I'm trying to "l2t" as you folk often say, and I've been stumped. This is not a LINK specific query, so please don't bury me in that poor, neglected subforum. I am interested in the engine components which could cause the behavior described.
My car has a wicked stumble which consistently causes the car to lurch as if I'm hitting a spark/fuel cut for 1 cycle of the engine. It is an instantaneous affair;
the lurch is less deceleration and more re-engagement of the driveline as the backlash is absorbed. I have not been able to find any other documentation of such an issue - most "stumbles" I've read about occur over a range of rpm.
Past the lurch, it will run to redline with no issue. It actually seems to gain power after the lurch, mimicking the behavior which some report of the stock ECU's transition from open to closed loop fuel management. However, my standalone LINK is set up with a wideband and uses closed loop all the way. It will also accelerate smoothly if the MAP is kept below the threshold.
I'd love to think that it was a simple electrical fault, such as plug wires, ignitor, etc., but it has an almost mechanical consistency of the failure mode. The lurch only appears if the MAP is above ~130 kPa and hits at an rpm between 3500-3700. It doesn't matter if I'm WOT or 3/4 throttle, it lurches all the same.
I've tried removing as many variables as possible from the equation, including turning off all boost management and running the ~6 psi spring wastegate only. No change. I've tried adding more fuel locally - no change.
Pictured is the data log of one such incident. As can be seen, nothing spikes but the AFR. The rpm wiggles up and down a bit as well. However, those look like side effects, not the causes. Ignition, fuel, MAP, TPS, everything remains changing at the same rate before and after the lurch.
I know the LINK is an outdated ECU, but the fault seems to be an engine issue, as the instructions the ECU is giving (or says it's giving) look OK. What could cause such a consistent issue?
Thanks for your help.