84 460 low compression
Tuned
Joined: May 2003
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From: Southern IL
Shoot some oil down it and then see what it reads. That will tell you about the rings or not. Blown head gasket should be putting water in your oil or oil in your coolant, most of the time. Remove the radiator cap and rev it a couple of times then check smell and for back pressure.
Last edited by Chiefrider007; Nov 22, 2006 at 03:13 AM.
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I was just looking around and I see where you posted about a bent pushrod. Is this the same motor? If it is, I would be concerned with a bent valve. It could have cracked the piston, but if it is the valve, you are running the risk of doing some major damage. Are you sure the air is coming from the block and not making sound through the intake? Either way, it sounds like it is time to pull the head to see what is really wrong.
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Yes it's the same motor, I drove it home from Kansas in this condition (750miles)
Not only could I hear the air but I could also feel it coming out of the breather.
It seems to be running on 4 cylinders, #3,4,7,8 wires can be removed with no effect on the idle.With valve covers removed while cranking engine everything moves like it should. It gets spark, good timing, I'm leaning towards blocked runners in the intake, I dont know how long it's been running without an air filter.
Not only could I hear the air but I could also feel it coming out of the breather.
It seems to be running on 4 cylinders, #3,4,7,8 wires can be removed with no effect on the idle.With valve covers removed while cranking engine everything moves like it should. It gets spark, good timing, I'm leaning towards blocked runners in the intake, I dont know how long it's been running without an air filter.
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From: Niceville, FL.
Sounds like more than one problem to me, the worst of which is the lack of compression on #3. I think you'll have to pull the head (at a minimum) to find out what's wrong. Never heard of "Blocked Runners".
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I have never heard of blocked runners either, but I wonder if the carb is getting fuel only on side or the other. In other words a plugged jet in the carb only allowing fuel in one venturi. While it is running take carb cleaner and spray it over the carb to see if it picks those cylinders up. If it does then you have a fuel problem. If you are getting air out of the breather, then it is a bent intake valve. I am assuming that you pulled the vacuum line to the pcv valve off and the fresh air side of the breather off, to the valve cover, so the leak can not back feed through them. I don't think the breather was left off long enough to cause serious problems since you only have low compression on one cylinder. A lean condition with the carb could have stuck the valve in the guide which would result in a bent valve. A lean condition is what fried two pistons in my 460. It still sounds like you will have to pull the head to see what is going on. Head and intake gaskets are cheap, don't take a chance. These really are a good old truck with a good motor, so don't give up on it. Let us know what you find.
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Previously I installed a carb from my 78 truck (Carter AFB) and it still ran the same. All ports from the carb are plugged except brake booster. If it has a bent intake valve wouldn't I here air from the carb/intake not the crankcase?
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From: lino lakes, mn
ok you've done the leakdown no bubbles in the radiator,no intake noise, no exhaust noise, you've got a leak into the crank case, put oil on the piston compression check it if it helps bad rings if it doesn't bads piston either way your pulling the motor. Fix this first then worry about your miss
Tuned
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From: Southern IL
That's what I said earlier. The oil can seal the rings and compression should go up, if that's the problem. Check the plug(s) for damp fuel residue, a no fire sign. I would also check and make sure the distributor is installed correctly. I've got them in off one notch instead of 180 and actually run so, make sure your timed right. If you have bent valves you should be able to hear it but, if you have dead cylinders it should be shooting raw fuel out of the exhaust pipe(s). Lean or rich will give you black or white smoke and if you have duel exhaust pipes you can hear which side is running rough.
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The only noise I hear sounds similar to an exhast leak at the manifold. If I start the truck cold and idle it about a minute, shut it off and feel the exhaust manifolds, the front sides are warm,(#1,2,5,6) the rest are fairly cold. I'm still convinced there is an obstruction in the intake manifold, something like a nest, dead rodents a drug stash etc. I have no history on this vehicle, the motor may not be original either. Due to the extreme weather here in WI I'm unable to persue this further this week. Thanks for the help
Tuned
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From: Southern IL
You should be able to see a black residue near to the sound if it's leaking. When you get a chance pull a plug or all of them that go with the cylinders that you think aren't running and let us know what they look like. I'm not sure and could check but, I would be willing to bet that some of the back two cylinders use the same ports in the plenum/intake. The back two, 4 and 8, have oil ports that sometimes get clogged on 460's. This is from under usage and the nature of the beast.
The oil and crankcase pressure can be caused by many things to include rings, oil seals on the valves inside the head, clogged ports, breather, oil filler cap, pvc valve, bad valve on that cylinder, head gaskets, internal cracks, and many more.
The oil and crankcase pressure can be caused by many things to include rings, oil seals on the valves inside the head, clogged ports, breather, oil filler cap, pvc valve, bad valve on that cylinder, head gaskets, internal cracks, and many more.
Last edited by Chiefrider007; Dec 4, 2006 at 10:29 PM.
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I pulled the intake last week, no obstructions. However most of the bolts were fairly easy coming out, maybe three where nice and tight. Could a major vacuum leak from the intake cause these cylinders to run so lean they don't even contribute? I'll put it back together this weekend.