1993 Chevy 3/4 ton 5.7 liter (350) knock on startup

[CCI]

1993 Chevy 3/4 ton, a little over 200,000 miles, 5.7 liter / 350 CID carbureted, there is a pronounced top-end knock on startup, then it goes away in a few seconds.

Any thoughts on what this is, what could be done about it, and if it even needs to be fixed?

 

[JavierH19]

I wonder if the oil is draining enough to where you get something of a dry start until oil pressure quickly builds. Oil filters usually have anti drain back valves and better ones use silicone. What oil and filter are you using.

 

[CCI]

I don't know what filter is on there, it's not my truck yet, but I will start there.

Looking on line with a web search, for whatever that is worth, it seems like this might be a common problem on the older 350s and relatively inconsequential. But if there is an easy and inexpensive fix I'd like to do that.

 

[JavierH19]

I don't know what filter is on there, it's not my truck yet, but I will start there.

Looking on line with a web search, for whatever that is worth, it seems like this might be a common problem on the older 350s and relatively inconsequential. But if there is an easy and inexpensive fix I'd like to do that.

Try a carquest premium and some 0/5w-40 since nm can get cold. You can look through the fill cap to see how it is and if it's clean use 0/5w-40 if not Valvoline restore and protect 5w-30 could clean in up in a few changes.

 

[JavierH19]

More importantly since it's getting hot does the ac work. It being an r12 system since it's a 93 and not a 94+ it's not gonna work that great with 134a though some got converted properly but some didn't and work poorly. It's what makes the older r12 vehicles worth less than the slightly newer ones.

I think there's some alternative called freeze 12 where it's mostly 134a but with something else to make it work better with the mineral oil and pressures but I'm not sure.

 

[Lawnman23]

Curious, are you sure its carb'ed and not TBI?

As mentioned, get a fresh oil change done, I'd run a 0w-40 like M1 with that many miles on it.

The noise you are hearing is most likely lifter clatter from bleed down and then quieting once pressure has filled them back up. Does it happen at every cold start or just once its sat for a few days? Same noise on hot starts or no?

 

[D60]

Curious, are you sure its carb'ed and not TBI?

As mentioned, get a fresh oil change done, I'd run a 0w-40 like M1 with that many miles on it.

The noise you are hearing is most likely lifter clatter from bleed down and then quieting once pressure has filled them back up. Does it happen at every cold start or just once its sat for a few days? Same noise on hot starts or no?

I wondered, too? I'm not that strong in GMT400s and I know the 3/4t+ often lagged in tech and body style updates, but a carb'd domestic in '93 is embarrassing.

I service an '89 K2500 with 350 that's TBI.

 

[Lawnman23]

I wondered, too? I'm not that strong in GMT400s and I know the 3/4t+ often lagged in tech and body style updates, but a carb'd domestic in '93 is embarrassing.

I service an '89 K2500 with 350 that's TBI.

All the 88-95 GMT400’s were TBI stock. I’m wondering if the OP is mistaking the TBI unit for a carb. Not uncommon, they look externally similar for those unaware.

 

[emmett442]

Likely a piece of debris stuck in a lifter check valve, letting it bleed down after shutdown and pumping up after a few seconds on startup.

It may resolve itself eventually. Else, you'll have to take it out, disassemble, and clean it out.

 

[LDM]

I wondered, too? I'm not that strong in GMT400s and I know the 3/4t+ often lagged in tech and body style updates, but a carb'd domestic in '93 is embarrassing.

I service an '89 K2500 with 350 that's TBI.

93 GMT400 would be TBI from the factory, not carb, even in the 3/4+ trucks. Only way it would be carb is if someone swapped out the TBI for a carb.

 

[Just Dropped In]

My 92 C1500 (5.7 w/161k) developed a startup knock last year, and soon got worse after engine warm-up around 5 or so miles. Oil pressure had been dropping a couple months before this took place. It ended up being main bearing/s. I went into the engine and replaced rod/main bearings & oil pump. My first time with this sort of repair. So far, 10 months later, it's doing fine. I think there's a thread here somewhere.

 

[Lawnman23]

I think there's a thread here somewhere.

Thread ''90s Chevy truck elusive engine knock. Rod/Main bearings, or what?'

Jul 17, 2024

Hi folks. I have a '92 Chevy C1500, 5.7 with 162k miles (owned since new). Been running Shell T5 15w-40 for a few years. A few months back, it developed an engine knock. Oil pressure in the past year has dropped somewhat, around 30ish at 50/60mph, and 10+lbs or so at hot idle. Not a ping or rattle type sound, a sort of deep knock. One shop said it might be the flex plate, but it checked out fine. They said it could be lower engine, but they don't do lower engine repairs.

Here's the things I can rule out causing the knock. All the front engine accessories are new (pow/steering original...

• Just Dropped In
• Replies: 127
• Forum: Mechanical/Maintenance Forum

 

[Just Dropped In]

Thread ''90s Chevy truck elusive engine knock. Rod/Main bearings, or what?'

Jul 17, 2024

Hi folks. I have a '92 Chevy C1500, 5.7 with 162k miles (owned since new). Been running Shell T5 15w-40 for a few years. A few months back, it developed an engine knock. Oil pressure in the past year has dropped somewhat, around 30ish at 50/60mph, and 10+lbs or so at hot idle. Not a ping or rattle type sound, a sort of deep knock. One shop said it might be the flex plate, but it checked out fine. They said it could be lower engine, but they don't do lower engine repairs.

Here's the things I can rule out causing the knock. All the front engine accessories are new (pow/steering original...

• Just Dropped In
• Replies: 127
• Forum: Mechanical/Maintenance Forum

Yeah, that's it!

 

[CCI]

Yeah, that's it!

Yes. The longer I drive this the more I'm thinking rod knock. 35 psi at highway speeds, roughly 10 psi hot idle with 10w-30. Bad knock at startup which goes away in seconds, slight knock at hot idle getting worse steadily.

The rest of the truck is in remarkably good condition so eventually it will probably get a new engine.

In the meanwhile, there really isn't any down-side to heavier oil, is there? If it makes the engine run a little longer before total failure it seems worth doing.

 

[Lawnman23]

5 quart jug of Valvoline MaxLife 10w-40 or 20w-50 is $20 at Wally World. Either will help your oil pressure situation, the 50 more so. The 10w-30 is too light for it at this point.

No, heavier weight won’t hurt anything and will protect the worn out girl as much as possible for the life it has remaining.