Gmc 454

[HobbyMan]

I know it isn't a cycle, but maby someone on this site will have some info. My Dad's 91 GMC 3500 (pre 88 body style) witht a 454 has a long exhaust tick from a leaking exhaust manifold. When it is dark out, you can see blue flames appearing out from the head/manifold gasket. Should I just pull off the manifold and replace the gasket and install new SS bolts, or just put in new bolts without the gaskett. I really need to fix this right because this a a very special truck. It is the very last unit that rolled off the assembly line before the GM workers went on strike. It only has 50k miles on it. The interior still smells like it did 10 yrs ago. Any ideas or help would be appreciated. Thanks guys.

[LoserShoes]

New bolts are unlikely to fix it. Do it right the first time and you only have to do it once.

Steve

[Dragonrider]

Definitely replace the gaskets. Just a smigen more work than the bolts, and the most likely problem. Also use sealer/antisieze on the bolts in installation, torque them in stages (your service manual may even have a pattern) and recheck the torque after running a while.

Also, since it's and older model, check out the exhaust system and make sure all it's brackets etc are okay and not putting undue pressue on the manifold (that can help cause gasket failures).

[HobbyMan]

Thanks for the info. Someone told me that these manifolds warp over time. Is this true?

[Dragonrider]

Generally the cast iron ones will not...but if it's missing bolts or something it could.

You can take them to an automotive machine shop if you suspect that and they can mill the surface flat. It doesn't cost all that much.

[HobbyMan]

thanks again for the info

[LoserShoes]

Yea, cast iron manifolds don't warp, they crack, so you might want to prepare yourself for that possibility too. Can be caused by a cylinder too lean. Good luck.

How's the clearance between the frame and the header you got to work in? Sometimes those big blocks are pretty tight, but since its a pickup it might be ok.

The early big block chevy cars, you had to pull the motor to remove the manifolds and oil pans. Lots of fun there.

Steve

[HobbyMan]

There is plenty of clearance on the manifolds. Thank goodness. When I replaced the timing chain the other year, I had too loosen the oil pan to get the timing cover off. I wanted to pull it completely off to clean the sludge out of it, but as you said, it doesn't come out. Sometimes I just want to put on some glass packs and side dumps. My friend rebuilt a 1971 SS 454 chevelle with all the goodies. When he pulls the dump covers off, it just about knocks you off your feet. And yes, when he punches the throttle, the car alarms start to go off. Well, i guess it is off to get some gasketts. I take it some/most of you have had this problem before?

[Sluggo]

We've had problems before when pulling off the exhaust manifolds that the bolt holes in the manifold refuse to line up again with the bolt holes in the head. After numerous times of drilling the manifold bolt holes larger,we got around the problem by just loosening the bolts enough to facilitate cutting the old gasket out, and on the new gasket, cut out the bottom of the bolt holes so the new gasket will slide into place between the head and manifold....being held in position by the manifold to head bolts. Dragonrider is correct, anti-seize the bolt threads that you can get to and torque to spec's and in order. Also check the exhaust pipes to frame rail space for interference.

[LoserShoes]

Up in this part of the country we don't bother with antisieze, cause by the time we need to do some work on the exhaust the bolts are just a lump of rust anyway.
We burn em off with a cutting torch and and twist em out with vice grips.

Steve

[DiverRay]

Hobbyman,
The 454 does have a BAD reputation for cracking manifolds. Make sure the old manifold isn't cracked BEFORE installing the new gasket.
I grew up on chevy's, and as you notice drive a 1990 old body for a cage.
Ray

[HobbyMan]

Yeah, I have heard from a number of peple now that have had cracked heads. I am going to pull the manifold completely off and check it. It is just a matter of time now. On the bikes, I can work on them without fear of not being able to use them. On the truck, it is always being used. Maby this winter break, I will get a chance to work on it. It has some other problems(fuel/sensor) related. It is getting to be like an XS. It is very old and the 454 puts out a LOT of heat in the summer. All of the wiring is old and brittle. Does anyone know if new maniflolds are still available?

[prometheus578]

"And to be more precise..."

grew up on chevy's, and as you notice drive a 1990 old body for a cage

Ray... you also drive an old body... for a body!

[mro]

Does anyone know if new maniflolds are still available?

If you can afford it, get steel headers. I have a 1991 GMC 3500 van with the 454 in it. Has a cracked exhaust manifold, but not sure that Im gone to keep it with gas prices the way they are so just livin w/it for now.


mro

[trbig]

Let's see.... 50,000 miles at 4 to 5 miles a gallon.... lol.

If you haven't gotten this done yet, I hate to be the bearer of possible bad news, but if it is leaking between the actual head and exhaust manifold, there is enough heat there from the exhaust gasses to actually cut through the metal.... either the exhaust port on the head, or the manifold. I have also heard about these big blocks warping the manifold.
If you do opt for replacing the manifold with headers.. I know this would take away from the stock stuff but it would make it breathe (And sound) better... you might check around. I have also seen people that cut the steel plate on the headers (where it attaches to the head) in between the center cylinders. It lets them expand and contract better in the heat without causing leaks. Good luck.

Tod