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Discussion Starter #1
Hello all,

I have the BWR supercharger with Charlie's tune, the Larini sport cat and exhaust. All was great until a couple of weeks ago when I got a lean code just as my exhaust went bad. So, I got a replacement 6" Larini under warranty and cleared the code. The lean code has not returned nor is it present now when I do a scan. Since that time, I have had intermittent engine stumbling at WOT. I wait for the car to get over 170 degrees and when I put my foot into it, I get a stumble towards red line. Today it was around 7000 rpm in 2nd gear. I checked the code today and I got a P0139 which is a slow 02 sensor - I cleared it, went for a drive and it has yet to return. The car feels like I'm hitting the rev limiter for a moment then it goes away. I emailed Charlie assuming it may be tune related and he said that the ECU should relearn the new exhaust so I'm not sure the new exhaust is the culprit even though it is the only change to my set up. Would an exhaust leak cause a stumble? Nothing has changed on the intake side so I assume at this point I have no leak on that end. Any ideas? Thanks guys...

Matt
 

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I've been having issues with the larini exhausts, the tune itself does not change or learn at WOT, so it has to be something external, especially given that it was working fine before you changed any hardware.

Somethings obviously gone wrong, either the larini exhaust is upsetting it, a bad O2 sensor, a leak in the exhaust system, bad injector, fuel delivery problem etc.

But there is no easy way to remote diagnose it, there are so many things it could be, even something as simple as the pre/post 02 sensors being swapped after the exhaust chamge, or they may have been contaminated during the swap.

I certainly wouldn't drive at WOT if its a bad stumble, as it may be running lean.
 

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yeah thats a good point, the MAF may also be dirty, i don't think its that since it'd idle poorly, a stumble is usually something else. Lets hope its not the cams too, though cams are unlikely, depends where and how the problem is.

Logic dictates if something goes wrong after you do a hardware change, then take a look at that first, and any bits associated with the change itself.

It also could be electrical, check the alternator and voltage supply under load.
 

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shay2nak
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...even something as simple as the pre/post 02 sensors being swapped after the exhaust chamge, or they may have been contaminated during the swap.
what do you mean contaminated?
 

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o2 sensors are fairly delicate beasts, so if you remove them and don't watch where you put them they can get damaged. the sensor is behind a pin hole so its easy to block, or get dirt in it etc.

Often it'll just burn off with the heat of the exhaust, but depending on what it is can make it clog up.

its just something to watch for.
 

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shay2nak
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^ can this contamination cause an idling issue like what a vacuum leak would produce?
 

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Discussion Starter #8
o2 sensors are fairly delicate beasts, so if you remove them and don't watch where you put them they can get damaged. the sensor is behind a pin hole so its easy to block, or get dirt in it etc.

Often it'll just burn off with the heat of the exhaust, but depending on what it is can make it clog up.

its just something to watch for.

Hmmm. I left the sensors hanging during the swap so there's not much chance of contamination... I do have the spark plug defouler mod on the post cat sensor.

I should just take it to someone to have a look. Anyone you'd recommend? I'm in West Hills/Canoga Park...
 

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Discussion Starter #9
Update... Still having issues. I took the car to Cisco at Famous Autosport and of course the car decided to behave. All of the numbers on his scanner were within spec and the stumble was gone.

He suggested we replace the Larini sport cat with the stock one anyway as his experience has been negative as well and in the process also replaced the post cat O2 sensor (it had become "one" with the defouler). I still have the 6" Larini exhaust on though as I do not have the stock exhaust. I thought all was well but the next day the stumble was back. I'm putting the car on a dyno in a couple of weeks so I can get the pertinent info. Total bummer.

Could it be something as simple as bad gas? This all stated after a full tank and a lean code.

Thanks,
Matt
 

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It could be, if its knock, try putting in 3/4 or more of 100 octane and see if it goes away,

knock doesn't usually occur at that high an rpm though, so although it could still be bad gas, i'm thinking more along the lines of fuel delivery issues, but something that would make it intermittent, pickup problems, voltage problems etc.

easiest one is probably checking that its getting enough juice, then fuel pressure tests, how are the plugs looking, injectors seated properly, check for bad o rings and pinched cushions ?

lots of data logging, bring it to vegas.
 

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most o2 sensors do not give input to the pcm under wot in my experience...look at the spark plugs, if nothing turns up...reset the pcm again and see if the problem returns in the first 10-20 miles. btw, it is not usually a good idea to go wot right after a pcm reset on any car with an adaptive learning strategy (96 & newer) until the pcm has "learned" a little. good luck
 

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As your attorney i advise renting a convertible white cadillac with a trailer and bring the car to vegas.

though you could probably fit an elise in the back seat of a caddy.
 

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Discussion Starter #14
Charlie,

Are you going to be in the LA area any time soon?

Matt
 

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I had intermittent "stumbling" issues, the rev limiter would kick in and I could not get above 6K. It turned out to be a dirty MAF, no problems since I cleaned it.
 

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shay2nak
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is it easy to remove & clean the MAF?
 

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Overall it is easy. To remove it with the air intake in place I use a small Phillips screwdriver that I had to cut down (the handle) so I could fit it between the air-intake and the coolant reservoir. Two small screws removed(don't drop them!) and it pulls right out. Disconnect it from the wire harness, and clean it with MAF cleaner.
I have cleaned mine with each oil change, it was quite dirty when it gave me troubles (first time).
Good luck.
 

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Discussion Starter #19
Overall it is easy. To remove it with the air intake in place I use a small Phillips screwdriver that I had to cut down (the handle) so I could fit it between the air-intake and the coolant reservoir. Two small screws removed(don't drop them!) and it pulls right out. Disconnect it from the wire harness, and clean it with MAF cleaner.
I have cleaned mine with each oil change, it was quite dirty when it gave me troubles (first time).
Good luck.
Thanks for the advice. I cleaned the MAF but the stumble is still there. It was dirty though so at least it got a good cleaning.

Matt
 

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shay2nak
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Same here. My MAF sensor was black on one side. Idling issue seemed better.
 
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