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Audi A4, Engine
Oct 15, 2003 (6:06 am)
"Hydro-locking can't bend valves, at least I dont think so unless someone can explain how"
On some interference engines the piston's have fly cuts and the MARGIN is only 20-30 degrees of timing [2 teeth on cam gear]rotation, the extreme pressure of hydro [gasoline or water]locking can cause the chain or belt to slip/jump enough so after/during/just before the rod bends the valves may hit the piston. 99% of the time bent valves mean timing slips.
We see lots of timing belts installed wrong [by independents or brothers in law] where timing is off 10 degrees [1 tooth].....whether they slipped after or just didn't do it correctly?]......the safety margin gets diminished.
Double whammy.........seen this on a few early Q45 [90-93] but the mileage was above 100k and the guides and tensioners were not in good shape.
I have a collection of bent rods from owners thinking their cars were boats [many with aftermarket cold air cone intake filters] and a few from independent shops not understanding to release the fuel rail pressure before changing a sidefeed injector.
55 cc of space can't accomidiate 56 cc [2 ounces of fluids]of non compressible fluids.
Oct 15, 2003 (8:14 am)
So, the engine hydrolocked, the timing belt slipped and then the valves were damaged after that. That sounds possible. But how did that engine get hydrolocked in the first place? Over-revving maybe. If that happened, is there a good chance the connecting rods are damaged/weakened? That makes me real nervous.
Also, maybe there was no hydrolock at all. The timing belt could of slipped during revving, the valves bent and then the cylinder filled with fuel afterward. That seems possible too.
Oct 15, 2003 (9:05 am)
Well the engine can't hydrolock while it is running unless you run through a creek or unless it has filled up with fuel or water overnight. You need a large volume of liquid to hydro-lock.
So did you run through a creek, or did you turn the key in the morning and hear a CLUNK and that was it? If neither of those scenarios, you didn't hydrolock, seems to me.
If your engine truly "hydro-locked", that is, was stopped dead by liquid in the cylinders, then the rods are almost definitely bent.
RE: valves bent--it still doesn't make sense.
If the engine truly "hydro-locks" the pistons aren't moving. They can't hit the valves even if the timing belt slips from the sudden seizure of the engine.
But I understand that the hydrolocking of an engine is a dramatic and unpredictable event, so maybe in a rare case the pistons could travel enough to just catch the valves if the timing belt slips a fraction of a second before seizure. It seems like a string of unlikely co-incidences, but stranger things have happened.
You'd think, though, that the slippage of the belt would relieve the pressure in the cylinders.
So how about this? Engine starts to hydrolock--the sudden deceleration of the pistons causes the timing belt to slip, the pressure is relieved on the pistons so the rods DON'T bend, engine doesn't seize, but valves are smacked by runaway pistons?
But if the engine came into the shop locked tight, then the rods almost have to be bent, otherwise there is no explanation for the seizure other than oil starvation---which means a totally lunched engine.
#37 of 43 How could it overrev?
Oct 15, 2003 (1:44 pm)
Most modern car have RPM limiters that kick in at redline don't they?
Oct 15, 2003 (4:49 pm)
Yep but not on downshifting...that's when most over-rev damage happens anyway. You accidentally slam the gearbox into 2nd gear at 75 mph and on most cars that's it for the engine if you don't catch it in time, or compression brake into a tree.
#39 of 43 RPM limiters
Oct 16, 2003 (9:36 am)
I agree that there are RPM limiters when upshifting, but with the Tiptronic transmission, there are also limiters when downshifting as well. When in Tip mode, I cannot downshift if it would take the RPM beyond the redline. I would have to slow the RPM down in the current gear before it would allow the downshift.
I don't know if shibs100 has a manual or Tiptronic transmission. I didn't see anything mentioned in this thread.
Oct 21, 2003 (1:42 pm)
I have a manual. When I heard the clunk i was in neutral. I just started the car , revved it, and "bang" it died.
The dealer called me again. They fixed the engine but when they put it together they found the a/c compressor was locked up. It won't turn. I need a new one. $1200. Could that have been the problem all along. I'm getting really frustrated. This is gonna cost me over 5K now. I don't know if i trust the dealership. Does what they are telling me and what happened to my car make sense?
Oct 21, 2003 (3:12 pm)
with the a/c compressor locked, you will spend a lot of HP burning up the serpentine belt, but that won't stop the car cold.
it might keep you from starting, because the starter motor is not as powerful as the one that burns gas.
but the a/c won't hydrolock you or spin the rod bearings, the only link is through the rubber drive belt.
Oct 22, 2003 (2:22 pm)
That the A/C compressor just happened to sieze at the same time the engine blew...
#43 of 43 if the engine kicked over hard against the mounts, it might have
Oct 22, 2003 (4:30 pm)
let all the smoke out of the a/c pipes, and we all know if the magic smoke goes away, stuff breaks. more to the point, if the charge is lost, the oil goes with it, and that is going to doom the compressor real fast.
it would be even more peculiar if the left taillight bulb also went dark at the same time. (cue spooky theremin music.)
but it's not specifically ruled out by Murphy's Law of Not Directly Connected Stuff