S4 NA wont start or rev unless AFM held open (Not fuel pump relay related)
Thread Starter
Sit and Spin
Joined: Mar 2001
Posts: 472
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From: Latrobe, Pa
S4 NA wont start or rev unless AFM held open (Not fuel pump relay related)
Good Day!
My buddy and I swapped my running / race winning S4 NA 13B drivetrain into his 1st gen racecar.
It ran well 1 time at Watkins Glen earlier this year.
Since then, it will not start and run reliably. We have run tests on the AFM, the spark, and fuel. It appears all parts tested are doing the correct thing. The engine has decent compression (90+ all rotors all faces).
I can get it to start and rev if I manually hold teh flapper door open on the AFM. I can change the AFR's based on how open I hold the flapper door. If I open it the entire way while holding the throttle plates open, it will rev normally. Not something I can test under load for obvious reasons.
If I do NOT hold open the flapper door, it will cough and sometimes start for a second, then die. The AFR's are LEAN! When it does this.
It seems to me that the AFM is NOT opening far enough for some reason. WE checked all vacuum lines (there are only the Boost sensor and a reference for the FPR and the brake booster hooked up). All other vacuum ports are plugged. In fact, we plugged the brake booster line as well. Same issue. WE removed the intake manifold and checked all flanges and gaskets. All looked good.
I tried unplugging the coolant temp sensor in the water neck. No difference.
The reason I KNOW this isnt related to the fuel pump relay and the AFM is that the fuel pump is not in the harness anymore. It is on its own power and relay. Therefore, the usual failure mode is not in play in this situation. Also, it did start and rev when we accidentally forgot to plug the AFM in. We redid all new wires to the AFM. We did all new sensor grounds and Vref wires to all the sensors as well. All sensor values check okay at ECU.
The fuel pressure is 37psi at idle.
Any help is appreciated!
TL
R
AFM tests good per FSM
Fuel pump on seperate circuit
Car will start and run and rev ONLY if I hold flapper door proportionally open to RPM(s)
AFRs change in relation to how far I open the flapper door
Not much left to have a vacuum leak (especially a large one!)
HALP!
My buddy and I swapped my running / race winning S4 NA 13B drivetrain into his 1st gen racecar.
It ran well 1 time at Watkins Glen earlier this year.
Since then, it will not start and run reliably. We have run tests on the AFM, the spark, and fuel. It appears all parts tested are doing the correct thing. The engine has decent compression (90+ all rotors all faces).
I can get it to start and rev if I manually hold teh flapper door open on the AFM. I can change the AFR's based on how open I hold the flapper door. If I open it the entire way while holding the throttle plates open, it will rev normally. Not something I can test under load for obvious reasons.
If I do NOT hold open the flapper door, it will cough and sometimes start for a second, then die. The AFR's are LEAN! When it does this.
It seems to me that the AFM is NOT opening far enough for some reason. WE checked all vacuum lines (there are only the Boost sensor and a reference for the FPR and the brake booster hooked up). All other vacuum ports are plugged. In fact, we plugged the brake booster line as well. Same issue. WE removed the intake manifold and checked all flanges and gaskets. All looked good.
I tried unplugging the coolant temp sensor in the water neck. No difference.
The reason I KNOW this isnt related to the fuel pump relay and the AFM is that the fuel pump is not in the harness anymore. It is on its own power and relay. Therefore, the usual failure mode is not in play in this situation. Also, it did start and rev when we accidentally forgot to plug the AFM in. We redid all new wires to the AFM. We did all new sensor grounds and Vref wires to all the sensors as well. All sensor values check okay at ECU.
The fuel pressure is 37psi at idle.
Any help is appreciated!
TL
RAFM tests good per FSM
Fuel pump on seperate circuit
Car will start and run and rev ONLY if I hold flapper door proportionally open to RPM(s)
AFRs change in relation to how far I open the flapper door
Not much left to have a vacuum leak (especially a large one!)
HALP!
OP - you didn't mention checking for cracks/vacuum leaks on the intake plumbing from the outlet of the AFM to the throttle body. Any leaks there would cause the symptoms you're seeing.
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