400gs99

Past 8 months were the same status quo. I just replaced the fuel filter day before yesterday. Also installed a high flow fuel pump. I reset the EFI by removing the EFI fuse for 30 minutes.

The car pulls pretty hard now, that's exciting. First fill-up, may be seeing a slight 1-2mpg improvement, probably closer to 1 than 2.

But, it's very noticeable, the original symptoms have not yet resolved. Hitting bumps often kills the motor, which quickly come back on. Not every bump, but it's still there.

I made a post where I cut open the old fuel filter, estimated 280k miles. I'm going to rebuild this 2nd fuel pump assembly with a new filter and cut that one open too.

Some of the original MAF sensors I sold on eBay and never heard anything back about it, so those were good.

Still trying to figure this out :-(

400gs99

Things were going more or less fine. Replaced the fuel filter, high flow pump, everything was jammin'. Ran great, even didn't really seem to cut out like it did.

I'd been filling the tank up to the tippy top. The top top. Suddenly I decided to stop. Now there seems to be issues. Got P0446, and was smelling a bit of gas in the cabin, but things were running ok.

Tonight things took a turn for the worse. Started hesitating noticeably with rough stuttering idle, and surging. I though at first maybe the fuel pump fell out. Nope, still solid. I swapped it out with my backup
pump to be sure. Same issue. Now I've got P0171 lean condition, which inspired P0420. I was fiddling with the PCV valve the other day because I thought it was leaking oil around there. I'm not convinced it
was, or maybe I fixed the leak. I sprayed out the PCV valve with brake cleaner, the ball bounces around in there freely. It was gummed up a bit.

I was assuming I had a bit of vacuum leak from the P0446 valve, maybe that got worse? I did read that suggested P0171 could be driver side vacuum leak. It must be substantial. Let me see what the service
manual says. Sure, check all the things.

Almost forgot, starting to get a bit of a ticking sound on startup. Thought I saw someone mention this is early sign of water pump going. I'm at 297k, so... shoot.

400gs99

opened hood, within 30 seconds noticed one of the clips had broke on the air box and the tube to the intake manifold was open to atmosphere. so last night's quest is resolved.

dwoods801

I was hoping you had figured this out, but then I saw your recent post and thought I would share a couple videos that I have found helpful about the fuel systems, but somehow u have got to determine if you have an electric problem and that’s what’s shutting off? Or if it’s a fuel problem and it briefly runs out of gas? That would help with diagnosing the problem. Check these and see if they give you any ideas.
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400gs99

thanks for sharing, i think i'm already getting some ideas. i've been leaning towards electrical for some time and this gives me some ideas what to do next.

this weeks saga involves the abs booster giving up entirely. replaced the whole assembly with one from a 2004 gs300, rewiring the new sensor into the old wires. wow, so quiet now! have to bleed the system to get things right but no more of that stupid buzzer. i still have a general abs malfunction code but cleared all the others except the wheel speed sensor (which came from replacing the lower ball joint) and a light sensor (which may be the shift interlock solenoid, i got that part at the junkyard this last weekend as well).

dwoods801

I wonder if it’s possible your ecu has a short in it or something and a hard bumb is causing a quick loss of power in a critical spot. Have u had your ecu service md yet?

400gs99

i have not, and so far don't see anything to suspect it. the problem has seemingly gotten better over time, although very weird. earlier this week closing the hood shut the car off, but the closest i've ever come to a replicatable symptom is running over speed bumps on the driver side at about 30mph

400gs99

let's see, over the past year i still haven't figured this out. i was using the car to run doordash. shortly after the last post, the brake booster motor went out on the way back from a trip to las vegas for my sister's wedding. that was easy to take care of by swapping a brake booster sourced from a junkyard, couple years newer car.

p0446 came back and i was not dealing with it. i changed out the vacuum valve by the charcoal canister, which last time fixed it. this time it did not. i have to suspect maybe there's some loose vacuum lines.

i also had p0763 come up and this was fixed by replacing shift solenoid 'c', which was a straight foward job.

i changed out all 4 o2 sensors. at first the long term fuel trims came way down. on acceleration they even out, where before one side was higher. except now for some reason, at idle they are stupid high, #3 is +25ish, #4 is +30ish. on moderate acceleration they equalize and drop to +12 ish

when i get some extra cash i'm gonna send in the ecu to sia to be serviced. i think maybe june.

going back to the original issue: still does it. highway bumps will kill power, which often returns after a second by holding down the gas pedal. this will often pop a p0100 code, but not always. a couple of the latches on the air filter housing had broken off and i kinda just had it using the intake tube to apply downward pressure to keep it closed. today i took some big zip ties and secured it down. i also took out the k&n filter and put the oem filter back in. i'm gonna clean the k&n. i read something about too much oil affecting the maf sensor so i removed that and sprayed cleaner in there. this didn't really do anything.

i'm also getting p0430 now, in addition to the on & off p0420.

the car runs good and i get 18mpg with my street/hwy daily commute of about 50 miles rt. i don't get this at all but i'm still trying.

i'm up to 325k miles <3

dwoods801

I was hoping you had this cured, your on the right path with having your ecu serviced. It’s not too often the gs400 has the leaking capacitor problem that plagues LS400 owners, but it certainly could be the culprit so it’s a good idea to get it serviced, so you can cross it off the list, and not have to worry about it again. It seems like there have been a few people with phantom problems like your car, and the ecu was found to be the culprit. It’s not to difficult to remove the ecu, and open it for a close inspection. But it’s not easy to identify a leaking capacitor. So keep us posted, good luck

400gs99

I think I inadvertently may have located the source of the problem, I've been sitting on this watching all week.

After putting the air box in, I noted the maf connector wiring was all twisted up clockwise, so I unwound it one rotation. I noticed there appears to be some wear on the wiring that may be shorting. Just this seemed to diminish the duration of the cutout, and the frequency with which it occurs. Today I was fiddling with the wires more and got the car to die when the hood was dropped shut.

There must be either the wire is leaking, or the connector pins suck after all these years. I have to spend next week in baja but when I get back I'm going to try and replace the connector and or a lot of wiring.

Will eventually get to replacing more connectors, jesus these things are brittle AF. i've owned a handful of 25 year old cars but none of them had the plastic deteriorating the way this lexus does.

dwoods801

I know exactly what you mean, I had to deal with mine causing problems right after I got the car. It was the first hint of problems to come. I am certain these wire harnesses must have been designed to be biodegradable! I removed mine last summer so i could replace all the plugs, and I replace all the old, brittle, corrugated, plastic covers. So I could replace it all with much more appropriate material for protecting the wiring in an engine replace. It was a major undertaking, but I am so happy every time I unplug something now and have the plug release nice and smooth, instead of having the release buttons just break off when i push it. Before the plug’s usually disintegrated before ever getting it reinstalled.
You could release each wire from the plug and then connect them directly to the sensor. so you can be certain they are all connected. Just make sure to take some photos before getting started so you know where each one goes. And then when your new plug arrives you can install it pretty easily. If that plug is broken at all, I would definitely order a good replacement and then reinstall it when you get back in town.

400gs99

not calling this fixed just yet, but;

i put a rag behind the maf sensor wire bundle running towards the ground to kinda cushion it from the engine compartment wall. spent a week in baja and despite many bumps jolts and pot holes, didn't have any notable issues. hit a pot hole at 60mph and got a flat tire, but the car ran great.

dctex

I have.a 2007 with 260+. It started hesitating poorly in heavy stop and go traffic in DFW and even died once. It threw these codes: P0430 and P2119. I had transmission mech verify transmission was good as I had the main seal replaced a year ago. He cleared codes and said trans was fine. I was on the road away from all my tools and drove to S Texas and back to Austin but it went into limp mode for my wife once in Austin. I think she gassed it when it was sluggish vs letting it go at its pace. So you know, her fault. That didn't go over well .. hahaha! Just took the neg battery cable off for 5 mins and it ran fine to place we were at. I then did clean the MAF air sensors and a tiny piece of oak leaf fell out of one of them. I also did a good clean of the throttle body which was REALLY dirty. That passenger side MAF air sensor clip/connector is very brittle from years of heat. So maybe validating your findings. I held together tightly and used electrical tape to sort it lock it together. Seems like it's 2 pieces snapped together vs a single solid one and one side seems to not snap together anymore. Anyways drove the car with WAY less issues from Austin to DFW. Runs like a dream on highway at high speeds. Just really seems to hate heavy stop and go DFW traffic in 200 degrees Texas heat. Seems to have no issues in cooler parts of morning or evening/night. I also noticed turning off AC seems to help sluggish behavior until I can get over 50 mph again. No codes are being thrown again and had a mech in Austin throw on rack and didn't see any visible cracks on exhaust in case the P0430 was crack related. He ran tech stream while running vehicle and he said all readings were in range and there were no codes. I'm going to look under heat shields once I get back home for cracks and wait for codes to reappear. Plugs have 60K on them so going to replace those and seals. I'll update if I find anything but really appreciate this long thread very helpful. Oh and I did notice oil leaking in passenger side breather tube (prob bad pcv) and oil around the cam sensor right on top on passenger side. I'm going to replace that sensor too because I don't like leaks and it's easy fix. Noticed leaking around both fuel pumps so those gaskets next. If PO430 comes back I might do O2 sensor. I do believe cleaning throttle body and MAF has helped a ton along with new engine filters. Peace!

​​​​​​Replaced plugs and seals. Found oil in spark plug wells 2 on each side. Since doing that zero hesitation and no engine codes. Mine is back to normal. Hope this info helps others.