So I was driving my 1987 S4 Tii the other day, and all of a sudden the car starts bucking like a madman. The afrs are going to rich to lean (maxing out both ends on my gauge 10, 16) and its pretty much undrivable, anyway, I had to make it home, so i kept driving softly. Then all of a sudden it quit doing the issue, and it was back to normal. This has only happened twice, once on a longer drive and the other on a shorter drive. Attached is my current tune, and im not sure how to post the .mlg datalog file. Send me a pm and I can email it or something. I am not sure what it is, below is the picture of the log



BAD AREA




GOOD AREA


I think its hardware related, and it must not be the AFR sensor, as i could feel the car jerking (badly). Last time it happened, my plugs were horridly fouled, but these plugs have only been in for like two weeks, and i cant imagine they are already toast. Im going to check them later today.

Grounding? I went through all the grounds on the car last year, they are clean now. The sensors all go back to the proper pin on the megasquirt, and anything that needs a non sensor ground goes to a fuse panel by the megasquirt, that fuse panel, and the megasquirt then go back to the battery. The only exception is the oem grounds, and the fuel pump is grounded to the chassis.

MAP? My map reads constant, so i dont think thats the issue

Injectors? They are new injectors, i dont know how to view each one on the datalog... Maybe its not getting signal for some reason?

Bad wire somewhere? IDK it would make sense with the problem happens intermittently

MAP? The map values read pretty much equal with the rpm, and dont really decrease.

Im at a loss, ill take any ideas

 

The two things that stand out to me is the Injector timing, with it all at 360 degrees, and the AFR/EGO control, you are using a flat 15% controller authority.

https://www.ausrotary.com/viewtopic.php?t=240934

For controller authority, would recommend turning on the table and set most of it to 0, but then 3-5% in the cruse section at most. you want boost tuned, not trying to pull fuel. 15% is a lot of fuel and would cause the back and forth you were getting at the 25 ignition events between changes, that is a change every 2 rotations of the eccentric shaft. I tend to use the delay table, which you have to find out the delay between a large burst of fuel and it getting to the O2 sensor and then the reading sent to the ECU.

Sorry did not see this because it was not in the Megasquirt section, only just realized there was an overall set of posts for the ECU section.

 

It looks a lot like a misfire. A misfire appears as a lean-spike, because a misfire consumes no oxygen and the AFR gauge reads air in the exhaust.

I would carefully inspect your plug wires and make sure they are all on securely. Measure the current to be safe. If you have a spare set of coils you can try swapping them out, although the stock coils are known to be pretty robust.

This is good advice. I don't think it's the cause of your issue, but once the issue is resolved you should refine your EGO settings.

The injector timing won't cause this issue at all, but you can use less fuel at idle and low-load while making equivalent power, so it's a good idea to tune it. This is what I arrived at, tuned via AFR gauge in the driveway:




Injector timing becomes irrelevant as duty cycle increases, so the idle / light load areas are the only places it will matter.