will determine after doing the tests this weekend. I still have not written off the ECU as potentially being faulty, and I have purchased the correct capacitor to do Yamaes ripple test this weekend as well.

djamps

Try starting it several times with the AFM disconnected. It might take a few cycles to hit the "default" map.

LeX2K

Here's a tip for OBD-I engines, it is common for codes to be stored but you never get a CEL or it could take months before you do. Easy to test the AFM theory disconnect it see what happens.