98 gst won't run worth crap.

[Firsts tuner]

Ok so here is the skinny. I just got this car after someone else gave up on it. It came to me with a bad turbo which I replace with a stock t25. The only aftermarket part on this car is a K@N air filter. The motor was just recently rebuilt (like 0 miles.) The idle is very poor (low) at best. I have found a couple vacume leaks which I have repaired. The only time the car seems to run right is when I rapidly press and release the the throttle multiple times and let off. In the brief moment when I release the pedal and the rpm's drop the engine smooths out and my short term fuel trim comes up. Unfortunately that's only momentarily. The fuel trim drops off to 0 and it's back to loping and running pathetic. As best as I can tell I have a severely lean condition that the computer is trying to compensate for by dumping obscene amounts of fuel? I have re placed the intake gasket, the tps, the maf sensor,
various relays, and even the ECM. ( btw its nice to have a complete running parts car). The timing is spot on, and the compression is 190 psi across all 4 cylinders, good fire on all plugs. I don't know what to try next. This is my first DSM and right now it will be my last unless I can get the thing going. Any help would be appreciated. I really like this little car. Other than the whole not running issue it's a cherry car, bone stock and pristine. Thanks guys, and sorry so long.

 

[Firsts tuner]

If the power was grounding to chassis wouldn't that jack up all the other sensors running off that same circuit? Or at least blow a fuse or two?

 

[delta448]

It could do either, or neither.

Do you have +12V at that pin? I suppose I shouldn't have assumed. The MAS may in fact be the problem, but I don't think we can know that for sure yet. It running well when you unplug it just means there's too much fuel going into the engine in the first place.

Are you absolutely certain there are no DTC's?

Do you have or have access to a datalogger?

 

[Firsts tuner]

The closest thing I have to a data logger per say is a blue point scanner that reads live data. It is showing the MAF as reading really high. The other MAF sensor I have reads a lot worse so I did kinda assume this one was good. I noticed the upstream o2 sensor was reading kinda low voltage so I am going to look into that tomorrow. At least I can start the damn thing now. That's a hell of a jump from last week. Sorry to answer your questions when I have tried to scan it I do not get any codes, except when I have unplugged sensors for test purposes. For the next question I am getting like 12.5 volts at the pin with ignition on. I also checked continuity between it and the corresponding pin at the computer harness and read acceptable resistance. Like .001 ohms.

 

[starfish]

maybe suckin air through the intake gasket.Def check all areas for vac leaks.

 

[Firsts tuner]

I have beaten that dead horse a few times. The problem now is that the MAF is reading a lot more going through it than what actually is. I am almost completely sure it's the sensor but I want to exhaust the electrical issue possibilities before dropping 170 on a new sensor. I did the DIY boost leak test, and also hooked the car up to a dealership smoke machine and only found a couple small negligible leaks. All of which have been fixed. My boost gauge is reading 20hg at idle which is spot on for these cars. Of course that's with the MAF unplugged. I am going to shoot the wires tomorrow and I have a guy that can get me a good used MAF so I may actually get to drive this thing by the end of the week. Thank you to everyone for the helpful advice. I will update when I get something figured out.

I messed with the maf today and when I tried to ohm out the iat I was getting an open line. If I plug it in and look at my live data it shows pretty accurate temp. The maf is reading 4.44 gps at idle. Does that seem high or low to anyone?i just want to make sure. I think I am going to try another boost leak test. Maybe I did something wrong here.

 

[Firsts tuner]

So the boost leak test is good. I put a new maf sensor in and nothing, no change at all. The only way I can get the car to run and respond to the pedal is to unhook the maf sensor plug. I did notice that the front o2 sensor was reading 0.040 volts. Seems low to me anyone else? It also remains there when I rev. I should mention that this is at operating temperature and the rear o2 sensor is switching between .08 and .06 volts. Can a bad o2 sensor cause this car not to run? One last thing is there a resistor somewhere to regulate voltage to sensors? I noticed while back probing that my supply voltage from the ecm flucuates with the alternator. As in if the alternator is putting out 14.5 volts there is 14.5 going to my maf, my o2, all that stuff. The lights in the car don't dim or brighten. This car just dosen't make any sense to me I really need help.

 

[ChiTownHussla]

Replaced front o2. It's cheap and youvwill knock that out. A bad o2 will make it run crappy. Also checkcfpe exhaust leaks before the turbo

 

[oneslowwdsm]

Not true^ I have had both o2 sensors on my car bad and it barely made a difference at idle. It will make a difference when you drive but only in closed loop operations. Did you try adjusting/replacing the CAS? Something else I may have missed, did you adjust the BISS screw to where the idle is high enough and what does it idle at when it starts to run bad? Also, a video would help greatly. Description of what's happening with words can only help so much.

 

[delta448]

Check the injectors to make sure they are stock 450's.
Check the CTS resistance.
Check for exhaust leaks.
Check for good spark on all 4 and note the coloring of the plugs.

AFAIK there is no voltage regulator for any of the sensors, especially since the alternator has one built in. The stock low impedance injectors do use a dropping resistor but there is also coding in the ECU to adjust their deadtimes based on system voltage.

Without a code thrown for the O2 sensor, I highly doubt replacing it will cause any improvement.

Also, do not blindly turn the BISS. You need a MUT command in order to hold the ISC centered when you adjust the BISS.

 

[Firsts tuner]

Ok so I had some free time and went to the junk yard today. I found a new o2 sensor and I went ahead and grabbed a iac. Came home and put them on and was able to run with the maf plugged in finally. I was about to pull the plugs and knock the carbon off when I noticed some smoke from the tailpipe. I just figured the exhaust had some residual in it from a previous failed experiment. I decided to try to clear it out, then noticed as soon as I started to make boost ( around 8psi according to the Autometer gauge) the boost fell flat on its face and I got more smoke from the exhaust ALOT more. So with all that said should I rebuild my turbo or just get a new one? Lol at least now it runs.( sorta)

 

[Firsts tuner]

Check the injectors to make sure they are stock 450's.
Check the CTS resistance.
Check for exhaust leaks.
Check for good spark on all 4 and note the coloring of the plugs.

AFAIK there is no voltage regulator for any of the sensors, especially since the alternator has one built in. The stock low impedance injectors do use a dropping resistor but there is also coding in the ECU to adjust their deadtimes based on system voltage.

Without a code thrown for the O2 sensor, I highly doubt replacing it will cause any improvement.

Also, do not blindly turn the BISS. You need a MUT command in order to hold the ISC centered when you adjust the BISS.

Kinda late to restart this thread but I wanted to say that it was the injectors. They were working like they were supposed to but they were too small. They were 250cc for a nt. delta488 you my friend are a genius