weird top speed cut-off + etc..

feeninpeece

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So although this is my first post on this site, I've been using it for much time now to work on my not-so-newly purchased 2002 Non-sport LS. I must say this is my first car, and it is a beast at that. I've been driving it for about 5 months now. I purchased it at 27k miles in pretty shitty condition (but nothing too bad), and its now touching upon 37k. Some of the things I've done to it include a front suspension rebuild (upgraded to the 16mm lca), sst swap, 19” jag wheels, ebay intake lol, cherry bomb muffler exhaust. Can’t think of much else right now. I’ll try to put some photos up. I am finally making my first post to address some things that I cannot solve. First off, top speed. I understand that the different top speeds of the various trim levels of this car are between 130 and 145 i believe? My car doesn’t go more than 119.5 on the clock and 117 tracked by gps. It is an extremely weird sensation, when the car basically is steadily rising in the power band and you can feel that it can obviously continue, but then the car cuts fuel or spark or something and the revs peel back. Does anyone have any idea as to what this could be.Secondly, I’m getting p0420 (bank 1), but i just passed inspection. so i used forscan to check the o2 sensor levels. I found that the bank 1 downstream sensor fluctuated completely between its max and min on idle. for reference, the bank 2 sensor stayed steady for the most part with only slight fluctuations. On a possibly unrelated note, both of the upstream sensors are constantly fluctuating between their complete range. Should it be this way? Shouldn’t the car find a good mix then hold it?Lastly(i think) I tried using the infamous x4 and bricked my ecu. After replacing it and not thinking about it for a while i’ve been attempting to tune again with the help of Torrie from unleashed tuning. Doesn’t talk much huh. After exhausting all efforts with the x4 and various tunes and fuses pulled, nothing would take. The tuner does not even pull the stock file off the car. I’m pretty sure that it got further the first time, but the battery died. Now i’m being sent an x3 device to try this again with. I understand it is more reliable. Hopefully something will come of it.Sorry for the long post but i Think this is it for now. Thank you all for your continued contribution to the support of this car and Happy Holidays

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Welcome to the forum friend. 27k miles should be almost brand new man, what makes you say it's in bad condition? That top speed cut off is a speed limiter, a lot of cars have it these days for safety reasons. It can be removed with a tune, but your aerodynamics and weight will hold you back more than anything, so eventually it will top out somewhere under 150 mph if stock.

As for your sensors, you should see fluctuations in voltage as you hit the throttle (in closed loop after sensors are warm), between 0.4 to 0.9 volts is normal. It's hard to tell you what's going on without looking at the graph but it's possible that your sensors are faulty. I've encountered something similar; I replaced both upstream and downstream sensors on bank 1 and it fixed my problem. P0420 indicates a catalytic converter efficiency issue though. If it's not the sensors then unfortunately you might need new cats. But I would start with the sensors for now. A bad o2 sensor doesn't always throw a code.

Anyways, this is just my amateur opinion, take it with some salt.. good luck
 
Welcome to the forum friend. 27k miles should be almost brand new man, what makes you say it's in bad condition? That top speed cut off is a speed limiter, a lot of cars have it these days for safety reasons. It can be removed with a tune, but your aerodynamics and weight will hold you back more than anything, so eventually it will top out somewhere under 150 mph if stock.

As for your sensors, you should see fluctuations in voltage as you hit the throttle (in closed loop after sensors are warm), between 0.4 to 0.9 volts is normal. It's hard to tell you what's going on without looking at the graph but it's possible that your sensors are faulty. I've encountered something similar; I replaced both upstream and downstream sensors on bank 1 and it fixed my problem. P0420 indicates a catalytic converter efficiency issue though. If it's not the sensors then unfortunately you might need new cats. But I would start with the sensors for now. A bad o2 sensor doesn't always throw a code.

Anyways, this is just my amateur opinion, take it with some salt.. good luck

Hello Sir,
I know that it is the car limiting my speed, but the limiter is supposed to be much higher for these cars. Around 135+ at least.. I am wondering if it’s possible that at the cutoff speed (117 mph), the engine is not getting enough fuel to handle all the load, so it runs lean for a while and then the pcm is retarding the spark or something to cut the revs back. I have no idea how exactly to diagnose this.


As for the second issue, i’m really hoping it’s not the cat because that would be a very expensive fix. That’s why i’m skeptical because it just passed the nys inspection no more than a month ago. I’ll try to record some footage of the graphs. What other parameters should i monitor besides the o2 levels
 
Also it is a great mileage to buy at, but the car was horrible maintained. It was basically sitting for about 18 years. For some reason the passenger side exterior was considerably worse than the drivers side. The ac didn’t work, the compressor made a horrible noise, clock spring was out, water in the passenger headlight, and a fat scrape/dent in the rear passenger door to this day. Lastly, all the rubber on the car was dry rotted to hell. This included all the suspension ball joint boots, tires, and basically anything that was exposed to the environment.
 
The factory speed limiter was set based on the speed rating of the factory tires. If the non-sport got T rated tires, then 117 would be exactly correct.
 
The factory speed limiter was set based on the speed rating of the factory tires. If the non-sport got T rated tires, then 117 would be exactly correct.
I’m pretty sure it got H rated tires, so wouldn’t it be 130?
 
You can find the speed limiter by raising up your rear wheels and doing a pull. Do so at your own risk
 
Funny this topic was brought up so I want to chime in.. I have noticed issue with my speed limiter also. Every 6 months I like to burn out my dust in my vehicles and run them to their ability. 02 LS Sport V8 early this year I did a run and I backed off at 145mph bc of safety reasons approaching the end of a straight away and other motorist (if you're gonna do stupid things don't do around other motorist obeying safety regulations..) About a month ago same stretch I opened it up and It refused to go over 130mph. I was under impression the sport V8 is governed at 155mph. Is this right or has anyone else had this issue or know what would cause the inconsistity of it?
93 octane shell is always used.
 
So it changed right? Weird thing is i vaguely remember doing 130 which would be over my 120 speed governor, but after all the sct drama and my ecu getting swapped, maybe that changed things.
 
My 2003 V-8 Sport topped out at 135. That is the maximum a stock LS can do. Very smooth and the car does have more in it but you need a tune to up the limit.
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