5.7 Vortec cold tap/knock HELP!

[99Classic]

So about a year ago, my head gasket blew on my 99 Classic. Had the heads professionally redone and had the lifters, rockers etc out. Fixed it up nice. Since then I have had a real bad cold start tap/knock. Engine didnt exceed 100⁰C when the gaskets went. The truck has about 330,000km (about 200,000 miles.) Tried marvel oil, synthetic, different filters etc. Only does it after sitting for 5+ hours. And there is a slight noise when warm, but barely audible. I'm thinking lifters? I'm quite mechanically inclined and prepared to replace them if need be.

Any help is greatly appreciated. Thanks!

Side note: the truck did not make any tapping or knocking noises before the repair

 

[Schurkey]

Short-out the spark to one cylinder at a time. See if the knock goes away when the spark to a certain cylinder is grounded.

At least that would give you a specific cylinder to do more testing on.

Killing the spark won't change the noise if it's a lifter or rocker arm. Should change the noise if it's a collapsed piston, bad rod bearing, scored cylinder, etc.

Would also be worthwhile to connect a scan tool, and look at the knock sensor. Does the KS pick up the noise you're hearing?

 

[99Classic]

Short-out the spark to one cylinder at a time. See if the knock goes away when the spark to a certain cylinder is grounded.

At least that would give you a specific cylinder to do more testing on.

Killing the spark won't change the noise if it's a lifter or rocker arm. Should change the noise if it's a collapsed piston, bad rod bearing, scored cylinder, etc.

Would also be worthwhile to connect a scan tool, and look at the knock sensor. Does the KS pick up the noise you're hearing?

I'll give that a shot tomorrow, no CEL at the moment. Never had a knock sensor code either. I'll be sure to get back to you on shorting the spark. This started happening immediately after the top end repair. Beforehand she purred like a kitten with no noises. So I'd doubt I spun a bearing or wrecked a piston lol. Wont hurt to check though!

 

[My 97 Silverado]

I have this same issue on my 97 with 168K on a 5.7 Vortec. It "knocks" but then goes away after the engine is warm. It also had a slight hiccup at around 50 mph. One of the Techs at work (I'm in the Parts Dept.) thought detonation. Upon further research I thought it was the camshaft position sensor. I decided to replace the whole distributor. The hiccup is gone and I'm getting better mpg, but the dang knock is still there! I'm 95% sure it's not a rod bearing. I'm interested in what you determine. Good luck!

 

[Supercharged111]

How gunked up was the engine? Gotta wonder if it's not a lifter.

 

[99Classic]

I have this same issue on my 97 with 168K on a 5.7 Vortec. It "knocks" but then goes away after the engine is warm. It also had a slight hiccup at around 50 mph. One of the Techs at work (I'm in the Parts Dept.) thought detonation. Upon further research I thought it was the camshaft position sensor. I decided to replace the whole distributor. The hiccup is gone and I'm getting better mpg, but the dang knock is still there! I'm 95% sure it's not a rod bearing. I'm interested in what you determine. Good luck!

I've been hearing a slight tap still at hot idle now so I've pretty much determined it's a badly adjusted rocker arm. Seems to be the cheapest thing for now. I'll do lifters after if it keeps happening. As for the distributor, I just replaced it with a new cam sensor. The truck was loping like it had a hawg a*s cam before haha! I'll be sure to keep you posted on what I figure out!

 

[99Classic]

How gunked up was the engine? Gotta wonder if it's not a lifter.

It was pretty gunked up. We let the lifters sit overnight in something, my dad was helping so not too sure what they sat in. I've flushed the engine lots so far, 3 filters and 4 changes. 1 of the changes was with fresh oil and marvel mystery oil. Now it has high mileage pennzoil 5w30 full synthetic. I do remember having to put marvel oil on and in the lifters after assembling it because they were sticking so bad it killed all the compression. Very well might be lifters but adjusting rockers is the cheapest choice at the moment.

 

[Supercharged111]

When I did my cam I had a couple sticky lifters. They'd already been soaked in Berryman's Chemdip and the outside looked like brand new but inside they were still full of sludge. The plungers didn't depress on those particular lifters when I was setting the lash which is why I cracked them open. To this day I regret not opening the rest up.

 

[99Classic]

So late update. The acceleration tap has slightly resolved. I readjusted the rockers and the lifters are definitely the culprit. The acceleration tap was partially because of preignition. I was getting 9-10 mpg highway. Set the distributor to the correct position using a scanner. The timing was -22⁰ on the cam retard! So 22⁰ of advance! No wonder the thing was sluggish. Got it dialed in to 4 degrees of retard instead. No CEL and no codes. Still a bad startup knock from lifters but power has increased and I have yet to calculate the mileage.

 

[Schurkey]

Set the distributor to the correct position using a scanner. The timing was -22⁰ on the cam retard! So 22⁰ of advance! No wonder the thing was sluggish. Got it dialed in to 4 degrees of retard instead. No CEL and no codes. Still a bad startup knock from lifters but power has increased and I have yet to calculate the mileage.

Turning the distributor on a Vortec does NOT change the timing.

It changes the alignment of the cam sensor and the crank sensor, so the computer "knows" which cylinder is firing; and it changes the alignment between the rotor tip and the distributor cap terminal, so the spark doesn't jump to the wrong terminal.

Has NOTHING to do with ignition timing otherwise. Base (initial) timing is handled by the CRANK sensor, changing the distributor position doesn't change the crank sensor location.

Ideally, the cam sensor alignment should be within 2 degrees.