Slight miss at idle

My '97, f355 Spider has developed a very slight miss at idle. Almost seems from passenger side. It starts and idled fine when aux air pump is running, then after it shuts off, rpm's drop to idle around 1000-1200, then the miss/small stumble is noticed. It runs fine on the road anywhere after say, 2000 rpm, not noticeable.
Aux air regulator is clean, exhaust bypass opens as should. It's this tiny almost hesitant miss it developed more noticed at idle. Put in a bottle of Lucas injector cleaner, will run that through over the week. No codes thrown.Havent checked plug wires under cover for arcing yet, as my buddy tr512 suggested.
Any ideas?

 

They really all do that. The first couple minutes while the engine is cold and the aux air pump is on, the engine runs open loop, no correction so it is rock steady. When the pump is off, the engine runs at closed loop and it is now adjusting the air / fuel ratio with the O2 sensor signal and other signals. Almost all 355 will have a slight rumble at idle. You can spend years chasing your tail on this one.

 

Lol! Thank you! I read from others saying same thing, but seemed odd as it ran better before, then just starts doing this.

 

As yelcab mentioned, most all 355s have some degree of misfire after warm up ..... I've been looking into this and collecting data for some time ...... some progress has been made but still working on it .....

vvv ... If you have not already done so, please participate in the poll listed in my signature .... vvv

 

I understand the idle portion, but wh would it just start doing it? When I go to accelerate, it almost wants to sputter like it's overloaded with fuel or a slightly bad if firing, then gone.

 

Most problems 'just start' at one point or another ....... there are many causes of a rough idle .... some known, some not so known and some a combination of both .....

vvv ... Did you check out the poll on my signature? This will help get us started ....

 

Will investigate further tomorrow night. If I find anything that cures it, will post. Thanks Gothspeed for your input and will get to your poll soon.

 

have you already done a compression check?

could be a valve not seating properly

 

One thing of note to mention, I just changed the pre-cat O2 sensor on bank1 a day before, did a power off reset of the computer as per specs (drive cycle etc), but no change. Or, does it take a few more drives to get the computer to get things in sync such as idle etc? My other vehicles seem to get in sync relatively fast.

 

hmmm ..... any codes?

 

wasn't Goth's ignition wire reroute supposed to offer relief on this?

my car is a 97, and once in a while from over night pure cold start
I will hear a little miss usually only after the air pump turns off.
but does not happen all the time.

and on the occasion it does happen it will be once and then nothing rest of day.
no codes nothing either.

 

true! ... the ignition wire reroute does make a difference for the better ...

im am not certain if the op has them though ...

 

You may want to check/clean the MAF and ICV, and check the for any vacuum leaks around the intake plenum I seemed to get a steadier idle after doing that to mine. However, I still have a slight burble to my idle that I think is from the tubi or exhaust leak. Swapping cats soon, so I hope to do a thorough check of the exhaust at that time.

 

Thanks emac! Just cleaned the maf and ICV, ran it at idle for 10 minutes, got better, then threw a P1448 code. It seems to do this once up to operating temp at idle or when engine is hit idling around town. So now what's the cure on that one, or is because I haven't raced engine to open bypass (it operates fine)?

 

Isn't p1448 the bypass ecu? if you were under there swapping the pre cat O2 sensor, maybe you dislodged the bypass ecu or TC. I would check all connections reset codes and see what happens.

 

Yes, but no, was a pretty easy swap and not near same connections. Will check anyway, have another ECU coming from tr512, will replace and see what happens.