1994 K2500 Strange Knock

[clevernickname]

I purchased a 94 k2500 a couple weeks ago. Runs fine, all that needed to be done was have a new clutch put in it. Put the clutch in and it runs fine... Except the knock.

 

Cold engine dosnt knock, it has 166xxx miles on it. After it warms up to operating temp it starts knocking. So i started pulling plug wires.. found that knock was coming from #2 cylinder. So i changed the #1,#2 main and #1 and 2 rod bearings, which looked pretty typical for the mileage, Knock still there. Put 5w-20 oil and a couple bottles of stp in it to boost the oil pressure and the knock sounds the same.

 

It seems like if i put the truck under load, imagine putting it in 2nd gear rolling slow then hammering it, the knock will go away. IE: more vacuum less knock.

 

The only other problem i am having is with the idle, More frequiently than not it will idle irratically, usually stays around 1500-2000 rpms. But sometimes it will idle at a perfect 650 and the knock is not noticeable unless you feather the gas a little.

 

NO Exhaust leaks

Dist looks ok

no smoke

no loss of performance.

No vacuum leaks

 

I tried testing out the knock sensor, unplugged the whole KS Module harness and the sound didnt change. But when i hit the right manifold the Ohm resistance increases from the sensor!

 

I AM STUMPED!!!! WHAT THE HECK IS GOING ON?

[Jsdirt]

Rod knock or spark knock? Sounds like you thought rod knock ... but that gets worse (louder) under load.

 

For starters I'd toss the 5w-20 for some 10w-40, at least, with that mileage. I'm skeptical running that in vehicles that recommend it (Fords), let alone a 17 year old engine. Way too thin ... unless you live at or above the arctic circle.

 

In the spark knock arena, you may have excessive carbon buildup in the combustion chamber & piston dome. Not unlikely given the mileage. Add the crap fuel we have today & it's a recipe for detonation / pre-ignition. Don't rule out a vacuum leak somewhere .. unless your 200% absolutely positive there isn't one anywhere between the heads & throttle body.

[clevernickname]

Yeah it calms down under load. Its absolute loudest if im going down my alley in 1st not giving it gas. But its not a double knock like ive heard with rod knocks. Ive ran a few cans of seafoam through it to clean carbon a bit still the same. I just put the 5w20 in it to see if it would quiet the sound, ie: more cushioning for rod knock. But it didn't.

 

Im not the best mechanic in the world but Ive never once had this much trouble diagnosing a knock. I dont think its a pully or anything because it stops when i pull the #2 Plug wire.

 

Can you make a list of things i can check?

[clevernickname]

One more note. If i shut the engine off while it is ideling high, between 1500 and 2000, i have to crank it for about 20 seconds for it to restart. If i shut it off while at normal idle it will restart right away. And it will always idle around normal until i hit the gas or it runs for a minute after restart then it will either idle normal or high.

[clevernickname]

 

 

Video of knock

and also i meant to say 20w50 oil not 5w20

[Jsdirt]

Definitely not spark knock there - you've got a mechanical issue. Sound seems to be going at the speed of the cam ... although it's tough to tell across the net. Might be time for an engine swap ....