Ok here's the deal: my '91 L98 rattles a bit on cold startup... it's not a heavy knocking or pinging, just a rattle that sounds like it's coming from the top end (I really hope). Once the engine runs for a half second, and oil pressure builds up, the rattle disappears, and I won't hear it again until I start the engine up. When it's warm, it will do this, but only a fraction of a second. There's no smoke, no loss of power, nothing like that. Can anyone offer any insight?

 

i had something like that, turned out my pistons were wobbling in the cylinder(s). my dad called it piston slap. its mainly from worn out rings. dont know if that helps you at all, that is just what i went through.

 

ok that's a good possibility. My next question: If it was bad piston rings, wouldn't it do it all the time?

 

most likely it will but you have to remember when heat is applied to metal it expands. so if you have a slight wear in the rings enough to start tapping the piston then it warms up it might expand enough to compensate and not let it rock.

 

Ok that makes sense as well... but the sound goes away while the car is still cold. Plus, wouldn't I see some smoke out the exhaust when I first fire it up? It will still do this when it's warm too, but like I said only for a half second until oil pressure builds back up. I'm thinking the oil pump may be getting worn (at least that's what I'd like in a worse-case scenario ) I mean that's just what I'm thinking, it really doesn't sound like a piston knock. I appreciate the info though and I won't rule it out.

 

my 88 sc done the same thing

use a filter that has a check valve in it like the ac-delco 1218 filter
if you use the truck filter or the 454 for the small filter

also when you take the oil filter off remove that late that the filter bolt too and check the bypaas hole there to see if carbon has got the bypass held open.

i cleaned mine and used the 1218 filter and my problem went away car has 151 k on it now still no rattles

 

I have the optional factory oil cooler on my L98, it goes right above where the oil filter mounts, so the filter mounts lower than it normally would. Is that going to get in the way? Thanks for the replies guys

 

I'll assume that what you call a rattling could also be described as a tapping. The key is that you say it goes away when oil pressure comes up.

Most likely this is caused by the hydraulic lifters bleeding down while the engine is shut off and cooling down. Without oil pressure to the lifters, there will be excess lash at the rockers, and they will tap. When oil pressure comes up, the lifters start adjusting like they are designed to do and the tapping stops.

Often this is caused by poor oil change maintenance. Dirt & varnish build up in the lifters and they bleed down when oil pressure is absent. Use of a filter with a check valve may help, but that only ensures the filter stays full while the engine is shut off. Since our filters are mounted straight vertical, that isn't a real problem.

 

Well I suppose it would be called a tapping, but coming from every rocker all at once it sounds like a rattle moreso. Yes, it disappears completely when the engine has pressure. When I first got this engine, it needed valve seals. New ones were installed, and proper lash was set. The oil ports were clogged with gunk, and they were cleaned as well. When I completed the swap with the new seals, I changed the oil (and filter) before I started the engine, then ran it for about 200 miles, then I changed it again, and it has been running on the same oil for about a week and a half now. Does this indicate a change of lifters would be a good idea?

 

Before you go that far, try an internal cleansing.

Do frequent oil and filter changes, say every 500 miles if mostly city driving, or 1000 if there is some highway involved - basically every couple of weeks. For the cleaner, use one quart of synthetic oil and the rest regular petroleum (all the same weight, 5W30 or 10W30). Keep a close eye on your oil pressure, because a heavily deposited engine can have that stuff break loose and form an unfriendly goo. The frequent drains/changes will help ensure you won't clog the oil pump pickup or oil filter.

If you want to hasten the process, AMSOIL makes an excellant crankcase flush that you run for 20-30 minutes before you change the oil (idle only, the flush is very thin and is not adequate for lubrication with the engine under load). I'd still do the one-quart-synthetic changes, though.

My guess would be that by the 3rd or 4th oil change, the problem will go away. If you used the flush, it might go away with the first use, but I'd change the oil sooner than normal anyway because of the gross nature of the deposits you described.

 

Ok I'll have to give that a try... I'll probably change the oil again this weekend. Where would I find that engine flush, and do I mix it with the oil itself before I drain it, or just run it on its own? I don't really think that the engine is suffering any damage at this point (I hope not anyway), but I still want to get it taken care of, it just really bugs me every time I fire it up. Think it would be a good idea to block off the oil filter bypass like SC2 suggested?

 

Ok if you do that with the oil you are also going to need to take the iol pan off and clean it out.. That gooy stuff will not flow out the drain hole with the oil 75% of the time and can still be picked up in the pump... Cleaning the oil like that is a good idea tho.. later...

 

Ok, is it hard to remove the oil pan with the engine inside the car? I just put that engine IN, and I really don't want to take it out I know it's possible to do, I'm just wondering the degree of difficulty that it will entail. Any insight appreciated.

 

It's not to bad.. I have to disconnect my header from the lower flange and take out the starter.. Both pretty easy...

 

hey quick

in my ealier post i didn't mean for you to block off the bypass.


i said to remove the plate and check to see if it is stuck open.

]cause if it is stuck open the oil is not getting filtered.

the reason the bypass is there is so you don't blow up the oil filter
if the pressure become to great (like starting up cold and going WOT down the road).
so just check it mine was stuck open and it was not filtering the oil.

 

are you running stock pistons? Some Forged pistons will make noise..clatter until they warm up and expand......maybe its that v-6 radiator..

 

SC2, that clears things up a bit haha. Although I HAVE heard of people blocking it off. I'm going to do that probably tomorrow and see what's up, then do the whole oil change thing, and then take the pan off and clean it out. Maybe I'll put a new oil pump in it while I'm at it, since I'm already there.

Riley's, I am running stock pistons (this year anyway). I didn't do any internal mods to the engine when I got it, just those valve seals. I was going to port the plenum this weekend, but I left the dremmel tool at work. For the record, I DID end up getting a new radiator to make that oil cooler work. You got me on that one!

 

Personally, I'd opt for the oil and filter changes, and possibly a quart of ATF about 50 miles before draining the old oil. If it persists, I'd be tempted to remove the rocker covers, then back off then relash the adjusting nuts a few cycles while the engine is running to force some hot oil through the lifters, potentially cleaning out the plunger check valves (likely your biggest problem).

Piston slap/skirt wear usually takes about 30-45 seconds to fade away on a cold engine, not just a few seconds.

 

Yeah it disappears as soon as oil pressure gets going. I'm going to take that filter adapter off and see if the bypass is stuck open.
Along with another oil change today hah.