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I was told to post this in here because ill have better luck asking in here, so its a repost if you already read it.
Ever since I've been making little mods to my fc (88) nothing major into the motor just the outside of the motor. My FC is starting to wine real bad. after hitting 4k rpms the motor starts to lose around 85% of its power and it gets real loud as if its clogged and no gas is getting into the motor. After it passes 4k rpms sometimes it gets back to normal. Sometimes it doesnt even hesitate at 4k rpms. but id say 9 out of 10 times it hesitates. Ne1 know what my problem could be ?
Im guessing its the injectors, but I dont want to play the guessing game just yet.
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it could be alot of things as he's made mods to engine peripherals... but he hasn't been specific enough as to what exactly gets "loud" and hasn't said what he's been messing with.
It's likely the 3800 RPM hesitation. It's a common problem, and is thus covered in the FAQ. Generally caused by bad grounds (at the pressure sensor and ECU ground on top of the engine), a bad TPS and a failing pressure sensor, in that order.
okay. I put a brand new Loranzar (spelled wrong) exhaust system. Brand new cat. Put a stainless steal custom intake piping from the dynamic chamber all the way to a K-N filter. drilled a hole in the intake to alow air to breath to the back of the motor. Removed Smog pump and sealed holes in the back for the pump. everything was removed in the back of the motor where the pump is bein used. Put a dual pully for the alt. Power steering pump is removed for the moment, only two belts are on the car. water pump belt, alternator belt. new NGK spark plugs and motor tuned. motor and tranny were regrounded and rewired Changed out AFM. runny gxl injectors in my gtu not sure if that matters. since there all 13b
It started to do this prob well within a week after everything has been done. I rechecked the grounds, the wiring all is fine. only thing I can think of that is the problem after checkign everything else is the injectors but wanted to see your guys OPINION in what you think my problem should be.
car has been sitting for periods of time when it wasnt running. when I would start it it wouldnt idle. I had to hold the rev and I would hear a poping noise where the injectors are. I would run it after that about twice a week to keep the motor from trying to freeze over. The motor sounds as if it is clogged at 4k rpms. But it has a brand new CAT. in it. it just starts to hessitate and like all the power goes away.
Removed Smog pump and sealed holes in the back for the pump.
Bad idea!!!!
The smog pump opens up your auxilary ports and VDI system if you have an 89-91. Either you put it back on there and all the shit you took off, or you custom rig an electric pump do the same thing. There are threads about it.
He just said its an S4, so the smog pump can be removed. Its exhaust backpressure that opens the 5th and 6th ports on an S4. I'd go with what Aaron said, and check the TPS and Pressure sensor, since you've already checked the grounds. My S5 would hit about 4k and would act like a "rev limiter". All I did to solve this problem was unplug the TPS until I got a new one put on. Oh, and if you do unplug the TPS, be sure to do so when the car is up to operating temps, because if you start it cold, it'll rev up to about 3k, and when you do the "throttle blip" to bring it back down, it won't go back down. I found out that its a really good way to have the fire department called if you need a rebuild, lol. LOTS of smoke.
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I saw GTU and didn't realize he was cool enough to own an 88 GTU like me Regardless, it still sounds like my car when my auxilary ports were stuck closed.
Are you running the split-air pipe still? I bet you don't have enough backpressure or you need to clean your ports.
as far as the different injectors go... did you have to do any re-wiring when you put in gxl inuectors? what i'm getting at is the fact that 86-87.5 and 87.5 to 88 (iirc) went from low impedance with a center notch on the connector to high impedance to an offset notch on their connector, respectively. to make things more confusing they look identical besides the connector and i think even have the same number printed on their necks.
i have run the wrong injectors without knowing the difference and ran into serious high rpm limitations and problems similar to yours.
so you're saying that it pops/backfires/stutters at 4k rpms? i would say that is definitely A/F mixture.
1. make sure your aux ports are opening. they need that pressure from your new catalytic conv. to open and if the cat was designed improperly it may not be happening. i put a magnaflow cat on mine and my ports are opening fine. try putting lithium grease on the actuators and running the car 4k rpm + and see if the grease was pushed on the rods by their opening motion. if they aren't you have a split air leak or a problem with your cat and the only other option than the right cat/backpressure is to set up a electric air pump with an rpm trigger or to wire them open.
With the back fire on the exhaust. fire comes out. LOL. Blue flames shoot out. Whats ironic is my gtu is blue. lol but thnx for all the help guys. I will try to do this around the weekend when I have more time. I'll get back to you all on this thread or ill make a new one with info on how everything went. Thanks for all the help.
sincerly,
Ivan <-> Tampa Slow P.O.S not running after 4krpms crew #000001
i dunno... that's wierd i didn't think they were purple... in fact i'm pretty sure they aren't. mine were red and i think that is a mis-type. or maybe i'm just totally confused. anyways... if you are running those84-85 injectors they are both the wrong impedance and capacity. you'll either need a set of the 88 non-turbo injectors or you can use S5 injectors if you solder in the right connectors, which is what i've done.
you can also use low impedance injectors if you wire in a 10 watt 10 ohm resistor into a lead on each one. but the ones you have now are probably already messed up. mine were.
Regardless, it still sounds like my car when my auxilary ports were stuck closed. 
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