Thread: 99 gtp sluggish after mods.. please help

looking for some help from some L67 pros

ok so first with a little background, 99 2dr gtp 102k miles stock with intake it went 14.39@97.4

now after speed daddy headers(no cat)and 3.5 pulley and tune it is no faster. using the same tune as all my friends have on there gtps. mine is just as fast as theres but i went 14.3 stock when they went 14.9@94 stock now we all run 14.0 makes no sense

so far ive swapped icm with no difference and put new front o2 sensor in.... i do have some small exhaust leaks but no enough to tick or anything at idle. leaves at crossover pipe and at dp with it meets catback

car knocks anywhere from 2-4.5 on a 2nd gear pull and LTFT's are -1.5 they are 9-10 at idle which seems high

thanks for any help

Got a Wideband?

Having the tune to look at could help a few of us with tuners see whats going on.

Honestly that car was a freak to go 14.3 with nothing else but an intake. Now what has happened is that when you tuned it you pretty much made it just like every other GP, and without a custom tune youre running what you pretty much should be running. 1 degree of timing can be worth 5hp, so that 2 to 4.5 degrees is where you are losing power. But in the end you are right where you should be. Maybe .2 tenths quicker if you can richen it up to get rid of that KR. What are your narrowband 02 values in that pull? Ive seen it too many times where people have factory freaks and think that they should still run faster than everyone elses after modding. 1+2 does not equal 3 in modding.

Put stock pulley back on, try again.

If you don't want to re-tune it that is.

Originally Posted by onalky87

Narrowband is in the 900 range. Ltft is -1.5 wot and +10idle stft is 0 +-3. It's just frustrating that I picked up maybeeeeeee .2 from headers and a 3.5 when everyone else picked up .8-1.0. I knew it was a freak. But I've seen many cars near by my that went 14.4 stock and now do 13.5 with my mods. Thats y I feel like I'm missing something.


The tune I have is the canned tune that my buddy has on his 98 gtp that went 14.0&98. He has a cat and I don't only difference in our cars. I have stock catback as well

Maybe my exhaust leaks r making it that slow? Ik it leaks at crossover on both ends which is before o2 sensor. Not bad u can't feel it or hear it really. But when seafoaming it smokes under good pretty good bc of it

I would start by fixing those leaks. The 02 readings will be wrong with those leaks and the car is probably running too rich because it thinks its too lean. I would fix those leaks and relearn the fuel trims. Then see what happens. You should be a lot faster.

Exhaust leaks really screw with the trims.

Fix those like ^^said...and try again.

I'm willing to bet that's what is causing the "sluggish" feeling 110%.

Seafoam is the only way I really know of or spray a light mist of water in the TB. You can use copper RTV. If you bought a set of ebay headers the clamps they come with are ****. I would buy better ones from the autoparts store. With fuel injection precision makes the difference. You want zero leaks because thats how the PCM knows how to adjust to get the targeted AFR.

2.25" stepped band clamps.

Get them from Napa locally online.

Originally Posted by onalky87

Those are actually the clamps I'm using. It's leaking at dp where it bolts to stock exhaust right after where the cat should be but I don't think that's an issue

Isn't the downpipe a V-Band clamp?

What kind of headers?

Yea anything after the first 02 doesnt matter. One thing you should think about getting is a wideband. That 900 could be rich or it could be lean. But if youre leaking at the crossover I would put my money on lean. Also you need to get rid of the 4 degrees of KR. 1 degree of KR is worth roughly 5hp. You could be losing up to 20hp due to detonation. Also, your fuel trims should lock in at zero. That -1.5 means that its pulling fuel when you go WOT, which means its leaning the car out, which is probably causing your KR.

Seal V-Band with RTV...have the race pipe section and the u-bend delete section that bolts to the catback welded up...I also recommend getting rid of the flange from the catback to the backhalf of the downpipe...that way that is all one piece...you're eliminating three possible leak spots, and can drop the catback with just the v-band clamp.

Not that those will affect fuel trims because they are post o2 sensor...so get the leak on the crossover or the rear header fixed first.

It's not.

Like Moore said...seafoam it...figure out where it's leaking from...it'll smoke from every leak.