Rough idle morning after

[nice1guv]

2012 3.6 Petrol GC 137k

Has anyone come across this, or have another idea:

First start in the morning after leaving overnight, car starts a bit rough on idle but sorts itself out after a few seconds.
Never does it again during the day, even when sitting at work all day.
Seems to need 12+ hours, ie overnight, to idle rough the next morning.
No codes, runs perfect, fuel good, air good, spark plugs replaced 20k ago.

My feeling is it might be petrol leaking past one of the injector o-rings when left for 12+ hours.
I haven't measured pressures at startup yet, not sure I can even do that.

Anyone seen this before?

Cheers.

[Fishframe]

Need to get fuel pressure measured. Probably the fuel check valve slowly losing pressure overnight. Should hold pressure while engine is off. That’s my best guess,

[nice1guv]

Thinking out loud here:

The fuel check valve is in the pump in the tank, if it is faulty, then the fuel bleeds back into the tank. So when it starts, it is rough due to initial lack of fuel? I assume this would need a whole new pump.

I was thinking the opposite, that it was rough due to excess fuel getting past the seals or from an injector. If there was excess fuel, would it run rough?

It doesn't crank any longer than normal and fires up straight away.

Would a leaky injector cause any driveability changes, as it runs as well as always.

Changing injectors or seals is doable at home, dropping the tank is probably in the too awkward basket for a home job for me.

[drover]

Check battery with a proper load test not just a voltmeter, a battery that looks great can often be the problem............. Check coolant level, a small leak will cause it after sitting overnight.....
Check air box seal, hoses and other valves etc hanging off the engine, I recall one of them giving me dramas on an old petrol job.

[OLMATE]

In regards to internal coolant leak, does the coolant in overflow bottle remain constant or is it slowly dropping for no apparent reason with no external visible signs

[drover]

Been thinking about this and again come back to start at the beginning, the KiSS application, rough idle on start up, plugs always comes to mind, remove plugs before start up and have a look at them they really should be dry, wet or moisture means a leak, fuel or coolant as condensation overnight, check gap. When electronic ignition first appeared, if the engine ran rough you threw in new plugs as the old ones were burnt to hell, anyway checks of plugs, cable/coil pack should be looked at especially if hidden under a plastic lid which have been known to wear through leads and cause shorts............ Do not try to short a plug or lead to engine nowadays to see the spark, it could/will kill you.

Also after a run have a look at the burn on the plugs, they really should be the same, its not uncommon for one to have an end burnt off because the apprentice didn't adjust correctly and with modern ignition, once warmed up the commputa will cover the issue..... a dodgy injector should come up with a code.

[nice1guv]

Legends! Thanks for the ideas guys.

Coolant has always been the same level, but I'll take another look and keep a closer eye on it, just in case.

I did the spark plugs 20k ago myself, replacing the original ones.
If they were faulty, I would have thought the normal running might be a bit different.
Not the easiest job to check them on the 3.6, but I might over Christmas.

Same goes for the coil packs, I would have thought there would be other driving symptoms, not just a rough cold startup idle.

I'm waiting for another OBD plug to turn up at the moment. Then I can try and monitor some things and see if i can narrow it down.

I'll be servicing over Christmas as well, so that will give me the chance to take a thorough look.

Just thinking if anything has changed recently, the only thing is I had a weak LED bulb in one DRL, so I changed them both to a different brand LED.
I can't remember if the bulb started going first or the rough idle came about, but if it was related to the bulbs (not that I think it could be really) it should do it every startup.