Broken AAZ Timing Belt: Causes & Repair

Yesterday, while driving on the highway at approximately 2,000 RPM, the timing belt on my AAZ (Golf 3 Variant TD, manufactured in 1998) suddenly snapped with a loud noise, causing the timing cover to partially burst.
Unfortunately, because I didn't think about the timing belt, I didn't immediately put the car in neutral, but instead let it run for about 200 meters under its own power, and then made another very short attempt to start it while it was stationary; in both cases, there were no unusual noises, except that it obviously didn't start and the oil pressure warning light came on.

The engine was in very good condition (220,000 km, excellent performance), and the timing belt change was due after 100,000 km (I was just waiting for a water pump to be delivered). The car was kept in a garage by a retiree and has virtually no rust.
What should I expect? At least two valves are bent and the hydraulic lifters are likely damaged. In addition to the new belt and water pump, the necessary work probably includes:
- Head down.
- Valves, valve stem seals, valve guides, and lifters (hydros) replaced.
- Grind in new valves, install seals and guides.
What else should I expect?
- Piston ring stuck on one or more pistons due to piston seizure? Therefore, check the connecting rod bearings, lift the pistons, and inspect the piston ring grooves, and rework them if necessary?
- Piston seized with a risk of later piston rod breakage or bearing damage?
- Is the camshaft bent/damaged?
- Any other risks?

Does an experienced mechanic know the possible and likely variations, and what to watch out for?
What would be the best options? A used engine (if I can find one that isn't completely worn out), a fully rebuilt cylinder head with or without a camshaft, or replacing the valves, valve guides, and seals, along with a new head gasket and hoping for the best?

Pistons are less likely, but you can never completely rule it out...

The best approach is to remove the cylinder head and inspect the damage. If the bearing shells on the camshaft are still in good condition (it has happened before that these have been broken due to the timing belt failure, or the camshaft has broken into multiple pieces), I would also consider reusing the cylinder head (but check for cracks in the combustion chambers).

Often, in the affected cylinders, the valve stems are "stuck" to the hydraulic lifters (hydros), meaning they have literally sunk into the tappets of the hydros.
"Use a welding machine to attach a larger washer to the top of the component. Then, use pliers or a puller to remove the component."

While the cylinder pressure readings can indicate which valve *might* be damaged, it's necessary to disassemble all the valves and check them for wear and proper sealing.
The valve guides may also be damaged.

I would remove the cylinder head so you can see how severe the markings are on the pistons. If necessary, you can measure the piston protrusion of all four pistons and compare them. Usually, the lower engine components survive this, though.

Then find a used cylinder head, clean it, re-grind the valves, install new gaskets, and put it back together.

Thank you for the tips - time to get started!
Actually, it wasn't a tear in the belt, but it came off to the side, damaged the timing cover, and then detached when it was worn out. All of this happened within 1000 km and a few days. I had just done an oil change and a visual inspection of the timing belt a week ago before a long-distance drive at 170 km/h, and everything seemed fine then. I didn't take the grinding noise seriously for the three days that followed, thinking it was just something else. Bad luck.

Hello!

Take a look at the timing rotor on the fuel injection pump.
When the storage is not properly aligned, things like this can happen.

Hi.

Have you checked the crankshaft bolt on the timing belt pulley?

/viewtopic.php?t=2564&highlight=aaz+crankshaft-screw

Best regards, Rainer.

Looking at the tensioner, I notice that the front edge is missing.
Is it made of plastic? What does the bearing look like? Also, a worn-out tensioner bearing can also cause the belt to "slip".

I've already experienced something similar in an Audi 80 B4 with a 75 hp diesel engine. The undercarriage survived.
"It turned out that only the hydraulic lifters were worn. After disassembling the cylinder head and inspecting it, the valves were still okay. The engine continued to run for another 6 years without any problems for the owner."

With the older TD engines that have a fixed tensioner pulley, I always replaced the timing belt kit after 80,000 km. These belts are heavily stressed when the engine is hot, because the fixed pulley doesn't compensate for the change in distance between the crankshaft and camshaft. Therefore, the belt is "looser" when the engine is cold and much tighter when it's at operating temperature.

Good luck.
Regards, Mpire.

Hi!

Yes, that's right, the edge on the tensioner roller is missing. I guess the roller is made of plastic.
I experienced that kind of misery over 20 years ago with my Golf 1 diesel.
In that case, the ESP wheel came into play, and the metal tensioner, with its lateral guidance, scraped off the ZR (presumably referring to a protective layer or coating).

Later, I once installed cheap parts during a timing belt replacement, and there was a series where the plastic tensioner pulleys would break. It looked similar to that.
It was a 1.9 TDI engine in a Caddy. After some back-and-forth, the manufacturer covered the damage and paid for a new cylinder head.
The car continued to run smoothly for many years after that.

It's probably not due to the few kilometers above the interval; spark plugs usually last much longer.
There must be another reason.

Thanks for the many tips. I'll check the crankshaft bolt, although I don't think the old "killer" bolt was used in a 1998 AAZ engine. I'll also check the ESP bearings.
It's being fitted with a Viton sealing kit due to the potential use of a PÖL (a classic swirl chamber engine with a mechanical turbocharger).

I'm focusing on the tensioner pulley. The previous owner obviously had an incompetent mechanic do the timing belt replacement. The four Allen screws on the vibration damper are stripped, so I'll need to use a chisel. It's not every day you encounter something like this.
The day after I bought it, I adjusted the timing belt tension. The belt was very loose, and the semi-automatic tensioner was tightened far beyond the mark.
Thankfully, he didn't jump over the teeth during the transport (500 km on the highway).
Most likely, the cheapest possible tensioner pulley was installed, and it may have become misaligned over time, causing the belt to slip off.
Fortunately, I managed to get a brand new cylinder head at a very good price, complete with camshaft. Hopefully, that will solve the problem. I'll let you know how it goes.
BTW, since the head is already off, I might as well change the connecting rod bearings. Is that a major job?

Quote:

Could I perhaps also change the connecting rod bearings? Is that a bigger job?

It always depends on how you can access the vehicle, specifically whether there is a lot or a little in the way when removing the oil pan.
There is no counter-wave module for the AAZ engine.

Replacing the connecting rod bearings does not require removing the cylinder head. As Rainer already mentioned, you only need to remove the oil pan for this. It should be quite simple on an AAZ engine.

LaurenzBo wrote:

BTW - since the head is already off, I might also be able to change the connecting rod bearings. Is that a major job?

Unlike the longitudinally mounted engines (found in various Audis, for example), the one in the Golf, which has a transversely mounted engine, is actually quite simple.

It fits almost all oil pan screws, except for the notorious problem screws on the flywheel (there may be recesses for mounting on the flywheel).

P.S.: The connecting rod bearings weren't a problem, but getting the oil pan off was difficult. The rubber gasket was firmly vulcanized on both sides, so nothing worked. Eventually, I had to cut the oil pan in half and painstakingly clean the engine block with acetone and a scraper.
Replacing the connecting rod bearings was very straightforward; there's good access. Since the cylinder head was already removed, I also installed new piston rings in the engine, which wasn't a problem either (using a piston ring compressor for installation).
Important: The repair manual stated that connecting rod bolts could be reused once, with a mark indicating their single use. However, I obtained the original workshop manual for the engine, and it clearly states: "Only use new ones!" And also, use a good quality torque wrench.

The engine has been running perfectly for 100,000 km since the overhaul. With an oil cooler, intercooler, and a new Chinese turbocharger, it's like new. It was worth it.