What is the horrible knocking exactly? Experts only.

Something out of the ordinary causing extra resistance to a piston moving in a cylinder. And the noise maybe the clutch plate between engine and transaxle.

No expert, but can’t resist.

 

Hydrolock - Wikipedia

TL;DR
Water does not compress very well, and the G3's coolant has a lot of water in it.
If the head gasket stops gasketing, and water gets into the cylinder bore the piston can "slam" into the fluid.
This can (and will) cause many bad things to happen.
Easiest way to diagnose is to look at the oil and see if your oil sump and your radiator are swapping fluids. Often you will see droplets of coolant on the dipstick.
There are those who insist that (among other things) looking at your oil more often solves lotsa problems but that's another topic.

I'm no "expert" but your info says that you have a 2004, which is a G2.
This particular sub-forum discusses G3 issues, which certainly include head gasket issues!
I'm NOT trying to be a forum sheriff but even if your car is "knocking horribly" from a non head gasket issue it bears IMMEDIATE investigation.

It's kinda like healthcare.
If you NEED IT but cannot afford it.....pretty soon you won't need it any more.


So....Pends on what you mean by an "expert."
Seems to me that the experts at Toyota are not very forthcoming about some of the design issues with the G3.
They WERE however "expert" enough to fix the piston rings and the EGR and.....even though they are uglier than an unmoderated political discussion, the G4s seem to be fairly reliable.

I fix phones for a living, and I'm not even very experty at THAT.
I get out over my skis a LOT....but I'm always trying to learn new things.

 

Definitely take it to an authorized dealership to fulfill your request. They’ll be happy to charge you a few hundred dollars for diagnostics.

 

So you now have a Gen3. You've already replaced the head gasket?
It's not uncommon for a failed head gasket to lead to a bent connecting rod/broken piston. All it takes is for that gasket to allow enough fluid to seep into a cylinder to make it hydro-lock during a startup. Did you check to see if you had a bent connecting rod? Usually all you need to do is place a straight edge across the top surface of the engine block and measure the gap between the straight edge and each piston when at full stroke. Makes a person sad when one piston is a mm or 2 lower than the others.

Also, don't forget to fix the obvious first. I wouldn't get too crazy with anything until I fixed the injector.

 

Some important things to keep in mind are what it's not.

The horrible knocking is not limited to cars with EGR. I had it in a 2001 Prius, Gen 1, which has no EGR.

The horrible knocking is not limited to cars with failing head gaskets. There was no sign of HG failure in the Gen 1 where I had the horrible knocking. There was no compression problem and no coolant being drawn into the cylinders.

The reason it sounds like it could be coming from the transmission, or from the engine/transmission interface, is because the transmission has an important role in creating the sound. Any car engine can misfire, but in conventional cars it doesn't sound as horrible. In a Prius, a misfiring engine has its unpredictable, changing crankshaft speed driving into an always-meshed transmission containing heavy motors being driven by a computer that is trying its best to synchronize them to the crankshaft speed.

You can read more about it on the fourth and fifth pages of this article (look for the heading "why the knock"). That article was also about a Gen 1, by the way, which has no EGR and did not have any head gasket problem. It simply had a misfiring engine, and in that particular car the misfire was caused by one bad spark plug.

Because the knocking was so horrible, that car's owner had let it go for scrap price before anybody diagnosed the misfire and found the bad spark plug.

If the knocking is prolonged, it can break the coil springs in the shock damper between the engine and the transmission. You can then reach through the hand hole into the bottom of the bell housing and find pieces of springs. Losing the springs makes the knocking even worse.

As another poster put it in another very recent thread, the cause is an engine misfire. Lots of things can be the cause of engine misfires. A failing head gasket can certainly cause engine misfires, but there are also many reasons for engine misfires that are not failing head gaskets.

 

exactly

 

The source of the knocking sound is the transmission input damper. The cause for that is usually engine misfire. The damper is a disc that bolts to the engine flywheel. It has a spring loaded hub that is splined for the transmission input shaft. Its job is to "smooth out" the torque pulses produced by each combustion event in the ICE. Those springs momentarily compress then relax twice per engine revolution. This is needed because the motors in the transmission can't work correctly when subject to irregular shock forces.

The motors have permanent magnet (PM) rotors (the part that spins) and stators (the "3 part" electromagnet that is bolted to the case). The computer (ecu) turns each of the 3 windings on (positive), off, or on (negative) in a dance to make the rotor turn. The rotor has a position sensor to tell the ecu where the magnets are in relation to the stator, because the TIMING of the stator control to the rotor is critical to make the motor move at all. Motors like smooth/ constant rotational force.

Now when there is a misfire, one of the torque pulses to the damper is missing. This allows the damper springs to unload more than normal, the next combustion event hits "harder" because you have now have more rotational momentum against the springs and the damper hits its travel limiter. That irregular torque shock transfers through the geartrain to the motors, causing irregular signals from the position sensors, and irregular control of the stator windings. This "spastic" operation makes the back and forth loading on the damper worse.

Posted via the PriusChat mobile app.

 

I read the article and I can't believe that they had all that gear, and they didn't really test the ignition until AFTER they had pulled the injectors? Sheesh.

Posted via the PriusChat mobile app.

 

Did you notice that was explained on the first page?

Because of the impossibility of recording primary ignition (as it's generated inside the coil-on-plug unit), and even the difficulty of getting a good secondary pickup because the coils are so deeply potted (I've tried even with purpose-built COP probes and a good scope, there is some signal there but boy it's faint), they initially went by the ignition-feedback signal generated by the coils, and trusted that. As one might.

Then they did the "having eliminated everything else, whatever remains, however improbable" business, and went back in case the IGF signals were lying to them, and wrapped wires around the COP igniters as a way to get usable secondary signals, and that was when they found the answer.

 

I did see that- if there was a problem that showed up on the IGF signal then it should have set a P035x code (assuming that it supported that code on that model). Coil on plug designs like Toyota's have been around for many years- yes you can't access the primary waveform, and yes, a getting a secondary reading might not work easily either (curse you Honda!!). However, pull the coil out and use an "extension" spark plug wire to connect the COP to the plug and you can use a conventional capacitive pickup. Depending on the failure mode, a fault can sometimes be seen in the COP amperage waveform.

Forgetting all of that, I'd probably test output capability of all the coils and throw a set of plugs at it before going too deep into heavy diagnostics. Just my personal approach- I prefer to find a "smoking gun" if I can, but will settle for a "TKO" if it fixes the car. Glad they figured it out.

 

The especially annoying thing about some Prius models is you can't get the coils out to use an 'extension' wire without fussing with the whole windshield wiper linkage and the cowl. You probably can lift them enough to wrap some wire around them the way the AECS guys did, though.

 

Now you got me thinking. I wonder what I can see on a scope if I use an INDUCTIVE (magnetic) pickup on the COP. Will have to try that sometime.

 

Did you get the cylinder head machined when you replaced the head gasket? Not sure if it’s true but I’ve been told that those head gaskets are pretty picky and it is best to get the head machined when replacing the gasket. If you did not and the head was not in perfect shape, then your new head gasket may still be leaking. I’m also not an expert, but I did just do mine, and I did have it machined and abs resurfaced for this reason.

 

I would get the head “inspected” and cleaned by a machine shop. Machining (planing the underside) may not be necessary. They will set a very accurate straight edge on the machined face, check what thickness of feeler gauge can slip under, at various points and directions, and only machine if it exceeds tolerance limits.