Aeroking
e.i.
I've been trying to solve this problem for sometime now.
Problem: In 3rd gear with the truck at full boost, the RPMs climb to 4000~4200 and stop. The RPMs hang at 4200, then drop to 3800, then bounce back to 4500 and sticks, then jump to 5000 (not climb to 5000, jumps to 5000) and continue climbing smoothly to 6000. Only in 3rd gear under a load. The power doesnt gradually fall off, it just hits a wall at 4200, and once it shifts to OD, the front end comes back up and it takes off again. But from about 90mph to 100mph it takes about 8-10 seconds :tdown:. Its not a tach problem, cause the datalog confirms the jumps in RPMs.
Solutions i have tried:
- Possible ignition issue. Replaced, the plugs and gapped to .028 (UR5s). MSD coil. Taylor 8.8 wires. Just rebuilt the distributor yesterday with a new pickup coil, and a new (good working) ignition module. The truck runs 18* at low boost, and 23* in high boost with meth. There's no ignition breakup, timing is solid up through the stumble, no knock.
- Transmission slipping. Its not, under light throttle eash gear is smooth. Trans temps stay around 120* during spirited driving. 3rd gear continues smooth once the stumble quits. Its a 700R4 built by a world class shop. 3400 9/11 converter. It locks up, and stays locked.
- Valves floating. The springs are rated for a higher lift then the cam. The valve guides tops look clean, so nothing it bottoming out. The seat pressure is well matched for the ramp of the cam. And again, the motor will rev clear to 6000, just not under load (in 3rd gear).
- Tuning. I've set my fuel and spark cutoff for 6200, and again, it'll rev clean to 6000.
- Airflow. I'm running a 4" cobra cutdown to fit my cramped enginebay. I'm running the stock IC, stock TB, and stock manifolds. Ported L35s with big valves. 72mm with a 3" exhaust. Boost is rock solid through the RPM stumble. I've tried removing the Cobra, thinking it was sucking closed, it wasnt, it takes alot of muscle to collapse the 4" cobra. The cam is a custom grind with a high powerband and a peak somewhere around 5000~5500, so i'm barely into the powerband when the engine stumbles.
- Fuel. I'm running twin intank HP Walbros, with plenty of pressure, and flow. 65# siemens injectors, at 10.8 AFR. Duty cycle ranges from 70-80%. I've run the truck without the methanol, thinking perhaps i was washing the plugs out, no good, it still stumbles.
Any ideas would be appreciated. I can provide datalogs, or a video of the dash. I've run out of options and ideas. Engine and transmission have roughly 3000 miles on them, and this problem has plagued the truck since the rebuild.
Problem: In 3rd gear with the truck at full boost, the RPMs climb to 4000~4200 and stop. The RPMs hang at 4200, then drop to 3800, then bounce back to 4500 and sticks, then jump to 5000 (not climb to 5000, jumps to 5000) and continue climbing smoothly to 6000. Only in 3rd gear under a load. The power doesnt gradually fall off, it just hits a wall at 4200, and once it shifts to OD, the front end comes back up and it takes off again. But from about 90mph to 100mph it takes about 8-10 seconds :tdown:. Its not a tach problem, cause the datalog confirms the jumps in RPMs.
Solutions i have tried:
- Possible ignition issue. Replaced, the plugs and gapped to .028 (UR5s). MSD coil. Taylor 8.8 wires. Just rebuilt the distributor yesterday with a new pickup coil, and a new (good working) ignition module. The truck runs 18* at low boost, and 23* in high boost with meth. There's no ignition breakup, timing is solid up through the stumble, no knock.
- Transmission slipping. Its not, under light throttle eash gear is smooth. Trans temps stay around 120* during spirited driving. 3rd gear continues smooth once the stumble quits. Its a 700R4 built by a world class shop. 3400 9/11 converter. It locks up, and stays locked.
- Valves floating. The springs are rated for a higher lift then the cam. The valve guides tops look clean, so nothing it bottoming out. The seat pressure is well matched for the ramp of the cam. And again, the motor will rev clear to 6000, just not under load (in 3rd gear).
- Tuning. I've set my fuel and spark cutoff for 6200, and again, it'll rev clean to 6000.
- Airflow. I'm running a 4" cobra cutdown to fit my cramped enginebay. I'm running the stock IC, stock TB, and stock manifolds. Ported L35s with big valves. 72mm with a 3" exhaust. Boost is rock solid through the RPM stumble. I've tried removing the Cobra, thinking it was sucking closed, it wasnt, it takes alot of muscle to collapse the 4" cobra. The cam is a custom grind with a high powerband and a peak somewhere around 5000~5500, so i'm barely into the powerband when the engine stumbles.
- Fuel. I'm running twin intank HP Walbros, with plenty of pressure, and flow. 65# siemens injectors, at 10.8 AFR. Duty cycle ranges from 70-80%. I've run the truck without the methanol, thinking perhaps i was washing the plugs out, no good, it still stumbles.
Any ideas would be appreciated. I can provide datalogs, or a video of the dash. I've run out of options and ideas. Engine and transmission have roughly 3000 miles on them, and this problem has plagued the truck since the rebuild.
