Boost question!! What else!?
12-18-03, 03:44 PM
#1
haiO
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Boost question!! What else!?
Here's the deal:
Car:
90 TII - I've got a 2.5" RB catback. Boost gauge. That's it.
Situation:
The car hits 10 psi at about 5k rpms at WOT. Sometimes 11. I've got no FCD unless one was installed into the ECU... BUT when it gets up to about 5krpms, the car seems to hit a wall. It's not violent, it just kinda stops pulling so hard. Revs go up just as fast as lower RPMs. Is my car not getting enough fuel? It feels faster when I drive it hard to like 5-6k rpms then shift. What's happening?
I don't think it's fuel cutoff to the rear rotor cuz i figure that would cut your power in half. That would make me lurch foreward probly, right?
Whatever the situation may be, the car is very driveable and i don't redline it often at all... if ever.
Ahhh.. there's a good description... backpressure. It feels and sounds like there's a lot of backpressure above 5k.
Let me know what you all think
Car:
90 TII - I've got a 2.5" RB catback. Boost gauge. That's it.
Situation:
The car hits 10 psi at about 5k rpms at WOT. Sometimes 11. I've got no FCD unless one was installed into the ECU... BUT when it gets up to about 5krpms, the car seems to hit a wall. It's not violent, it just kinda stops pulling so hard. Revs go up just as fast as lower RPMs. Is my car not getting enough fuel? It feels faster when I drive it hard to like 5-6k rpms then shift. What's happening?
I don't think it's fuel cutoff to the rear rotor cuz i figure that would cut your power in half. That would make me lurch foreward probly, right?
Whatever the situation may be, the car is very driveable and i don't redline it often at all... if ever.
Ahhh.. there's a good description... backpressure. It feels and sounds like there's a lot of backpressure above 5k.
Let me know what you all think
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12-19-03, 07:47 AM
#12
Seduced by the DARK SIDE
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My stock S5 will hit almost 12PSI in the cold air we've had lately, but It's because the pressure sensor is dead.
The AFR gauge says it's rich enough to 11PSI, but starts towards lean at 12PSI.
I just have to back off WOT in cold air.
The AFR gauge says it's rich enough to 11PSI, but starts towards lean at 12PSI.
I just have to back off WOT in cold air.
12-19-03, 08:42 AM
#13
haiO
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how do you tell if your pressure sensor is dead? my stock boost gauge doesn't register anything above like 200mmHg... (looks broken)does that mean the pressure sensor is dead? If it's dead.... is that bad? Could the car be leaning out? I know that's bad
Finally, what is the easiest and cheapest way to fix it so i don't blow my car up?
Finally, what is the easiest and cheapest way to fix it so i don't blow my car up?
12-19-03, 11:07 AM
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1. Get on pin 2H on the ECU with a meter. Put pressure on the nipple on the pressure sensor. Does the voltage rise and fall with pressure on it. Does the voltage rise above 3.5 volts when over 8 to 9 psi is put on it (ball park figures).
2. Or put the meter on 2H and go for a drive. How high does the voltage go under boost?
3. Pull the vac hose off the sensor. Plug the vac line. Idle the car. The output should be approx 2.2 to 2.5v. The needle on the boost gauge with this setup should show zero.
4 On a series four car the pin on the ECU would be 2B.
5. It sounds like your car is just not boosting very much. Buy a aftermarket boost gauge to double check the output of the turbo.
2. Or put the meter on 2H and go for a drive. How high does the voltage go under boost?
3. Pull the vac hose off the sensor. Plug the vac line. Idle the car. The output should be approx 2.2 to 2.5v. The needle on the boost gauge with this setup should show zero.
4 On a series four car the pin on the ECU would be 2B.
5. It sounds like your car is just not boosting very much. Buy a aftermarket boost gauge to double check the output of the turbo.
12-19-03, 11:57 AM
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I have an aftermarket gauge. It reads 9-11psi. The stock gauge doesn't zero properly when the car is turned to the on position. It kinda moves when under boost but only to 200mmHg. Still confused.
12-19-03, 02:30 PM
#19
I came, I saw, I boosted.
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I'd say clogged up cat, but that probably wouldn't result in that much boost.
How's your air filter??? Clogged to hell?
Check your timing!!!!
Other things-
Possible a dirty fuel system. If I were you I'd change the fuel fliter (easy job, it's on the firewall), and even take a look at the fuel pump. Check the voltage at the fuel pump plug under the rear carpet, make sure it's at least 12v or so. If it's 10v, then you need to rewire it.
Also- check the screen inside the tank- could be clogged.
You get at that by popping off the cover under the rear carpet. Screws will be rusty so might have to hit them with the corner of a chisel to turn them.
Another thing you can do, which will tell you if you are leaning out (different from fuel cut), is to measure the voltage of the o2 sensor while you are flooring it!
Very easy, just take a voltmeter (digital, or an analog one with a 1v scale), and scrape a little bit of insulation off the o2 sensor wire right at the ECU under the passenger side carpet. I can't remember what pin it is.. find it, or Hailers will probably chime in here! When the car is FULLY warmed up, check the voltage under WOT... it shouldn't drop below .8v at most. ANy lower and you know you have a fuel problem. That is dangerous to your engine though, since you can blow a seal due to detonation.
Good luck.
How's your air filter??? Clogged to hell?
Check your timing!!!!
Other things-
Possible a dirty fuel system. If I were you I'd change the fuel fliter (easy job, it's on the firewall), and even take a look at the fuel pump. Check the voltage at the fuel pump plug under the rear carpet, make sure it's at least 12v or so. If it's 10v, then you need to rewire it.
Also- check the screen inside the tank- could be clogged.
You get at that by popping off the cover under the rear carpet. Screws will be rusty so might have to hit them with the corner of a chisel to turn them.
Another thing you can do, which will tell you if you are leaning out (different from fuel cut), is to measure the voltage of the o2 sensor while you are flooring it!
Very easy, just take a voltmeter (digital, or an analog one with a 1v scale), and scrape a little bit of insulation off the o2 sensor wire right at the ECU under the passenger side carpet. I can't remember what pin it is.. find it, or Hailers will probably chime in here! When the car is FULLY warmed up, check the voltage under WOT... it shouldn't drop below .8v at most. ANy lower and you know you have a fuel problem. That is dangerous to your engine though, since you can blow a seal due to detonation.
Good luck.
12-19-03, 02:46 PM
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will check fuel... the stock gauge is not accurate. It reads below the bottom of the gauge when it's started and doesn't zero like it should in the on position.
Since the stock gauge IS inaccurate, does that mean bad pressure sensor, and if it does, is that bad?
Since the stock gauge IS inaccurate, does that mean bad pressure sensor, and if it does, is that bad?
12-20-03, 02:47 PM
#22
I came, I saw, I boosted.
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If the pressure sensor is bad, giving you such a high reading, then a few things are happening.....
1- You are getting way TOO much fuel, since the computer thinks more boost (hence; air) is going into the engine. Too much fuel-low power
2- Completely contradictory- You SHOULD be getting a fuel cut at that level though.. which also would explain the lack of power, though it would be more sudden.
Final analysis- replace the pressure sensor. Get one from on this board for cheap.
Done.
1- You are getting way TOO much fuel, since the computer thinks more boost (hence; air) is going into the engine. Too much fuel-low power
2- Completely contradictory- You SHOULD be getting a fuel cut at that level though.. which also would explain the lack of power, though it would be more sudden.
Final analysis- replace the pressure sensor. Get one from on this board for cheap.
Done.
12-20-03, 03:59 PM
#23
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****Since the stock gauge IS inaccurate, does that mean bad pressure sensor, and if it does, is that bad?***
You should show more interest in the voltage output of the sensor to the ECU AT the ECU. The signal at the gauge is the same signal that is SUPPOSED to go to the ECU. Look and see what the output is at 10-11 psi boost. Fuel cut happens at apporx 3.7 volts output from the sensor.
I'd buy a FCD if I were you. Your probably hitting fuel cut on the rear rotor.
Also make sure you have a turbo sensor and not a non-turbo sensor on your car.
You should show more interest in the voltage output of the sensor to the ECU AT the ECU. The signal at the gauge is the same signal that is SUPPOSED to go to the ECU. Look and see what the output is at 10-11 psi boost. Fuel cut happens at apporx 3.7 volts output from the sensor.
I'd buy a FCD if I were you. Your probably hitting fuel cut on the rear rotor.
Also make sure you have a turbo sensor and not a non-turbo sensor on your car.
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