Strange RPM jump in Tunerstudio

Hey guys, hoping someone has seen this before. First a quick run down on the car. The car seemed to be running well, before we pulled the motor.

Fresh 347 (100 miles), Summit B303+ cam, Systemax II intake, Accufab 65 TB, 36 lb injectors, Promaxx 180 heads. I bought the full Megasquirt kit from LMR with the wideband and gauge.

Car runs awesome and pulls hard at mid to upper throttle range, idles decent. Has a persistent "miss" at part throttle, which seems to be heat aggravated. Symptoms slowly get worse, to the point of not drivable after 30 ish minutes. If the hood is off, it can be driven seemingly as long as you want, with only minor issues. Symptoms were classic to a failing TFI module. What I've done since to "fix" the problem

New distributor, with remote mounted new TFI module, new plug wires, cap, rotor, coil, and new TPS. Scoured for vacuum leaks, removed any and all unused vacuum lines, EGR , and other emissions controls.

When driving, or even idling, with Tunerstudio running, you can see that the computer thinks the engine RPM is jumping way high at the exact moment of "stumble".

I'll include a datalog, and a Youtube video where you can see the RPM jump.




If this doesn't belong in this forum, I apologize in advance.

After much digging, I think I have this sorted out. The initial problems of the car becoming undriveable after time were I think the failing TFI and or PIP. After replacing those, the problem was better, but the "miss" was still there. While messing around in tunerstudio trying to figure something out, I noticed that when the RPM jump would happen, the "AC on" box would light up green, even though the HVAC knob was set to OFF. I had at some point turned on the " AC idle up" feature, which apparently is a no-no. Disabled that, took it for a 30 mile drive last night, no issues even with the hood on.

Now if I could just get it not die at the stop lights when the AC is on lol. Always something....

86HO5.0 said:

That part was challenging. I added a few degrees extra timing at -150 rpm+ to help recovery, added 3% dashpot and tweaked the IAC initial values tables to remedy the issue in my car. It dips to 450-500 occasionally with the A/C on, but doesn't stall.

Click to expand...

Thanks!