I'm observing a new issue since a few days : after driving for a little while, engine warm, the idle is a little rough and when starting from 0 it stutters a bit like it's going to stall (but doesn't).
The symptoms are not felt at higher rpm.

Among the possible causes for this would be a compression issue (haven't tested that yet), however this morning I had my obd reader plugged in (an iCarsoft 930) and exploring the data streams I inadvertently triggered a P1798 error, all errors cycling on the dashboard : gearbox fault, low brake fluid, etc.
This disappears when restarting, so I wasn't worried, however: the rough idle symptoms disappeared when it happened.

So I'm guessing this is related to a sensor and isn't a mechanical problem.

- I've checked the air intake for leaks -- looks airtight. (including the britlle breather tube)

- At idle, the MAF reads 8+ g/s when the engine is cold, around 5.6 g/s when hot, which seems normal.

- The values I'm getting for the coolant and air intake temperatures are respectively -40° C and +66° C, which sounds ... not right. But I suspect it's more of a reader issue, because the engine temperature on the dashboard is normal, and when hitting the "Ext" button on the A/C panel I'm getting the correct value as well.

- At idle, LTFT are -2.76% on one bank, -0.79% on the other. Is that normal ?

What other sensors and numbers should I check to properly identify the culprit ?

 

Temperatures definetrly doesn't sound right and be easily just code reader. At warm engine anything between + and - 5% of trims is perfect. What downstream lambda singnals looks like when engine is cold and warm ?

 

I'll try to check that today.
I changed both heated ones just a few years ago (they're Denso).

 

Values seem to be normal, about 0.08-0.1 V on warm idle.

I unplugged the MAF sensor while the engine was running with that rough idle problem, which caused the engine to stall instantly. It started just fine afterwards, but I'm surprised : I expect a "restricted performance" amber light on the dashboard, no more.

 

Downstream lambda voltages should be somewhat steady somewhere 0.5V-0.8V. Throttle blip will throw it away for a while. Juping from 0.1 to 0.8V constantly is bad and blown out cat.

 

Hum
I'll recheck, may have missed something or misremember the values. Numbers on each sensor were similar.

 

So, downstream lambdas where steady at around 0.8V, looks about right.

From another thread, I figured it might be related to the coil packs because a few months ago a coolant hose exploded and flooded the engine. And indeed, the driver side coil packs and spark plugs holes were pretty dirty with half dried coolant.

I cleaned that up and it seems to be running better, not quite perfect yet but a significant improvement. I'll remove the plugs and clean more thoroughly (didn't want to have coolant fall into the cylinders and was in a hurry so it's not really pristine down there yet)