[Chutzpah]

So, I've seen a few posts related to engine misfire and subsequent repairs (collapsed afm valves and bad injectors) I recently started having intermittant misfire shortly after start up. Just as a note, I always let the car rpm idle down on start up before taking off. Anyway, instance #1 lasted all but 5 seconds, the car stumbled heavily between 1-2 on a 90f day. I assumed this was an anomaly, based on a 40 outside air temp swing from the day prior. Well, 2 days later, same thing, but lasted for about 5 miles. Attempts at light acceleration gave way to stumbling and jerking. When I finally came to a stop sign, the car was shaking pretty good. I was like oh crap, I have to get this puppy home. Not even a mile later and all is back to normal. There was no check engine light and I'm certain the gas is not the issue. Any thoughts or experience with this? Hoping it's not a sticky afm valve at 1700 miles...

[nukemustang]

I have experienced a similar thing. It isn't consistently crappy enough to take to the dealer and hope that it happens to them the next morning and if it doesn't I am sure it would cost me $50 for the dealer to say "could not replicate". Some days its fine, others.... not so much. I switched gas brands (I was stopping at a BP station religiously, but always 93 octane) to Shell, added a bottle of techroline, and no real change. I subscribed to a few other threads and some have mentioned about the injectors going bad, but taking several thousand miles to actually "fail".

[Blacksapphire]

I noticed the car acts differently with Exxon gas vs. Mobil. A little rougher with the former and I've experienced what the op stated with Exxon. Very strange to me.

[Vyncynt]

Use high octane gas, preferable without ethanol if you can find it (more expensive, but worth it).

When I bought me 2016 2SS from the dealer, they filled the tank with 87. I noticed it doing the same thing as I drove home (2 hours away). The next tank of gas was 93 and I have not has the problem since.

[nancygrl]

my 1st thought was bad gas or low octane gas. I haven't used exxon since the early 90's when my t-bird turbo coupe would run like sh*t on exxon. never put it in since.

[Burly]

I recently put 89 in, usually only run 93, and had this misfiring for first time yesterday. After a few miles it went away. Car had been sitting for few days.

[RCF]

Just a quick note on brands of gas... You can't use that as a basis. Exxon could be getting their gas from Chevron, or Shell, or ... Gasoline is a traded commodity, just because you go to Exxon, it's better than 50% chance that that gas is coming from somebody else, and changes. from time to time. Or if you live here in Houston, it maybe all (Chevron, Exxon, Race Track, etc.) loaded out of the SAME TANK at a terminal. The additives packages are different but are very small in volume and would not cause this stuff.

Now, the station might have issues with water in their tanks, which could cause the ethanol to separate out of the gas, etc.

I have worked for a refiner and have blended, scheduled and traded gasoline and diesel - so speaking from direct knowledge here, not what my brother's landlords friends son said...

[Garrett52]

Did you ever find the problem?

[Chutzpah]

Originally Posted by Garrett52

Did you ever find the problem?

No I didn't, bought a pretty expensive scan tool. Saw a couple missfires, but nothing else. Still sputters when the engine is cold. Other than that runs fine.

[tecmec]

For what it's worth, I have the same issue. Only happens before the car is completely up to temp, and seems to happen more often if it's colder out, or if the car has been sitting for a few days. I too took it to the dealer. They say they couldn't reproduce the issue, though they did acknowledge that there were stored misfires.

Has anyone had this fixed? Is it a bad injector?

[vtirocz]

After a cold start, IF I drive off right away and on a very specific drive cycle (very light load), it will stumble a little bit for maybe 5s. It's only does it during a specific manuever (very low pedal %) in the 1-2k RPM window.

Mine has done this since the beginning and hasn't gotten any worse.

I noticed other SS cars that I test drove had a similar "rough idle" on a cold start, so my assumption is this is "normal" and the dealer will not be able to find anything wrong. I think the cold adjust tuning could probably be optimized, although the cam is pretty aggressive. My '00 Z28 (LS1/M6) never did this, but obviously was certified to a much more lenient emission standard.

[tecmec]

Mine is definitely longer than 5s, maybe closer to 5 minutes if it's not particularly hot outside. It's not just a little rough, either. It's noticably missing, with significant hesitation and loss of power. The first time it happened, I got at few blocks from home, then I thought I wasn't going to make it to work. I questioned whether I should actually get on the freeway or not. The engine felt like it could have stalled at any moment.

Sent from my Nexus 6P using Tapatalk

[avalonandl]

These SS have a lumpy cam and take a while to warm up. Those of us old enough to remeber what a real Z28 (1969- 302 w solid lifters) was like at start up... you could;t drive those for the first 5 minutes.... without bucking and snorting.

[Garrett52]

Mine has a procharger on it and after the tuner installed new plugs I had a bad misfire that would randomly show up. Finally figured out that they went 2 step colder plugs and that was causing the problem. I put the procharger suggested plugs and gap and its a lot better, but I'm still seeing misfires on a cold start and in the low rpm range. From talking to the tuner and chevy it seems like this is fairly common even stock. I may try opening the gap on the plugs a little to see if it makes any difference since the colder plugs were fouling and causing the car to run horrible.

For those that say it isn't a real misfire. I have a gauge that is showing the misfire count, the tuner has seen them in HP tuners, and you can definitely hear and feel them at times.