Carpokes Porsche Forum

whalenlg wrote: Mon Jun 22, 2026 10:17 pm My simulations don't show any extra tach pulses at idle.
I do see some idle variations with engine warm for cases where tps has failed, coolant temp fail, air temp fail, afm open circuit, high altitude enabled.
They all kind of look like the 2nd picture.

The 1st pciture is a case where o2 is stuck lean and the simulation shows rpm going to 0 at about 13 seconds after start (see 2nd pic)

Does this happen at cold or warmed or all the time? How long is it between the idle slowing?

soxnail wrote: Tue Jun 23, 2026 9:28 am

Tom wrote: Mon Jun 22, 2026 2:25 pm Is this a factory 951 DME and KLR? If so, are you acquiring those signals from the PCB itself or in the harness?

Sox: It is a factory DME and KLR, the speed and ref are being acquired at the dme connector pin 25 and 27.

If you are probing/logging directly off the DME board, the probe wire can itself introduce or alter fast transients, especially with a loose lead on a sensitive 8051 interrupt pin. That loose wire can act like an antenna. I would not conclude that “no spikes visible on the scope” definitely means there are no spikes affecting the circuit. Probably not, but not definitely not.

Sox: nope

Where exactly are you picking up the ignition pulse in that scope shot? Assuming it’s the DME pin 21 tach signal, that would suggest the glitch occurs before the ignition signal splits off inside the DME to the coil driver, since you see it in the logs and feel an actual stumble.

Sox: At pin 21 DME. Yup before the KLR processing.

I see that the short glitch pulse appears to starts where you would expect the full ignition pulse to start, but then stops quickly, before the full pulse appears seemingly late. That sequence would make me suspect the KLR first. One of its main jobs is to delay timing, so if there is an issue with that circuitry, I could imagine it causing your symptoms. I’d be tempted to bypass the KLR as a short diagnostic test (no hard driving) to see if that’s the source of the issue. If the problem goes away with the KLR bypassed, I’d start by looking for cracked solder joints in the KLR. Let us know. Sorry it's frustrating, but hopefully we can help think it through.

Sox: It is happening before the KLR. I have already done the bypass test. No need for hard driving it happens at idle.

Thanks Sox