rparker888

Good day all,
Finally signed up after years of lurking as I have a very puzzling problem. Car is a 1990 UCF10RGKW with a possible plugged fuel return line. My question is if there is some sort of safety valve that shuts the fuel return line (in case of accidents etc.). Currently my fuel pressure is pegged at 48psi with or without vacuum applied to the Fuel Pressure Regulator (FPR).

Disconnecting the Return Line and returning it to a separate fuel can, I get 42psi w/o vacuum applied and 35psi w vacuum applied which is to spec.

Applying compressed air (via duster) into the banjo fitting reveals that initially there is some gurgling at the fuel tank for maybe 2 seconds then it become completely blocked and a huge rain of fuel back through the return line on me. Yes, I opened the fuel cap. Nice. This is what leads me to believe there is some check valve that might have been triggered. Even now, it can't receive anything into the return line.

Any thoughts from those who have touched the fuel lines before?

For reference, what prompted me to do this search is a slightly rough idle and a miss on sweeping the rev range. I haven't been able to solve this for years. I've since decided that I'm going to have to do it myself since the local mechanics can't even feel the problem or tell me that its an old car and shouldn't be expected to perform flawlessly. I can't accept this.

Thank you for your help and time.

Regards,
Ryan E. Parker

CELSI0R

Welcome rparker,

I haven't worked with the fuel system extensively before, but I don't believe that there is a check valve for the return line.

See for 1990 USDM LS models:


There is however a cut off valve, but my understanding is that this device is to prevent you from overfilling the gas tank. I don't believe it would cause the symptom you are seeing

https://parts.lexusstevenscreek.com/...tem-components

What symptoms do you see for the rough idle? Is your car a Traction Control car or a non-Trac car? Have you checked and cleaned the throttle body in case there are carbon deposits? What else have you tried so far?

rparker888

It's a non-trac car. Yes, have indeed cleaned up the throttle body, ultra sonic cleaned the injectors and even changed the fuel filter. Opened up the old filter and it was nearly perfect. Other things done include new plugs, rotors, ignition packs and spark plugs. No vacuum leaks using the ol cigar trick. EGR system removed and plugged. ECU caps recently done by Jim Walker.

I have also been doing some research into the cut-off valve and would agree that it doesn't seem related to the return lines.

By rough idle, its almost imperceptible to non LS drivers. I can best describe it as a buzzing that concentrates right at the hip of the driver very similar to having a bad transmission mount, however I have just changed that together with the engine mounts rear-subframe bushings and diff bushings, so it rules it out. Visually, would be the same buzzing frequency at the engine that can be seen on longer hoses that would vibrate. Also visually, leaving any of the front doors slightly open would show the same cadence/frequency of buzzing via a vertical oscillation in the tip of the door furthest from the hinge. It happens intermittently but did indeed not happen when the fuel pressures were back to spec and the engine did feel more "correctly tuned" at the 35psi fuel pressure at idle.

Cheers.

Ryan

CELSI0R

It's hard to tell what's causing that from here, but I did have a similar experience with a 2001 Camry that might be worth noting

Some of the exhaust hangers failed on me, so the car was running fine but the exhaust was moving up and down causing the whole car to shake at idle. It was especially noticeable when I had the door open, as it is with yours. I could see the edge of the door visibly shaking.

Perhaps check underneath the car while it's running and see if anything is moving around when it shouldn't be.

Does this happen regardless of gear or only when you put the car in Drive, Reverse, etc, and are moving?

rparker888

Hah! Off to the exhaust shop I go and new hangers all round! Anyway, I'm going to try rigging up something to use the engine vacuum to suck anything from the fuel return line via a brake vacuum bleeder can, lets see what I get out of it and indeed the mmHg that it can pull whatever is blocking the return line. Was looking for powerful vacuum pumps last whole night and realized that I have a 250hp vacuum pump right next to the return line, duh.....

Thanks for the suggestion. And am also hopeful that there is someone who has redone or has further experience with the fuel lines might chime in.

CELSI0R

Sounds like a good plan.

For the exhaust, can't tell if it's a joke or not because it's online, but definitely see if it's shaking underneath the car first before doing that

If I find something more on the fuel system that would be useful, I'll share

rparker888

No jokes on the exhaust hangers. It's a 32 year old car and in the 20 years that I've owned it I've never changed one hanger. I think its about time to have them changed. My thinking is that even if they are still working, they have hardened, compromising some of its function and will inevitably contribute to some level of vibration no matter how small.

This is my experience with the change my of rear sub-frame bushings, diff bushings and the prop shaft couplings a couple of months ago. On looking at them, they seemed fine but my sense after 32 years, no matter how good a material engineered by Lexus, it would have at least hardened. I was indeed proven right when we did a flex test on the old flex disks, haha, pun. Now the car engages gears very smoothly and you get the nice controlled take-off from standstill. The N-D transition is still rougher than desired however Lexus did put a bulletin out on a change of one shim's thickness if my memory serves.

Cheers.

CELSI0R

That's a good point. I would still double check beneath the car first to see if that's your major issue before replacing them. I guess it depends on how intense the vibration is that you're getting in the car

fondu

The bulletin they put out added dynamic dampers to the rear stabilizer bar bushing brackets. It does not apply to any 90-94 models, only 95 & up. I reduced a majority of the N-D lurch in my '94 with new rear stabilizer bar bushings + links and replacing the rear upper control arm bushings/ball joints. Badly worn CV joints or really anything badly worn in the rear suspension will also make it worse.

rparker888

An update, hired a hoist for a day and blew compressed air through the tank end of the return line. A bit of old rubber hose came out of the banjo joint. Finally some progress. The fuel pressures are back to normal. The annoying thing is that the vibration is still there!!

Diagnostics reveals Code 21 and 28 telling me that the main O2 sensors are dead. Voltage check on Ox1 reveals small voltage and Ox2 is 0.00v. Ox1 fluctuates voltage up and down on accel and decel of throttle telling me it might be marginally working or intermittently working. Off to get some O2 sensors from Amayama again. Will report back to document this for those who still keep their survivor cars.

Also checked the exhaust hangers and tighten what I could since I was down there. Indeed, the exhaust hangers were original but still soft! So am going to put off changing them.

Ryan

400fanboy

Is that pre or post-cat O2 sensor? I think your 1990 is like all other 400's and has 4 of them right? 2 per bank, one before the cat and one after the cat. Just in case... just be sure you're not accidentally reading the downstream sensor on the same bank because there are situations where that can return a constant value (though if it's reading 0.00v then it would be super lean which would be very strange to not have other issues, and bad cat would mirror the upstream sensor reading once the car is in closed loop)

rparker888

By the looks of the TIS manual, It's the "Main Oxygen Sensor Circuits" which are just before the cat, after the exhaust manifolds. I didn't read the ones after the cat, but thinking logically, I think those might be secondary in purpose, to check the cat and confirm that it is running correctly. I have however already removed the cats and replaced them with straight bits of pipe as an "Air Flow Meter Event" that happened years ago which basically killed the cats. I still have the cats but they are so tightly blocked that taking a leaf blower to them outputs minimal air through the other side.

400fanboy

Ahhhh I gotcha. Then yes the secondary O2 sensors won't be doing anything, because my understanding is the same as yours, they are just there to measure the performance of the cats and throw a code if they fail.

Well good luck on your journey! thanks for updating the thread, it's always great to have completed answers.