Stumbling After Cold Start

**90Vette** · 09-27-2018

Recently, my car has started running really rough when it first starts in the morning. The throttle is very unresponsive, and under light load below about 1400 RPM (i.e. going up a hill, lightly accelerating, etc.) it stumbles pretty badly. It's weird, at around 1400 RPM it instantly picks up and feels fine. For testing purposes, I've pulled timing way back below a certain RPM, and that felt very similar to this - bog, bog, bog, then instant smooth acceleration. Also, it feels like the engine kind of jolts immediately after I add throttle (possible related to the acceleration enrichment function?). After the engine runs a few minutes (haven't been able to watch the datalog, but it seems like it's when the engine goes to closed loop), it seems to go away entirely.

I initially thought it was running lean, so I gradually increased startup enrichment to about 15 percentage points higher than stock, but that didn't seem to help or hurt. It doesn't smell abnormally rich, so I'm hesitant to blindly decrease enrichment.

I've made a few changes recently that could have caused this. I put long-tubes on it just over a month ago, but I don't recall seeing this behavior immediately after. A few days after adding the headers, I drove out from CA to school in VA, and I'm wondering if the humidity or cooler morning temps could have something to do with it. Lastly, a few weeks ago I discovered my PCV system was set up backward (the check valve was on the wrong side, causing the air to flow the wrong way and a strong vacuum in the crankcase), so I switched it to the proper orientation.

Looking back at my BIN dates, it looks like the stumbling popped up shortly after I fixed my PCV system.

Do you guys have any idea where I could start with diagnosis?

**sturgillbd** · 09-28-2018

Can you record a datalog and post up?

**90Vette** · 09-28-2018

Well, I took a couple datalogs where it was stumbling really badly, but ALDLDroid didn't save them for some reason (looks like there's a hidden 256MB limit for all saved datalogs?).

I took another this morning, but it didn't seem to be stumbling at all, which could have been for one of several reasons: it took a few minutes to get the datalog started, so it warmed up a little before I started driving; I left the house a bit later, so it was warmer; and it felt quite a bit less humid than previous mornings. It still sounded like it was idling rough though, so I'm attaching the datalog I took. (I attached a ZIP that has both the CSV and the XDL.)

I'll try to go for a drive early tomorrow morning to see if I can get a better log.

**90Vette** · 09-29-2018

Here's a better datalog. Still wasn't doing it real bad for some reason, wonder if it's weather-related.

**sturgillbd** · 09-29-2018

I viewed the first datalog this morn and didnt see anything that stood out. I need to find the more complete adx for 8D and will review both logs in more detail. I'll try to do it this eve but it may be late before I get a chance.

Brian

**90Vette** · 09-29-2018

Thanks Brian, really appreciate it.
What do you mean by the "more complete" adx?

**dzidaV8** · 09-29-2018

I have checked the log, you are running a little rich with BLMs around 120, and the moment you tap the throttle it gets momentarily lean from what I can see. Check your fuel pressure and have a look at the spark plugs.
If you still have stock injectors it's a good idea to check them or replace them anyway to D3 type injectors. Stock ones tend to finish their life after those 30 years.

EDIT: I read on other topics that your car is far from stock... I'd say to give it more AE on low temperatures in: "AE Delta TPS Pulse Width Factor Vs. Temp. ".
And work on the tune, log shows rich everywhere.