Random miss when warm

[Michael]

Now that my cold idle problem has been solved I have one more problem I want to fix. I first noticed this when charging my ac system last month after the compressor replacement. There is a random miss. It's misses very often, but there is no pattern to it that I can tell. In fact I never noticed it unless you just sit in the car and hold it at around 1500-2500 rpms where it is most prevalent. It's very hard to feel when driving but it is there under a load as well.

Now here is the thing, when it's cold in open loop it will be flawless. I can hold 2k and not one hic up. It's only when it goes into closed loop that the miss happens. The miss is there throughout the rpm range, it's just most noticeable and more common at 1500-2500 rpms.

I have checked the vacuum solenoid and it seems to be working correctly. I even unhooked the vacuum line and applied my own vacuum to the dist. advance to make sure it wasn't leaking.

So is there something that is screaming red flag to anyone? Plugs are fairly new but I have yet to pull them. I'm wondering if it running rich or lean once warmed up? Wish I had a dwell meter right now.

[Spittybug]

These are the tougher ones...... random misses like that could certainly be the ECU, but I'd be more inclined to look for cracks. Cracked plug insulation or cracks in the rotor or cap. Check your plug wires too. Make sure nothing has worn through and is shorting. A coil going bad (especially as it warms up) can give these symptoms too.

[Michael]

These are the tougher ones...... random misses like that could certainly be the ECU, but I'd be more inclined to look for cracks. Cracked plug insulation or cracks in the rotor or cap. Check your plug wires too. Make sure nothing has worn through and is shorting. A coil going bad (especially as it warms up) can give these symptoms too.

Thanks Owen. When I say "random" I mean it's always there when warm, it's just not a dead cylinder. You can hear a miss, half a second later 2 misses, 1/4 second 1 miss, runs fine for a second then misses...very sporadic but always there.

[NightFlyer]

To determine if it's related to the lambda system, you could always run it open loop by unplugging the O2 sensor and seeing if the problem persists or not.

[hmcelraft]

To me random means on a different cylinder each miss - where intermittent means the miss occurs at the same cylinder just not each time the cylinder is suppose to fire.

No misses when cold makes me think it is fuel mixture related first then, heat related activation like in wiring or fuel/air mixing.
Those related items could be - leaking fuel injector seal(s), bad or cracked spark plug, bad or loose sparkplug wire at plug or distributor cap, vacuum leak or mixed up vacuum lines at distributor and/or FPR, vacuum valve backwards, vacuum line going to fuel canister from bottom of throttle body is open -- stuff like that. From what you have said it is probably something that happened during the compressor change?

[Michael]

I disconnected the O2 sensor when warm and the miss is still just as bad. Side note, the miss is related to temp, but is there when cold too, just not as bad. It does get worse and more common when warm. I am not sure if it's the same cylinder or not but this video will help convey what I have here. In the video you can see the tach moving when holding a certain rpm level....The throttle position does not move once I hold it. You can see the rpms change and the engine straightens out for a second, then misses a few seconds(tach drops about 100rpms), then back to normal.

[DMCMW Dave]

I disconnected the O2 sensor when warm and the miss is still just as bad.]

Plug wires/cap etc. is where I'd start. See if you have a wire loose on the cap or plug. Usually running problems are fuel. This one sounds like ignition.

[hillbillydmc]

Hi, Sounds like it is running lean when warm, I would check the control pressure cold and warm to see if it is in spec.

[NightFlyer]

I like both suggestions

As a side, your alternator appears to be rock solid, regardless of engine RPM. Definitely not that way with the Ducey

[dustybarn]

I had a warm miss that turned out to be a bad ignition coil. It was pretty obvious, though, because it was leaking oil around the screw terminals.