Och

iTrader: (3)

No, read it again. It increases the pressure. I've bypassed this VSC by running the hose directly from the fuel pressure regulator to the intake manifold, and the car does seem to have more punch, especially at midrange, but still not 100%. I'd say its a 20% improvement over how it was.

However it does have a strong fuel smell from the exaust pipes now.

On a different note, I do have a leaking valve cover gasket, that I'm planning to take care of in the nearmost future. Could that be causing an air leak or some sort and cause my car to perform poorly?

nthach

Leaking valve covers usually cause a mess of oil spewing. It could affect PCV. And our cars also use a drop-down resistor for the fuel pump - the pump voltage drops down to 6-9V at cruise, 12V at full tilt.

Amskeptic

It can contribute to lean running since it is allowing unmetered air into the engine.

A couple of notes after reading this excruciating saga:

Older cars should accelerate faster than newer cars due to diminishing frictional losses.

EFI engines do tend to run leaner than original fuel calibration due to intake valve guide clearances opening up.

Your LS400 was stripped at one point, Och, what this does to the sophisticated interactions of these ECU-crazed cars is anyone's guess. If the wiring got goobered, you're in a world of hurt. The Lexus factory manuals have matrix charts to help guide you through troubleshooting. If there are 20 symptom rows and 12 systems checks, you have 240 possible starting places with 57,600 possible outcomes.

The only way you can possibly nail this down is to follow the paths set out in the factory flow charts, do not skip steps.

I had a 1990 LS400 that gave me a flawless (minus upper control arms and an idler pulley) 177,000 miles.

Just sold a '93 BMW 5 Series that was also flawless with the original clutch plate still in good shape at 228,000 miles, to buy a 1992 LS400. At 95,748 miles, it is perfect except for a goofy tilt/telescope set up. Every car I have bought has had to have an unmolested electrical system. Even an aftermarket stereo rig can wreak havoc on an engine management system, and in the case of the Lexus, it can wreak havoc on the engine/theft-deterrent/transmission/trac/abs ECU interaction (read up on how the trac ECU can dog the engine down trying to prevent a "skid" that is caused by a faulty resistance value in an ABS wheel sensor, for example).
Colin

KMinAF

A couple of things I haven't seen in this post that could contribute to the symptoms:
Low fuel pressure
Cam timing i.e. did someone install the timing belt 1 tooth out on one of the cams?
Ignition timing (although I am not sure how you would check)
EGR valve stuck open

mr.ajandkj

Any updates on this most interesting thread?

Amskeptic

I too am most curious.

I have lost my innocence since I last posted, I had to take out my camshafts and adjust the valves and replace a couple of lifters and the timing belt, and these cars make me a little woozy.


P.S. That hot start solenoid works *by removing vacuum signal* to the fuel pressure regulator in order to increase hot start fuel pressure. If you bypass the solenoid, you get normal pressure reduction with intake manifold vacuum, thank-you.
Colin

slvrboolet

Looking at a 94 ls with 147k guy sed all it needs is a maf anyway to tell if thats all it really needs> I havent asked how he determined it needed that. The car is pretty cheap though at 1500obo and looks pretty good from the couple pix i saw.

nervekiler

"Then the light went off, and the car is running pretty much the same as it used to be. The acceleration under very low throttle seems jerky, it was somewhat jerky before, seems jerkier now."

my car does the same thing.. and after reading several pages here i am a little discouraged as to what it could be. sometimes my CEL comes on and says its the TPS sensor.. and adjusting it definately changes the way it runs. guess ill try to switch it out with a new one but if that doesnt fix it i have no ideas.
the car will run fine when i first start it up but after it warms up (been driving for 15-20 minutes) itll start running rough under part throttle. of course i can give it more gas till the tranny downshifts and its got a little power but nothing like before when i bought it a few months ago.

how frusterating.

Amskeptic

How frustrating indeed . . . when you consider the incredible sophistication of these cars, and the tight interdependence of systems like the transmission ecu and the engine management ecu and the trac ecu, you are setting yourself up for overwhelming frustration if you think you can just wade in to this mess with an idea that it might be this part or that. A TPS code tells you to start your diagnosis in that circuit, not to replace it.
AFTER you check VTA - E2 for .2 - 1.2 Kohms fully closed,
AFTER you put a .40mm feeler blade between the stop screw and the lever and check IDL - E2 for less than 2.3 Kohms,
AFTER you put a .85mm feeler blade and check IDL - E2for Infinity,
and AFTER you fully open the throttle
and check VTA - E2 3.3 - 10 Kohms
and check VC - E2 4-9 Kohms

THEN you can think about replacing the TPS if it fails the above tests.
The workshop manual is big on checking connectors and harnesses, you know why? Because the connectors are the more usual place for symptoms and throwing codes. I use De-Ox ® gel on every electrical plug I visit.
Colin

sam12345

iTrader: (1)

OCH Are you sure that your catalitic converter is not plugged?

Och

iTrader: (3)

Really no updates. I did replace the valve cover gaskets and cam seals so there are no more leaks. I just really don't use this car much anymore as I have two other cars, in fact my inspection expired back in august and its pretty much been sitting in my driveway, I haven't bothered to get it inspected. Eventually I still want to get to the bottom of this problem just to satisfy my own curiosity. But I'm not spending another dime on this car.

e57820

At this point I would try to physically monitor some engine conditions. If you hook up a fuel pressure gauge you would be able to easily detect a drop in fuel pressure which could cause a lean mixture. Or, hook up an LC-1 or something similar to be able to monitor your AFR. If both those things check then it would likely be a timing issue. And yes the factory maps suck, they have a lot of timing retard under 3500rpm. I also wonder if for some reason you are stuck in a limp map. If that were true you would have a CEL, but only if the ECU is good. And just because the ECU is "rebuilt" doesn’t mean it's good, even if they test bench it. I have a 92ls and the computer won't display or store CEL's it did at first but then as I modified the car it stopped displaying CEL's. Yes the CEL light is good.

Anyways, maybe if you can detect a problem physically then that will get you back in the right direction.

Good luck!

Amskeptic

At this point, I would unload the car.
The challenge of these cars is to go in with some loyalty and determination to dis-cover the problem. The factory manuals help guide you to a successful outcome.

If you have read of the complexity of these cars, and the interdependence of all of these systems, and you still want to bush-whack through hopeful guesses, be my guest, but do not diss the car for being a piece of crap that you're not spending another dime on. If you love the car and can't cough up another penny, well, I can understand that, but please be aware that your very cheapest way out of this predicament is to follow the damn directions at the very least.
Colin

Och

iTrader: (3)

Well, when I dissed the car, it was mostly out of frustration, lol. I'm sure you know what I mean.

As far as repairing it, I tried to do everything, replaced a bunch of parts from good to new. Right now I'm not willing to spend anymore money on it, my registration and insurance on this car is expiring in Feb, and I don't plan on renewing them. I will be storing the car in my driveway, as I just don't have the heart to sell it for a measly 2-3k after putting so much work into it, especially the interior.

Och

iTrader: (3)

So if any of CL members are willing to do me a favor, send me a PM or respond in this thread please.

After all this time, the only thing that can contribute to my problem that I haven't replaced is the ECU. I'm not willing to spend a grand on a new ECU without knowing 100% that this is whats causing me. So if anybody has a properly running 93-94 LS400, and would be willing to test my ECU, I will mail it to you to connect it to your car and see if it affects the way your car runs. Let me know.