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Thread: Crazy Air/Fuel issues

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    Crazy Air/Fuel issues

    So I have little problem that has been bothering me for a few months now. I haven't really been able to take care of the problem because I have been in school, but now I am done so its time to take care of the situation.

    Alright first thing first I drive a turbo converted 95 3000gt sl, factory sl motor with stock compression. I am running wastegate boost still (5PSI), and i decided to go with a 93 JDM GTO ECU. I am using a MAFT Gen II for tuning. My engine now has 103,xxx miles, and the car is having some weird tuning issues.

    When you first start the car it high idles for a while, wether it is at operating temp or not, usually at about 1100RPM, and stays there for a long time. My wideband reads about 15-16 A/F ratio. When you drive the car it generally does pretty good, still has all the power it needs under full throttle (11-12.5 A/F ratio) and just fine under mild acceleration (14.1-14.9 A/F ratio). The problem comes in under mid throttle, (boost gauge reading 0 boost maybe one to five in/HG vacuum depending) the car leans out (15.5-16.8 A/F ratio) and bucks just a little, kinda like knock or maybe small misfires (it could even be my tune). Also the idle after a while just gets weird, when it settles to about 800 RPM it idles at 14.7 for a while then gradually riches out to about 12.5, snap the throttle and it starts back at 14 and works its way down. When cruising on the highway, or other steady throttle conditions, the car has had a tendency to "lean rich dance" staying most of the time around 14.7 but occasionally jumping to the 15s and then dropping to the 13s. Now after replacing my TPS under low throttle (doing 30 in town) the car starts reading 16.1-17.1 A/F ratio, but doesn't slow down or lose any power. Then when i try to engine break the car under 3500 RPM it riches to 13.1, then when i push the clutch in the RPMs dip down to about 300 RPM and climb back up to 800 or 1000 RPM and stay. The longer I engine break the car the lower it dips, with no engine breaking sometimes it dips a little sometimes it just gradually falls to idle RPM.

    Since I have been done with school I decided to replace some of the old sensors (IAC and TPS) Both of those did nothing to fix the problem, though after replacing the TPS the car is more responsive. I used to have the notorious front header exhaust leak, but i fixed that. I have checked for more exhaust leaks but all my exhaust bolts are tight and both headers don't have any more cracks. I have listened for vacuum leaks and there is no hissing. The car on a cold start and drive ticks a little loud for the first ten feet of driving but quickly goes away (I am assuming lifter tick). I have owned the car for about 3 years and I have been able to take care of most problems but this one is just freakin frustrating. Any help would be greatly appreciated!!

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    Are you running O2s? They'll mess with your tune in closed loop and if they're bad they could probably make weird stuff like this happen.

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    yeah I am running an O2 sensor, I went and got a new one today and will be putting it on tomorrow to see if that works. I am not to suspicious of it since i replaced it when i did the conversion 10k ago.

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    Have you had this problem since converting or did it run normally for a while? Do you have a datalogger? Hopefully the new O2 helps out, mabye you just got a bum one the first time.

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    yeah it ran fine after the conversion, I tuned the car myself and gradually continue to do so. I had an exhaust leak on the front header and fixed that. After I fixed that it ran perfect for while. The intake hose right over the manifold burned a hole in it, after I fixed that it ran fine. I don't have any real data logging other than the lc-1 and my MAFT II. I have been suspecting ignition problems (mostly plugs) for a little while because the car seems to run and drive fine just a little underpowered on occasion(possible mild misfire)

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    just changed the o2 sensor, no go on a fix. I did however figure out the no engine braking problem, I check the sensor monitor on the MAFT II and saw my TPS was reading really high at closed throttle, so I went and reclocked the sensor and yay I can engine brake again. The other problems still persist though. Gonna change out my plugs on monday when they come in, and hopefully that fixes the problem.

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    You may want to reset your ecu by disconnecting the battery for a few minutes and unplug the O2 and run it like that for a bit to see if it helps. I would highly recommend getting a datalogger. In this case it would let you see if your fuel trims were getting wacky, but in all cases it would let you monitor knock which is very important when trying to tune a boosted high compression engine. If you were misfiring, I would expect the wideband to read rich rather than lean (unburnt fuel).

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