Stumble 3 seconds after cold start

[DaddySS]

I have an annoying symptom:

The car starts fine, idles up to 1600 RPM and then about three seconds in, it stumbles down to 600, recovers back up to fast idle and then runs fine. It only does it on initial start up.

Looking through the possibilities;

Start up airflow delay
Startup friction airflow decay

Spark idle flare control

any thoughts or experience on how to tackle it?

[WeathermanShawn]

Usually the Start-Up Friction Airflow Correction B4343 cures everything.

If you have a Tune & Log you can post up we can probably figure it out..

[DaddySS]

Here's the tune file and the log, you can see where it stumbles at frame 41

[WeathermanShawn]

A couple of questions..

Are the following Tables stock or modified?

1. B3605 Commanded Fuel in Open Loop
2. B5936 Idle Underspeed Error

It looks like you are staring our very rich on your Start-Up AFR. Your Spark is very low. Some is being cut by the ECT Spark Table..some perhaps Catalytic Converter. In any case that is not a lot of Spark (11 degrees).

I worked a lot of the Learning Airflow parameters as described in the Idle Tutorial. I might try a leaner start-up AFR, and zero out any multipliers that may be pulling Spark down.

Let us know about #1 & 2 (questions) and if you made any recent and drastic changes.

I take it that once you get into Closed-Loop it runs fine. You might also want to not log more than 24 channels if possible..

[DaddySS]

Are the following Tables stock or modified?

1. B3605 Commanded Fuel in Open Loop
2. B5936 Idle Underspeed Error

B3605 is 5% reduced from stock
B5936 is stock

It runs really good except for this little annoyance....and thanks very much for the help BTW.

[WeathermanShawn]

You might want to take a look at B5902. Both the stock tune (and yours) has a multiplier of '1' for those ECT temperatures you are having problems with. It then references Table B5901 which is subtracting ~ 12 degrees of Spark from your Base Spark. The theory is that the retarded spark will light up the Catalytic Converters quicker..hence assist cleaner emissions.

I wonder if that is just pulling your Spark to low. At Idle, you should be at 22 degrees per Table B5932 'Base Spark in Gear'. Your log shows only 12 degrees of Spark at 70F (21C).

I might change the Table (see attachment) to a multiplier of zero (0) for every ECT..

Hey, it is just a theory..but for Idle it is all about Spark, Fuel, and Air. So, one by one you attack each parameter.

Good luck..

[DaddySS]

I increased the start up friction by 20% which solved the stumble but now it is harder starting - takes more cranks to fire it up. Thoughts?

[WeathermanShawn]

A little limited on time right now..without going through all the Logs and Tune again, what is your AFR when you are starting? I have found that the richer it is (in summer) the harder to crank. I.E., it bogs.

Have you messed with the Cranking VE or any other Start-Up parameter that might be causing a tune-related problem. Of course, there could be a mechanical-related problem..plugs, etc.

Hard to diagnose without knowing if you did any tune related change that instigated the problem. It really just came on with warmer weather?

[DaddySS]

OK, I think I got it - I kind of answered my own question as I was typing before. It started right up but stumbled, and after adding 20% to Startup friction it cranked over several times before starting but didn't stumble. So I split it, made it 10% and it started right up, no stumble.

As background I haven't changed anything tune wise or engine mecahnical wise since changing the fan settings last year. It runs great, it was just that annoying little stumble right after start up that bugged me.

[joecar]

Cool good job.