Rich on start. Then normal. Then lean.

Hey all, I'm encountering a fun condition and wondering if anybody has a guess at the cause based off of the description or logs. This is for an EJ20 on LinkECU.

I've been doing street tuning with a tuner to dial everything in before hitting the dyno and seem to have encountered a mechanical issue.

As background, the other day I had the stupid idea to play with launch control - pretty sure I heat soaked the engine which led to running very lean. Let it cool down and it was back to normal until I idled for a long time and started running quite lean again. After cooling the car down, I managed to drive it 30 minutes home. Eventually driving around it hit a sudden lean condition (~1.4) with no power and I had to get towed home. Discovered I blew a chunk out of the up-pipe to turbo gasket so repaired that, plus corrected some warping in the turbo downpipe flange due to poor fabrication on my part. I can't find much when leak testing the exhaust or intake (but don't have real testing equipment). I've changed spark plugs and the lambda sensor (old spark plugs came out white confirming true lean condition).

On startup the car runs fairly rich, then normalizes with a lambda correction ~-5. After getting up to temperature and playing with the gas pedal a bit, lambda jumps to 1.4 and car starts running poorly again. With the old sensor it would remain at 1.4 when off the gas (seemed to stabilize when off the gas in my latest run up).

I've included log files with the old lambda sensor on the 10th and the new sensor on the 11th. Would be super grateful for any suggestions of next steps in diagnosis!

I would suggest you first determine the mechanical condition of the engine is good. Replace the plugs, run a compression / leakdown test, verify the ignition timing, perhaps look at the cam timing, check valve clearances, smoke test the intake manifold, check /replace fuel filters, remove injectors and check for debris on intake screesn, verify fuel pump pressure and flow.

Looking at the log from 6-11, you definitely have too much post start enrichments. You should only need this for say 5-10 seconds. Yours is way too much (113% additional fuel) and I think that is fouling the plugs, which then mis-fire indicating lean. The Warmup Enrichment (changes with coolant temperature) won't even become active until after the post start finishes. I would suggest you change post-start to only be 10-20% for 5 seconds.

Your fuel table needs some tuning -- at 800 RPM / 38 kPa, you are 20% too rich. 1000 RPM, 41kPa is 5% rich. Those are just the cells used for idle and they seem pretty far off.

Thank you so much David!!

I replaced the fuel filter (shouldn't be the issue with good pressures but cheap to do) and added some fuel system cleaner to the gas tank and managed a short drive yesterday without any issues.

I think I screwed up the fuel trims by fixing the exhaust leak upstream of o2 haha. Will get on a dyno soon to get those values properly dialed!

I've reverted to the plugs in the car when the problem started and still running happily.

Thank you for your help David.

I guess it was something gummed up in the fuel system causing the issues despite good pressure readings.

Hopefully this helps someone else some day!