lucasLS400

Hello all,

I'm making a post because I'm at the end of my rope. My 1990 LS400 developed a stalling issue at the tail end of October last year, and I haven't been able to solve it. After sitting overnight, it will start up fine and idle correctly for about 5 minutes while it warms up. But once it's at operating temperature, the rpms drop WAAAY low and it starts to stall and then catches itself. It does this until either A. it stalls out or B. I give it some gas. After it stalls it starts back up effortlessly, and goes right back to low rpms/verge of stalling again unless I give it gas.

It's been to three different shops now, and they've all had differing suggestions. I've replaced the IACV and fuel filter and tested the MAF sensor, which seems to be alright. The best tech that's worked on it says there is a cylinder misfire that is only present at low rpms, and I believe him. But the other shops said it could be the IACV (I replaced that first, no luck), spark plugs, spark plug wires, and even the distributors.

At this point, I've spent about $500 trying to fix this issue, with no luck. I am really tired of it, and since I'm going to be headed for college soon, I don't have much more money I can spend on it. I know enough about cars that I am comfortable to work on them myself, but I don't have any experience diagnosing problems, especially ones like this. So I'm coming to y'all for help.

Has anyone had this problem or anything similar? What could or should I try to do? I currently have the car up for sale, but I hate the thought of parting with it.

Thanks for the help in advance.

AlaskanLS

When your car warms up and it starts stalling, unplug AFM/MAF and check if it shuts off. If it don't then it is faulty AFM. If it does turn off when you unplug AFM then move on to throttle position sensor and test if it is functioning normal.

AlaskanLS

It has a vacuum leak which under load goes away. I 60% suspect it is a bad AFM.

YODAONE

Glossed over post, but did OP replace his ECU capacitors?

Moarpower

Just a heads up, this is exactly what mine was doing. Mine was starting fine, then after warming up the idle would drop low then bounce back up a little. This is called "hunting idle". I tested the AFM and it tested fine. However it just felt like an AFM issue to me. Replaced it with a known good, used part. and it was the problem

JonnyO78

YOU BEAT ME TO IT!

Moarpower

I replaced mine when I had the exact issue...Didnt fix it. THen everyone told me that I must have used the wrong caps, which I didnt. It was the AFM.

Might not be in this case, but the issue is the same.

lucasLS400

Thanks for the help everyone. It's still at the shop but I asked the mechanic to check out the MAF sensor. Still waiting to hear back from him.

And no, I haven't replaced the capacitors yet. I was planning on doing it sometime this spring once it is warm enough for me to go outside.

basilpluss

aptoslexus

I'd suggest checking the o2 sensors. It sounds like it's fine when it's in open loop at startup, but when it enters closed loop, the ECU doesn't know how to regulate the fuel mixture, possibly because of a faulty o2 sensor.

Moarpower

If the mechanic doesn't know these cars you will pay a ton.

head to a junk yard and pull a used afm

oldskewel

iTrader: (1)

This sounds like a tough one. Too bad you don't have another car you can swap parts with to do some easy debugging.

I'll say that it could be just that your idle is too low for some reason, and it then lets some minor deficiency in another component kill it.

So I'd look into things that can raise your idle (not in any particular order):

1. Don't believe your tach. It's just a gauge with a needle, which could be off. Measure the idle some other way if you want to aim for a specific number.
2. I believe the idle on these cars is set by the ECU, so not easy (not possible?) to adjust it from there.
3. When the AC is on, it should bump up the idle a little. See if that helps.
4. The Power Steering idle up air valve allows some air to flow from the air intake to the intake manifold. It is supposed to only do so when PS is activated, such as when parking. But you could try bypassing that air hose with an always open connection. Usually when people bypass that valve (it is problematic and some choose to block it off completely, including blocking the air hoses, but this would kind of be the opposite; it should constantly add some air flow, raising idle).
5. Perhaps something as simple as tightening the throttle cable will give you a little boost in idle.
6. You can adjust the throttle position sensor (TPS) to adjust idle. This page has a pretty detailed explanation of the Lexus way vs. the clublexus way: http://www.lexls.com/tutorials/EFI/tps.html (and BTW, it could be that TPS mis-adjustment is the only problem you've got)

And anyway, the concept is that if you can boost idle a little, it will mask whatever problem is killing the engine. I know a targeted fix is better than a mask, but this might help you get on your way until the actual problem either fixes itself or makes itself more apparent so you can more easily find and fix it.

Good luck.

lucasLS400

Hello again everyone,

Sorry for the long wait. I've been waiting for the car to get back from the shop, and as soon as it did I had a brake caliper seize. So I've been working on that instead of the stalling issue.

But now that the brakes work again, I've been able to dive into possible causes for the stalling issue.

I pulled the AFM with it warmed and running, and it kept chugging along. Not a good sign. The mechanic that worked on it most recently said that the TPS could be faulty and cause it to stall, as well as affecting the AFM and coolant system (coolant light is on too). After testing the TPS, the resistance seems to be low all across the board, which leads me to believe it is bad. Any opinions? Someone is going to take a look at it Sunday, so I may end up selling it and not having to worry.


In the meantime, I am going to try to raise the RPMs at idle as a temporary solution. More updates to come I'm sure. Thanks again all!