mtnickel

Hi guys. Thought I'd come here for help first.

So I've purchased a 1998 GS400. It has about 90k miles on it. Does not burn a drop of oil. Idles smooth. Bone stock.

Here's the problem. The car seems quite sluggish and slow.

I've timed 0-60 runs with power braking at various RPMs, after resetting the ECU, etc. and the best I could ever pull was about a 7.1. Typical is lowish 7.3s (60ft's about 2.6 seconds). To me this seems unacceptable since new the car should should pretty close to a sub 6 second. Even 6.2-3 I'd be fine with, but I'm averaging a FULL second slower than that.
My g-tech dyno (actually in my iphone, but reasonable accurate) puts the horsepower at around 180whp using a 4000lbs calculation.

I already checked the ODB reader hoping to find a code (thought the car was running in a safety map/sort of like limp mode. Other cars do this if the knock sensor and or o2 sensors are bad). Unfortunately no codes were thrown . Though i guess that is good. haha.

I'm guessing the car just needs a decent tune up.
Can anyone think of a good place to start?

I'm leaning towards,

Spark plugs
Clean MAF sensor
New Air filter

Any other issues with these cars that can affect their performance? I'm fairly confident the compression is good. Perhaps maybe a bad VVT valve? Is there a TPS on these? It almost seems to run more responsive/powerful at 1/2-3/4 throttle. Going WOT isn't working.

If these clean-ups don't do much, i suppose my next step would be to look at purchasing a decent obd datalogger to see what's going on ignition and fuel wise. Perhaps also hook up my wideband o2 and or do a dyno run to see what it's putting down.

This was my first lexus purchase and I know the car is capable of more. Any thoughts would be appreciated!

lexusben

iTrader: (3)

Clean the Throttle Body too while you are doing the MAF. I did my MAF a while ago. I did the Throttle body, too but there was still some gunk left over so I'm going to give it another go sometime. Cleaning the MAF and TB helped a lot with idle problems I was having, idk how much it helped with over all performance, though. I'm looking at getting new spark plugs soon, also.

Och

iTrader: (3)

The car is 12 years old. Its probably a mix of slightly clogged injectors, slight loss of compression, cat converters might be a bit clogged, some parts are worn. Nothing to worry about.

Salil022

iTrader: (1)

7.1 secs def seem slow....I have about 260k miles on GS400 and car feels just as quick as Gs with less than 100k miles...I will recommend a full tune up including plugs, new fluids.

BossGS4

iTrader: (2)

Fuel Filter and Fuel Screen

Stop powerbraking your GS or you WILL eventually destroy all 3 engine mounts

werewolf

How much do you weigh? lol j/k

2nd on cheap fixes (new filters, MAF/t body clean, maybe some techron for your injectors)

I wouldn't change out sparkplugs unless you're close to a change interval. How is the oil? Any chance of sludge?

400DGRZ

iTrader: (2)

All the info you posted about the performance sounds like the numbers close to a GS300. I'm not knocking your knowledge about cars bc I don't know you, but can we see a pic of the engine bay?

We all know how people like to switch badges on vehicles; particularly our models. You don't sound like someone who wouldn't know the difference between a V8 and an I6. I just hope you didn't get scammed by someone claiming a GS400 that was really a 300 with 400 badges.

At 90k miles, the engine shouldn't be in terrible shape. Mine has waaay more miles than that and still pulls very hard.

JBrady

Assuming it is a 400:

What octane fuel are you using?
What altitude are you driving at?
Of course confirm that all normal service issues are solid.

Exhaust flow restrictions such as a catalyst problem could cause the issue.

It is possible that the throttle is not opening completely. Get a vacuum gauge and check at full throttle in 2nd or 3rd gear to redline and confirm engine is getting full throttle.

These engines have a dual mode intake manifold. At low RPM they operate with full length runners. Above 4000rpm they have shortened runners via internally plenum located 8 throttle plates (one per runner). If this mechanism is not working and stuck closed you will loose top end horsepower. If stuck open you will loose power under 4000rpm.

It is also possible that one or more of the COPs (coil on plug) is weak and not able to give a strong spark at high RPM. I would expect this to show a missfire code.

mtnickel

Age shouldn't be as much of an issue as mileage is. I'd highly highly doubt any loss of compression on such a low mileage 1uz. These engines are bulletproof and of the lowest wearing engines anywhere. I don't and won't chaulk this one up to "wear".

Injectors. Possibly, but you'd think this would lead to a poor idle and cruise condition.

Clogged cat??? I haven't heard of Cat's clogging these days (perhaps back in the days of Leaded fuel). Or perhaps if it was SPEWing oil.

Good insight on fuel, yet I'm skeptical it'll affect performance that much.
And thanks for advice on mounts. Perhaps I'll have to swap them for poly if this is a common wear point on the gs.

200lbs...haha. Car is about 4000lbs on the scales fully loaded.
Ya, I'm skeptical of plugs as well, but it definitely feels like CPU pulled timing (which comes from knock which comes from misfires/plugs).
On that note, it could be poorly functioning knock sensors, but i'd hope that'd come up on the OBD read.

Oil is great, about 2500 miles on Penzoil Platinum 5w30. Did lots of research on Bobistheoilguy.com. It's a great low wearing oil good for 8k mile intervals fairly easily. Even after 2500 miles it's still wonderfuly clean and golden (pleasantly surprising).

All i can say is "wow". DEFINITELY a 1uz-fe.

94 Oct (and this definitely makes a difference. Runs much worse on 89).
Sea Level
I was totally thinking about the throttle plate. It's concerning running a throttle by wire. I'd be tempted to rip the thing apart and modify it to manual just to have full control. I didn't even think of throwing my vac gauge on it to check all these systems. will definitely head out tonight and check it.

Also thanks for info on Butterfly system. I'm familiar with them and will check for operation with volt meter. Hopefully the system is active when in park, otherwise it'll be more difficult testing this.



All in all thanks for all the ideas guys. I'm gonna take all these into consideration and definitely report back. Hopefully to help someone with similar problems.

JBrady

20 years ago I bought a head for my Thunderbird Turbocoupe from a guy in Canada. Can't remember mileage but I want to say 60-70k miles. I was shocked at the amount of carbon buildup in the intake ports and on the valve stems and heads.

Recently a lady friend of mine with an 07 IS250 had her o2 sensor go out at 64k miles and in researching discovered many engines from Audi, Porsche, VW and of course now Lexus are experiecing rapid carbon buildup due to the direct injection. Some as low as 10k miles.

While our engines are not direct injection they due use the VVTi to create overlap creating EGR. This is a definite factor in carbonization.

See if you can find a shop that has a video scope, pull an injector and look inside the ports.

Heavy carbonization restricts airflow and power. It can also build up on injector tips reducing atomization. Car would idle and cruise fine but be choked at higher RPM as well as more likely to send unburned gas into exhaust system which is not helpful to catalysts.

Here is an 2007 Audi RS4 (V8) with 40k miles.

Och

iTrader: (3)

This is the first time I hear of this. Is this present on 1UZ and 3UZ? How exactly does the system work?

ElitistK

iTrader: (4)

That is not carbon build-up, but rather pcv system flooding the intake tract with oil.

Newer toyota engines that employs direct injection also has a port fuel injector.

the reason why they kept the port injector is to keep the intake tract "wet", so the pcv system will not clog the intake valves like pictured below.

JBrady

Actually it IS carbon buildup. Carbon buildup can occur due to PVC problems but that is not this reason for the above picture buildup. Lexus also has this problem on its IS250 with direct injection only. While a port injector tends to reduce this problem it is only one of the variables and buildup has been occuring in engines for YEARS. The buildup in the turbo head I referenced had port injection. It also occured in many carbureted and throttle body injected engines.

The MAIN culprits for carbon buildup is reversion and EGR. Our engines have both and increase overlap for EGR. This is most common during light throttle operation.

If you doubt this condition exists look inside your throttle body and ask yourself if THAT buildup is not carbon and ask if that is a PVC problem.

JBrady

Yes, it is on the 98-00 1uz with VVTi and the 3uz.

Each runner has an openning at the bottom of the plenum. From their it curves up over the top of the plenum then curves down directly to the intake port. Total length is roughly 16 inches. The butterflies create and opening between the plenum and the runner mid way and when open shorten the path to about 8 inches. Longer runners are better for low end power while short are better for top end power. The butterflies are opened above 4000rpm WOT.

Och

iTrader: (3)

That is cool to know. Is this system reliable? Like the op, I do feel that I've lost some of the low end torque, but high end is as strong as ever.