454 Engine problems - The 1947 - Present Chevrolet & GMC Truck Message Board Network

JAYBEE · 08-10-2006, 01:39 PM

I have a 490Hp 454 crate motor in my pickup. It is around 4 years old with 10,000 miles on it. All at once with a steady RPM the motor lost 20% of power. A carburator malfunction thinned down the oil with gas. After oil change, carb rebuild, and plugs, The engine is still running very rough. I did a compresion test and cylinders 2,4,6,8,1,3 red around 195psi , cylinders 5,9 had no compresion at all, the needle did not move at all, I added oil to the cylinders, and there was little or no change. The exhaust is smoking on both sides. Appears to be gray. What is the next test I should do to narrow down the bad news?

Longhorn Man · 08-10-2006, 03:14 PM

I'm assuming you meant 5 & 7... not 9.

regardles if it is melted pistons, or broken valves, ir just a head gasket the head is gonna have to come off.

MrC1 · 08-10-2006, 05:48 PM

Not the best news, but I'd pull the head. Doesn't sound like there's anything you can do until it's off anyway.

For giggles, pull the valve cover and watch the rockers when cranking. Do they move normally? Anything broken? Pushrod poked through a rocker? Unlikely at best, but I'm tryin to help here.

MrC1 · 08-10-2006, 05:50 PM

Maybe drain the oil first and see what kind of metal you find. There's gotta be either piston or piston ring in there, right? Even a piston ring that completely cratered would make SOME compression while cranking. Sounds alot like holy (but not in a good way) pistons to me.

Let us know what you find!

dwcsr · 08-10-2006, 08:38 PM

That was my first thought, a bent jammed pushrod hanging the valve open. Any time I've seen a burnt piston it didn't dump all of a sudden, there was some gradual hints that pending doom was coming. of course it could have dumped the piston. that would be fast and 0 PSI on the gage. You have to pull the covers to get at the heads anyway so start there and may be you'll get lucky.

JAYBEE · 08-11-2006, 08:57 AM

Thanks for your help, I removed the valve cover and found a broken posi-lock nut on the number 7 intake rocker arm. (full roller rocker arms, appear to be crane cam brand) The stud was also damaged. After changing the stud, I used another nut from a different rocker. When I tightened the nut down and opened the valve alittle, a pool of oil came poring out the spark plug hole. After spinning the motor over without any plugs in, the oil became a light mist. I redid the compression test and both 5,and 7 came up to 200 psi. Am I out of the woods? The rocker studs seem to short for the full roller rocker. I only have about 1/4 inch engagement with the posi-lock nut. This seems wrong. Can you get longer studs for more engagement? Should this motor have stud girdles? Where can I get just 1 posi-lock nut?

ed455 · 08-11-2006, 09:20 AM

Yes you need longer rocker studs. See ARP's website they have different lengths available.

08-10-2006, 01:39 PM	#1
JAYBEE Registered User Join Date: Oct 2005 Location: FLORIDA Posts: 7	454 Engine problems I have a 490Hp 454 crate motor in my pickup. It is around 4 years old with 10,000 miles on it. All at once with a steady RPM the motor lost 20% of power. A carburator malfunction thinned down the oil with gas. After oil change, carb rebuild, and plugs, The engine is still running very rough. I did a compresion test and cylinders 2,4,6,8,1,3 red around 195psi , cylinders 5,9 had no compresion at all, the needle did not move at all, I added oil to the cylinders, and there was little or no change. The exhaust is smoking on both sides. Appears to be gray. What is the next test I should do to narrow down the bad news?

08-10-2006, 03:14 PM	#2
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Re: 454 Engine problems I'm assuming you meant 5 & 7... not 9. regardles if it is melted pistons, or broken valves, ir just a head gasket the head is gonna have to come off.

08-10-2006, 05:48 PM	#3
MrC1 Dork For Days Join Date: Aug 2005 Location: Dallas Posts: 407	Re: 454 Engine problems Not the best news, but I'd pull the head. Doesn't sound like there's anything you can do until it's off anyway. For giggles, pull the valve cover and watch the rockers when cranking. Do they move normally? Anything broken? Pushrod poked through a rocker? Unlikely at best, but I'm tryin to help here. __________________ 1972 C20, 402/700R4 - My first classic truck, bought in September 2005. 85% transformed from "Farm" to "Cool" status! 1970 Dodge Challenger 440, 4sp (my other toy) 12.67 @ 117 MPH, with a pathetic 2.022 60' 2007 Dodge Charger SRT-8 2008 Cadillac CTS All-American garage, baby!

08-10-2006, 05:50 PM	#4
MrC1 Dork For Days Join Date: Aug 2005 Location: Dallas Posts: 407	Re: 454 Engine problems Maybe drain the oil first and see what kind of metal you find. There's gotta be either piston or piston ring in there, right? Even a piston ring that completely cratered would make SOME compression while cranking. Sounds alot like holy (but not in a good way) pistons to me. Let us know what you find! __________________ 1972 C20, 402/700R4 - My first classic truck, bought in September 2005. 85% transformed from "Farm" to "Cool" status! 1970 Dodge Challenger 440, 4sp (my other toy) 12.67 @ 117 MPH, with a pathetic 2.022 60' 2007 Dodge Charger SRT-8 2008 Cadillac CTS All-American garage, baby! Last edited by MrC1; 08-10-2006 at 05:51 PM.

08-10-2006, 08:38 PM	#5
dwcsr Hollister Road Co. Join Date: Apr 2005 Location: Houston Posts: 6,131	Re: 454 Engine problems That was my first thought, a bent jammed pushrod hanging the valve open. Any time I've seen a burnt piston it didn't dump all of a sudden, there was some gradual hints that pending doom was coming. of course it could have dumped the piston. that would be fast and 0 PSI on the gage. You have to pull the covers to get at the heads anyway so start there and may be you'll get lucky.

08-11-2006, 08:57 AM	#6
JAYBEE Registered User Join Date: Oct 2005 Location: FLORIDA Posts: 7	Re: 454 Engine problems Thanks for your help, I removed the valve cover and found a broken posi-lock nut on the number 7 intake rocker arm. (full roller rocker arms, appear to be crane cam brand) The stud was also damaged. After changing the stud, I used another nut from a different rocker. When I tightened the nut down and opened the valve alittle, a pool of oil came poring out the spark plug hole. After spinning the motor over without any plugs in, the oil became a light mist. I redid the compression test and both 5,and 7 came up to 200 psi. Am I out of the woods? The rocker studs seem to short for the full roller rocker. I only have about 1/4 inch engagement with the posi-lock nut. This seems wrong. Can you get longer studs for more engagement? Should this motor have stud girdles? Where can I get just 1 posi-lock nut?

08-11-2006, 09:20 AM	#7
ed455 Captain Ed Join Date: Jul 2003 Location: Port Charlotte, FL Posts: 1,241	Re: 454 Engine problems Yes you need longer rocker studs. See ARP's website they have different lengths available. __________________ If you ain't bleedin', you ain't workin'! 1999 C3500 LS 7.4L



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