The intake cams are driven by the timing belt. The exhaust cams are gear driven by the intake cams. ON the VVT motors there is a "camshaft timing tube" on the front of each intake cam.
Broken Timing Belt Confirmed
- Thread starter jgray
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The intake cams are driven by the timing belt. The exhaust cams are gear driven by the intake cams. ON the VVT motors there is a "camshaft timing tube" on the front of each intake cam.
Dan, on the VVTi, it only advances the intake cams, correct? Exhaust is static timed? Seems like there were later variants that included exhaust timing adjustment as well as lift.
A picture is worth 1,000 words...
Correct, intake only on 4.7. The 5.7 and 4.6 are dual VVT.
I'm confused as I agree with Hoser's post, so please explain how I'm wrong...
On a typical V-8 engine there are four crankshaft journals which are offset from the crankshaft centerline. Each of those four journals has two connecting rods located on it. Those four journals are offset by 90 degrees. So when the engine is at TDC, two pistons are at the top of their stroke (one piston is on the compression stroke, the other is on the exhaust stroke). If you rotate the crankshaft 90 degrees two other pistons are at the top of their stroke (and so-on every 90 degrees through 360 degrees of rotation).
So unless the engine was within a few degrees of one of the four different 90 degree points, no pistons were near the top of their respective strokes. As such if any (or all) cams were rotated it's possible they could rotate freely without valves hitting pistons.
I agree with OregonLC that one would procedurally rotate the engine to TDC prior to a preventive timing belt replacement, and it would be necessary to do so prior to actually installing a new timing belt in the case of a belt failure, but after a belt breaks all of the required engine dis-assembly (and re-assembly to the point of timing belt/cam timing) could be done regardless of the engines rotational position.
I would also agree that the camshafts would eventually stop at their point of spring pressure equilibrium, which would most likely be will all valves partially open, and none fully open. So one could rotate the crankshaft through a complete revolution and no pistons would contact a valve.
All of the above is my understanding of the way a V-8 is designed. "Real world" I would think when jgray's belt broke at 55 MPH the engine was somewhere around 2,000 RPM. I just can't imagine that the four camshafts would slow their rotation at the same rate as the crankshaft and no damage would occur. That would strike me as a lottery winning luck level.
I think TrunkMonkey is correct that the pre-VVTI engines are non-interference engines.
Again PLEASE correct any (and all) mistakes above...
On a typical V-8 engine there are four crankshaft journals which are offset from the crankshaft centerline. Each of those four journals has two connecting rods located on it. Those four journals are offset by 90 degrees. So when the engine is at TDC, two pistons are at the top of their stroke (one piston is on the compression stroke, the other is on the exhaust stroke). If you rotate the crankshaft 90 degrees two other pistons are at the top of their stroke (and so-on every 90 degrees through 360 degrees of rotation).
So unless the engine was within a few degrees of one of the four different 90 degree points, no pistons were near the top of their respective strokes. As such if any (or all) cams were rotated it's possible they could rotate freely without valves hitting pistons.
I agree with OregonLC that one would procedurally rotate the engine to TDC prior to a preventive timing belt replacement, and it would be necessary to do so prior to actually installing a new timing belt in the case of a belt failure, but after a belt breaks all of the required engine dis-assembly (and re-assembly to the point of timing belt/cam timing) could be done regardless of the engines rotational position.
I would also agree that the camshafts would eventually stop at their point of spring pressure equilibrium, which would most likely be will all valves partially open, and none fully open. So one could rotate the crankshaft through a complete revolution and no pistons would contact a valve.
All of the above is my understanding of the way a V-8 is designed. "Real world" I would think when jgray's belt broke at 55 MPH the engine was somewhere around 2,000 RPM. I just can't imagine that the four camshafts would slow their rotation at the same rate as the crankshaft and no damage would occur. That would strike me as a lottery winning luck level.
I think TrunkMonkey is correct that the pre-VVTI engines are non-interference engines.
Again PLEASE correct any (and all) mistakes above...
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Dan had his crank at TDC to do his timing belt. He took the belt off while the crank was at TDC. He then rotated the cam (sans belt) 360 degrees. One of the four pistons on that side would have been at TDC. This would be the best possible method to intentionally induce contact and none was observed.
A running engine would indeed be a crapshoot of timing, although my suspicion is that the cams would stop way before the pistons do (inertial mass) so some of the valves would be stopped in the path of the still moving pistons of contact were an issue.
So, yes, we're in agreement that an interference engine is not the likely case for the non-VVT 2UZ.
A running engine would indeed be a crapshoot of timing, although my suspicion is that the cams would stop way before the pistons do (inertial mass) so some of the valves would be stopped in the path of the still moving pistons of contact were an issue.
So, yes, we're in agreement that an interference engine is not the likely case for the non-VVT 2UZ.
APKhaos
Unfixing the unfixable
Dan's experiment is the only reliable evidence for non-interference. If others could confirm this by repeating the experiment [Crank @ TDC, no TB, rotate cams through 360 without valve blocking on a piston crown] it would help put this question in the "answered" column.
The OPs experience with breaking a TB on the road has too many variables to be reliable:
- The cams spin down and come to rest due at spring pressure equilibrium, where no valves are fully extended
- The crank spins down at some rate, and completely independent of the cam position
It could have been sheer luck that the variables of cam and crank position never saw a specific piston at top of stroke when its specific valve [either one] was fully opened. Even so, the conventional wisdom is that if cam timing fails on an interference engine its a lead pipe cinch that valves will meet piston crowns. How hard and how many is variable, but its going to happen. The fact that it didn't happen in the OPs case creates more questions than it answers. Dan's report suggests an answer that matches the two cases we are discussing here.
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Muddy Bean
Breaking something or fixing something
Can someone explain to a dumb newbie why he blew both head gaskets when this happened?
APKhaos
Unfixing the unfixable
No head gaskets were blown. His shop pulled the heads to look for valve damage. It would have saved a lot to have done a compression and leak-down test to see if there actually was valve damage, and to look for signs of valve contact on the piston crowns with a $150 video borescope, but that's water under the bridge.
APKhaos
Unfixing the unfixable
My bad - I missed that statement. Can't see how a TB failure would cause both head gaskets to fail, but stranger things have happened. I'd thought they just pulled the heads expecting to see valve damage, but its easy to confirm valve damage without lifting the heads so something else must have been going on with the gaskets.
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The pistons are still in motion while the vehicle coasts to a stop due to the direct connection to the drivetrain. Losing the timing belt meant that the valves (both intake and exhaust) would lose timing with the piston's stroke. You begin to lose power on the highway - what's the instinct? You push the accelerator pedal (more fuel delivery). At some point, you'll have a compression stroke, fuel delivery, ignition, and then re-compression all without lifting an exhaust valve. You fill a cylinder completely with air and fuel, ignite it, and the pressure is what gives you the torque at the axle - quite a bit of it. Now compress that on the next stroke without opening an exhaust valve and you exceed the limits of the head gaskets. They're supposed to blow in that scenario, so you don't sacrifice a connecting rod instead. A pair or head gaskets is MUCH cheaper than losing a connecting rod and poking ventilation holes in the block.
The thing to take away from this is the strength of the forged 2UZ connecting rods in the LC/LX 4.7L engines. The head gaskets are nearly indestructable - unless you lose timing. The fact that the head gaskets are not inherrently weak, yet became the weak point in this scenario is a true testament to the build quality of the components inside this engine.
However, if I were the owner of this truck - knowing what happened inside those cylinders to blow the head gaskets - I'd drop the oil pan and see if I could get a close enough look at the lower end of the rods and the crankshaft and I'd do it pretty quickly before I sank a BUNCH of money into it just to find out that it needs a full tear down. Just because the rods didn't break doesn't mean they didn't bend...
The thing to take away from this is the strength of the forged 2UZ connecting rods in the LC/LX 4.7L engines. The head gaskets are nearly indestructable - unless you lose timing. The fact that the head gaskets are not inherrently weak, yet became the weak point in this scenario is a true testament to the build quality of the components inside this engine.
However, if I were the owner of this truck - knowing what happened inside those cylinders to blow the head gaskets - I'd drop the oil pan and see if I could get a close enough look at the lower end of the rods and the crankshaft and I'd do it pretty quickly before I sank a BUNCH of money into it just to find out that it needs a full tear down. Just because the rods didn't break doesn't mean they didn't bend...
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Plausible, but as soon as the crank angle sensor and cam shaft sensor disagree fuel and ignition events cease. It's not like and old school system which ran only off the crank.
Further, if air is coming in and you're getting a good compression stroke, it's because an intake valve is open. If all 4 are shut, your just making a vacuum chamber.
Lastly, I'm still on the fence as to whether the head gaskets were blown or rather the removal was precautionary. I know jgray indicated they were blown, but I'd like to learn more of how the shop knows this. Compression test? The other confusing item which hasn't been discussed is why the cams were removed prior to pulling the heads. AFAIK, you leave all of the top stuff intact and just pull the heads as a unit.
Further, if air is coming in and you're getting a good compression stroke, it's because an intake valve is open. If all 4 are shut, your just making a vacuum chamber.
Lastly, I'm still on the fence as to whether the head gaskets were blown or rather the removal was precautionary. I know jgray indicated they were blown, but I'd like to learn more of how the shop knows this. Compression test? The other confusing item which hasn't been discussed is why the cams were removed prior to pulling the heads. AFAIK, you leave all of the top stuff intact and just pull the heads as a unit.
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2001LC
SILVER Star
Plausible, even if only one cylinder pulled in fuel then cam stopped and valves stopped in closed position, while crank spinning through a few more revaluation. This could happen and take out a weak spot of head gasket, but is a stretch IMHO.
Unless we get confirmation, I'm leaning towards the word blown was used loosely, and heads were pull based on assumption of interference engine damage.
Unless we get confirmation, I'm leaning towards the word blown was used loosely, and heads were pull based on assumption of interference engine damage.
2001LC
SILVER Star
Thought; mechanic may have pulled heads to measure if all pistons extend to top of cylinders, rather than pull bottom end.
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APKhaos
Unfixing the unfixable
If one cylinder was charged, the compression pressure is orders of magnitude less than firing pressure at ignition which the head gasket handles every day.
Don't think we'll ever know why they pulled the heads and [even stranger] removed both cams from the heads unless they were planning a skim or other machine work on the heads.
Inspection for valve damage or even rod bend can be done with the combo of compression/leakdown test and a visual [which would also look for pistons not returning to the top of the bores as a sign of rod bending] using a borescope.
Lets assume they had a good reason for pulling the heads and cams, but we can't figure out what that good reason was.
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. . . and as long as you knowledgeable types are explaining things to newbies;
If the timing belt is removed, and you rotate one of the camshafts, how do you know how to get things all perfectly exactly lined back up again before the new belt goes on?
The marks will line up.. . . and as long as you knowledgeable types are explaining things to newbies; If the timing belt is removed, and you rotate one of the camshafts, how do you know how to get things all perfectly exactly lined back up again before the new belt goes on?
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Timing marks on crank pulley, cam pulleys and belt.
. . . and as long as you knowledgeable types are explaining things to newbies;
If the timing belt is removed, and you rotate one of the camshafts, how do you know how to get things all perfectly exactly lined back up again before the new belt goes on?
2001LC
SILVER Star
If crank has been turned; the way I've tested to make sure crank back at TDC is with compression gauge on #1 or pull plug from # 1 cylinder and drop in a screw diver then watch rise as come up to crank timing mark while turning crank IIRC.
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