Misfire after cold start

In my reading that has to do with the computer testing the octane rating of the fuel. Im not sure how true that is, but I have read it in a few threads on this forum.

Its been a regular occurrence on my car, both pre and post tune.

I have had this random misfire code along with misfire first 1000rpm atleast 5 times over the past 2.5 years. I just delete the code and go about my day. never seemed to be a problem. I have changed my plugs 3-4times at this point and they never look worn out so not sure what causes it.

Did you guys ever find a resolution? Started getting a code for this after Cobb tune. Sometimes idle will drop to 600 for a couple seconds but otherwise car runs fine.

This is can be caused by carbon coked intake valves on direct injection engines, especially if it's only on cold start and goes away. VW guys have been dealing with it for years. Pull the intake manifold and do a walnut blast.

ice445 said:

This is can be caused by carbon coked intake valves on direct injection engines, especially if it's only on cold start and goes away. VW guys have been dealing with it for years. Pull the intake manifold and do a walnut blast.

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I've been researching walnut blasting and may consider it. Most people I've spoken with don't believe its the valves and claim they rarely see the valves get that dirty on these vehicles. I've yet to see a pic where someone has removed the intake manifold on our car to display dirty valves that warrant cleaning.
Why would it cause intermittent rough idle that only lasts a few seconds?

IIRC the 1 step colder plugs are more suited towards tuned cars to allow more of a safety net from added timing/boost. Stock tune/ bolt-on cars are better off with the stock heat range and gap to avoid fouling.

BoostedNate said:

I've been researching walnut blasting and may consider it. Most people I've spoken with don't believe its the valves and claim they rarely see the valves get that dirty on these vehicles. I've yet to see a pic where someone has removed the intake manifold on our car to display dirty valves that warrant cleaning.
Why would it cause intermittent rough idle that only lasts a few seconds?

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Any DI engine can have coking on the backside of the valves. With that said, what is the specific issue you're having? Coked valves usually causes a P0300 on cold starts only which clears up (and reduced power but it's usually not noticable). If you're having a random one that comes and goes then it's probably something else. I'm not offering this advice from a position of knowledge of these specific engines, but moreso just giving something to check because it affects lots of others cars.

My 18 is in the shop now. Got a random...a cyl 1 and a cyl 4. They're thinking bad coil packs because it only occurs on cold starts but who knows. But that's what the dealer is telling me right now. I've got densos pre gapped from tune+ and the FP tune.

I saw misfire on cyl4 in the first 1000 revolutions, but only after the car sat for 2 days or more. I had a tiny coolant leak into cyl 4, but it took 2 days to leak enough to cause the misfire.

I'm on day like 5 or 6 in waiting for the dealer to hear back from Ford on what to do because they can't replicate the issue on any other cyl's and they apparently haven't gotten the ok to do a compression test. So I'm not real thrilled and wish I could get a GT.

Cmw458 said:

I'm on day like 5 or 6 in waiting for the dealer to hear back from Ford on what to do because they can't replicate the issue on any other cyl's and they apparently haven't gotten the ok to do a compression test. So I'm not real thrilled and wish I could get a GT.

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How does your coolant look? If its the slightest bit low, ask them to let it sit for a day, and visually look for moisture. they can do that shining a flashlight in with the plugs removed.

BoostedNate said:

I've been researching walnut blasting and may consider it. Most people I've spoken with don't believe its the valves and claim they rarely see the valves get that dirty on these vehicles. I've yet to see a pic where someone has removed the intake manifold on our car to display dirty valves that warrant cleaning.
Why would it cause intermittent rough idle that only lasts a few seconds?

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I have 87k no catch can. I didn’t have a bore cam. But my valves were coked. Stoked them in acetone and worked a toothbrush then walnut blasted them. The car recovered quite a bit of power to the point I had to relearn my shift timing from a response perspective. There was a rough idle symptom as well BTW. See video below explaining the cold start misfire. I’ve only had one Misfire since which came at the end of a half tank of VP additive+ 93 to make it 100 octane. Could have been the octane relearn theory. Before they were happening a lot frequently, like every 7-8 starts.



Have Cobb stage1 91 tune. Run 93octane. Intake tube and filter. ATM IC.

I do have a leak as the coolant resovoir slowly seems to drain over months of time. Maybe the weeping head gasket leak...

DUNDEM said:

I have 87k no catch can. I didn’t have a bore cam. But my valves were coked. Stoked them in acetone and worked a toothbrush then walnut blasted them. The car recovered quite a bit of power to the point I had to relearn my shift timing from a response perspective. There was a rough idle symptom as well BTW. See video below explaining the cold start misfire. I’ve only had one Misfire since which came at the end of a half tank of VP additive+ 93 to make it 100 octane. Could have been the octane relearn theory. Before they were happening a lot frequently, like every 7-8 starts.



Have Cobb stage1 91 tune. Run 93octane. Intake tube and filter. ATM IC.

I do have a leak as the coolant resovoir slowly seems to drain over months of time. Maybe the weeping head gasket leak...

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mid your coolant is disappearing, and it’s not on the ground under the car, then it’s getting in the motor or in the oil. Time for a compression test. Or, if your out of warranty and want to do yourself this is how I did it. I pulled the plug on the cylinder that was misfiring, put a towel over the fender to protect the paint, put a small 3 step ladder next to the car, climbed up and shined a flashlight into the cylinder. I could see a tiny puddle clear as day. I taped a qtip to the end of the straw, and stuck it down to get a sample of the liquid, and the color matched the color of the coolant. Since I’m under warranty still, I took it to the dealer and told them what I found.

RichBrew said:

mid your coolant is disappearing, and it’s not on the ground under the car, then it’s getting in the motor or in the oil. Time for a compression test. Or, if your out of warranty and want to do yourself this is how I did it. I pulled the plug on the cylinder that was misfiring, put a towel over the fender to protect the paint, put a small 3 step ladder next to the car, climbed up and shined a flashlight into the cylinder. I could see a tiny puddle clear as day. I taped a qtip to the end of the straw, and stuck it down to get a sample of the liquid, and the color matched the color of the coolant. Since I’m under warranty still, I took it to the dealer and told them what I found.

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Thanks for information. I’m scared to check the compression... gonna throughly check for fluid leaks with fingers crossed.

My issue is a sometimes rough idle, lasting a few seconds. The codes P0300 & P0316 take a day or two to pop back up after being cleared. Never had any codes before updating to a new Cobb tune that netted more power. Considering my plugs are new and the mileage I am starting to wonder if it is in fact the valves...