Cold engine knock

[300zxt]

I started up the 88 Turbo yesterday and there was a slight knock at idle when the engine was cold, I changed the oil (20w50 like always) and oil filter even though I'd only done maybe 3,000kms since the last oil change. But today it did the same thing when it was idling cold. It does clear up once the car is warm, or when driving off or if you just give it a little gas when it's idling. I'm thinking (and hoping not) possibly piston slap? It's hard to pinpoint exactly where it's coming from, it's not lifters as it's clearly a slight knock.

The obvious thing would be to user a thinner weight oil but I've used 20w50 forever and I'm actually living in a hotter state now than I was before and never had this problem... Another less scientific cause is that my driveway is on a bit of an angle and the car doesn't sit flat when it's parked. It sounds stupid but could this be contributing?

Obvious things like oil pressure is good, the car still drives good and doesn't knock when cruising and it doesn't return throughout the entire day but I suspect that when I start it up tomorrow morning it's going to do it again. Any suggestions besides a tear down? This car is my daily driver so it would not be cool to have to rip into it to fix it at this point.

[Steve88t]

Very common problem is gummed up lifters taking a few seconds to a minute or so to pump up. You can pull them out and "rebuild" them which basically involves cleaning them and checking the contact surfaces. You can also polish them to remove any wear or very light scarring.

You can also try Dexron in place of 1 qt of oil or a product such as Berrymans in the oil.

[OR-Zman]

I installed all new lifters when I rebuilt my heads and they want to knock when the car is cold or has been sitting for a couple days. Its really no big deal as long as the "knocking" stops in about 10 seconds.

Basically once the oil pressure gets up and the knocking lifter(s) refill with oil they stop their noise.

If it lasts longer than 10-15 seconds I would say you need to consider some action before they really cause a problem.

[300zxt]

That's the thing, I know what the typical lifter "tick" sounds like and this just doesn't sound like same thing... It's more of a rythmic knock rather than a lifter tick and it sounds more internal.

[OR-Zman]

But it goes away after the engine warms up?

[300zxt]

Yeah it does, when idling you can even give it a bit of gas and it will go away but returns when the revs come back down. I'd describe it as more of a piston slap type sound though rather than lifters as it sounds much more internal. I wouldn't normally be worried, my NA's lifters used to tick alot when it was cold while the oil pressure was building up but this just doesn't sound anything like that. Typically with lifters you can hear it clearly coming from either side of the valve covers, with this you cant.

I've also noticed once or twice when shutting the car off sometimes there's a slight knock aswell. Im concerned it's something a bit more serious internally. Another thing I should add is that you can only really hear it when you're in the car, it's quite difficult to pick it up when under the hood again nothing like lifters. With that said driving is perfect and once it's at temp it doesn't return which makes me think it is piston slap or rod bearing.

[Andrevas]

Wassman has teh same problem, a very audible knock from a sticking exhaust lifter, can clearly see it with a flex LED light thru the oil cap at idle

[300zxt]

Yeah I did actually read that after searching but I'm just not 100% sure that it's the same sound. I'm quite familiar with what noisy lifters sound like, and this is different. It didn't actually do it today so it's a bit of an annoyance more than anything at this stage, I'm just hoping it's nothing more serious.

[Steve88t]

Well I've never seen a VG30 with piston slap that you could hear. In fact the only noises I've heard from one were lifter tick, exhuast leaks, and rod knock (only one btw).

[homestar]

you may not have a lifter tick if it doesn't sound like it. It could be an exhaust leak that seals itself up when the engine warms up and the metal expands.

[300zxt]

Steve88t wrote: Well I've never seen a VG30 with piston slap that you could hear. In fact the only noises I've heard from one were lifter tick, exhuast leaks, and rod knock (only one btw).

Yeah I'd much prefer an exhaust leak lol, a rod knock would certainly be worst case scenario but I think the car runs too strong for it to be that really I'm just preparing for the worst. Fingers crossed anyway, Im giving it a full tune up this weekend aswell as pulling the plenum to get the injectors cleaned and I'll inspect the exhaust while I'm at it so we will see.

[Jason84NA2T]

I'm guessing rod bearing, though you would usually hear it pretty clearly free-revving the engine. If you put a pipe against the head/block and put your ear on it you should be able to hear more closely where it's coming from, and more accurately what the sound sound's like (if that makes any sense?)

Steve88t wrote: Well I've never seen a VG30 with piston slap that you could hear. In fact the only noises I've heard from one were lifter tick, exhuast leaks, and rod knock (only one btw).

At least not stock... mine piston slaps like a mofo. Obviously worse on cold starts, it sounds like a diesel truck.

[Jason84NA2T]

[quote]300zxt wrote:

Well I've never seen a VG30 with piston slap that you could hear. In fact the only noises I've heard from one were lifter tick, exhuast leaks, and rod knock (only one btw).
Yeah I'd much prefer an exhaust leak lol, a rod knock would certainly be worst case scenario but I think the car runs too strong for it to be that really I'm just preparing for the worst. Fingers crossed anyway, Im giving it a full tune up this weekend aswell as pulling the plenum to get the injectors cleaned and I'll inspect the exhaust while I'm at it so we will see.

A rod knock would not usually change the way the engine runs, or power. Given you don't beat on it when cold, you sometimes can go a LONG time on a knocking rod. I know a guy with a Z32TT that had a rod knock for about three years (maybe 15K miles) before it finally went.

This all is NOT to say you have a rod knock. I obviously can't hear your car making this noise from here in Michigan. If you can't figure it out yourself, there are always experienced mechanics that will listen to it for free and tell you what they think it is.

[Steve88t]

Jason84NA2T wrote: At least not stock... mine piston slaps like a mofo. Obviously worse on cold starts, it sounds like a diesel truck.

Really? Either I'm deaf or mine doesn't make a sound. Skirt coatings probably help that though.

[300zxt]

[quote]Jason84NA2T wrote: [quote=300zxt]

Well I've never seen a VG30 with piston slap that you could hear. In fact the only noises I've heard from one were lifter tick, exhuast leaks, and rod knock (only one btw).
Yeah I'd much prefer an exhaust leak lol, a rod knock would certainly be worst case scenario but I think the car runs too strong for it to be that really I'm just preparing for the worst. Fingers crossed anyway, Im giving it a full tune up this weekend aswell as pulling the plenum to get the injectors cleaned and I'll inspect the exhaust while I'm at it so we will see.
A rod knock would not usually change the way the engine runs, or power. Given you don't beat on it when cold, you sometimes can go a LONG time on a knocking rod. I know a guy with a Z32TT that had a rod knock for about three years (maybe 15K miles) before it finally went.

This all is NOT to say you have a rod knock. I obviously can't hear your car making this noise from here in Michigan. If you can't figure it out yourself, there are always experienced mechanics that will listen to it for free and tell you what they think it is.

Yeah worst case scenario would be a rod knock I think, although I must say it is really hard to hear it under the hood and is far more audible when inside the car... I'm gonna give it a full tune up and possibly switch to synthetic oil just on a whim if it keeps it up.