Looking for ideas my 2003 ss is having a cloud misfire while idling and driving at low rpms, one it kicks to higher rpms it doesn't really misfire, I have changed coils, plugs, wires, fuel pump, fuel filter, fuel regulator, there is no check engine light and it does not come on while it misfires, I hooked my code reader up to run a health report and it is saying there is a ground issue, but I am having issues finding it, was wondering if anyone else has had an issue like this as I have run out of ideas.

 

What is a cloud misfire?

Are all cylinders missing? Are they randomly bouncing around or are some worse than others?

How do the fuel trims look down low?

 

cloud misfire is multiple cylinders misfiring, nothing severe, small misfires that cause power loss, then when it drops to a lower gear and the rpms go up it stops or barely misfires. haven't checked the fuel trims, Im not very mechanically inclined I just know some stuff. Check engine light does not come on though.

 

I only ask because its not really an automotive term, so I wasn't sure if it signified something unusual. If its from the scan tool, it could just be their mistranslation of an English word.

When looking at individual cylinder misfire data, is it evenly randomized across all cylinders? Heavier on one cylinder or on one bank? Still present / worse at idle? When the rpm go up, does it gradually fade away or is it an abrupt change (ie 2000-3000 is heaviest, 4k+ is almost perfect).

When did it start? Was it tied to a recent tank of gas / maintenance performed, or has this been occurring for some time.

From a general maintenance perspective, do you know the age / condition of the spark plugs?

The downside is a random misfire probably has the longest diagnostic tree of any engine issue as there could be so many causes. I ask about fuel trims to try to narrow things down.

My gut feeling is it isn't ignition as problems there are either present across the board (signal issues) or get worse with load (spark power issues).

Same with fueling. A weak pump or a partially plugged injector will show up as more misfires with rpm, not less. This also plays into the question about which cylinders though.

My initial guess with the limited data is a gasket/hose vacuum leak. The engine consumes little air volume at a high vacuum at low throttle. So a tiny leak hole post MAF can skew the AFR more substantially. As the throttle opens, vacuum goes down (less sucked through the tiny leak hole) and the tiny leak hole makes up a smaller and smaller % of the air inlet area as the throttle opens so it has less impact.

Thats why I ask about fuel trims. If its adding a bunch of fuel down low, but shifts back towards 0 as rpm / throttle go up, then that could warrant some smoke testing to check for leaks.

Just trying to help focus the investigation so you're not having to run through the entire general misfire diagnostic tree from top to bottom.

I don't think that really helps or hurts anything here. Clearly there's an issue if youre feeling it, theres power loss. It just means it hasn't tripped a reporting threshold yet. Even if it did, P0300 is the likely code if it is misfiring randomly all over which doesn't really help your diagnostic process at all here.