SC400 possible Timing belt slip?
04-14-11, 10:23 PM
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SC400 possible Timing belt slip?
After much effort and frustration under the hood & car, I have come up with a theory for why my SC400 has died (that the timing belt has slipped), but I would benefit from another set of eyes, experience, and opinion to confirm or refute my logic, hence I'm posting this.
My 1994 SC400 died (i.e., engine stalled) on the freeway, wouldn't restart afterwards (90K miles, original timing belt). No symptoms at all prior to the stall - just died suddenly. It turns over, but doesn't start. There is a slight hint of a partial "cough" (though very subtle) when turning it over, though nothing even comes close to catching. The cough is so subtle, I'm not entirely positive it is there, but it does sound like there is a small (partial) firing when turning the key. But mostly my first impression from the sound was that it wasn't getting fuel.
Towed it home - checked for fuel and spark - both seem to be present. For those tests, I disconnected fuel line going into fuel pressure regulator on injector bar -- pumping strongly to that point, which I think absolves the fuel pump, filter, pump ECU, and fuel line. Pulled a spark plug wire and arc'd it to the negative battery terminal. Looked good - I think that exonerates most the ignition system components.
I removed the air assembly assembly that runs from the air filter to the intake manifold, and removed the distributor covers, exposing the gears behind the distributors and the timing belt. In there I found several unexpected things that led me to my diagnosis -- please tell me if you agree.
(1) The timing belt appears to have moved from the center of the gears toward the front of the car. The forward edge is actually now partially off the gear (by about 1/2" at the right side distributor) and on top of the non-moving assembly along the side of the gear. The front edge of the belt is frayed. However, when turning over the starter, the belt and gears turn as one would expect, there is no obvious sign of teeth missing in the belt.
Because the belt appears to be on solidly, even though it has obviously moved and frayed a bit, the idea that it would have slipped didn't seem feasible to me for quite a while. I now theorize that the belt rode up on the non-rotating edge, causing it to stretch/lift and allowing the gear to slip a few notches.
BTW, why would a belt precess forward from its ideal position in the center of the gear? Does this indicate a bad bearing or something?
(2) There was a very small amount of oil in the intake manifold.
How does oil get into the intake manifold? I am theorizing that a valve is open at the wrong time, due to a timing belt slip, and somehow this opens up a path for the oil from the cam to the intake manifold?
(3) With the air assembly removed from the intake manifold, I turned the key to starter position to turn it over, and air was blowing out of the intake manifold opening. Normally I would expect there to be a strong suction of air into the intake manifold. What would cause air to blow out in the reverse direction?
Here I am again theorizing that if the timing belt had slipped, the intake valves would be open as the piston was compressing, causing air to blow out the intake manifold.
I don't have a compression tester that can be used on this car, so I'm limited in those tests. However, I invite all of you that I am sure are far more experienced in these sorts of things than I to tell me if you think I am on the right track here.
Thanks,
Lonnie
My 1994 SC400 died (i.e., engine stalled) on the freeway, wouldn't restart afterwards (90K miles, original timing belt). No symptoms at all prior to the stall - just died suddenly. It turns over, but doesn't start. There is a slight hint of a partial "cough" (though very subtle) when turning it over, though nothing even comes close to catching. The cough is so subtle, I'm not entirely positive it is there, but it does sound like there is a small (partial) firing when turning the key. But mostly my first impression from the sound was that it wasn't getting fuel.
Towed it home - checked for fuel and spark - both seem to be present. For those tests, I disconnected fuel line going into fuel pressure regulator on injector bar -- pumping strongly to that point, which I think absolves the fuel pump, filter, pump ECU, and fuel line. Pulled a spark plug wire and arc'd it to the negative battery terminal. Looked good - I think that exonerates most the ignition system components.
I removed the air assembly assembly that runs from the air filter to the intake manifold, and removed the distributor covers, exposing the gears behind the distributors and the timing belt. In there I found several unexpected things that led me to my diagnosis -- please tell me if you agree.
(1) The timing belt appears to have moved from the center of the gears toward the front of the car. The forward edge is actually now partially off the gear (by about 1/2" at the right side distributor) and on top of the non-moving assembly along the side of the gear. The front edge of the belt is frayed. However, when turning over the starter, the belt and gears turn as one would expect, there is no obvious sign of teeth missing in the belt.
Because the belt appears to be on solidly, even though it has obviously moved and frayed a bit, the idea that it would have slipped didn't seem feasible to me for quite a while. I now theorize that the belt rode up on the non-rotating edge, causing it to stretch/lift and allowing the gear to slip a few notches.
BTW, why would a belt precess forward from its ideal position in the center of the gear? Does this indicate a bad bearing or something?
(2) There was a very small amount of oil in the intake manifold.
How does oil get into the intake manifold? I am theorizing that a valve is open at the wrong time, due to a timing belt slip, and somehow this opens up a path for the oil from the cam to the intake manifold?
(3) With the air assembly removed from the intake manifold, I turned the key to starter position to turn it over, and air was blowing out of the intake manifold opening. Normally I would expect there to be a strong suction of air into the intake manifold. What would cause air to blow out in the reverse direction?
Here I am again theorizing that if the timing belt had slipped, the intake valves would be open as the piston was compressing, causing air to blow out the intake manifold.
I don't have a compression tester that can be used on this car, so I'm limited in those tests. However, I invite all of you that I am sure are far more experienced in these sorts of things than I to tell me if you think I am on the right track here.
Thanks,
Lonnie
04-15-11, 11:04 AM
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One additional thing I forgot to mention in my posting was that I read the computer diagnostic codes (connecting pin Te1 to E1) and there were none stored (the engine light just flashed continuously with no pauses when the jumper was connected).
04-15-11, 10:36 PM
#3
Have you been able to remove all the timing covers to check if the timing marks all line up properly?
It is possible the belt jumped a tooth or two, it happens more than one would think. Typically a car that has jump a tooth would still fire up and run, runs bad though.
For the belt to move forward on the gears leads me to believe you may have an idler pulley or tensioner bearing going bad on you, I've seen this happen as well. The bad bearing may have tilted the rotational axis of the pulley causing the belt to shift forward. Or, the bolt that holds either one of the pulleys may have backed out causing the same issue.
It is possible the belt jumped a tooth or two, it happens more than one would think. Typically a car that has jump a tooth would still fire up and run, runs bad though.
For the belt to move forward on the gears leads me to believe you may have an idler pulley or tensioner bearing going bad on you, I've seen this happen as well. The bad bearing may have tilted the rotational axis of the pulley causing the belt to shift forward. Or, the bolt that holds either one of the pulleys may have backed out causing the same issue.
04-16-11, 06:32 PM
#4
Have you been able to remove all the timing covers to check if the timing marks all line up properly?
It is possible the belt jumped a tooth or two, it happens more than one would think. Typically a car that has jump a tooth would still fire up and run, runs bad though.
For the belt to move forward on the gears leads me to believe you may have an idler pulley or tensioner bearing going bad on you, I've seen this happen as well. The bad bearing may have tilted the rotational axis of the pulley causing the belt to shift forward. Or, the bolt that holds either one of the pulleys may have backed out causing the same issue.
It is possible the belt jumped a tooth or two, it happens more than one would think. Typically a car that has jump a tooth would still fire up and run, runs bad though.
For the belt to move forward on the gears leads me to believe you may have an idler pulley or tensioner bearing going bad on you, I've seen this happen as well. The bad bearing may have tilted the rotational axis of the pulley causing the belt to shift forward. Or, the bolt that holds either one of the pulleys may have backed out causing the same issue.
you don't need any super special tools to at least examine the timing belt marks... Just pull the front covers and look at it...
It is normal for the intake manifold to have an oily residue in it - that is oil that has come through the crankcase PCV to be burned in the engine... Even if the timing belt has not slipped, you need to find the cause of it not being aligned, which, could be as simple as the tensioner not keeping it tight...
Stop looking for alternatives, go with your gut, and look at it with your own eyes!
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