Cold weather fuel knock

[phantom309]

I have a 97 tahoe 4x4 egr motor.
In this cold weather we have had, it starts ok for old spark plugs, but it has a slow idle,. and misses on a couple till they warm up usual bit of smoke then it clears once shes popping all 8.
BUT,..for the first up to a minute -ish, i have one hole that sounds exactly like it has a stuck open injector, my friend was listening and he was alarmed and started with "its got a rod knocking"
Give the motor a minute or so and the rap/fuel knock dies slowly down untill it sounds all systems normal, except for the low idle. Oil pressure is good and consistent hot idle is 35 or so. Engine is you garden variety happy little 6.5 when it up to 180 deg or so all sounds are normal. I did unplug the temp sensor to see if a bit more timing would help or maybe trick the ecm into raising idle but it did rattle a little more but no rpm change.
Conjecture what the problem might be?
Thanks for your thoughts.

[DmaxMaverick]

Spark plugs? There's your problem.

Excessive cold can cause issues that would not normally be an issue. The miss could simply be a very cold injector with a tighter tolerance that takes a bit of heat to act normal again. It could also be increased fuel system restriction due to increased viscosity, and a really cold IP. The idle speed on an EFI engine is 100% controlled by the PCM, so it will run at the speed it's commanded. Making the PCM "think" it's -40 with no change in engine speed only means the PCM "thinks" that is the correct speed. The timing advance is expected and may be helpful, but has nothing to do with the engine speed that I'm aware of. The PCM does have a "high idle" feature, which can be accessed with programming and/or external switching. It's part of the PTO feature, and is independent of climate condition controls. If you want a higher idle, under any condition, outside what the PCM commands by default, you'll have to change something, either the program or the command.

[phantom309]

spark plus,. aw c'mon u knowwutimeen!

i have a nice high idle switch on my burban only set for 1100,.phantom309ish on youtube.
https://www.thedieselpageforums.com/tdpforum/blob:https://www.thedieselpageforums.com/00abda35-a2cf-48e3-a23f-5003847c7be7
i'm thinking its an injector,. i am leaning towards a slightly open positio dropping pump pressure causing slow idle?

[DmaxMaverick]

It's EFI, right? How/why would/could it NOT run at commanded speed? If, for any reason, it couldn't, be sure it would complain (DTC's). IF pump pressure is dropping, it is compensating. IF pump pressure is dropping enough it couldn't compensate, it would complain (a lot). A single (or multiple, for that matter) injector won't drop pump pressure any more than it would normally expect (minuscule). Try identifying the suspect injector by loosening the line. If it's overfueling, the knock will stop when that injector is loosened. Swap that injector with that of another cylinder, to see if the problem follows the injector, or stays at the cylinder (compression issue, perhaps). Of course, it will be the least-accessible injector, so expect that (there's actually a real-world reason for that, in most cases).

[phantom309]

ok,, some better s'plaining,.
cold start,. no throttle input,. it cranks over starts to fire, and then it starts to 'hunt" just like a blown alky motor, rpm varies between 350(ish) to 750(ish), this will last 5-10 secs,. then it will find its idle rpm,. still takes 20-30 secs to pick up the last hole thats not firing because of bad "plug"but thats right around 550 ish. To be honest i haven't cracked any lines because i'm thinking its one of the back two,
Temps here are running between 15-35f at the moment,,
I'm hoping at that a good dose of the stanadyne additive might smooth things over,..

[DmaxMaverick]

Maybe it's getting a little air in it after sitting cold. Try loosening the fuel cap when you park it overnight.