knock when cold

[metallica21156]

ok. i know alot of people might have this. when i start the jimmy after sitting all night i notice it. it will start and everythings fine then after its been running for 5 secs or so it starts knocking. the engine will knock for like 5-8 secs and then go away. thats if the truck just idles. now if i start it and take off easy it will be good and then after i just pulled away it will be knocking but louder then if its just idleing and then after 4 secs its gone. happens everytime. does it the most with it cold. the colder the longer it knocks. when its summer it doesn't do it but every now and then i can almost hear it. and no its not a valve tap. much to loud and solid for that. i've been told its piston slap. don't know. any ideas? i never hear it again except when starting. sometimes it will do it also if it sits for a few hours and then i go out and drive it but the knock time is much less. its a 95 jimmy with the 4.3 CPI motor. and oil pressure is right up there at the next mark past 40.

[wolfox]

What kind of oil filter do you have? Oh, and almost forgot to ask, what weight of oil and what brand are you running in the sump? I bet it sounds like sharp, metallic ping-a-tap-tap-tap that you can hear in the cab, right?

[refrigerationdude]

Sounds like a classic Fram filter knock to me.

[metallica21156]

well its done it ever since i've had the truck. 2 years. the oil is modil 1 true syn 5w-30. i used to run a hastings oil filter but the place around me stoped selling them so now i'm using a wix.

[metallica21156]

well its done it ever since i've had the truck. 2 years. the oil is modil 1 true syn 5w-30. i used to run a hastings oil filter but the place around me stoped selling them so now i'm using a wix. as for the sound it almost sounds like it could be a rod. but it goes away and once its gone you never hear it again.

[rlith]

could just be a sticky lifter

[wolfox]

Yep, if you're feeding it good oil, and a good filter (You made great choices, BTW) I would say that is just may need a little cleaning out. Before your next oil change, for like 200 miles, try half a quart of Marvel Mystery Oil in the crankcase. It's a light petroleum carrier oil with a little bit of solvent. (Pretty close to kerosine) I would not run it longer than 200 miles, then drain and refill. Or, remove a quart of oil, and do a Gunk engine flush before changing the oil. Do it on a cold engine, letting it idle until warm for about half an hour. Drain and replace oil and filter. If it *still* ticks along like a sewing machine, there is the very slight possibility you are dealing with contamination in a lifter that is not allowing it to pump up very quickly at cold start. THe only cure at that point really is to replace the lifter, as it also may be partially collapsed. Anyone else with thoughts? Remember, these pushrod style engines are not very quiet by design. Even when fully warmed up, and feeding it fine oil and filters, mine just softly rickety clatters away which I am resigned to believing is simply the nature of the beast. You can't hear it when it's super cold out and the fan is spinning at engine speeds until the viscous coupler starts doing it's work... she just smoothly idles along with that typical Chevy rattle-clattery-click. In fact I would be worried if that noise suddenly stopped - cause then I would know the engine quit.

[metallica21156]

its got 79,940 on it so a flush prob won't be to vad. but i thought running syn cleans the motor out. i do have to say that i put that gunk engine cleaner in my mustang with a 302 with 121,000 miles on it and after the oil change the engine ran alot better. better pick up, better idle. the exhuast didn't pop any more like it did before. its worth a try. next oil change is in the end of feb.